non destructive testing for reactor core components and pressure ...

IAEA-145

DESTRUCTIVE TESTING

REACTOR COMPONENTSPRESSURE VESSELS

REPORT OF A PAN ELSPONSORED BY THE

INTERNATIONAL ATOMIC ENERGY AGENCYAND H ELD IN VIENNA

29 NOVEMBER-3 DECEMBER 1971

A TECHNICAL REPORT PUBLISHED BY THEINTERNATIONAL ATOMIC ENERGY AGENCY, VIENNA, 1972

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TABLE OP

Abstract

I.II.III.IV»V.VI.

IFFRO.DUCTIOÏÎ

STATE~OF~ART SURVEYCONCLUSION'STECHNICAL OBSERVATIONS

PANEL RECOMMEÍTOATIOÍTSPRESENTE!» PAPERS

59

*84346

PL/477-1

PL/477-2

PL/477-3

PL/477-4

PL/477-5

PL/477-6

PL/477-7

PL/477-8

PL/477-9

Techniques at the CJJEEf Hot Laboratories" 49by T. Boszoni, Italy"An Inspection System for Use Prior to and During 69Service for the Pressure Vessel in the Atuoha Reactor1*by Juan ÎSF. Baez, Argentina"Use of Nondestructive Testing in the USA During 79Fabrication and Surveillance of Pressure Tubes*1by Mr. McClung, USA"Neutron Radiography, An Attractive Method for the 87,HOT of Irradiated Fuel Specimen"by W«J. Stara and M. van der Kleij, letherlands"Post Irradiation Inspection of Fuel Elements 93(Existing Methods and Equipment Used in the StudsvikHot Cells)» L. Jansu»,Sweden"Base Line and In-service Inspection of the Steel 107Pressure Vessels in Sweden"Sweden, Tefcniska Rontgencentralen AB"Periodic Inspection of Oskarshamnsverket Reactor 123Vessel"by G. Ahleberg et al, Sweden"Hondestructivs Testing of Irradiated Fuel Elements 155in the USA"by Mr. McClung, USA"Ultrasonic In-service Inspection of Reactor Pressure 169Vessels"by H.J. Meyer, FRG

Page

PL/477-10 "SDT Facilities Applied to Zr Base Alloy Can 183Fabrication"by L. Ljungberg and J. Wiklund, Sweden

PL/477-11 "The Non-destructive Testing of Fuel Elements and 193Their Components as Applied by the BelgianManufacturing Industries . Review of Methods"by R. de Knock, N» Mostin, J. Gérard, Belgium

PL/477-12 "Non-destructive Testing Procedures During 203Fabrication of Steel Pressure Vessels"by R. Trumpfheller, FRG

PL/477-13 "Contrôle des Gaines en Acier Inoxydable et Alliages 225de Zirconium pour les Eléments Combustiblesby A.C. Prot, Prance

PL/477-14 "Chaîne de Contrôle Non Destructif de Tubes Minces" 243by G. Boulanger, JP.Dufayet,A.SamoeltB. Spriet,A.SlosselFrance ("One Line Non Destructive Testing of Thin Walled Tubes

PL/477-15 "Contrôle par Ultrasons des Tubes de Force du 271Reacteur SL 4" par A. Prot("EL-4 Pressure Tubes Ultrasonic Inspection"by A. Prot)t France

PL/477-16 "Some Aspects on Radiographie and Ultrasonic 275Inspection of Butt Welds in Steel Reactor PressureVessels"by S. Daim, J. Osterberg, Sweden

PL/477-17 "Non-destructive Testing of Coated Particle Fuels" 293by J. Holliday, M.S.T. Price, OgCB

PL/477-18 "Méthodes de contrôle non Destructif Durant la 349Fabrication et 1*Inspection en Service des Caissonsen Béton» de Leur Peau d'Acier et des EchangeursIntègres" par R. Roche, France

PL/477-19 "Trino Nuclear Power Station In-service Monitoring 357of Reactor Internals"by M. Calcagno, F. Cioli, A. Gadola, G. Possa,G. Varoli, Italy

PL/477-20 "Hbn~destructive Testings for the Reactor Components" 333by Y. Ando, Japan

PL/477-21 "Non-destructive Testing in the Fabrication of 411Stainless Steel Cans for Nuclear Fuel Pins"by C.A. Mann, U.K.

PL/477-22 "Technique for Inspection of Li$rfc~Water Reactor 417Pressure Vessels"by B. Watkins, U.K.

PL/477-23 "3ne Post-Irradiation Inspection of Fuel Elements 419in the UK"by V.tf, Eldred, UK

PL/477-24 "On Ban-destructive Examination of Nuclear Fuel 427Pellets in the USA"by R.W. MoClung, USA

PL/477-25 "Techniques de Controle non Destructif au Cours 433de la Fabrication des Enclintes sous Pression enAcier* par fi. Roche, ("ÏTDT Procedures*DuringFabrication of Steel Pressure Vessels" by R. Roche)Prance

PL/477-26 "Examens non destructifs des Eléments Combustibles " 475Irradies'* par M. Watteau ("Fuel Elements PostIrradiation Inspection" by M. Watteau) Prance

PL/477-27 "Contrôles ïïon Destructifs Mis En Oeuvra Lors 487de la Fabrication des Elements Combustibles'*"by M. Watteaut Prance

PL/477-28 '"Phe Inspection of SGflWR Pressure Tubes" 493by R.B. Cockaday, UK

PL/477-29 "Opération Surveillance of lîuclear Power Plant 501Components by Vibration and Noise Analysis"by J.W. Ehrentreich, Euratom

PL/477-30 "A îîote on Acoustic Htatission Measurements at 5^7REML"by F. Kiî->by and P.G. Bent ley, UK

PL/477-31 " "îîie Prevention of Fracture Initiation in 525Reactor Structural Materials"by R.W. SFichols and A. Cowanf UK

PL/477-32 "On Jfondestructive Testing in the USA of Pre- 527stressed Concrete Pressure Vessels for lîuclearReactors"by R.W. McClung, USA

AÎDSïSX A. List of Items Proposed for DiscussionAAUïEX B. Meeting Arrangements and Time Table 542AÏÏKBX C. List of Participants and Observers 544

IAEA PANEL OUT "HST FOR HEAOTOR CORS COMPONENTS AITD PRESSURE VESSELS"

ABSTRACT

1. A Panel on ÏTondestmotive Testing for Reactor Core Components and PressureVessels was held in Vienna between November 29 - December 3 1971» i'ts scope andoutline agenda having been laid down at a meeting of Consultants held at Harwellt(UK)on 22 - 23 March 1971.

2. Une Panel was attended by 36 delegates made up of nominated representativesand observers from 12 countries, OECD and CBC (see appendix C). 'She Panel Chairmanwas Mr. R. Sharpe (UK).

3. îîie object of the Panel was to prepare a critical report and make recommendationson nondestructive testing in the nuclear power contert, since this is a specializedbranch of reactor technology in which there is évidence of growing interest andconcern at all stages from reactor core and containment fabrication to in—servicemaintenance and inspection.

3.1 ïfee detailed scope of the Panel was to:a) Survey present practice and identify areas of strength and

weakness in the technology.b) Highlight any evidence of convergence towards common test

procedures and comment on points of divergence.c) Specify needs for international or regional educational

and training programmes.d) Define programmes of underlying or support work suitable for

coordinated international research.e) Indicate methods of improving international information exchange.

4. In order to rationalize presentation and discussion of papers, the subjectmatter of the Panel Proceedings was divided into 11 areas each devoted to aseparate aspect of the subject. Por some sessions coordinated papers were presented

summarizing parallel work in different countries and in other sessions anational delegate was invited to review the individual contributions that hadbeen submitted which were relevant to that session.

4.1 The areas separately reviewed (with a cross-reference to the relevant

papers) were :1) Coated fuel particle evaluation (paper no. 17)2) Fuel oellet production (papers 24 & 20(para. 2.2) )3) Stainless steel can fabrication (papers 13, 21 à 20(para. 2.1) )4) Zirconium-based can fabrication (papers 10, 14 & 20(para. 2.1) )5) Assemblies manufacture (grids, sleeves, soacers, etc.)

(oapers 11, 2? & 20(para. 2.3) )6) Post irradiation inspection of fuel elements and assemblies

(caoers lt 4, 5, 8, 23, 26 & 20(oara. 2.4) )7) NDT procedures during fabrication of steel pressure vessels

(papers 12, I6t ?5, 30, 31 & 20(paras. 3.1 & 3.2) )8) Base-line and in-service inspection of steel pressure vessels

(papers 2, 6, 7, 9, 22 £ 20 (para. 3.3) )9) HOT techniques during fabrication and in-service inspection of

pressure tubes (papers 3» 15 & 28)10) WDT techniques during fabrication and in-service inspection of concrete

vessels, steel liners and incorporated heat exchangers (papers 13 & 32)11) Reactor internals (papers 19, 29 & 2Q(para»4) )

5. 32 papers were formally presented to the Panel and discussed in the PanelMeeting in this way. Separate Working Groups to report on the areas of(l) Fuel Elements (items 1-6) and (il) Pressure Containment and Reactor Internals(items 7-11) were convened respectively by Mr. R. McClung (USA) and Mr, A. de Sterke(Netherlands).

Tîie reports of these two Working Groups were reviewed, modified and acceptedby the Panel. Their common observations, conclusions and recommendations areoutlined in the following sections.

6.Arising from the discussions, the following needs were emphasized:

a) Better discrimination between the dj fferent types ofdiscontinuities *

b) Better understanding of the significance of the variousdetected discontinuities and their probable effect on theservice life of components and structures.

c) Intensified investigations into the application of someof the newer HUT technologies.

d) More quantative results on data evaluation from ïfDT andbetter agreement on terminology and definitions in this field.

e) Harmonization of codes and specifications together withdetailed writings on the applied procedures and programmes.

f ) Improved communication among those involved at the variousstages of fabrication and operation.

7» Out line ._of JPs-iel .Conclusionsa) The state of art of ÎTDT is satisfactory in the area of metrology

of coated fuel particles, but further studies are necessary toevaluate a satisfactory method for measuring the voidage or porosityof fuel kernels.

b) Automatic measuring techniques are generally available for economicaland reproducible measurement of the desired properties of productionfuel pe31ets.

o) ÍíDT used during the fabrication of ceramic fuel and metallic cladelements are satisfactory although HTR or other new fuel elements callfor the development of new tests.

d) It is highly recommendable to develop further and use mechanizedscanning systems for testing steel pressure vessels.

e) It is considered worthwhile to apply acoustic emission techniqueswhenever posssible at the stage of proof testing of vessels,

f) TTDT techniques used to date are satisfactory for fabrication controland in-service surveillance of pressure tubes though some developmentwould be necessary to detect early cracks.

8. Ctotline of Panel ReoommendationsIn view of enhancing the exchange and dissemination of information on the

available nondestructive technology, it was proposed to:a) Establish a permanent international working group.

.$(.b) Sponsor educational programmes in order to accelerate the spread

of this particular technology.c) Encourage coordinated research work on problems of common international

interest »d) Sponsor symposia related to specific nuclear KÜT problems at about

3 year intervals.e) Publish the Proceedings of this Panel.

Training courses, seminars, study groups.

IAEA PANEL 01 *TOT FOR MâCTOR GORE QOIffiOlCTTS AID PRSSSUKS VESSELS"

Ï.

1. Th.e nuclear context, with the associated high demands placed on performanceand component integrity by operational safety requirements and codes, has alwaysbeen one of active progress in the development and application of WDT techniques.The Agency has already recognised the significance oi" this specialized area ofnuclear technology since it encouraged international exchange of information bymeans of a Symposium held on the subject in Bucarest in 1965*

2. With the expansion in siae and importance of nuclear power programmes, ithas "become obvious that the complexity of reactor systems, theiroverall costs and consequences of service failures and malfunctions are becoming sogreat that it is imperative that more effective nondestructive testing "be developedand xxsed, preferably with the "cradle-to-grave" concept of continuous monitoringand surveillance of quality and integrity,

3. Testing should be included an the early stages of design so that appropriateUBT techniques can be applied to successive stages of the fabrication processwhere defects are most likely to be introduced and where there is a bigger economicincentive to detect them. Inspection departments would also then be in a betterposition to perform their function throx^-* the various manufacturing departments inorder to guarantee the required quality of the component in its finished form.

4. However, the correct interpretation of the significance of defects, in termsof the probability of in-service failure, requires considerable skill; this isbecause many of the primary properties {such as embrittlement), which ultimatelyinfluence service performance cannot be monitored directly so that one has to findsome relationship between these properties and those structtiral defects which canbe monitored either during fabrication or during service.

5- Síondestruetive testing is a rather specialized technology embracingengineering, materials science and physics, and formal university-leveltraining for the personnel involved is rare. Accordingly, the level ofcompetence that has been achieved is the result of many years of activeparticipation and dedicated practice in developing and evaluating the variousmethods.

6. There is some evidence that the level of education and scientifictraining of WDT engineers is often insufficient for the demands of inter-pretation and evaluation nlaced on them and that NOT codes of practice are ofteninadequate or leave too much to individual interpretation. 'ÏMs is particularlytrue when facing newer problems of defect characterization, residual stressand fatigue detection, or bond strength evaluation.

7. Encouragingly} new concepts, techniques and equipment are being developedand evaluated in this field of testing^ although in most instances they arenot well enough known or industrially utilized to replace the well establishedmethods ordinarily used in conventional manufacturing plants and which havebeen modified by practical experience for the more demanding requirements ofreactor components testing.

8. It was the hope of the Agency that in convening this Panel with thefollowing terms of reference

a) Survey present practice and identify areas of strength andweakness in the technology

b) Highlight any evidence of convergence towards common testprocedures and comment on points of divergence

c) Specify needs for international or regional educationaland training programmes

d) Define programes of underlying or support work suitablefor coordinated international research

e) Indicate methods of improving international information exchangethe papers presented and the discussions on the subject would provide thelatest picture regarding both the current and the more advanced 3EÏOT techniquesfor nuclear oore components and pressure vessels.

9. Interest in these fields was evidenced lay the large participation andnumber of papers submitted; attendees from 12 countries and two internationalorganizations totalled 36 participants and observers submitting 32 individualpapers, three coordinated papers and four report papers,

10. However, among the Member States there is a broader interest in nuclearbased ÎTDT technology. Fowadays, many of the NDT specialised laboratorieswithin nuclear research establishments move to extend their interests intonon-nuclear activities, realising that there is a wider market both for themore highly developed equipment and techniques and for specialized experienceand technical competence of the staff concerned. Already, the IAEA isresponsible for developing a major Î3TDT project in Argentina and is takingpreliminary action for a similar Par-East region project based on Singapore.

11. Accordingly, this change of emphasis was noted by the Panelsince it could point the way to a developing structure on which implementationof some of the Panel*s recommendations might "bo based.

The IAEA Scientific Secretaries are grateful to the authors,to the rapporteurs and to the coordinators of the papers and to all membersof the Panel for contributing to the discussions and conclusions.

They are particularly thankful to the Chairman, Mr. R. Sharpe, forguiding, in the most productive manner, the work of the Panel.

They address their sincere thanks to the Conference Coordination Unitas well as to the Section Secretaries who have made it materially possibleto fulfill, without delay, all the items proposed for discussion.

ii. smTE QP_Agy .SURVEY

A. Qn foated Particle. ..Fuels, ..for. ff

Al. Paper lío. PL-477/17 was discussed and arising from the discussion,the following points are considered to bf of interest to the Panelî

Al.l) As far as nondestructive testing methods are concerned, it isimportant that the techniques used are sufficiently sensitive to examinetypical defects, which can be expected to occur in microspheres. The speci-fications, therefore, must indicate those areas where defects could cause afailure in service. Ihe nondestructive and other tests selected, must showthat the specification has been met on these important points.

Al«2) A fuel charge contains a very large number of coated particles,for example, in one fuel compact of 40 ram in length by 35 nun in diameter thereare about 10,000 microspheres. This necessitates a very careful samplingscheme as it is obviously impractical to test 100$ of fuel production as,economically, this would be unacceptable. A statistical approach to thisproblem is discussed in the paper (paragraph 2.3. and associated references).

A2. As a relatively small sample is tested to represent a very largepopulation, the choice of test methods imst be restricted to techniques whichare technically sound and sufficiently accurate to measure the parameters withconfidence, ïîiis has led to the adoption of x-ray examination of a very high

•*quality to establish the measurements of coating layer thicknesses to a highdegree ot accuracy. X-ray projection or contact microradiography can be usedfor this purpose (réf. paper 17, para. 3-3.2,}. Contamination measurements(réf. paper 17, para. 3-3.3-1) are also made non-destructively by alpha-counting.Eofch the radiographie,metrologica], and the contamination measurements areeasily automated.

A3. The fuel compact is tested radiographically for fuel dispersion,(réf. paper 1?, para. 3.5} by gamma spectrometry and gamma absorptioraetry forUp.,,- content and heavy metal loading respectively. (Réf. paper 17, paras.3.5.1 and 3.5.4}- Eddy current testing is also used to detect cracks andother flaws.

A4. The most important test is that of the determination of the failedparticle fraction in the fuel compacts. At present there is no routine non-destructive test available but important developments are exnected if the radonemission monitoring technique proves successful (ref, paper 1?» para. 3.5«5«2).This is the most urgent problem which must be solved, if reliable informationon oredictable fission product release from coated particle fuels is to beachieved. Future developments in the nondestructive testing field for coatedparticle fuels are, at present, concentrated on this problem.

A5. The fuel element nondestructive tests have been the examinationof ¿rraohite fuel tubes by eddy currents and the .radiography of whole fuelelements. The latter technique is only used to identify location of compactsand the positioning of thermo-couples.

A6. The techniques of post-irradiation examination, where they differfrom methods used for other types of fuel, are reported under Section E.

A7. In freneral, the nondestructive tests referred to in this summaryprovide a rapid feed-back, of information to the fuel production line. Anytest method chosen must have this capability unless it is only concerned withresearch and development.

A<3. Apart from the radon emission technique already mentioned, muchwork needs to be done on a nondestructive method for determining the porosityof the fuel kernels to within + 2"k* It is also anticipated that the use of

10

gamma absorptiometry will displace gamma spectroraetry and panoramic radio-graphyt as it is cheaper and faster. There is a possibility that a stresswave emission technique at present under investigation, may prove successfulin identifying failed particles at the coated particle stage.

A9»

A9.1) The state of the art of UDT is satisfactory in the area of metrologyof coated fuel particles. It is reasonably satisfactory in the measurement offuel content and dispersion in fuel bodies.

A9.2) There is need of urgent development to satisfy the requirement fordetermining the amount of failed fuel particles in fuel bodies before irradi-ation. Further studies are necessary to evaluate a satisfactory method formeasuring the voidage or porosity of fuel kernels.

A9«3) Finally any testing organization dealing with coated fuel particleproduction must be prepared to provide suitable training in the appreciationof statistical sampling and an understanding of its importance in the economicTOT of coated particle fuels,

B. On PuerlrPrelTletsBl. Several nondestructive évaluât .ons are made of uranium- and plutonium-

bearing fuels to assure conformity to specifications (see paper number 24)» Theprincipal procedures are:

- Visual comparison with physical standards that containminimum acceptable conditions

- Mechanical measurements of dimensions and density- Measurement of fuel content by analytical chemical

methods, and- Measurement of the characteristic emitted radiation,

11

Por fast breeder reactors, an additional requirement is the measurement ofthe amount and distribution of plutonium in a mixed fuel pellet» Electronrnicroprobe and alpha a,utoradiographic techniques are "being studied as anapproach to this problem.

B2. A distinction was noted in the examination of pellets between thoseconditions due to fabrication (e.g» causing difficulties in can loading orundesirable pellet/cladding interaction in reactor service).

B3. The general conclusion is that techniques are generally availableto measure the desired properties (that are presently recognized) ofproduction fuel pellets. Advanced automatic measuring techniques (see e.g.paper no. 20) with integrated data processing by computer offers the potentialof faster as well as more economical and reproducible measurements.

C. On FDT in the Fabrication of Pans JTor Jguclear; Fue PinsCl. ÎTOT techniques used in testing cladding tubes have reached a

technical standpoint where one can say we have means to reveal most of thedefects present.

G2. From the presentation of the papers 10, 13, 14» 20 and 21, itfurther became apparent that the main concern today is to make a technical/economic optimisation of the testing. In writing specifications one must becareful not to demand too strict testing since modern techniques permit moresensitive testing than may be necessary.

03. This leads to the question whether the specifications on testingare relevant or not. Concerning defects, standard defects of a given size arenormally specified. These are necessary tools when setting up test equipmentand maintaining constant sensitivity but it must also be clearly understoodthat this does not mean that defects of that size will be revealed andrejected.

12

G4. Tn the above case, there is a strong need for further investi-gations on the relationship "between indications and actual discontinuitiesas well as between different types of discontinuities and their effect onsafety in the operating fuel element» In the latter case this can be donsby simulated destructive testing; however» one must consider the relevanceof the simulation methods used so that the siae, character and location ofdefects that are significant to service will be identified»

05. Hie techniques used include ultrasonic and eddy current methodsfor defect inspection, ultrasonic wall thickness measurement and e.g. pneumatic,mechanical, electromechanical and capacitive methods for different kinds ofmetrology»

C6. Concerning defect inspection there has been much discussion aboutthe relative merits of ultrasound and eddy currents. Investigations madeindicate that they are in many cases complementary and in these cases bene-ficially used together. The main weakness in the eddy current method is thelarge number of active parameters! on the other hand? its sensitivity tospecific types of defects can benefit the inspection. In test-ing of finnedtubing, it has been particularly useful; however5 for the same case ultrasoundcan also be used to test for cracks at the base of the fins.

G?. In using different techniques for ÏÏDT» such asultrasonic and eddy current methods, there is a need for better knowledge onthe effects of the instrumentation characteristics on the test result. As anexample, the ultrasonic transducer can be mentioned. If the test results areto be relied upon, there must be assurance that the sound beam characteristics

i,

are as specified.

C8. Sometimes the scope of ÏTDT is somewhat limited* Being a part ofquality technology, the possibilities in feedback of the test results toearlier stages in the manufacturing procedure roust be considered, Another

13

important receiver of this information is the fuel element designer tobenefit the understanding between those involved in nuclear fuel can fabrication,

09- GonqlusionsC9.1) The relevance of testing specifications for defects as well as

dimensions must be carefully examined to obtain a safety level giving atechnical/economic optimum. Also there is a need for better understandingof the indications obtained regarding the underlying actual defects and theirrelevance to safety in an operating fuel element.

C9.2) Prom the presentations and discussions, a need also became apparentfor tests of testing tools as e.g. the ultrasonic transducers,

C9.3) To improve the overall quality technology it is of interest tostimulate the contacts and hence the understanding between different partiesinvolved in writing specifications and performing tests,

T). On TOT Used Puring _ Fabrication .of L Fuel Elements jfficl AssembliesDl . Pour contributions-1/ on FD11 used during fabrication of fuel elements

were submitted and in addition fir-. H. Meyer (Federal Republic of Germany)presented i film on the testing of Zircaloy shrouds.

D2. These contributions demonstrated the range and diversity ofWOT concerning namely:

- fuel pins and their coroponente- structural parts- assemblies,

B3. After discussion, the Working Group agreed, for the sake of brevity»to limit this summary to tests for flaws liable to leadto cladding failures under irradiation conditions.

_!/ Gontribiitions Hbs, 11, 20, 27 and Mr. Prot*s contribution on MTRFuel Elements.

14

D4« Apart from the cladding tests summed up separately (Section C), the mostrelevant comments dealt with:

a) 100 HDf of rods used for end plug machiningb) weld KDT of weldsc) NOT for corrosion behavirur of Zirconium alloy cladding» andd) leak-tightness NOT.

D5« Short comments on each follow:55.1) Ultra-sonic and eddy-current techniques are used industrially

for ÎÏDT of end plug rods.D5.2) To check the quality of the weld between the cladding and end plug,

radiography, together with destructive examination of samples during processingand the close monitoring of the welding parameters, is the technique mostcommonly used.

D5«3) Testing may be facilitated if, for pin design - especially forend plugs ~t account is taken of the technical problems involved in takingand analyzing radiographs. Modifications to the design could also allow theuse of other techniques e.g. ultrasonic or eddy currents.

D5»4) ÎTOT for corrosion behaviour of Zirconium alloy cladding isaccomplished through autoclaving. Paper no» 21 draws attention to theneed of adapting processing conditions to the purpose. The present trendis to test samples of tubes» plugs and welds only.

D5*5) Helium mass spectrometry is a satisfactory test as regards theleak-tightness of fuel pins.

B6. Other fabrication defects of the fuel elements which may leadto failures under irradiation conditions» e.g. deformation of pins orstructural parts, saust be detected e.g» by raetrological tests.

15

B7• ConclusionsD7.1) Generally sneaking, ÎTOT used during the fabrication of sintered

ceramic fuel and metallic clad elements is satisfactory.D7.2) The investigation and development of new methods would "be

.justified only if it is expected to lead to a simplification or a savingof time and money. HTR or other new fuel elements, however, call for thedevelopment of new tests.

ÏÏ7.}) iftie efficiency of TTDT tni$it be enhanced if there were bettercommun!oations between designers, fabricators and inspectors.

D7.4) The constant improvement of TfDT instrumentation is obviouslydesirable, as long as it does not entail technical complication of theprocesses.

E. On Tfon-destructive Testing in the Post Irradiâtion Jfeajaination ofFuel ElementsT31. The papers contributed on this subject described the range

of techniques used in this field; as might be expected, agreement is evidenton the techniques considered useful. The reasons for performing the examinationare apparently more varied, but are - we believe - based on a commonreason, that of maximising the efficiency of nuclear cower generation.

8?. Before considering techniques in detail, this latter pointmay be further analyzed. Fuel elements are, in fact, examined in theirradiated state in at least three situations:

a) In the reactor (which generally, at present,must beshut down to pewit this).

b) Discharged from the reactor, but in such a mannerthat they can, if desired, be re-charged in it.

c) Discharged at the end of their irradiation life.

16

E2,l) Clearly in (a) and (b) any tests carried out on the elements must"be rigorously non-destructive and must "be done quickly. In (c) non-destructivetests will be used in the early stages of examination? when destructionof the element might "bring about the loss of information on structure, flaws,deformation^etc.?of diagnostic value» Since it is not possible to separatethe destructiva and non-destructive operations completely in descriptions ofprocedures, the tests are not all I!ÏÏDT". This is indicated in the list oftechniques below»

S2.2) The aim of examination in situations (a) and (b) will be to verifywhether the elements are suitable for further irradiation, which ,if so,is ofimmediate economic benefit to the reactor operator» Even in situation (c),the examination of an element may give useful gai dance on whether- similarelements still in the reactor may safely be further irradiated, with directbenefit. The ultimate justification of post-irradiation examination of anyfuel element other than these must be the economic benefits which will derivefrom the improved design and operation of future fuel»

17

E}.

(Condensed from Papers 1, 4, 5, 8, 20, 23 and 26}

Tvt>e Details

Vi suai

Gamma-scanning

"Gamma-snectrosoopy

Gamma Absorntiometry

X-Radiography

Neutron Radiography

autoradioi^rapby

Ultrasonic

Direct(Stereo) microscope(Stereo) periscope(Peri) photographyTelevisionPh ot o graiTim e h ry

DirectScanningProjection (for coated

partióles)

ReactorTNeutron generator,Isotopic source

General observation offuel elements, shape»deformation, debris. May becarried out Jn reactor, pondor hot cell.

Measurement of stack lengthsmeasurement of fission prodtictmigration and variation ofburn-up along with measurementof density and thickness.

Shax>e of fuel element anddisposition of components

As above, and alsomeasurement of hydride inzirconium, and distributionof burnable poisons,

Various applications, e.g.finding "hot spots" ingraohite HTR components

Flaw detectionBond determinationLocation of water in pin.

18

Bdày Currents

Leak Detection

Stripping and otherexamination ofsurface layers

Metrology

Piercing

Analysis

#Metallography

Mechanical testing

BubblingTracer Gases (He, SP )"Sipping* (wet and dry)

A very wide range ofmethods

ChemicalX-ray diffractionX-ray fluorescenceMicro-proba

Conventional sectioningand visual examinationElectron & Scanningmicroscopes

Flaw detection (preferredover ultrasonics, becausea "dry" method)Measurement of spacingand cladding thickness

Verification and locationof leaks as first step indiagnosis of failuremechanism. "Sipping9* may"be in reactor, pond orelsewhere.

Information on cruddeposition and corrosion

General information ondimensional behaviour

Gas measurement and analysisMeasurement of free volume

Determination of changesduring irradiation

Determination of changesduring irradiation

Tensile strengthBurst strengthCreep strengthImpact strengthMiorohardness

* Indicates test which is, by definition, destructive.

19

B4« Gonelusi onsThe Panel agreed that directions in which development might usefully

be directed are:a) ..¡proved methods of rapid collection and analysis of dafca - e.g.

holography in metrology.

b) Improved methods of location of cracks and other defects incladding and fuel.

c) Methods of identification of a failed tdn in an assembly,without the need to break the latter down.

d) Where possible, post irradiation tests should be comparableand coordinated with pre-irradiation tests.

e) The further development of tests capable of being conductedin the reactor during its operation.

F. QjiL JTDT Procedures^ Jtaring Fabrication of Steel._ React or• Pr essureVesselsPI. gie _rPerasibil it;y; of JEnspeqtin^ Austenitic Welds and

Welds in lii i ífickel Allots

Pl.l) Usually such welds are bested oy a radiographie method.But in naany cases, it is necessary to use another method because, for instance,the radiographie inspection cannot be carried out owing1 to the shape cf theTo test welds in larije wall thicknesses it is desirable to use an additionalmethod.

PI.2) If possible, in some cases 3iquid pénétrant tests are performedat several intermediate states of filling the weld grooves. But the ultra-sonic method seems to be more important because the whole section of theshall be inspected in i te. final state.

20

PI.3) An ultrasonic inspection by an angle beam technique with shearwaves is mostly impossible with acceptable sensitivity and must be limited tothe parent material beside the weld - probably including the heat affected acme.Special techniques with longitudinal waves have shown good results in somecases » îTormal and angle beam probes have been applied, mostly with focussedor semi-focussed beams.

F2. The Aavôuritages and. Pisadyantages of ir i ji. ^ o phic,

Inspection

P2,l) Principally ultrasonic inspection is more suitable to detecttwo-dimensional defects than radiographia examination oi-ring to the differentphysical characteristics of the two methods. [Sie advantage of ultrasonicinspection increases with the wall thickness to such an extent that the usuallymentioned disadvantages of this method soon lose their importance.

F2.2) Ine restriction of manual ultrasonic inspection and the need -fora hand written report from the ultrasonic operator is amply compensated by theadvantage given in the detection of dangerous defects.

P2.3) Evidence has been quoted to show that radiographs did not indicatedangerous cracks in many cases, particularly in thick sections .

P2.4) Conversely, it was pointed out that radiography offers the potentialof distinguishing between different foriss of three dimensional defects. Thisadvantage may be particularly useful in avoiding unnecessary repairs.

F2.5) However, the conditions of the complete ultrasonic inspectionand calibration to a high sensitivity make this method a most reliable one.

F2.6) Complete inspection requires the application of several scanning1

directions and of several angles of incidencesand for welds having wallthickness above 100 mm the additional performance of the tandem techniqueinspection. On small thickness of welds, both of the methodsj ultrasonic andradiographie inspection»should be applied.

21

F2.7) On large thicknesses some members of the Panel felt that ultrasonicexamination with magnetic particle or liquid pénétrant inspection can "be regardedas sufficient, provided that the tests are made by highly skilled operatorsusing suju<..i,ble equipment and that the completeness of the scanning proceduresare fulfilled. Others felt that radiography is a necessary part of thisinspection and should be regarded as a complementary method.

F3. A cous tia.In most countries acoustic emission as a procedure of nondestructive

testing still seems to be in a, state of investigation with the aim to find cracksas sources of emission during the pressure test. In the USA and the UK thismethod has been applied on several pressure vessels. To increase experienceof this method it is recommended^ whenever possible ¿to perform measurements ofthe acoustic emission during the pressure test as an experimental research inother countries too. Other ÎTDT - aethods, ultrasonic: inspection for instance,can give information about the extent of the flaws which have caused acousticemission.

jt itP4. Autojaarfc i c Met h , odeFor ultrasonic examination of the base metal, autornized equipment having

sufficient sensitivity could be built and used today. The successful effortsin the field of in-service inspection lead us to expect less expensive testingof welds by automatic ultrasonic methods»

At the present time the sensitivity of ultrasonic examinationduring fabrication must be higher than that of the in-service inspectionbecause of the reasons outlined under F2.

** See also Item Gil.

22

p 5 . JS«g-Jj£.lgíiQiij[k^of. Defects

P5«i) 5 e identification of the type of defect by the appearance ofthe indication causes some difficulties in the application of ultrasonicmethods» In normal cases the operators only have to measure the positionand the sizes of the flaws and their density. In other cases ultrasonicoperators are called upon to distir guish between two-dimensional and volumetricdefects. It should be observed that in steels in common use today for pressurevessels, small cracks may be acceptable if their density is not too large.

P5«2) It is highly reeomraendable to develop further and use mechanizedscanning systems, not only for economic reasons but also with a view toimproved reliability, presentation of results^ reproducibility and resolution.

5*5*3) It is believed that the extent of SDT procedures mentioned in item P2could be reduced for welds in pressure vessels made of the most commonly usedsteel if one or both of the following criteria are proved to be applicable:

- the critical orack sizes from fracture mechanics are large enougheven for all possibly occuring deviations from defined states ofweld metal and heat affected soné;

- the welding process^ including the heat treatment^ hag beencontrolled with such a reliability that no severe deviationin the state of metal can occur»

F6 . ffiie TO er e ce oj ^ Various .Code5he Panel urgently recotranends further unifying of the various national

codes especially with regard to the arguments under F 2.

23

' Base-Line and .Yes sels

Gl.The ASME Gode Section XI is considered the guide line lor planning

and performance of in-service inspection. The Panel felt that the mostcritical area of the reactor system from the point of view of safety was thepressure vessel itself. Discussion was restricted to that topic. However,it -was recognized that external circuits, mpework, etc» were of importanceand the Panel acked that existing reported work on this fcopio be added asreferences and recommended that the agenda for any future Panel discussionsinclude these topics.

G?. The areas requiring most critical examination during in-serviceinspection are those possessing the least tolerance to defects. Typically,these areas are those in which high stresses exist e.g. nozzles or where therecan be a significant dégradation of material properties due to metalluï-giealchanges on neutron irradiation. The standards of inspection necessary willvary according to the critical size of defect and inspection should be basedon these standards; this is discussed further in para. G 11»

G3. It is believed that there are two major differences for in-serviceinspection compared with inspections during the manufacture of pressure vessels.The first is the tyne of defect and orientation. Due to reactor service, ifat all, only cracks or separations are to be expected,, i.e. two-dimensionalreflectors for the ultrasonic testing, which run, more or lesss perpendicularto the surface according to the stress and strain conditions. Por obviousreasons three-dimensional defects ( slag ? porosity, etc.) will not be introducedin service. The second difference to manufacturing inspection is in the sizeof defects» Defects of interest here are only from a size upwards which is

well below the critical crack size at the end of reactor life.

24

G4« fhe design of the vessel, its insulation and its shielding dictatethe approach necessary in the design of equipment for manipulation of inspectiondevices. For inspection from the inside of the vessel a central mast is commonlyused being supported from a bridge,seating either upon the vessel flange or thetop of the pond, Several variants of mast design have been used. When all ofthe vessel internals cannot be removed partial inspection has been possibleusing manipulators attached to the refuelling machine. Access to the vesselfor ultrasonic examination has also been possible by the design of equipmentto fit into a 50 ÏÏIBÎ gap between an internal thermal shield and the inner wall of? <the vessel» Each specific design appears to present its own specific problemsin access and manipulation of equipment»

G5» Inspection from the outside of the vessel is usually possible onlyif provision has been made at the design stage of -che reactor system. One ormore parts are made through the concrete shielding thus permitting access forinspection equipment. This equipment may be mounted on rails surrounding thevessel or may be rotated on a, framework capable of traversing the vessel in thevertical plane circumference, The requisite gao should be at least 250 mm »

G6. A unique approach for inspection of nozales has been reported in theexamination of Oskarsham 1» An access port for ultrasonic equipment wasprovided into the elbows attached to the nosales and the equipment was positionedon a centering cone attached to the vessel internals. This technique was theonly example of nozzle inspection not being made from the inside of the vessel.

G7. This technique and indeed all those from the outside of the vesseldemand close collaboration between the designer of the reactor system and theoperator of the inservice inspection techniques. Inspection from the outside

25

of the vessel offers advantages^not only in -terms of FÛT capability but alsoin reduction of reactor off-line time. Continuing close liaison between ÏÏBToperators and reactor designers is essential so that the most effectivecurrently available HDT techniques cars be applied.

G8. Visual inspection of the vessel surface uses closed circuittelevision, still cameras and introscopes. The use of colour is recommendedto aid detection of corrosion products from the ferritic steel.

G9. Breaks in austenitic cladding have bean quantified using aresistance (SMECK) gauge and depths of cracking in ferritic ¡?urfaces measuredusing an ultra high frequency conductivity apparatus.

G10. Ultrasonic examination is the only method currently being usedfor volumetric examination of the vessel. Multiprobe systems are mostfrequently used and varying degrees of sophistication have been applied toprobe travel, to digital readout systems, to chart recordings and to stop-motion photograohy. All incorporate methods of correlating probe (and defect)position in terras of vessel coordinates» Formally specific indexing pointsare marked on the vessel. One system uses a variable angle probe capable ofcovering 7^° from one side of the weld, through compression to 70° from theother side of the weld. A cross sectional image of the area being scannedis produced on a storage oscilloscope and recorded, along with positionaldata and a visual record of the surface being examined, on videotape.

Gil. Defect standards are generally based on a flat bottomed hole ofabout 10 mra depth or semi elliptical slits of dewth 5 mi?* a d UP %*> a totallength of 30 ram. These standards may be significantly less stringent thanthose which can be applied in pre-service inspection»

26

Panel thinks -that it would be very useful to design a specialreference block with articificial reflectors for ultrasonic in~service inspection;in this appropriate specification references to specifications should be madeto establish the orientation, position and size of different reflectors inthe specific "block for individual pressure vessel block.

Gil.2)It is considered that the defect standards to be aimed for duringin-service inspection should be based on a fracture mechanics assessment ofcritical sizes. Thus, for example» defects in the barrel section orientad inth« longitudinal direction would require more critical examination than thosein the circumferential direction. It is reeognieed that such an approach wouldbe necessary in the fabrication stage and would demand establishment of fracturetoughness data and possibly different (probably more rigorous) methods ofifuality control of materials, welding techniques,, etc. than are currentlypractised.

G11.3)lt is considered that this approach should be the long term objectiveand designers, specification making bodies, fabricators and licensing authoritiesshould be encouraged to achieve this objective (See also P5)«

G12. The inspection of studs also utilizes ultrasonic methods from theinside in the case of hollow studs. A 30 kHz eddy current flaw detectr (AMLEC)is also used to detect cracks at thread, rootc. Similar techniques are appliedto nuts.

G13. Detailed comments on the results of in-service inspection were givenin only one paper (paper no. 6). Delays due to minor malfunctions of equipmentv?ere found^emphasizing the necessity for comprehensive pro-inspection trials.During the inspection only minor flaws were found; the sizes were below theacceptance level at the fabrication inspection. Apparently significantindications wore given in some areas. For example, at the inspection of therecirculation noasles indications above the reference level were recorded.

27

G14. The sensitivitv ofultrasonic inspection is extensively influencedby the austenitic cladding (external surface, grain structure and the contourof the fusion zone "between base metal and cladding). Due to this fact theexperts have the opinion that results of in-service inspection, i.e. theminimum flaw size still detectable, depends to a great extent on the ultra-sonic behaviour of the cladding. Depending upon welding process and claddingmaterial, differing conditions can exist with individual pressure vessel manu-facturers and even within one pressure vessel. It is, therefore, exceptionallyimportant to have at an early stage,reference blocks available for preliminarytests or to be able to perform at the pressure vessel itself transfer measurementsin order to find out the existing influence. In some cases it might even bepossible that the surface from which the volumetric ultrasonic scanning has tobe done will be dictated by the cladding influence. Por the volumetricinspections it is normally sufficient to have access to either the outer or inner(cladded) surface of the pressure vessel. In close co-operation with the designerof pressure vessels, a maximum of access to either the outer or inner surfacefor the ultrasonic orobes should be obtained.

An annulus between the outer surface and insulations may be useful forTV inspection and other special inspection purposes, but, in general.not mandatoryif full access to the inner surface for ultrasonic volumetric inspection isprovided (an exception could be, as mentioned before, an extremely high influenceof the cladding),

Other problems presented by surface finish, extraneous matter such aspaint, brush hairs etc. have been reported by Dr. Meyer and emphasize theneed for the greater attention to be paid, at all stages in fabrication to therequirements of ÎTDT met hods.

28

G15« îïone of the techniques described permit coverage of all significantareas of the vessel. Gaps in coverage exist even in systems designed for in-service inspection; for example, areas can be masked by rails necessary to holdinspection equipment on the outside of the vessel; the bottom dome may beinaccessible and techniques for the inspection of the inside edge of nozzlesare still under development. In some instances these gaps in coverage may bereduced by closer liaison with designers (para. G?)» In the case cf reactorsalready designed many areas will always be inaccessible for inspection usingconventional techniques.

G16. The technique of acoustic emission is suggested as being mostpromising for giving a significant increase in the extent of inspection coverageof the vessel. The possibility of continuous in-service inspection to monitorfor crack growth is the roost attractive but presents the greatest technicalproblems because of background noise» high temperatures and the effects ofneutron irradiation. An intensive study of these problems is being undertakenin the USA by the electrical utilities^and invaluable practical evidence onthe feasibility of continuous surveillance should be forthcoming on completionof noise and defect signal tests planned on the EBOR vessel in early 1972.When the results of these tests a.re known it would be more opportune for- aPanel to make a rea .listic appraisal of the feasibility of continuous monitoring.

G17. Meanwhile, it is considered worthwhile to apply acoustic emissiontechniques whenever possible at the ta j f jgroof Jtesting of vessels (asrecommended in Item P). Experience built up in this way would not onlyendorse conventional SfDT methods but would also be invaluable in establishingan acoustic "fingerprint" if in-service continuous monitoring was consideredvaluable at some future date.

29

G18. It is well known that in the field of NDT in-service inspection

development a number of base-line and in-service inspection has been performed.

These were not discussed by the Panel and references are given.

1) Wylie, R.D.; Surveillance of ïïuclear Povíer Plant Components.Kuelear Engineering and Design, Vol. 8, 1968, p. 117-124.

2) Gross, L.B. and C.R. Johnson; In-service Inspection of ïïuclearReactor Vessels Using an Automated Ultrasonic Method, MaterialsEvaluation, Vol 28, July 1970, p. 162-167.

3) Hornbuckle, J.D. , C.E. Laut-zenheiser, K.P. Baskin and H.K. Hendricks;In-service Inspection of San Onofre Unclear Generating Station-Unit 1,ASME publication 71-PVP-51.

4) Holt, R.G. and A.M. Hubbard; Design for In-service Inspection ofBoiling Water Reactor Pressure Vessels, ASME-publication 71-PVP-59-

5) Norris, B.B.t P.D. Watson and ÏUD, Wylie; Repair of PrimaryPressure System Piping in a Nuclear Power Plant; ASME publication71-PVP-50.

6) Weissert, L.R.; In-service Inspection of Nuclear Reactor CoolantSystems, Reactor Technology, Vol. 14? no. 1, Spring 1971> P* 44-6? •

7) Seli#, 3.J., fi.P. Kosky and O.P. Hadden; In-service InspectionProgramme for a PWR. American Nuclear Society, Power Reactor Systems*»nd Components Meeting Williamsburg, Virginia, Sept. 1-3, 1970.

3) de Sterke, A; Ultrasonic Incpecti m of Welds in U'uc'ear Reactor PressureVessels (Hewly developed equipment for automatic scanning and recordingof butt weldo) "The British Journal of Ion-Destructive Testing" vol.12 Ho. 4.

H« On yPT a^^Sjirveillance pjF rPr&ssure Tubes

HI. Only three contributions were given on this subject - one bythe USat relating to the "Plutonium Recycle Test Reactor" (PRTR) in operationat the Hanford Laboratories,

- the second by the United Kingdom relating to the "Steam GeneratorHeavy Viater Reactor" (SGHWR)

- the third by Prance relating to EL4 Reactor (Heavy Water TypeReactor) .

30

H2. This means that the conclusions drawn from these papers could notbe considered as universal, several other reactors being in operation or at adesign stage, in other countries.

H3» From the three reactors mentioned above, only the French one is ofthe cold-type pressure tube, that is to say that the thermal insulation betweenthe cooling gas and the moderator is inserted inside the pressure tube. Inthis way» e temperature of the latter is reduced to about 80 C. As far asmetallurgical nature and sta te, as well as dimensions are concerned, it maybe said that the differences are not significant for the ED? techniques involved.

H3.1)All tubes were inspected ultrasonically for longitudinal defects and

wall thickness measurements (resonance method) ~ sometimes transverse defectswere also considered» Radiography and fluorescent pénétrant examination werealso used in the USA.

Dimensional measurements were equally carried out for length, outsideand inside diameter and for straightness. In fact, all of these examinationswere made several years ago and the techniques were perhaps less sophisticatedthan now, but it seems that, even so, no major difficulties were to be overcomeat the moment of the reactor design and building»

H3«2) Post Installation :

This inspection seems to be restricted mainly to dimensional variations.The device's used largely utilize the techniques of L»V*"D«T» (Linear VariableDifferential Transformer). Visual examination "is carried oat using eitherconventional Borescope or remote controlled T«?. cameras. Sometimes (USA, UK)a further measurement of discontinuity depth is also made.

<£!a& presence of an external shroud tube around the PR'TR*s pressure tubehas led to the development of a special eddy current device to measure the

31

annulus betiíeen the pressure and shroud tubes, without any noticeableinterference due to minor variations in the wall thickness of fche pressuretube or the aluminium shroud tubes.

H4. ConclusionsH4.3) The Panel considers that, except for further details eventually

coming from other countries not present during the Panel sessions, it seems theNOT technicjues aised to date are satisfactory.

H4.2) In terms of known-service behaviour, in-service inspection appearsto have been adeofuately covered» However, if a more rigorous examination wasconsidered to be necessary, for example to detect cracks in the tubes, somedevelopment would be necessary.

I. On ITDT _For Concrete Pressure VesselsTl. Only two contributions were presented on this item. One concerning

a prestressed concrete pressure vessel (PGPV) for a high temperature reactor,the second concerning several carbon dioxide cooled reactors.

12. Reactor types differ but ÏFDT of PCPV are quite similar. However,from a general point of view, ÎTBT problems in PCPV are very different from UDTproblems ¿.n conventional metallic nuclear pressure vessels.

12.1) First of all, normal behaviour of PCPV shows phenomena whichare not expected in the normal behaviour of a metal oressure vessel. Forinstance the cracking of the concrete is normally anticipated and is acceptablewithin specified limits. Obviously, this is not the case with metal pressurevesselr, for which cracking is generally not acceptable.

12.2) In current practice the proper design of PCPV implies intensiveuse of ëcale models that are often tested to failure before construction of

32

the actual full scale PCPV. Testing techniques cf this type on materialsamples or scale models, are not really nondestructive. Fevertheless, theyare of prime importance in PCPV testing.

13. PCPV are generally supplied with a permanent instrumentationthat is more important than a metal vessel instrumentation. The continuousmonitoring of strain? temperature, tendons load, etc. provides an instantawareness of the current state of the operating vessel. Some of theseexaminations and monitoring technqiues admittedly do not fit the so-calledfield of classical WDf, However, they aref in fact, real ÎSTDT techniques.

14. Qo/iffij^^jjSisL^Por concrete and prestressing tendons, generally no ÎJDT techniques are

•used. Only conventional destructive tests on samples are carried out.Concrete is not gas tight. Tightness is achieved by using a steel liner

as an internal skin to the PCPVa This liner extends from the main PCPV cavity upto the penetrations. For the purposes of ÏTDT, it is considered as a steelpressure vessel and generally examined in-accordance with the ASME Code SectionIII for Class A or B.

14.1} The liner cooling circuit is embedded in the concrete and weldedto the liner to control liner temperature. This circuit must be leak-tightand welded joints are radiographed and checked with soap bubble techniques.Good circulation inside the circuit is also tested.

15. Whole PCPV testing takes place during proof testing which isrequired by national regulations. The proof tesiing pressure must be higherthan peak working pressuret from 1.1 times to 1.2 times the peak workingpressure. During this test, the concrete must remain in a compressive stateand general cracking must not occur. Deformations must be low and of theorder of magnitude of scale model deformations»

33

15.1) The main goal of examination during proof testing is tocharacterize the vessel "behaviour in order to compare it with the "normalbehaviour". "Normal behaviour ic. determined from specifications, stress andstrain analysis and,above all,by scale model test results.

Ï5-?} The following tests are conducted during proof testing:- leakage tests- crack pattern recording- overall deformation measuring- local strain monitoring- temperature monitoring.

Î5.3) "The visible crack pattern on POPV external surfaces is recordedbefore and after proof testing. It is compared with scale model crack patterns.Surface cracking can occur during construction» Cracks with a lateral otteningof a few 1/10 mm are generally considered acceotable. They are marked on theappropriate PCPV drawings for comparison at later inspection.

Ie),4) Overall displacements of particular points of the vessel are measuredduring proof testing. Concrete temperature and moisture content are alsomonitored during proof testing, especially if the proof test is not carried out atroom temperature.

16. Periodic in-service inspection is often required by nationalregulations. Testing systems described for use through the proof testingare also used for in-service inspection. These include visual examinationand mapping of cracks patterns, measurements of deformation and strains»tendon load, leakage( concrete temperature and moisture.

I?. {/onclueipnsi!

The Panel feels a need for more information on this subject and aneed for other examination techniques to be considered for in-service

34

inspection. For example, new techniques win oh are gaining in interest are:- visual examination "by closed circuit TV- ultrasonic examination as with steel vessel examination- acoustic emission- ultrasonic leak testing.

J. On Jfo^jUigsJijiTjg^

Jl. Surveillance programmes have to be conceived in such a way tominimize and possibly avoid arty interference with normal power plant operation.Thus, the installation of special sensors on irradiate'd structures inside thehostile environment of the pressure vessel to detect stress levels or vibrationamplitudes, is generally believed to be unpractical» Until now, surveillanceprogrammes have generally been based on techniques -utilizing sensors installedoutside the reactor pressure vessel.

J2. The three main techniqu.es for surveillance which are presentlyunder investigation are based on:

- audible noise- neutron T¿oisef and

- coolant pressure noise.Audible noise was found r^uite practical because the operator's analysis

of the signal is simply made by hearing» The neutron noise signals containmuch useful information, however, it is difficult to trace back the uhysicalmechanisms causing the useful noise signal * The coolant pressure noise isattractive since it gives reproducible results when the noise signal isanalysed by spectral analysis. Based on the results availablef these surveillancetechniques appear to the Panel to be promising for in-service monitoring andincipient failure detection of reactor internals,

35

JÍ. However, their validity and usefulness are -not to be over-estimatedbefore significant experience has been accrued,

.13.1) Tn «articular, the so-called "signature approach" has been mainlyUEQd UP to -tow. However, this approach may only give alarm to the reaotoroperators, afterwards, it is difficult to establish whether this alarmoriginates from an actually dangerous situation, or by a departure from someconditions of no inroortance to reactor operation.

J"}.?) Therefore, the "si feature approach" cannot be considered fullysat i sfact ory.

J4« Tt is believed that full acceptance of any surveillance techniquewill come only when the physical mechanisms causing a departure from the referencecondition are well understood. The main research efforts on surveillancetechniques baoed on noise have to be made in this direction.

J5« By initiative of the Commission of the European Communities,the experts working in this field in the Member States have been broughttogether for cooperation under a study contract with the title "CriticalcomnariEon of operational surveillance techniques for components of nuclearpower piaras by vibration and noise analysis for the early detection ofdamage and theoretical studies for their further development'. This should helpavoid duplication of efforts and present a possibility to exchangeresults.

J6. Gonclusion

The Panel considers it would be quite valuable to compare the resultsobtained from the above-mentioned study with equivalent ones available inother countries.

36

J7» .Refereîiceg

(l] J.A, Thie "Reactor ïîoise" Rowman & Littlefield Inc., New York (1963)(2) R.E» Uhrig "Random SFoise Techniques in ïTuclear Reactor Systems".

The Ronald Press Co., ïTew Yorè (19?0).(3) M. Calcagno and P. Cioli "In-Sarvioe Monitoring of Gore Structures

and Reactor Internals by Neutron Noise Measurements" Report ENEL-CPNC3.R1/0.9.70 (August 1970),

(4) D.ff. Pry "Experience in Reactor Malfunction Diagnosis Using On-LineNoiae Analysis", Nuclear Technology 10 273 - 282 (1971).

(5) R.L. Randal "Applicationc of Hoiso Analysis Techniques to Detectionof Incipient Malfunctions" COW-C71011 (1968).

(6) V. Rajagopal et al. "Nuclear Hoioe Measurements on Reactor GoreVertical Motion" ANS Transact!one, Vol. 12, ÎTo. 2 (1969).

3?

III. GOgCLUSIONS ..PROM SURVEYS OUT;

Coated Particle Fuel. s f çxr Higft, Temperature Reactors

1. The state of the art of ÏÏDT is satisfactory in the area of metrologyof coated fuel particles. It i? reasonably satisfactory in the measurementsof fuel content and dispersion in fuel bodies,

2. However, there is need of urgent development to satisfy the requirementfor determining the amount of failed fuel particles in fuel bodies beforeirradiation. Further ctudies are necessary to evaluate a satisfactory methodfor measuring the voidage or porosity of fuel kernels.

3. Any testing organization dealing with coated fuel particle

production must be nrepared to provide suitable training for the appreciationof statistical sampling and an understanding of its importance in the economicTIOT of coated particle fuels.

Fuel. Pellet s

4. Techniques are generally available to measure the desired properties(that are oresently recognized) of production fuel pellets. Advanced automaticmeasuring techniques (see e.g. paper ¥o. 20) with integrated data processingby computer offers frUe potential of faster aj well as more economical andreproducible measurements.

Pabricat j .ÇYI of ' £ores for Tfuclear Fuel Pins

5. The relevance of testing specifications for defects as well as dimensionsmust be carefully examined to obtain a safety level giving a technical/ economicoptimum. Also, there is a need for better a better understanding of the indicationsobtained regarding the underlying actxial defects and their relevance tosafety in an operating fuel element .

38

6. A need also became apparent for tests of testing tools as e.g. theultrasonic transducers.

7. In order to improve the overall quality technology, it is of interestto stimulate the contacts- and hence the understanding- between differentparties involved in writing specifications and performing tests.

Fabrication of Fuel JBlejmentLS and¡Assemblies

8» Genrally speaking» FDT used during the fabrication of sintered ceramicfuel and metallic elements is satisfactory. HTR or other new fuel elements,however, call for the development of new tests.

9. The investigation and development of new methods would be justified onlyif it is expected to lead to a simplification or a saving of time and money.

10. The constant improvement of ITDT instrumentation is obviously desirableas long as it does not entail technical complication of the processes»

11. The efficiency of EOT might be enhanced if there were better communica-tions between designers, fabricators and inspectors.

Post irr di at i on Examination of Fuel Eleinents

12. The Panel agrees that directions in which development might usefullybe directed are:

a) Improved methods of rapid collection and analysis of data -e.g. holography and metrology.

b) Improved methods of location of cracks and other defects incladding and fuel.

c) Methods of identification of a failed run in an assembly,without the need to break the latter down.

39

d) Where possible, post-irradiation lests should, be comparableand coordinated with p re-irradiation tests.

e) The further development of tests capable of being conductedin the reactor during its operation.

Fabivicat ion of Steel'Reactor

13. It is highly recoromendable to develop further and use mechanizedscanning systems, not only for economic reasons but also with a view toimprove reliability, presentation of results» reproducibility and resolution.

14. It is believed that the extent of HOT would bereduced for welds in pressure vessels made of the most commonly used steelsif one or both of the following criteria are proved to be applicable:

- the critical crack sizes from fracture mechanics arelarge enough even for all possibly occuring deviationsfrom defined states of weld metal and heataffected zone;

- the welding process^ including the heat treatment hasbeen controlled with such a reliability that no severedeviation in the state of metal can occur.

15. The experts attending the Panel would urgently welcome further unifyingof the various national codes

Ba_ -JLiri_e_. _d_ f3ft

164 The Panel thinks that it would be very useful to design a specialreference block with artificial reflectors for ultrasonic in-service inspection;in xts specification, references should be made defining the orientation,position and size of different reflectors in the wall of the specificindividual pressure vessel block.

40

17. It is considered that the defect standards to be aimed for duringin-service inspections should be based on a fracture mechanics assessmentof critical sizes; this approach should be the long term ob jective., anddesigners, specification making1 bodies? fabricators and liciensing authoritiesshould be encouraged to achieve this objective.

18» ïhe technique of acoustic emission is suggested as being most promisingfor giving a significant increase in the extent of inspection coverage of thevessel. The possibility of continuous in-service inspection to monitor forcrack growth is the isost attractive but presents the greatest technicalproblems because of background noise, high temperatures and the effects ofneutron irradiation.

19« Nevertheless, it is considered worthwhile to apply acoustic emissiontechniques whenever possible at the stage of proof testing of vessels.Experience built up in this way would not only endorse conventional M>T methodsbut would also be invaluable in establishing an acoustic "fingerprint" ifin-service continuous monitoring was considered valuable at some future date.

Pressure .Tubes W&brioa.tion ana Survei llano e

20. The Panel considers that, except for further details eventually comingfrom other countries not present during the Panel sessions, it seems the NDTtechniques used to date are satisfactory.

21. In terms of known service behaviour, in-service inspection appears tohave been adequetly covered. However, if more rigorous examination wasconsidered to be necessary, for example, to detect cracks in the tubes, somedevelopment would be necessary.

41

.Concrete Pressure Vessels22. The Panel feels a need for more information on this subject and a needfor other examination techniques to be considered for in-service inspection.Por example, new techniques which are f*aining in interest are:

- visual examination "by closed circuit TV- ultrasonic examination as with steel vessel examination- acoustic emission- ultrasonic leak testing.

React or Internals Qri-1 ine Sur\rei.lla.nce

2^. ïîie Panel considers it would "be quite valuable to compare the resultsobtained from the mentioned study in paper no. 29 with equivalent onesavailable in other countries.

42

IV, ygCHJreCAtL OBS3

As a result of the presentation of papers and subsequent discussionof the current etate-of-art of non-destructive testing, certain commonneeds became apparent throughout the broad scope of the meeting1. Bie listof needs (although not exhaustive) includes:

1 _ More quantitative results on data evaluation from non-destructiveexamination and, concurrently, better discriminating capability betweendifferent types of discontinuities, (in the containment area this wouldimprove flaw identification and avoid uneconomic and unnecessary repair work.)

2 - Improvement?; in item 1 will call for improved scanning systems, betterinformation and presentation and higher speed data processing; perhaps to allowcross-correlation between complementary inspections and certainly to allowfeedback for process control procédures.

3 - Better understanding of the significance of the various discontinuitiesthat can be located and their effect on the service life of components andstructures.

4 - "Development and increased use of nondestructive testing methods tomeasure material properties, e.g. the degree of cold work in tubing, grainsize, conductivity, et.al.

5 - Improved, long endurance and reliable méthode and equipment for themeasurement of strain.

6 - Better knowledge of the development of flaws and other operationalmodifications, during service so as to make specification relating to in-service inspection more reliable.

43

7 - Valuable information eventually obtained from specially nreparedtest samóles made from reactor vessel steel (or reactor internals) andmaintained during oroprammed periods in the reactor vessel under opera-tional conditions.

8 - Intensified investigations into the application of some of thenewer "tiDT techniques such as:

- neutron radiography, -acoustic emission- ultrasonic spectroscopy- colour W for inspection of reactor internals

o, _ Better methods of testing the inspection equipment and instru-mentation to improve reproducibility and, in some instances, thesensitivity and selectivity.

10 - Harmonisation of codes and specifications in the field of nucleartesting technoloi^y regarding:

a) Techniques,b) Sensitivity settings, Thresholds for recording,c) Reference to state of product, heab treament, oressure

tests, etc.d) Necessary reference pieces,e) Extension of the examination to be performed with

regard to the areas, directions and coverage,f) Preparation of base metal and weld surface in the

case of weld testing,g) Surface nreparafcions of cladding in nuclear vessels,h) Interpretation of results,i) acceptance levels of defects.

44

11 ~ Î3TDT procedures and programmes to be written in d&fcail and criticallyexamined by safety committees in the same way as all other operationsperformed "by suppliers and manufacturers.

12 - Unification of terminology and definitions in the field ofnuclear technology to prevent misunderstandings and uncertainty.(See para, V of Panel Recommendations,)

13 - Awareness of the fact that some so-called TSDT techniques, incorrectlyapplied, can be the origin (source) of new defects or incidents, e.g.overloading by pressuref mechanical damages on fuel pins, chemicalpollution (contamination) of cleaned components, losing test equipmentin reactor vessel»

14 - Improved communication among those involved at the various stagesof fabrication and operation such as designer, fabricator, inspector, user .This would help to answer such (questions as:

- Can the design be altered to improve the probability ofa successful (or more economic) examination with higherconfidence in the achieved quality?

- What is the actual property desired or for which inspectionis to be done? (Many times a specification may be phrased interms that reflect a misunderstanding on the part of thedesigner about aspects of the test to be performed.)

45

V.

The following recommendations are emressed to the agency:

1. Include ill the paoers nresented at. tne Panel as? well as thestate-of-art surveys, technical observations, conclusion? andrecommendation?: of the Working Groups in an IAEA Proceedingsoubli cat i on .

?+ Allow for a wide notification and dissemination of the Agency'spublication so that these proceedings become more available tointernational professionals and to others wishing to utilizethe contained information to their best advantage.

^. Serve as a focal noint for ex-change of information on currentand advanced TTOT technology.

A, Ax-range to assenble and disseminate detailed information onthe sneoif i cation and nerformance of NOT equipment» This couldenable a potential user to select the most appropriate techniquesand equipment for particular applications.

^. Collect experience on the use, mi su ne and abuse of WDT standardsand codes of practice for nuclear apnli cat ions; assist inactions to develop a more common approach to these matterswithin the framework of nuclear practice.

t „ Sponsor educational nroprammes in developing countries and regionsin order to accelerate the spread of nondestructive testingtechnology; this may include:

6.1. fjeminars and class-room type presentations to assembled groupsof selected technical students (these may be mature professionals),

46

6.2. Fellowships for qualified professional personnel to work fora period in an established laboratory where a high level ofcompetence is recognized.

7. Include into the field of nondestructive testing covered byÎAEâ seminars or training programmes, such related topics asflaw significance based on fracture, mechanics principles,inter-relationship between design and inspection, discussionon operating experience of standards and codes cf practice,economic significance of a "cradle-to-grave" philosophy ofquality maintenance (a more expansive term than WDT might thenbe more appropriate).

8. Encourage the establishment of coordinated research work onÏÏBT problems of common international interest.

9» Sponsor international symposia related specifically to nuclearÍFOT problems of common international interest.

10. Establish a permanent international working group on nuclearnondestructive testing to serve as a panel of specialists whowill provide continuous guidance and consultation on fundeddevelopment programmes on matters and subjects related to theabove recommendations nos. 3 to 9- (©•§*> identification andrecommendations of subjects of coordinated research works.)

11. Attempt to ensure that bodies (such as International NDT CoramitteetInternational Standards Organization, International Institute ofWelding), where the framework of international collaboration

already exists in some of the fields of ÏTDT applica-tion discussed by the Panel, are made aware of any subseopientAgency's activities in this field, when and where such cooperativeinterchange of intent seems appropriate,

47

HDT Techniques at the CHBN Hot Laboratories(Italy)

T> BOZZOUTI &Û. Cochi

CSM - Gasaccia, noine, Italy

A survey dî-the N0*F techniques presently available at the CNEN Hot Labora-tories in Casaccia (Rome) is presented, with short description oJÇ the variousexperimental apparatus and of some applications to special problems aroseniâlhé'-pâst ten-year exploitation.

Development work is being done on other NDT techniques in some areas asneutron radiography and eddy currents; possible applications of those techniquesto fuel performance evaluation in research programmes of national interest areoutlined.

1. Forewpra

She CHEN Hot Laboratories at the Casacoia Hesearch Center nearHome constitutes the major single system in Italy of integratedfacilities,, techniques- ana labour skillness for the examinationof irradiated materials. • . , -Work has been, performed since 1962 on many different fuel andstructural materials in the frame of the GNM programmes withNDT techniques playing a major role in assessing their relativeperformance»Recently, the expanding nuclear programme has shown the need foran increased capacity in examining irradiated materials, with,emphasis on reactor fuel elements.A new independent line of Hot Cells, capable of handling theplutonium fuels required for future -fast and thermal reactors,

2is presently under construction with a 30 m area specificallydevoted to KDT techniques.Great advantage has been taken in the design stage from theexperience gained during the ten years exploitation of the existingfacilities*

49

?. ?0)g available ffechinques

2.1. V isual ^jxaminat ion (fig. 1)

CoBipleee fuel alements as well as small irradiated samples canbe examined visually either directly through the Cell windowsor through a remoted Bausch & Lomb stereomicroscope for moredetailed investigations.Photography provides the necessary recording of visual observations»

2*2.Longitudinal gamma scanning is performed on single fuel pins, (fig.2)carried by a variable speed motor in front of a collimator.The gamma radiation is detected by a Nal (Ti) 3í'x3" crystalfollowed by a conventional electronic equipment.The apparatus is 'being extensively used for determining therelative burnup and burnup profiles as well as for investigatingthe longitudinal migration of fission products.For burnup studies, the usually long cooling and transport timeshave prevented the choice of Ba 140/La 140 as the isotope to becounted; Zr 95/Nb 95 activity is i.ormally used, waile for stilllonger irradiation and cooling times Ce 144/?r 144 is the onlychoice»Sometimes, an integral counting is performed (gross gamma scanning)with a low energy discriminator set at 400 KeV, to get rid ofthe scattered gammas and of gamma shelf ~shielding in the rods.Fission products and other materials diffusion has been .studiedin the past using the garoma scanning technique, as in experi~mental fast fuel pins with venting of fission products to thecoolant sodium.Here the efficiency of the venting mechanism has been studiedby looking for ïîa 22 activity in the fuel and blanket, and forCs 137 in the blanket and the other components. !

50

Other applications of gamma scanning are summarized in (j "^.A new mechanism is being loaded into the Cells, where the fuelpin, which is carried _by a stepping motor,can be translatedand rotated continuosly or discontinuosly according to selectedprogramm es jjj? J.It would then be possible to perform the gamma scanning on lowactivity samples, when a point scanning (fig.3} and a long countingtime are needed» The gamma detector will be a Ge(Li) 50 CO,with a resolution better than 3 KeV on the Co 60 peaks.Future improvements of this technique will be directed towards :a) an extended use of data handling by punched cards and inte-

grated computer programmes, andb) absolute burnup determination by suitable calibration of the

collimator and detector system.In addition to the longitudinal activity distribution, the concentrât ion of fission products along the fxiel radius is measur

. ed by a radial' gamma scanning technique,The apparatus is installed in a lead cell equipped with a smallX-Y optical bench driven by a stepping motor.Two lead colliraators may be used (0.8 and 2 mm); the activityis counted by a Ge (Li) 50 CC crystal and pulses are-sent to amultichannel analyzer (LASEN 4096 channels), ana recorded on apunched tape for subsequent computer", programmes.

2.3. Metrology.Only individual fuel pina can presently be measured on the me-trology bench, (fig.4)Diameters and profiles of fuel pins up to 1774 mm length, -withdiameter ranging from 6 to 30 ram can be continuosly recordedalong different generatrices with a praci&ionof -!0.010 ram. (fig.5)Smaller benches are in use for samples of reduced size.

51

The surface roughness and other localized surface deformationson the cladding (ridging, bulges etc.) will be measured .by areplica technique on a POSTER profilograph to a precision ofsome microns, A metrology bench for complete fuel bundles, capable of operating both in vertical and horizontal positionwill be installed next year,

2.4. Gamma densitometryThe gamma densirfcometry apparatus partially utilizes the sameequipment in use for gamma scanning, the gamma emitting sourcebeing in this case a Cobalt rod, 9.5 mm diameter, 152 mm long,encapsulated in stainless steel fsj.Co-60 has been selected because its photopeaks fall in the re-latively low background existing in an irradiated fuel in theregion of energy immediately higher than 750 KeV. (fig.6)When fuels have to be analysed after a long cooling time, asource of Cs 137 can be used, allowing for a greater sensiti-vity for small diameter fuel rods.The advantage of the method, which is complementary to othernon destructive tests, consists in its rapidity in detectingunambigously fuel cracking and dislocations, while its maindrawback lies in the inherently low sensitivity (less than ifiT.D. of UQ- in absolute measurement of fuel density on 10 mmdiameter fuel rods). (fig. l-io}

2.5. Leak J3? estLeak testing of fuel pins is routinely performed with the aimto provide information for the subsequent destructive tests:the emission of gas bubbles from cladding craks and holes ondepressurizing after immersion in water is used. However, onoccasion the method has proved unsuccessful, giving no failure

52

indication on fuel pins which later proved to have been defected by other destructive tests (fission gas analysis and metal-lography). We have in fact evidence that some kind of defects,at least at the early stages of their formation, have a tendency to close on further limited reactor exposure»On the other hand,surface porosity may also produce erroneousinterpretations„

3. TOT Techniquesunder developmentOther HDT techniques have been brought to different developmentstages by the Research Group of the Hot Laboratories,Moot important are X-rays, eddy currents and neutron radiogra-phy techniques»

3.1.The experimental apparatus is now being tested -as a mock upout of Celia.By using a radiodiagnostic type generator, capable of 1000 mAat 100 KV or 300 ító at 150 KV, and short exposure and transfertimes (respectively 1-2 sees and 0.1 sees) we expect to obtainsatisfactory results in exítmining; even large portions of thecladding*The film is therefore e-'-l^etoc* to have a high sensitivityX(100 KeV) to gamma (Co 60).Particular care has been given to the design of the transferequipment, which uses a spring-acted carrier for the cassette,with an electromagnetic locking device during the exposure time.(fig.ii)

3*2» Eddy cúrrentemMm&*0g&ta*m!ii ivmttnaantaiitimi «u umeim «««ma»

3?he eddy currents method is under study for possible applicationsto detect failures and corrosion attacks on the cladding, aswell as to give indication on the contact aone between fuel andcladding.

53

The expected advantage over ultrasonics is the easy of handlingrequired in the Hot Cells.

3.3. Heutron radiographyNeutron radiography has already been used to examine nondestructively irradiated fuel capsules containing burnablepoisons (Gd (X), in the form of small particles embedded ina U02 matrix, (fig. 12-14}The TRIGA reactor of the Casaccia Nuclear Center has been usedas neutron source; however work is now directed to build asuitable facility in the swimming-pool RAM reactor, with anincreased available volume and better ease for handling andtransport of samples.We expect neutron radiography to provide considerable resultswhen applied to some research at ertis of particular interest inour Programme, as:a) measure longitudinal burnup of burnable poison uniformlydistributed in a fuel pinb) detect and give quantitative data on the corrosion attack

of Zircalo sheaths from hydrogen, both from the coolant andfrom internal contaminants

c) study the fuel dislocation in vibratorily compacted fuelpins irradiated at high power and/or burnup.

4. References

G. Pugnetti, C. Cochiî "Some applications of the GammaScanning technique"XII Colloque de Métallurgie (June 1968)

F2j C. Cochi, L. Babiloni: "Apparecchiatura per il comandoreraotiasato di un gamma scanning per celiacalda" fiT/ING(7l)l

C» CesaranOj c.Ooohi: "Gamma Densitometry Technique onIrradiated Fuel Samples" RT/ING (71)6

54

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Neutron radiography of capsules containingCapsules 11 e 12

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66

Neutron radiography of irradiated capsules containing Gd203 in ÜO,Capsules 13 e 15

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67

Neutron radiography of capsules containing Gd 0 in UOCapsules 16 e 14

Fig. 14

68

m INSPECTION SYSTEM POR USE PRIOR TO AND DURING SERVICE POR THEPRESSURE VESSEL IS THE ATUCHA REACTOR

Juan N. BáezArgentine national Atomic Energy Commission

A B S T R A C T

The Atucha reactor, ñrst Nuclear Power Plant in

Argentina, is scheduled to produce 319 MeV to 1973. The in-

service inspection system for the vessel is described in this

work. The scanning device, the transducers, the inspection

technique and data acquisition system are outlined and general

characteristics discussed.

1. lîiTROBUCTIOîJ

The first Argentine nuclear power station, for which the CNSA concluded acontract with the West German firm of Siemens in 1968, is at present underconstruction at the Atucha. site, on the River Panara, and is due for start-upin 1972. This station, with a natural-uranium fuelled, heavy-water cooled andmoderated pressurized reactor, will be able to supply 319 MW(e).

The original design of the station aid not provide for the inclusion ofan inspection system for checking the operation of the primary circuit. Inview of the experience gained to date on the international level, and theengineering codes and regulations already approved or provisionally in force inthe United States and Europe, the GNEA decided to incorporate a system whichwould permit volumetric inspection of the welding of the reactor pressure vessel.

The first snag arising out of that decision was the fact that there couldbe no question of inspecting welds from the inside of the pressure vessel.Bearing this limitation in mind, the CNEA, after considering a variety ofproposals, decided to commission the Southwestern Research Institute (SWRl)(United States of America) to study, design and "build automatic equipment forultrasonic inspection of the welds in the pressure vessel, nozzles and lockingholts that could be operated from the outside. The commissioned equipment

69

includes transducers, inspection modules and an electromechanical system forpositioning and moving them. The CM3A kept for itself the work of designingthe data acquisition system and the inspection programme, in due course thecontracting firm was requested to introduce certain modifications into the designof the insulation system and the containment for the pressure vessel so that the•inspection system could be installed and operated.

?. CHARACTERISTICS OF THE PRESSURE VESSEL

Material:Forged steel 2? Ni Mo Or'37 (ecfuivalent to ASTM, A50Ô class 11}containing 0.9$ Wi in the core area and 1.5$> Ni in the flange andhead. The inside is lined with 5«5 nm» of arc-deposited austeniticsteel.

Dimensions of vessel body:Internal diameter 5300 ramKadius of hemispherical base ?735 nunInternal height 9730 moiHeight of flange 1850 nunHeight of cylindrical shell 5695 nunThickness of cylindrical shell 220 nanThickness of flange - 455 «unThickness of hemispherical "base 135 naa

!

Dimensions of vessel head;

Internal diameter 4900 mmHadius of concave section 3000 nanHeight of flange 6*00 mmThickness of concave section 340 ma

The head is secured by 60 bolts. The flanges and cylindrical sectionsare composed of three segments welded together.

Working conditions:

Pressure 115.0 atmMean temperature ?67.8°C

70

3. AREAS TO BE INSPECTEDThe design of the reactor does not permit complete access to all the welds

that require examination in accordance with the recent codes. In thesecircumstances the CHEA had to consider the extent to which maximum volumetricinspection of the welds in the pressure vessel was feasible, bearing in mindhow accessible and how critical the different areas were. The inspectionsystem designed by SWR1 enables ultrasonic inspection to be made of:

All of the welding between the cylindrical shell and flange;All of the longitudinal welds in the cylindrical shell;All of the circumferential welds joining the two segments making upthe cylindrical shell:All of the welding between the shell and hemispherical base;All of the welds in the nozzles used for the coolant and moderator;All of the joins between the piping and nozzles}Locking bolts for the head»

4. DESCRIPTION OP THE INSPECTION EQUIPMENTThe inspection e<ïuipment designed and constructed by SWRI consists of

three separate systems (for vessels, nozsles and bolts) and a console thatalso supplies data relating to the positioning and movement of the transducers.4*1- System for inspecting the vessel

This item is designed for ultrasonic testing of the circumferential andlongitudinal welds in the pressure vessel, located between the flange andhemispherical base. It consists of the following parts:

(a) A permanent structure situated in the circular space, approximately300 mm wide, left for this purpose between the pit insulation andvessel ;

(b) An inspection module containing the ultrasonic transducers, permittingthe selected areas to be scanned;

(c) A drive unit for the inspection module;(d) A drive unit &r rotating the permanent structure»The permanent structure consists of a circular rail fixed to the walls

of the pit, and supporting a rotating section fitted with a vertical rail, alongwhich the inspection module can travel. This structure was installed in the pitbefore the pressure vessel was lowered into it.

71

The inspection module and its drive unit were mounted during each inspectionat the top of the pit "by removal of one of the "blocks forming the insulation.

•The drive unit for the permanent structure is hooked up during the inspectionthrough a tunnel providing access in the lower part of the pit.

Scanning of the areas requiring inspection is accomplished by combinedmovement of the module up and down the vertical rail and of the permanent rotatingstructure for horizontal motion. In this way the transducers can be set in anyposition at all on the cylindrical body of the vessel, below the nozzle line.

The inspection module was designed in accordance with the inspectiontechnique applied by SWHI, which requires three transducers arranged in a line,of which the two end ones can shift relative to the central one and be turnedat different angles. The central one is fixed and normal to the tested surface.The arm supporting the three transducers can rotate 9^ about its mid point in aplane parallel to the surface in question, thereby permitting inspection ofboth horizontal and vertical welds. The transducers are lodged in receptacleswith a pressurized water feed for coupling purposes.

The use of three transducers in the module enables one to apply differentexamination methods, and the possibility of varying the angle provides maximumindicating of a flaw and the greatest amount of information relating to it whendetected.4.2. Arrangement for•inspecting nozzles

This consists basically of an annular rail mounted permanently around thenozzle at a convenient distance from the weld, and an inspection module similarto the one used for the pressure vessel. The rail is fitted over the nozzle bymeans of Inconel springs, which absorb the difference in thermal expansion.4.3. Arrangement for inspecting bolts

The head locking bolts have an axial aperture running through which makes itpossible to inspect them ultrasonically from the inside by means of a specialself-centring probe which is guided by a rod from a device on the outside, and fixedto the end of the bolt during each inspection operation.4.4» Console and positioning controls

There is only one console for all the control and positioning devices inthe system, and there are separate independently operating units for thefunctions relating to the different devices. Those for the drive unit of the

72

permanent structure and thé drive -unit of the inspection module indicate andcontrol the following: on/off, direction of movement, attitude, couplingmechanism and speed.

The unit for the inspection module provides for indication and control 01the following: on/off, transducer angle, translation of transducer signal inthe module, rotation of module, and pressure of transducer receptacles.

5. INSPECTION TECHNIQUES

The pulse-echo technique is applied to the longitudinal and circumferentialwelds in the cylindrical shell in the following form:

(a) Scanning of the weld from "both sides at an angle of 45 ;

(b) Scanning of the weld from both sides at an angle of 60 ;

(o) Scanning of weld zone and adjacent area with normal beam (90 ).

The scans in (a) and (b) are carried out consecutively, whereas (c) iseffected at the same time as one of the other two.

The tandem technique is applied at the same time as the pulse-echoscanning, an additional transducer being used as a receiver for this purpose.

For the welds between the cylindrical shell and the flange, and in thepart where the cylindrical shell is joined to the hemispherical baset we use onlythe pulse-echo technique and scan at 45° and 60° from just one side of the weld.Scanning with normal light is effected in the same way as for the cylindricalshell.

Por the nozzles and bolts we only use the pulse-echo technique, at suitableangles for the geometry of each area.

6. DATA ACQUISITION SYSTEMWhen commissioning SWRI to design and construct the inspection equipment

in service, the GNBA detailed an ultrasonic test expert from its END service towork at the Institute so that he could get familiar with the design andconstruction of the equipment, undergo training in inspection procedures priorto and during service, and work on the design of the data acquisition system.

The equipment supplied by SWRI described above provides informationrelating to the ultrasonic inspection in analogue form, and digital data on thepositioning of the module.

73

The data acquisition system designed by CKEA is designed to provide apermanent record of all inspect-ions and to enable the recorded data to besubsequently processed for handling by computers.

Two sets of ultrasonic equipment are used to actuate the three transducersin the inspection module» One of them operates simultaneously with twotransducers - an emitter-receiver and normal angle of incidence instrument -and the other acts as a receiver (45 or 60 ). The data (in analogue form)sampled by four gates plus the infoimation received from the X, Y positionindicators on the control panel are recorded on magnetic tape by an FM recorderwith multiple channels for simultaneous recording.

This FM recorder, which provides a recording that can be processed forsubsequent computer analysis, is supplemented by simultaneous recording on amultichannel oscillographic recorder. The latter is thus used as a graphicdisplay system which is simultaneous with the inspection and also permitscompensation for the magnetic ink recording, since the signal is received, aftera certain delay, from the playback head of the same magnetic recorder.

Incorporated in the magnetic recorder is an audio recording channel so asto include all the additional information required, for example changes inangle of inspection, number of scan and so forth.

Finally, the system includes a video tape recording of the console andoscilloscope screens.

74

Fig 1.- View of the permanent rotative assembly in the Vessel Pit , -

i r>

O3 T 2N,\\\ 03\

X\

T 3

C2

TI , shear wave transducer (emitter-receiver)T2 , longitudinal transducer (emitter-receiver)T3 , recever transducer (pitch & catch with Tl )C1,C2,03,04, gated areas

U.S. Unit1

U.S. Unit2

Cl 02

03

i ' i

Fig. 2.- Draw illustrating the inspection technique

U.S. Transducers

Emitter1

& Receiver

11-J — *~\\I

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————— » ——

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Ii!!1t

1IT"i

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——————— |B»*_

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F MMagneticrecorder

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Fig. 3.- Instrumentation and Data Acquisition System

USE OF NONDESTRUCTIVE TESTING IN THE U.S.A. DURINGFABRICATION AND SURVEILLANCE OF PRESSURE TUBES*

R. W. McClungMetals and Ceramics Division, Oak Ridge National Laboratory

Oak Ridge, Tennessee 37330

ABSTRACT

Nondestructive testing techniques veré applied to Zircaloy-2 pressuretubes for the PRTR both during fabrication and at intervals after operation.Techniques for inspection of the as-fabricated tubes included ultrasonicsfor both vail thickness and flaw detection, radiography, fluorescent péné-trants (on both inner and outer surfaces,}, and visual examination. Examina-tion at intervals after reactor service included (1) visual examinationVising, closed-circuit television and a bore scope; (2) measurement of flawdepth with a device incorporating a dial indicator; (3) measurement of innerdiameter with a probe containing linear variable differential transformers;and_(4).a»asurement with an eddy-current gage of the eccentricity of theannular space between the pressure tube and the surrounding shroud tube.

RESUMEDes techniques de contrôle non destructives ont été e.ppliqxiées à

des tubes à pression construits en alliage Zircaloy~2 pour lependant .leur fabrication ainsi que par intervalles aprbs utilisation,ÏJÔB techniques poxir l'inspection des tubes ainsi fabriqués utilisaientles ultrasons pour la nesure de l'épaisseur des parois et la détectiondes défauts, la radiographie, les huiles pénétrantes fluorescentes(sur les surfaces aussi bien internes qu'externes) et l'examen visuel.ï-es contrôlée par intervalles après leur util! pat-ion <?.mis le réacteur

*Research sponsored by the U.S. Atomic Energy Commission under contractwith the Union Carbide Corporation.

79

comprenaient (1) ira examen visuel utilisant, un circuit fermé de tele-vision et une jr.uge visuelle ; (2) des wesuros cio la profondeur desdéfauts avec un dispositif incorporant un indicateur à cadran ; (3) desir.eoures de diaiabtre intérieur avec uno sonde contenant des transforma-teurs différentiel*; lluoaires variables ; et (4) des recoures, avec unejauge à courants de l'oucault/de l'excentricité de l'espace annulairesEtendant entre le tubo à pression et le tube de blindage qui l'entoure.

INTRODUCTION

Pressure tubes for nuclear reactors in the U.S.A. were used in connec-

tion with the Plutonium Recycle Test Reactor (PRTR) at the Hanford Laboratory

(Richland, Washington). In this application, high-pressure process tubes of

Zircaloy-2 passed through the reactor core in a low-pressure moderator tank

and operated at temperatures up to 542°F and pressures up to 1050 psig (réf. l)

The pressure tubes were 17 ft 5 in. long and each tube consisted of

large and small diameter portions connected "by a tapered center section. The

large diameter section was 14 ft 3 in. long, with a 3.25 in. inner diameter

and a 3.564 in. maximum outer diameter. The minimum wall thickness for both

the large diameter and tapered section was 0.146 in. A transition tapered

length of 17 in. led to the 1 ft 9 in. long small diameter section for which

the minimum inside diameter was 1.54 in.; the outside diameter was 2.065 in.,

and the minimum allowable wall thickness was 0.220 in. The tubes were so

designed that conservative engineering properties of Zircaloy-2 could be used

with the requirement for high-quality tubes and in-service monitoring. The

latter two considerations included extensive use of nondestructive testing.

Since much of this work was accomplished about 1960-61, the techniques are

cited for historical record and not as being indicative of current recommended

practice.

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IKSPECTlCHf DURING- FABRICATION

The physical integrity of the tubes was assured by extensive nondestruc-tive testing at Hanford to detect any significant defects such as seams,cracks, pits,, embedded material., and dents that could produce stress concen-tration or local thinning of the -wall.2 The techniques included ultrasonicsfor both wall thickness and flaw detection,, radiography., fluorescent pénétrants,and visual examination of the surfaces.

Measurement of Wall Thickness

The wall thickness of the large diameter portion of each tube -was mea-sured with a resonance ultrasonic technique, with the transducer in contactwith the tube. A linear translation of the transducer along the tube at arate of 6 in./min with the simultaneous tube rotation of 12 rpm produced anoverlapping helical scan. The values of vail thickness were recorded con-tinuously on a strip chart. The observed variations in the wall thickness wereapproximately 0.010 in. (as eccentricity) in the tube, which had a designminimum thickness of 0.146 in. Accuracy of the measurements was stated to be±0.002 in-

Ultrasonic Flaw Detection

The iaaaersed ultrasonic technique was applied to detect longitudinali

flaws with a l-in.-diara 10-MHa lithium sulfate transducer. The referencestandard was a machined notch with a nominal depth of 0.003 in. on the innersurface of a tube. Any discontinuity on either the large diameter or smalldiameter portion of the tube that produced an indication equivalent to orgreater than that from the reference standard was considered objectionable.A few of the tubes originally examined had indications of small but objection-able discontinuities. However, upon re-examination after pickling andautoclaving., all of the tubes were free of discontinuities.

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Fluorescent 'Pénétrant Examination

Post-emulsification fluorescent pénétrant was used to examine "both theinner and outer surfaces of the tubes. It was not used for rejection butrather to pinpoint areas for radiography and visual examination. The stepsin the process included precleaning in an alkaline solution, followed byrinsing and drying; a 20-rain dwell time of the fluorescent pénétrant; ¿-minemulsification; thorough washing of both surfaces with a water jet; 5-minapplication of a wet developer suspension; and finally 10-min drying with hotair. The outer surface was then examined under a bank of mercury lamps, andthe inner surface was viewed with a borescope with a 90° viewing head andultraviolet illumination. The tube was rotated at 10 rpm as the borescopeadvanced at about 12 in./min. All indications were recorded for subsequentevaluation. To avoid spurious signals, particular care was taken to providecirculation in the emulsifying bath, to assure thorough water rinsing afteremulsification, and to aàniadze contamination between baths during handlingand transfer of tubes.

Radiographie Examination

The tapered sections of all tubes as well as all areas with fluorescentpénétrant indications were radiographed for discontinuities with 23Q-Wx rays. Type M film was placed inside the tube to allow radiography throughonly one wall thickness of the tube. The penetrameter for the large diameterand tapered sections of the tube was a 0,003-in.-thick piece of Eircaloy-2containing three drilled holes 1/16 in. in diameter. A Q.005~in.-thick 'piece was used for the thick-walled small-diameter section. A few small pits(less than 0.003 in. deep) were detected in addition to a longitudinal seamthat was also detected by both ultrasonics and pénétrants.

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IN-SERVICE

Since the PRTR represented one of the first major applications ofZircaloy-2 as a load-bearing structural material at elevated temperatures andbecause of the limited experience in comparable operating environments, asurveillance program -was -undertaken.3 As part of this program, remotelyoperable experimental equipment was developed and used in the PRTR for(l) visually examining the inner surface of the pressure tubes, (2) measuringthe inside diameter, (3) measuring the eccentricity of the annulus betweenthe .pressure tube and its surrounding shroud tube, and (4} measuring the depthof gouges, dents, corrosion, or other attack. The monitoring schedulerequired examination of-different selected tubes at the end of each ten-dayoperating period.

Visual Inspection

Initial attempts to couple a television camera to a borescope werediscouraging because the light losses in the borescope prevented adequateillumination for a good-quality picture. Therefore, the initial inspectionswere conducted with a remotely controlled television camera that was suffi-ciently small to fit inside the nominally 3.25-in.-ID pressure tube. Laterexperiments with a special vidicon camera more responsive to low illuminationlevels coupled to a good-quality borescope showed that improved (less costlyand more easily operated) viewing .was possible. The latter system was thenused.

Measurement of Discontinuity Depth

Each surface discontinuity observed during the visual examination to bea partial penetration of the tube wall was measured for depth. A dial indi-cator device was mounted on the end of the borescope. A shoe, curved to

83

match the curvature of the tube, was attached to the dial Indicator, and theplunger of the indicator passed through a hole in the shoe. A helium actuatormoved the dial indicator in a radial direction so the shoe would fit againstthe wall. The spring-loaded plunger would thus be depressed to provide a zeroreference reading. Positioning of the device over a gouge or other .discon-tinuity allowed direct measurement of depth to be observed through the borescope.During the course of the examinations several wear corrosion marks wereobserved that were prodxtced at contact points by the end brackets and otherexternal featxtres on the fuel elements.

Measurement of Inside Diameter

A special gage was constructed containing four linear variable differen-tial transformers (LVDT) spaced at 90" intervals. Opposing LVDT1s wereelectrically coupled to provide two orthogonal measurements of the innerdiameter. Each LVDT contained a spring-loaded, tapered pin whose hai'denedtip was in contact with the wall. The gage was adjusted to null at the nomi-nal value of 3.250 in., and both positive and negative displacements in diame-ter were recorded on a strip chart. The measuring accuracy was ±0.002 in.

Measurement of Annulus Surrounding the Pressure Tube

An eddy-current device* was developed to provide assurance than an ade-quate insulating annulus was mintained between the pressure and shroud tubes.The probe assembly contained four sensing coils spaced at 90° intervals. Thediametrically opposed coils were electrically connected in two legë of anac bridge. The operating frequency for the eddy-current bridge (5 kHz) wasselected to allow optimum transmission of the electromagnetic signal throughthe Zircaloy-2 tube wall (with minimum effect due to small changes in wallthickness of the Zircaloy) and yet maintain maximum response from the aluminum,

84

which has a higher electrical conductivity. The frequency is sufficiently highthat the minor variations in aluminum thickness are insignificant on the eddy-current response.

After the ac bridge is balanced, any difference in spacing between atcoil and the aluminum shroud tube, when compared to its opposing coil, will

produce an unbalanced condition that is related to the change in spacing (oreccentricity) in the annulus between the Zircaloy-2 and aluminum tubes. Aphase detection circuit determines which coil is nearest the aluminum.Opposing pairs of coils are alternately activated at 5-sec intervals. Theaccuracy of the system under the worst acceptable condition of variation inZircaloy vail thickness (from 0.140 to 0.170 in. under matched coils) isapproximately ±0.025 in. near zero eccentricity of the annulus and improvesto ±0.010 in. at eccentricities greater than 0.100 in.

The inner diameter and annulus gages were both mounted on a single probe;measurements veré made and recorded simultaneously.

SUMMARY

Nondestructive testing techniques were applied to Zircaloy-2 pressuretubes for the PRTR both during fabrication and at intervals after operation.Techniques for inspection of the as-fabricated tubes included ultrasonics forboth wall thickness and flaw detection, radiography, fluorescent pénétrants(on both inner and outer surfaces), and visual examination. Examination atintervals after reactor service included (l) visual examination using closed-circuit television and a borescope; (2) measurement of flaw depth with adevice incorporating a dial indicator; (3) measurement of inner diameter witha probe containing linear variable differential transformers; and (4) measure-ment with an eddy-current gage of the eccentricity of the annular space"between the pressure tube and the surrounding shroud tube.

85

REFERENCES

1. R. L, Khechtj Fabrication of the PRTB Zircaloy-2 High Fressure ProcessTubes, HW-6Q358 (July 31, 19597";

2- R. L. Knecht, The Physical Integrity and Corrosion Resistance of the2irealoy-2 Pressure Tubes for the PRTB, HW-67677 '(December 196ÏÏJT

3. D. R. Doman and P. J. Pankaskie, ln-Eeactor Monitoring of Zircaloy-2 PRTR-Pressure Tubes, Part 1, Sept ember~3560-May 1962 [, HW-73701 (Rev.j

4. J, C. Spanner, An Eááy-Cixrrent Technic|ue for Measuring the Frocess-to-Sbroud Tube Annulus in the PKCR, ffi-i-70453 (July

86

KEUTROH RADIOGRAPHY. AN ATIPJICTIVB MEflHOPFOR m TON-JESTRUCTIVE fBSCTG Off IRRA-DIATED FUEL SPECIMEN

W.J. StammIT. van der Kleij

ABSTRACT

Unlike X~radiography, neutron radiography can be a useful method for thenon-destructive testing of irradiated fuel specimen. In this paper a sum-mary is given of the principles and possibilities,^and also a short des-cription of the neutron radiographie facilities present at RCN in Petten.

'The principle of neutron radiography is essentially similar to that of'X-radiography in that a beam of particles is attenuated OR passingthrough matter, and the attenuated beam forms an image of the objectunder inspection on some suitable recording material. Neutrons are usedin a similar manner as X-rays, although in some circumstances they givea poorer radiograph. Their advantage is that the difference in absorp-tion between portions of a specimen may bo much greater for neutronsthan "for X-rays, and since it can be unaffected by interfering gammaradiation, the neutron radiograph can be superiod in contrast.Unlike X-rays, neutrons are not directly detected by photographic film;this material is far too insentive. The method normally employed usesan intermediate foil in contact vith the film. The foil converts theneutron image into alpha, beta or gamma radiation, and it is this sec-ondary émission which is detected by the film.One variation of this method (the transfer technique) docs nor requirethe film to be placed in the neutron beam and thus avoids the problemof gamma fogging of the film during radiography of radioactive matat-ials. This advantage makes neutron radiography an attractive methodfor the non-destructive inspection of encapsulated irradiated reactorfuel and other materials undergoing irradiation tests.

87

For example» the inspection of reactor fuel specimens after receivingknora burn-ups is uecessary for the development of reactor fuel?. , Ear-ly detection of physical changes is esscncial; the measuremeiits ot" therat*» of .swelling of a small fuel raniplc within a can is o typical prob-lem. The fuel could be rcraoved from the can and measured, out chis ef-fectively prevents further irradiation of the sample under unchangedconditions. However» neutron rotliography by the- transfer technique iswell suited for carrying oi*t medsnrcrxinfs of tbir, naf.arti., although inpractice it is not capable of such high resolution es X-radiography.Further principies oí psutcon rarliogrsphy can be found in tef. \\\As neutron source the High Flux Reactor (HTP.) of the Euratom Centre inPetten is used. This is--a light-watsr moderated and cooled materialstesting reactor (réf. '(2J)1, In ooe of its neutron irradiation facili-ties, the poolside facility,'an apparatus for neutron'Tadiography islocated. This apparatus is designed 'primarily for the non-destructiveinvestigation of fuel pins and construction materials in the Hi'R.The apparatus can be devided into the following main parts (fig.I):collimator» diaphragm, objectholder, cassette system and a valve sta-tion. The length of. the conical colliniator is 190 cm, the diameter ofthe diafragm can be 2, 4, 6, 8, 11 or 18 nua. The object holder consistsof a rectangular tube, cross section inner dimensions of J3 ens x 18cmand length of 81 cm. The cassette system contains a dysprosium conver-ter foil. A valvû station is needed because all movements must be con-trolled hydraulically. The thermal neutron flux on the converter foil

5 7 —2 ~1can be varied from 5 x 10 to 6 x !0 cm s by changing the diafrsgm.Further details are described in réf. J 3 J .The apparatus has recently been put in operation, and the first resultsare qu.ite promising. In fig.2 a neutron radiograph is shown of a testcapsule which contains a number of different materials.

Up till now neutron radiographs of fuel pins at the )1FR have been takenby moans of a provisional installation. Fig.3 shows a neutron radiographof ao irradiated fuel pin. It is easy tro see that the central part ofthe pin is molten. The irradiation time of the 100 ym thick dysprosium

7 -2-1converter foil vas 8 min. in a thermal neutron flux of 2 x 10 cm sAfter irradiation the foil has been put in close contact for 2.5 min.with an Agfa-Gevaert D7 film.

88

A second possibility for neutron radiography work at RCN consists ofthe Low Flux Reactor (LFR), while probably also a beamtube of the HFRcan be used for this purpose. In both cases mainly non-radioactive ob-jects can be investigated.

In order to utilize all possibilities of the neutron radiographie tech-nique research and development of detection methods will be necessary»

|ij P.J.de Munk, NEUTRON RADIOGRAPHY, A SUMMARYOF PRINCIPLESAND POSSIBILITIES. Report RCN-138, Reactor Centrum Neder-land, Petten, May 1971.

J2J J.J.M.Snepvangers, S.H.Woldringh, EXPERIMENTAL FACILITIESOF THE RCN MATERIALS TESTING AMD RESEARCH REACTOR. ReportRCN-Ext~i097, Reactor Centrum Nederland, Petten, Septem-ber 1959.

|3[ P.J.de Munk, H.P.Leeflang» AH APPARATUS FOR NEUTRON RADIO-GRAPHY. Report RCN-135, Reactor Centrum Nederl.and, Petten,March 1971.

89

FIG.1 APPARATUS FOR NEUTRON RADIOGRAPHY

90

PIG.2. Neutron radiograph of atest capsule taken "by meansof the new apparatus forneutron radiography in thereactor pool of the HPR

91

FIG. 3. Neutron radiograph of an i r r a d i a t e d

fue l p in.

92

POST IRRADIATION INSPECTION OF FUEL ELEMENTS(EXISTIMO METHODS AND EQUIPMENT USED Iff THE STODSVIK HOT CELLS)

L. Jansson, AB Atomenergi, Sweden

IntroductionThe hot laboratory a" Stvdsvik wf.s planned firstly for th". studyof tha physical,and suitailurgir.al properties of materials irradiatedin the materials testing rtacuor R2. Secondly it was intended forexamination of large fuel elements irradiated In reactors otherthan R2, and besides to fill the need for a general service facilityfor high active handling of hot material e.g. change and repair ofisotope irradiation sources.

The floor plan of the laboratory is given in Fig. 1. The cell blockconsists of 'two large ce)Is with dimensions (length» width, height)4x2,5x4 m separated "by e moving wall, five small cells havingdimensions 2x2x4 m and a tnicroucope ..ead brick cell at the end ofthe cell row. Each large cell is fitted with tt u pairs of Etascerslave manipulators (Nuclear Limited No. 9 and 8) one lifting craneand one window. A General Electric Arm manipulator works in thetwo large cells, which if necessai-y can be joined to give a total 3working area of 9x2.5 it. The shielding is 1.05 m concrete (3.7 g/cmoor 2.4 g/cm ) providing adequate protection against y irradiationfrom sources of at least 10 curies 1 MeV y photons (large cells).

The cells are the g, y-type, thus primarily not intended for exaairia-tion of a~active material. It has been shown, however, by theoreticalinvestigations and practical experiments that irradiated ï'u-enrichedmaterial can be examined in the cells without heaJth hasards usingtha accompanying y-activity for monitoring.

A description of the use of the various' c^lls is given in Table 1.

1. Visual inspection and disassembly1.JL_^AssembliesVisual inspection is made through the cell windows by means of thenaked eye or through binoculars. Photographs are taken usually withLinhof or Hasseublad cameras and oftcti with Polaroid film (neg. +pos.).

93

For the fli.{î<':>si'<:-ri<bly vaiious tools are «sea, the most, conasoply usedbeing tï»c "FCÍTI" síiv «ílv'ch is an electrically driven hadk-saw withcompressed- -air fe^U. M&chiues tor milling, drilling «met turning arealso available.

Visual inspection is aadc through the Bausch & Lomb Steieo Periscope«(magnifications: 5, 7, i 4 ot¡d 40x) . The rods are placed under themicroscope on a table wïiich is moveable in all three coordinatedirections. To facilitate the inspection a roller mechanism rotatesthe rod. The illumination is arranged with one ring-shaped neon, lampand two moveable 300 W spotlights. Fnotographs are taikcn through theperiscope, Polaroid film (ncg. +• pos,} being used. The possibilityof taking stereo photos is usually tiot being use<?,

2» Surface replicationNot performed.

3. Surface^ deposits

Usually a sufficient quantity of the surface deposit is received bywhiping the rod with a piece of cotton soaked in alcohol.

Such crud deposits which attach very firmly to the canning havesometimes been loosened by heating the sin-face with a type of"soldering irofj".

Applying a piece of tape on the surface aacl stripping off has usuallynot given a sufficient quantity of material adhering on the tape foranalysis.

3,2Analysis is made with purely chemical methods. X-ray diffractionidentification and analysis has also been performed.

4 . Metrology

AB AtomeuergiTs metrology rig Goliath has been designed specificallyfor accurate dimensional Tueasureiuents of nuclear ftu±l cléments. The

94

standard instrument can perform diameter and profile nieosuretoenl's offuel bundles and individual roda as well ¿>s length if«.i«£uteir.£mt£ ofindividual rods. Specialised accessories have been developed tor,among other things, measurement of inner diameter of canning tubes,hollow fuel rods and spacers.

All measurements, w:th the exception of the length measurements,are continous and thñ results are registered on a chart recorder.The. instrument has, ir, order to make automatic data processingpossible, been equipped with an extra output for connection to ananalogue-digital converter.

An adapter consisting of an analogue-digital converter and tapepunch has been developed.

A¿2__GeneralThe metrology rig consists of a cast bed along which the measuringhead can be moved on ball bushing guides. The measuring head canalso be moved in two directions at right angles to the axial movement,The head movements are achieved by synchronous motors.

The top end of the specimen is attached to the spindle of a headstockand the bottom end rests against a holder which is springloaded andmoves in a ball bushing guide in the tailstock. The specimen isrotated to the measuring position by a synchro-torque receiver. Toavoid distortion of slender specimens, the measurements are carriedout in the vertical position. For this purpose it is possible totilt the whole rig from the horizontal to the vertical position bymeans of an oil-stabilized pneumatic cylinder. An instrument unitconsisting of a recorder, a feeding system ror the transducers, anda control unit with switches for movement of the Measuring head androtation of the specimeti, is included in the equipment.

The outer rod diameter is measured continuously with two metal edgesalong the. specimen in the vertical position. The edges have arelatively, large radius of curvature at-the contact points ana,owing to low interfacial pressure of the edges on the specimen, notraces in the form of scratches are left on the fuel tube, rod orelement. The calibration of -the measuring head is performed bysetting pieces.

95

Simultaneously with the maasurüment of the outer diameter, themeasurement of the profile (bow) can be tuarle. The results «rerecorded automatically along the length of rhe tube.

Measurements of the inner diatnoter of fuel tubes, hollow elementsor spacers» may «be carried out using the rig in the wrtiealposition. With auxiliary equipment the inner diameter can becontinuously measured along the whole length»

The measurement of length after disassembly of the bundle with tharig in a horizontal position takes place in a V-groove in the castbed between a fixed and a moveable measuring plane. The positionof the raoveable plane is indicated by a transducer. The method ofdetermination is thus a comparison between a known length and theunknown specimen length.

a) Measuring head for diameter and profileMeasuring capacity

AccuracyTotal 0-115 mmProfile œeasurements * 0.02 nanDiameter measurements * 0.005 mm

b) Length measurementsTotal measuring range 0-3200 ramRange of measuring head * 0.5 mmAccuracy * 0.01 mm

5. Gattgna scanning

A study of a typical fission product spectrum will show the presenceof several photo-peak superiapor °.d on a broad Compton background.these peaks are found at 0.14 MeV (Ce-144)» 0.51 MeV (Ru-103, Ru-106) ,0.76 MeV (2r-95, Nb-95), 1.6 MeV <La-MO) and 2.19 MeV (Ce-144).

96

The choice of suitable fission product io. scanning energy isgoverned by several factsa) The fission product must emit photons of sufficiently

high energy to reduce self-absorption inside the fuel.This rules out the 0.34 MeV peak.

b) The energy resolution of the peak should be high. Thisrequirement makes the 0.51 MeV peak less suitable toNaJ(Tl) "detectors.

c) The fission yield should be high and accurately known.

d) Migration of the fission product inside the fuel or lossby evaporation must not happen. This is a very seriousdisadvantage with Cs-137.

e) The half life of the fiesion product must be suitablecoorçared to the irradiation time. Short lived fission.products will only remember the late part oí the irradiation,

A list of merits for interesting fission products follows:144 144Ce -Pr has the advantages of a long half life and high

photon energy. Low branching ratio will howevergive too low activity for scanning of experimentswith short irradiation time»

Ru-106 has the same advantages and disadvantages. In thosecases where it can be used it can however give ameasure of the fission rate fro-n Pu™239.

140 140Ba -La has the disadvantage of a short half life.95 95Zr ~Nb is commonly being used if the irradiation is of

moderate length.

Ce~137 has the disadvantage of migration.

5.2.1. Relative burnup measurements,, axialWhen Y"scanning is made in the hot cell the fuel rod (max. lengthof rod section is 750 mm) is driven by means of an electric aotor

97

past « colllmator slit placed in a plug hole in the cell wall. ANaJ(Tl) scintillation crysi.al i 3/á"x2'* is used as a detectortogether with a single channel analyser. Alternatively a Gc(Li)detector and a maltichau^eT ¿in^lyscr can be used but NaJ derectoris the si-anOard equipment, Collimatm- slit mostly 0.5x10 mm. Therod is driven at a constant speed of 2 Î/2" per min. This speedhas sometimes León found too high,

A new y-scauner which is being designed will allow a speed variationfrom 5 to 500 uaa per min, This scanner will also give the rod arotation during scanning.

Wuere more exact results are necessary a second equipment isavailable. This consists of a fuel element transport flask whichhas been modified as a facility for gamma scanning irradiated fuelelements up to a length of 75 cm. By means of a Ge(Li) detectorsatisfactory activity profiles along the specimens have beenobtair -a. An annular plastic detector surrounding the Ge(Li)detector allows cyaratioti of the spectrometer in the anti-coinci~dence mode and a 50 % reduction of the Compton background has beenobtained.

5.2.2 Relative burnup measurements, -diametralDiametral scanning of thin fuel slices is being made stepwise witha racking device which permits movement of the slice past thecollimator hole in short stages.

6. ^ y densi tome tryY-densitometry is not being performed. Neutron radiography canhowever be made, at the R2 reactor on fuel rods up to a length ofabout 500 mm»

7. Piercing

The system for collection of gas from the pierced fuel rod is inprinciple only a volume which can be evacuated. The gas is let toequilibria in the system and the pressure is measured with a McLeodgauge. The volume, including the free volume inside the rod is

98

determined by letting in a knoi*r- Miami t of gas £.nd measuring thepressure.

For pi-srcing we use a high spf*d stainless steol drill driven byan asynchronous motor. The motor has been modified with the incorporation of a thin-walled sleeve between the rotor and the ota tor.In this way all moveable parts in the puncturing device bave beenplaced on the evacuated side of the system. The sleeve contimtcswith a bellow and an cud piece with a ring-formed neopren gasketwhich will tighten against the rod to be punctured.

One sample and a spare sample are brought to masspectrometricanalysis. Kr, Xe, Ar, N2> 0»» CO and CO^ are analyzed from thespectrum. H2 and D? cannot be analyzed in the spectrometer used.

A sample of known volume and pressure is analyzed for Kr-85 byY-spectroroetry through comparison with a Kr-85 standard of knownactivity.

8. Leak testingCanning leaks are usually detected by immersing the rod which isput under overpressure of air into water. A film of soap-wateron the surface is an alternative to immersing the rod in a waterbath. Equipment €or helium leak detection is available,

9. Absolute bornut» measurements— - - - - - - - - - - — - - - - - - ~ - - - • - - —

Development v?ork is going on to determine burnup by means of gammaspectroraetry using Ge(Li) detectors. SChere ¿re several inherentadvantages in this method:a) It is a non-destructive method.b) It is possible to differentiate between fissions in U-235

and Pu-239 by studying the Ru-106 activity.c) The method is suitable for analysis of diametral distribution

of fission products in the fuel, which is a problem ofincreasing concern in connection with a study of fuel swelling.

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9.2 Mass £PsS'í:£Mass speci:i:onif trie analysis of the stable l*.o isotopes in a samplevhicU hcs been diluted wii;h a known quantity of natural Mo hasbeen fhe standard uic>.rbod In use in fîtudsvik, Natural Mo is nowreplaced by <? Tsiixtur« of Ho-94 and Mo-96.

r>evelopment work is going on with the neodymiuin method.

Cutting and mounting of samples follows one of the alternativesbelow,}.) A section of about, a pellet length is taken by means of a.

tube cutter. The cpeciraen is put in a brass tube niouldhaving a paxspex bottom. The mould bas been covered witha thin fun» of silicon greasfc. The mould is filled with

2038 epoxy lesin and hardener 3416. After 16 hoursdening the sample is am

and pressed out of the mould.fr hardening the sstnple is annealed for 3 hours at 65 C

2) The rod is cut in lengths of max. 290 satn by means of a tubecutter. The canning is perforated by 2 ran holes. The specimenis placed in a mould where it is treated with Hysol 2038 andhardener 3410 after which it is transferred to an autoclave.The autoclave pressure is reduced 30 ran Hg below ambient for2an hour after which it is slowly raised to 125 kp/cm . Atthe same time the ter^perature is raised to 65-70°C. Pressureand temperature are held for 1C-18 hours. Suitably longspecimens are cut with a diaraond wheel cutter (300 r/min)under water cooling.

II. ^fetaljographic reparationAll grinding and polishing is made on Atcherly machines,

Grinding: 220, 320 and 600 grit silicon carbide paper using wateras fluid. 240 r/miu.

Coarse polishing; Hypres 6-vi diamond paste on Hyprocel Pa-K clothand Hypres Fiuid, 160 r/tnin»

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ainte polishing: Hypres 3 \> diamond paste on Hyprocctl Pí>~K.cloth and Hypres Fluid, 160 r/min.

Fine polishing: Hyproc 1 v diamond paste on Hyprocel Fa-K clotharid Hypi.es Fluid. 160 r/min.

Final polishing: Attack polishing with a. mixture of 100 cc H^O +10 cc HNO + 2ce KF + 15 g Al.,0. 0.05 u on a Polimetal-B cloth.3 ^ -j160 r/inin.

Etching: Chemical etching with a mixture of 5C cc H^CU ^^ W/P)* 40 cc HNO, •* 30 ce RF for 15-20 sec. will reveal the zirconiumhydride structure.

After microscopy and evaluation of the Zircaloy structure the L'Ostructure is studied after etching in a mixture of 10 cc tUSO, +90 cc H202 (30 Wo) for 2-3 lain.

Ultrasonic cleaning is made between all'the above steps. A Grundigtelevision equipment is installed for raacro examination.

12._ MetallographyAll metallography is made on a Reichert Telatorn microscope.Magnifications: 75 to 1500 times. Polaroid filia (neg •*• pos) isused. The distribution of grain size in a specimen can be madeby an attached linear analyzer.

13.Performed on the Raichert microhardness tester.

14. AutoradiographyAutoradiography is only in a development stage and is not yetused regularly.

.15. Specimen replicatioaReplication with plastic film has been used occasionally forelectron microscope replicas. The methods are not used regularly.

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Betennirtat-icm of the hydrogen content in Zircaloy canning is aroutine iss&sutemcnt.

The canning samples are small discs, 6 mm in clinmetor, v?hich arepressed out from the canning tube. The samples are heated in ahigh-frequency furnace which is part of an evacuated system. Ihehydrogen (and other gases i,e. deuterium and nitrogen) diffusesout oí- the Zircaloy as the temperature is raised. The gas mixtureis in contact with a palladium fitter, through which hydrogen anddeuterium readily passes while the remaining gas quantity doesnot. The amount of hydrogen •*• deuterium is finally determined bya pressure/volume measurement.

11.1 Micrqprobe analysisMicroprobe analyses have occasionally been performed on irradiated.canning and fuel material » The instrument used (Cameca MS 46} ishowever not properly equipped for active work and therefore thereare severe limtaticns as far as activity levels are concerned. Thabackground radiation level from the specimen must not exceed afairly low level, Fur that reason only minute specimens can be usedand the preparation cost is considerable.

17.2 Mechanical tes tingAt the adjacent Physical Metallurgy Laboratory mechanical testingof constructional materials is performed. The equipment used islisted in Table 2.

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Table 1 - P&KC ci.p-ímT. of use of the various cell-,'

Coll Dimensions Typs of work EquipmentNo. ^ ________________________ ____ _ __ _ _ _ ______

7 4x2.5 DisassemblyVisual, inspection Stereo periscopeDimensional measure- Metrology rigmentaGamma-scanning Gamma scanner

6 4x2.5 Disassembly Fein sawPiercing

5 2x2 Cutting of samples Diamond wheel cutterExtraction of Comoni tors

A 2x2 Radiochemistry • Mettler balance

3 2x2 Disassembly of Cutting machineirradiation rigs for „ .... ., . „,& Small tensile test macnineconstructionalmaterials (Phys.Metallurgy Section)

2x2 Measurement of Comonitors

1 2x2 Metallographic Grinding and polishingspecimen preparation apparatus

TV-equipment for macroexamination

0 Microscopy Keichert Telatoin microscopeEquipment for linear analysis

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Tub! f- 2 - Mechanical^Tgsting.

Cel 1 forReception of Cutting devicesaraples

Micro hardness Volpert Diatestor

Tensile testing 10 ton AMBLER machine5 ton INSTRON machine, active and non-active testingin the temperature r?nge -200° to 80Q°C

Microscopy Sample preparation: FEIN sawDISC ATOM cutting vheel

Coarse grinding: Grinding machines with 180, 400and 600 grit paper

Polishing: Polishing machines with Kicrocloth A f.,Electropolishing: DISA ELECTROPOLEtchingMicroscopy: Reichert TEL ATOM 62 (75-1500x)

Impact testing Mohr & Federhaff» 0~5 kpcmJiiniaturized Charpy test specimens: 3.33x3,33x27,5 ismTemperature range: -195°C to 365°CVolpert, 30 kpcmCharpy test specimens 10x10x55 ramTemperature range: ~195°C to 365°C

Creep testing 7 Mohr & Federhaff creep machines, temperaturerange 0-800°C

104

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1 I

1LJL1_L

i

Jii J.

i* »* 2.;^«

5«ó.?.O *

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Large cellSmall cellKicrosooee cellOperating areaCrair.ary ohar.jte rc^TiFrog suit change roomKilter roomLeading areaActive store

i^ion areaService roen? fer rc«err.ar.írulator

Pig. I Floor plan of the laboratory

IAEA Panel on NDT.Cor Reactor Core Components and Pressure Vessels

6* "Post Irradiation. Inspection" (Sweden)aimmmmmmmmsta"tmmmai'*'O'>'<»<*ata'm**'Bztit*'**t**'**"meti*'*ie

Neutron Sadiography in Sweden by B. Sokolowsky, AB Atomenergi, SwedenNeutron radiography ha» been used routinely as ana SOT tool at the Atomicenemy laboratorv at Studsvik for some years, primarily for investigatingreactor fuel elements in connection with irradiations in the materialstesting reactor. Neutron radiography is a F.-ISC method for surveying defectsin the fuel, such as cracks, "orosity migration and dimensional chances,prior to more <let«ilcd rictallojvranhic examination in hot celia. For reactor-irradiated components it is indeed the onlv radioaranhic method available,since X-radiation vould be completely masked by fMKwa radiation frow theobiect. Non-nuclear apnlications of neutron radiography are also beingconsidered. These are primarily based on th« hi<»h sensitivity of themethod to hydrogen. Thus, neutron radiography has been used €or detectingthe onset of hydride formación in a isctallm-<v?^.t i«t«dy of the "sun burst"phenomenon in zirconium, and neutron radio!>,rani-uc studies of the moistureuptake in building materials are being planned.

The swimming-pool reactor *?2~0 (1 'ft?) supolie» the neutrons in both neutronradiography facilities in operation at Stu'Ssvik. One of these is completelyimmersed in the reactor r-ool, vhich provides tfdemiate shieldninR forradioactive objects. The othc^r extends tbrou<rt> tho concrete shield of thereactor and is more var^iJtiîc? with regard to obî^ct s?ae and shane.Furthermore, this latter facility i» designed to give a .low Rawraa background.Both radiographs use conical collitnators that pive a geometric unsharnness

7 2of about 0.1 ira» and a. flux áensitv at the oí->}e<*t oí «bout 10 neutrons /. em a.

Some development work on SIM. 11-source neutron radiography is in progress atthe Department of Reactor Physics at Chalmers Technical University, where aradioactive source (mnericium/beryIlium) and an electrostatic generator areused for neutron production. Emphasis is lii<« on detection methods.

106

BASE LINE ATO INSERVIGS INSPECTION OF THE STEEI» PRESSUREVESSELS I» SWEDEN

ABSTRACT

The comprehensive nuclear reactor program in Sweden has called for an extensivestudy of possibilities and means for periodic inspection with the aid of îJDTmethods» The base line inspection of the Oskarshanra BWR unit 1 pressure vesselwas carried out during the period 1969-1970* The experience gained in connectionwith this inspection work has formed a basis for determination of inspectionsystems for other BWR units now being erected in Sweden. Special designconsiderations have been taken for the inservice inspection of the bottom areaof these vessels, using a special design ultrasonic manipulator. Various typesof outside channels will provide accessibility to the vertical and circumferentialwelds in this area» For a PWR unit in the 800 MW range, now under erection»the same ultrasonic inspection technique will be employed as for the BWR pressurevesselso In this case a universal positioning fixture based on a telescopic,central mast design, will be used,, With this fixture all of the vessel andprimary inlet and outlet nozzle welds will be reached for inspection from theinside of the vessel.

By 1980 there is expected to be installed in Stieden about 10 nuclear powerunits with the rating between about 400 and 1000 MW. It is anticipatedthat one unit will be taken into operation each year in the middle of 1970sand two units per year towards the end of 1970s.

With a view to the high demand for safe operation and high availability ofnuclear systems this comprehensive programme has called for an extensivestudy of the possibilities and means for periodic inspection with the aidof non-destructive test methods. This has necessitated a very close co-operation between Swedish authorities, reactor plant owners, reactor system

107

suppliers and inspection organizations. The planning of the inspectionwork and the development of inspection programe has to a large extent beenhandled by these parties in co-operation.

The main work in the field of inservice inspection has been related topressure vessels for boiling water reactors. Work in connection with baseline and inservice inspection of pressure vessel s has been carried outduring recent years for a,o. Agesta Power Station, Marviken Power Stationand Oskarshamn Nuclear ower Station unit 1. At the moment work on designand manufacture of equipment for inspection of reactor pressure vesselsis in progress for three more boiling water reactor plants now under erectionin Sweden.

Oskarshatansverket unit 1, a BWR station ordered in 1966 from Asea-Atow,is currently (Autumn of 1971) approaching commercial operation, Gskarshamns-verket is the first nuclear station with a true base line inspection of thereactor pressure vessel on record. This so called fingerprint operation wascarried out with the same mechanizedr remote controlled inspection systemthat will be used in tuture inservice inspection operations.

The Swedish work on developing means to inspect reactor vessels started in1966, subsequent to the Pilsen conference, with a study carried out by theSwedish inspection and testing organization Tekniska RcSntgenceritralen AB(TRC). This investigation confirmed that ultrasonic scanning throughstainless steel cladding is a feasible inspection technique.

The next step was taken the following year when Southwest Research Institute (SwRI)was brought into the picture. This organization had a limited amount ofexperience in inspecting reactor vessels, and its personnel were activelyengaged in committee work with the aim of developing & draft for an inserviceinspection code.

A preliminary inspection programme was drafted by a group of people represent-ing the utility and the two consulting organizations mentioned above, Basedíon the recommendations in that report Oskarshamnsverkets Kraftgrupp ÂB (OKG)ordered a remote handled ultrasonic inspection system.

108

The development of inspection techniques as well as the task to design andmanufacture inspection equipment was entrusted to TRC and SwRI in collaboration,TRC was engaged to perform the base line and subsequent inservice inspectionoperations.

When the inservice inspection programme was» being prepared, the fabricationof the pressure vessel had already started and the reactor building andpiping adjacent to the reactor were designed. Even though some of theproblems associated with inservice inspection already had been consideredin the original design, of the system, problems of accessibility for inspectionbecame apparent and some design changes had to be -made to facilitate inspectionat some specific points of the vessel.

One design parameter that could not be changed was the 50 mm annulus betweenthe thermal shield and the vessel wall. In order to enable inspection ofthe vessel without removal of the thermal shield and the moderator tankthe ultrasonic module assembly for the inspection of the vessel shell had tobe designed to pass through the annulus, This caused considerable troublein equipment design and called for a very cautious and precise handlingduring positioning and inspection operations.

The technical considerations of the inspection programme were discussed soas to take into account the most probable failure locations based on designcalculations, the potential failure mechanism and metallurgical changes dueto irradiation. It also became apparent that a complete base line inspectionwould be necessary in order to collect data for reference purposes. Theinspection programme thus laid down must be considered as a first real attemptto achieve, within reasonable technical and economical limits, an inspectionaimed to be as complete as ever possible. In Sweden as in many other countriesthe requirements concerning inservice inspection of nuclear reactor coolantsystems have now been modelled after the known US requirements, the ASME Code,Section XI, but the philosophies laid down in the inservice inspection pro-gramme for the Oskarshamn unit 1 pressure vessel in principle still can beconsidered applicable.

The task of design and manufacture of equipment to perform an ultrasonicinspection of the vessel was, as already mentioned, entrusted to TRC, workingin close co-operation with Southwest Research Institute. The inspection»

109

which was to be carried out by TRC, would comprise the vessel shell and theclosure head including the welds, the steam outlet nozzles, the recirculationnozzles and the studs. Equipment had also to be designed ana raatmfacturedin order to operate the ultrasonic modules and a remota controlled TV-camerainside the vessel.

For a complete description of the inspection programme and related equipmentreference is made to the earlier published paper, appendix 1, "Periodicinspection of Qskarshamnsverket reactor vessel". In addition, to this picturesfig. 1-5 show some of the equipments used during this inspection.

The base lins inspection was carried out during the Autumn 1969. However,some time-consuming alterations had to be made of the centering device forthe recirculation nozzle equipment. The inspection of these areas had there-fore to be postponed and was performed at a much later stage during thestart-up period in December-January 1970/71.

Due to some minor malfunctions of the mechanical devices» which had to beadjusted during the inspection, the calculated time for the operations wasexceeded. The time used for the actual inspection was about 3-4 weeks,sometimes with work on two or three shifts.

The experience gained during inspection work pointed out the need ofdesigning and manufacturing of a new rail ring, which easier than theprevious one could be positioned on the vessel flange.

The ability of accurate repositioning o£ the equipment to assure reproducabletest results was investigated at the final stage of the inspection of thevessel. The universal positioning fixture and the vessel shell inspectiondevice were completely brought out of the vessel. After complete dismountingand a following reassembling of the equipment, repositioning accuracy wasfound to stay within - 3 mm. This accuracy was considered to be satisfactoryin view of the conditions laid down in the inspection programme.

For calibration purposes a set of reference test blocks with artificial defectswere used. The blocks were chosen so as to represent the major geometries indifferent parts of the vessel, such as flange, nozzles and vessel wall» Drilledflat bottom holes 12 œm in diameter and half-moon shaped slots with a max.

110

depth of 5 ŒBI and a total length of 30 nan placed on i.he oxiter and inner sur-faces of the blocks were chosen for setting c£ appropriate scanning levels,

The inspection information was displayed and recorded by a data acquisitionsystem, utilizing stop motion photography of screens and positioning read-outsystems together with XT-recording of information above predetermined level,appearing in the gated region of the ultrasonic instruments.

During the inspection operations all indications which produced responsegreater than 18 db below the predetermined scanning level were recordedwith the XY-recorder and in addition to this with stop motion photographicrecording o£ indications from a level 12 db below the scanning level.Indications equal to or greater than 6 db below the scanning level wereinvestigated during inspection. The built-in analysing capability of theultrasonic Inspection modules proved in this respect to be of utmost value,

Only minor flaw indications below fabrication acceptance level have beenrecorded. Other indications received could be shown ns false indicationsdue to a.o. geometrical conditions. For exasiplo, at the inspection of therecirculation nozzlas indications above the reference level were recorded.

The reflectors giving these indications could be located to the area ofthe weldment between a flow deflector plate and the stainless steel claddingof the vessel wall. Accessibility to that region was severely restricted.The investigation work had partly to be carried out using a reference block,representing the geometry in the actual area, A lot o." different ultrasonicbeam directions had to be applied for the purpose of this investigation. Itcould be shown that the indications did not originate from the base materialand it. was stated that these indications roainly were due to geometricalconfigurations and thus considered not to be of significant importance,

The experience gained in connection with the inspection work carried out onthe Oskarshamn unit 1 pressure vessel have formed & basis for the determinationof inspecting systems for other BWR units now being erected in Sweden, viz.Binghals unit 1, Oskarshatan unit 2 and Barseback unit I, The inspectionprogramme laid down for thesa pressure vessels have as far as possible beenaimed at the fulfillment of the. requirements of the ASME Code, Section XI.

I l l

It has been foreseen to use in principle the same inspection system and thesame type of equipment as for the Ogkarshamn unie 1 vessel» Some major partsof the existing equipment, such as the universal positioning fixture andthe data instrumentation, will be utilized also for the inspection of thesevessels. Some of the equipment parts are modified to cover the variousdimensions in the new vessels. Due to variations in the design of theindividual pressure vessels some other equipment parts have to be speciallymade for exclusive use in a particular vessel.

The vessel wall inspection module used for the Qskarsharan unit 1 vessel isplanned to be held exclusively as an analysing tool in the future. The mainultrasonic scanning work will be performed with a new specially designedinspection device consisting of a number of fixed ultrasonic probes selectedfor various scanning purposes. The use of such an inspection module and thefact that there are no such restrictive circumstances with regard to theannulus between the vessel wall and the thermal shield, as in the Qskarshsmnunit 1 vessel, will give a possibility to reduce the inspection time forthese new vessels.

The task of modifying and completing the existing equipment an.d to perfonainservice inspection of the vessels in question has been entrusted to TRC.As already mentioned, the design and erection of the unit 1 pressure vesselat Oskarshamn took place at a time "before the present philosophies of in-service inspection were outlined. The accessibility of the bottom area ofthe reactor vessel was severely restricted and no practical solutions forthe inspection of this area could "be found at that moment.

For the BWR units now being erected special design considerations have beentaken for the inservice inspection of the bottora area from the outside ofthe vessels using a special design ultrasonic manipulator. Various types ofoutside channels will provide accessibility to the vertical and circumferentialwelds in this area. All weld areas can, however, not, be covered. The bottomdome including penetrations will still not be accessible and at places wheree.g. the channels reach the vessel wall and at the channel junctions theinspection possibilities will be limited.

The bottom ares of the pressure vessel for the Ringhals unit 1 contains onecircumferential weld between the vessel shell and the spherical ring,

112

vertical welds in the spherical ring and one circumferential vessel skirtweld. In addition to this the bottom area of the pressure vessels forOskarshamn unit 2 and BarsebMck unit 1 has one circumferential weld betweenthe bottom dorae and the spherical ring, which can be covered by the ultra-sonic manipulators to be used for the inspection.

For the Ringhals unit 1 vessel one upper and one lower inspection channelis located close to the upper circumferential weld and the skirt weldrespectively. It has been decided to use three entrances, each servingabout one third of the circumference. Each entrance tunnel is provided withtwo rail ¡systems for remote handling of the ultrasonic manipulator, one forthe upper and one for the lower channel, The rigid tunnels with the railsystem are fixed to a steel structure separated from the vessel. Thecircular tunnels have openings at the vertical welds to make provisionsfor inspection of these welds by extending the module test arm.

The figure 6 gives a general outline of the position of the inspectionchannels for the Ringhals unit I vessel.

The design of the inspection channel system for the Oskarshamn unit 2 andBarsebSck unit 1 vessels is slightly different. In this case the ultra-sonic manipulator is transported through the entrance tunnel on a transportrail and fixed to the inspection rail system with a loading mechanism. Theinspection rail system is fixed to the vessel wall arid the skirt to providea remote controlled ultrasonic inspection of the skirt weld, the circumferentialwelds and the vertical welds. The ultrasonic manipulator, figure 7» includesa motorized tractor equipped with a swinging arm assembly containing a multi-probe ultrasonic device. The probes are outlined for gap scanning technique.The probe system and the individual probes have the necessary flexibilityto follow the scanning surfaces. Different probe set ups can be used with aview to beam directions and beam angles necessary for various scanningcombinations»

In case of the bottom area inspection the reactor plant owners have engagedthe main contractor for the actual stations Asea-Atom and SwRI and TRC tocarry out the various works involved.

113

A mock up check of the tunnel system and the ultrasonic manipulator willtake place in January 1972, For the Ringhals unit 1 vessel a mechanicalcheck of the proper functioning of the system as a whole is planned to besade at the site in February 1972,

The basa line inspection of the bottom area of the Oskarshamn unit 2 vesselis planned to be tnade during late Spring 1972.

A PW& unit in the 800 MW range, Ringhals upit 2, is now being erected, andwill be taken into operation during 1974, For the inservice inspection ofthe pressure vessel the same ultrasonic inspection techniques as for theBWR pressure vessels will be used.

In this case TRC will provide a universal positioning fixture based on atelescopic, central tnast design. With this fixture all of the vessel andprimary inlet and cutlet nozsle welds will be reached for inspection fromthe inside of the vessel. All inspection operations are to be performedunder water with the equipment placed on the vessel flange. To facilitateinspection of the various parts of the vessel, special ultrasonic inspectionmanipulators will be connected to the fixture,

The base line inspection of the Ringhals unit 2 pressure vessel will beperformed during Autumn 1973.

Although considerable efforts have been made to improve the MDT techniquesand procedures used for inservice inspection, there are still many problemsthat have to be solved. In Sweden, as in many other countries engaged inthis new inspection technology, there is a continuous interest in theimprovement of the inspection methods used. A new NOT method with anexciting potential in this field is the acoustic emission ïnethod, which sofar has not been applied for practical inspection purposes in Sweden. Thereis, however, a growing interest in our country to incorporate this methodamong other NDT methods used for ínservice inspection purposes and investi-gations to this respect may be assumed to start in the near future.

114

Picture 1

POSITIONING FIXTURE LOCATED ON VESSEL FLANGE IN OSKARSHAMN UNIT 1

115

Picture 2

LADDER SECTIONS WITH ATTACHED ULTRASONIC VESSEL TESTING DEVICE(OSKARSHAMN UNIT l)

116

Picture 3

LOADING MECHANISM FOR RECIRCULATION NOZZLE TESTING DEVICE(OSKARSHAMN 1)

117

Picture

FRONT PART OF RECŒRCULATION NOZZLE TESTING DEVICE WITH REMOTECONTROLLED PROBE

118

Pi cture 5

MANIPULATOR POR INSERVICS INSPECTION OF REACTOR HEAD CIRCULAR WELD

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121

PERIODIC INSPECTION OFOSKARSHAMNSVERKET REACTOR VESSEL

Goran Ahlberg*Clarence JLautzenheiser**

OHe Sandberg***

ABSTRACT

The Oskarshamnsverket is a boiling-water system operating at 70 barspressure and with an output, of 440 MWE. The system was designed andconstructed by ASEA oí' Sweden on a turn-key basis. Early in the designphases, efforts to formulate a periodic inspection program were initiated.This program was based on the design analysis and utilized ultrasonicinformation generated during fabrication of the vessel. Problems ofaccess for inspection became apparent and ASEA made changes to allowinspection of selected points. Equipment was designed to perform anultrasonic inspection of the vessel shell including the welds, the steamoutlet nozzles, the recirculation nozzles¡ and the studs. Eqxxipment wasalso designed to maneuver the ultrasonic apparatus and a remote televisioncamera within the vessel. All of the equipment was designed for precisepositioning and repeatability so as to accurately reproduce the initialinspection during future inspections. The overall inspection programand equipment included a Data Acquisition System to accumulate informa-tion relating inspection position with ultrasonic indications. A simulatedinspection of the reactor is to be made using the equipment to determineaccess and operation problems. The final data generated will be a"fingerprint" inspection of the reactor pressure vessel for use as a refer-ence basis for future inspections.

J. INTRODUCTION

The Oskarsharrm Nuclear Power Station is located on the east coast

of Sweden, inside the city limits of Oskarshamn. The plant -was ordered

from Allmanna Svenska Elektriska Aktiebolaget (ASEA) in 1965 on a turn~

* Tekniska Rontgencentralen AB, Stockholm 50, Sweden.** Southwest Research Institute, Post Office Drawer 28510, San Antonio,

Texas 78228.

*** Oskarshamnsverkets Kraf tgrupp AB, Stureplan 19, 111 87 Stockholm,Sweden.

123

key basis and commercial operation is scheduled for August 1970- The

reactor is a direct-cycle forced-circula tí on BWR, and net plant rating is

440 M"\VE.

The reactor pressure vessel, fabricated by Gutehoffnungshutte (GHH),

is constructed oí A 30E Grade B steel with a shell plate thickness of 125 mm,

internally weld, clad with austenltic stainless steel. The vessel has an

overall height of approximately 18 m and has an inside diameter of 5000 mm.

There are four 600 mm recirctilation loops and eight 300 mm steam outlets.

The bottom head has approximately 200 penetrations; the largest of these

are the one hundred and twelve 100 mm control rod nozzles.

Tekniska Rontgencentralen AB (TRC) was employed for full-time

surveillance of the pressure vessel fabrication at GHH and, in addition, to

perform certain inspections in order to achieve adequate quality control.

Southwest Research Institute (SwRI) was retained to perform an engineering

analysis of the manufacturing processes and to aid in resolving any manufac-

turing difficulties.

The overall quality assurance program, outlined in the early stages

of the project, included provisions for in-service inspection of the reactor

vessel throughout its operational lifetime. As this program was being

developed, it became apparent to Oskarshamnsverkets Kraftgrupp AB (OKG)

that prompt action would allow some design modifications to improve

accessibility for inspection and to obtain an initial ultrasonic "fingerprint."

TRC and SwRI developed a periodic inspection program, shown in

Appendix A, and proceeded to design the required equipment. Personnel

124

from these organizations, together with, design personnel from ASEA, evolved

the periodic inspection program. In addition, ASEA was able to make design

changes to facilitate inspections.

When preparing the inspection program, the following factors were

considered in the selection of inspection methods, areas to be inspected,

and frequency of inspection for the various areas:

1) Stress levels

2) Probability of failure with respect to material properties and

their changes during operation of the reactor

3) Consequences of failures.

The inspections relied primarily on ultrasonic techniques with pre-

cise, mechanized scanning in order to make reproducible inspections.

Accurate, reproducible inspection would allow determination if defect growth

was occurring. Mechanization would minimize radiation exposure to per-

sonnel. Approximately one-ha3f of the equipment was designed and is being

fabricated by TRC and the other half, by SwRJ. All of the equipment is to be

assembled at the site in late summer 1969 for a final checkout and a

"dry run" simulated reactor inspection, A pro-service inspection is to be

made on the reactor pressure vessel in autumn 19¿»9 and data secured to

act as a "fingerprint" for future inspections,

This paper will describe the general inspections performed by TRC

during fabrication with emphasis on problems which would affect in-service

inspection, the design and fabrication of the inspection equipment, and the

proposed dry run and pre-service inspection.

125

U. BASIC,DESIGN, PROBLEMS

When it was decided that a program should be prepared and equipment

fabricated for periodic inspection of the reactor pressure vessel, the fabri-

cation of the vessel had already started and the reactor building and piping

adjacent to the reactor was de-signed. Even though some of the problems

associated with periodic inspection had been considered in the original de-

sign of the system, at the time that the formal program was initiated, it was

only possible to make limited design changes to facilitate inspection, As

an example of design considerations, the ¿tainless steel cladding had a sur~

face finish specification that would provide a suitable finish for 'ultrasonic

inspection. As an example of changes in design that were made to improve

accessibility, an inspection access port was added in the forced-recirculation

loops for insertion of equipment for inspection of the forced-recirculation

nozzles and adjacent welds.

One design change that could not be made and caused considerable

trotible in equipment design was the 50 mm nominal annulus between the

thermal shield and the vessel wall. This width of annulus could not be assured

for the full circumference, if the vessel was minimum diameter and the

thermal shield was maximum diameter, ASEA did inform the suppliers of

the problem. Nothing could be done about the x*essel dimension or orienta-

tion. The thermal shield dimension was fixed but the orientation and

alignment within the vessel could be varied,

126

HX. INSPECTION AT GUTEHOFFNUNGSHUTTE

TRC has witnessed all inspections performed by GHH during fabrica-

tion of the pressure vossel in accordance with a detailed inspection program.

In addition, TRC performed -ultrasonic inspection of all material and weld

seams in the pressure boundaries, the moderator tank stand, and the vessel

support skirt. The acceptance limits to be used for the ultrasonic inspection

of the welds were set by ASEA and TRC. As an example, the following

•were the acceptance limits used for butt welds in the pressure boundary:

jDefects that will not be ace eptcd;

1) Root defects

E) Lack of fusion

3) Cracks and other notches

4} Slag inclusions with a greater length than 25 mm

Shorter but closely situated slag inclusions will be judged

from case to case,

5} Pores which are larger or closer situated than what is allowed

according to IIW character blue

6) All flaws, with the exception of isolated pores, situated

within a distance of 1 5 mm from the inner and outer face

of the weld.

The ultrasonic inspection of the weld seams consisted of a 100-

percent scan using a 45~degree and a 60-degree shear wave from all four

sides of the weld and a longitudinal inspection from the inside and outside

surface of the weld. These inspections were performed after the initial

127

stress relieving operation of a part, and selected areas are being inspected

after the final stress-relief operation and again after the final hydrostatic

pressure test. All of the inspections are carefully documented» including

detailed sketches of acceptable flaws remaining in the vessel. These sketches

could, be of value for comparison when performing the pre-service inspec-

tion.

During inspection of the welds in the moderator tank stand, large

echoes were obtained around the entire circumference of the weld joining

the Inconel segment to the vessel and to the stainless steel extension of the

stand. An ultrasonic inspection of the welding procedure qualification plate

gave similar echoes. This plate was sectioned and mctallurgically examined.

No defects could be correlated with the ultrasonic echoes. This phenomenon

is not unknown in the ultrasonic inspection of Inconel and stainless steel

welds, This problem prohibited the ultrasonic inspection of these welds

during fabrication and also forced a change in the plan for in-service inspec-

tion. Liquid pénétrant and radiographie techniques were used during

fabrication and the in-service inspection plan was changed to use visual

inspection by remote television camera.

The stainless steel cladding surface finish requirements had been

rautuaUy agreed to by ASEA and GHH and defined by a workmanship standard.

The finish of the stainless steel cladding was not up to the workmanship

standard in several places. Instead of local improvement by grinding, GHH

decided to grind the entire internal surface. This relatiycly smooth ground

surface allows good possibilities for in-service ultrasonic inspection.

128

TB.C performed laboratory investigations to determine the sensitivity

of inspection that could be obtained through the stainless steel cladding.

These tests were to detenrdne the sensitivity obtained when, in special

cases, fabrication inspection had to be performed through the cladding and

would also be useful for in-service inspection information. These tests

were made using standard manual techniques and on the various metal thick-

nesses and geometries of the vessel. These tests showed that a satisfactory

signal/noise ratio can be obtained and that the quality of the stainless

steel weld cladding is satisfactory for inspection. As an example, a small

flaw with a 10-20 sq. mm reflecting surface could be detected 170 mm

below the clad surface by a 45-degree shear-wave technique.

IV. POSITIONING LANDMARKS

Due to the importance of inspection rcproducibility, it was decided

to incorporate positioning "landmarks" on the O. D. surface of the pressure

vessel. Such "landmarks" would allow the inspector to correct for expansion

of the inspection device dvie to temperature, check the accuracy of the

positioning indicators, and also serve as ultrasonic parameters to check

on inspection sensitivity. Dxiring fabrication of the vessel, GHH applied

20 Inconel weld build-ups in a vertical line down the vessel, 550 mm apart.

TRC, using a stainless steel clad test block supplied by GHH, welded

on similar Inconel pads. These pads were machined to various configura-

tions in order to determine an appropriate shape for proper reflection of

the signal and to determine the precision with which the "landmark" can be

located. It was found that a "landmark" dimension of 17 mm by 17 mm

129

sqxiare with a height of 14 mm above the vessel surface,, with the inner cïad

surface ground to a smooth finish, allowed reproducible location of position

within ± 1 mrru

V. STANDARD BLOCKS

tStandard blocks with artificial defects will be fabricated for calibra-

tion of the various pieces of ultrasonic equipment. There will be standard

blocks representing the major geometries in all parts of the pressure vessel,

euch as the flange, nozzles, platCî etc. These standard blocks will become

a permanent part of the inspection record of the reactor pressure vessel,

VI- INSPECTION^ EQUIPMENT

There are six separate inspection devices and a general-purpose

positioner designed for the Oskarshamn reactor pressure vessel in-service

inspection. This equipment is in the advanced stages of design and con-

struction. Insofar as possible, all of the equipment can be controlled and

operated from a central panel which also contains position-indicating de-

vices» The equipment is briefly described in the following sections:

^* F or c e à -Reci rculati on N o z z 1e Insp cc tion Device

This device, shown in Figures 1 and 2, is designed to inspect

the Bozzle-to-shell weids inner nozzle-to-vessel radii, the nozzle

forging material, and the nozzle-to-pipe welds of the forced re-

circulation inlet and outlet nozzles. The device i's unique and required

ASEA to provide an access port in the elbows attached to the recircxi-

lation noazles and a centering cone on the vessel internals in front

of each nozzle,

130

The principal features of this device are:

1) A central bar utilized for centering an ultrasonic in*L. ac-

tion module within the nozzle. The bar is aligned -with a guide

cone attached to the vessel internals and the access port

attached to the nozzle.

2) A retractable ultrasonic module is attached to an outer

tube which utilizes the central bar for guidance.

3) The equipment is to be inserted through a valve welded

into the access port. Due to the limited space behind the access

port tube, the inspection equipment \vas made in five sections

which are joined together successively as the inspection assem-

bly is inserted.

4) The insertion of the equipment against the water pressure

in the water-filled reactor is provided by a device with two

rack and pinion drives powered by an electric motor.

5) Translation of the inspection module along the nozzle

bore is by the rack and pinion drives. Rotation of the ultra-

sonic module is by means of a separate electric motor located

on the outside of the access port tube.

6) An automatic mechanism, to adjust transducer angle to

compensate for the changing vessel/nozzle geometry as the

transducer is rotated.

7) The ultrasonic module contains a transducer adjustable in

two planes for inspection of the nozzle-to- shell weld, the nozzle

131

material, and the circumferential welds ox the noz,zle-to~safe

end and the safe end-to-pipe connections.

B, Ste : am Outlet Nozzle, Inspection

This device, shown in Figure 3. was designed to inspect

the nozaie-to- shell weld, the inner nozzle-to-vessel radii, the nozzle

forging material, and the welds between the noazle and the piping.

The device is self-contained except that it is positioned by being attached

to the boom oí the Universal Positioning Fixture described in para-

graph G of this Section.


1} Inspection is performed by two separate ultrasonic

modules attached to a central bar. The bar provides support,

alignment, and rotational and translation capability.

2) One ultrasonic module contains a transducer adjustable

in one plane. This transducer is to be used for accurate

centering of the eqxiipment in the nozzle and inspection of the

nozzle-to- shell weld.

3} The other ultrasonic module contains a transducer

adjustable in two planes. This transducer is to be used for the

inspection of the nozzle forging material and the circumferential

nozzle -to-piping welds.

4} An automatic mechanism to adjust transducer p.ngle to

compensate for the changing vessel/nozzle geometry as the

transducer is rotated.

132

5) The circumferential and lateral positions of the transducer

modules are adjustable from the control paneL

C. Closure Head InspectipB Devic^

The welds in the closure heac* will be inspected from the outside

surface when the head has been removed for refueling. The longitudinal

welds will be inspected using manual techniques. The circumferential

welds will be inspected using the device shown in Figure 4,


1} The xjpper portion is fixed in a bearing located in the

central nozzle. This bearing is used as part of the device for

removing the stud bolts.

2) The lower part of the arm is positioned around the cir-

cumference of the head on a track.

3) The inspection is to be done using a Sperry wheel contain-

ing a transducer with adjustable angle.

4) The wheel can be positioned at any point along the arm

so as to inspect the circumferential welds.

5) The circumferential motion is mechanized and is adjustable

from the control panel,

D. Stud Inspec tion fie vi c e

The stud inspection device will be similar to that shown in

Figure 5r The device shown was designed for use on the Agesta

reactor and has been successfully used. The principal features of

the device are:

133

Ï) The transducer is manually adjustable in all directions

in order to be precisely positioned in relation to artificial de-

fects in a standard block stud bolt.

¥,) The transducer is contained in a captive-water chamber

so as to utilize the immersion inspection technique.

3) Once the transducer has been positioned, rotation of the

device allows reproducible inspection from stud bolt to stud

bolt.

The inspection will be performed from both ends of the bolt

for maximum sensitivity.

E. Vessel Shell Inspection Device

The device shown in Figures 6 and 7 is designed to inspect the

welds and the base plate material of the reactor pressure vessel. The

design of this device had a major restriction in that it was required to

pass through a 50 mm annulus between the thermal shield and the

vessel wall, This was an OKG requirement so as to be able to in-

spect the vessel without removal of the thermal shield and moderator

tank.

Major vertical movement and all circumferential movement of

the device is by means of the Universal Positioning Fixture. Incorpor-

ated in the device is a precision vertical movement which is slightly

more than the distance between the "landmarks" built into the exterior

of the vessel.

134


1} Two modules, each containing one transducer, indepen-

dently movable toward and away from each other with a minimum

separation of 85 mm and a maximum separation of 300 mm.

Both transducers are on the same line of translation to allow

inspection by the pulse-echo and the pitch-and-catch techniques,

either independently or simultaneously. The position of the

transducer module is adjustable from the control panel and

accuracy of position is ± 0 . 1 mm.

2) Each of the transducers has angulation control in the

direction of translation of the module. The adjustment is

sufficient to enable a surface wave to be propagated in either

direction along the translation line. The angle is adjustable

from the control panel and the accuracy of position is ±6 minutes

of arc.

3) The ultrasonic module assembly can be rotated so that the

translation line of the ultrasonic modules is horizontal or

vertical or any desired position between these two points.

Stops are provided at the horizontal and vertical positions and

an indicator light is provided at the 45-degree position. Loca-

tion between these points can only be estimated.

4) A fixed longitudinal-wave transducer is built into the

corner of the ultrasonic assembly. This transducer can be used

to determine the distance of the ultrasonic assembly from the

135

surface being inspected and also perform a longitudinal inspec-

tion if desired,

5) The ultrasonic assembly can be translated vertically within

a yoke* The position it. adjustable from the control panel and

the accuracy of position is within d=0, 5 mm.

6) Since the ultrasonic assembly had to pa&s through a 50 mm

annvilus, it was desirable lo provide a means to move the inspec-

tion module away from th<; wall once the module was below the

thermal shield. This would allow a sufficient watcrpath distance

so multiple echoes would not appear within the inspection zone.

This "kickout" feature is performed by the use of hydraulic

cylinders located at the top and bottom of the assembly. A water

jet system attached to the module yoke provides the necessary

force to maintain module contact with the vessel wall.

F. Remote Television System

A remote television system, shown in Figure 8, was designed

around a Model 2500 COHUf radiation-resistant, television camera.

The basic camera is 75 mm in diameter and the rotatable mirror assem-

bly increased the diameter to 94 mm.


1) A rotatable mirror in front of the lens. This mirror can

be rotated 360 degrees and is adjustable for angle.

2} Interchangeable lenses and built-in lighting allow a general

view or critical observation. The lenses can be focussed as

close as 200 mm from the face of the camera,

136

3} A gimbals system attached to the rear of the camera to

maneuver the camera 90 degrees in the vertical plane. This

feature, plus the rotatable mirror, allosvs viewing upwards

beneath an object or in any direction parallel to the longitudinal

axis of the vessel.

4) The supporting column can be rotated and gimballed up to

10 degrees in the attachment to the Universal Positioning Fixture.

These movements allow the maximum flexibility and enable re-

inote viewing of any point accessible through an opening larger than

94 mm. Position of the mirror and boom rotation is visual. Posi-

tion of the gimbals is maneuvered and indicated on the control panel.

G. Universal Positioning Fixture

The fixture shown in Figures 9 and 10 was designed to position

various devices within the reactor pressure vessel cavity. This fix-

ture is essentially a rotating bridge crane with precision control.

For use of the fixture, the circxilar rail is installed immediately

after removal of the head. The remaining portions of the positioner

can be installed remotely under water by means of the reactor build-

ing crane. Thus, the positioning fixture can be removed for installa-

tion of the steam outlet nozzle inspection device, the vessel ultrasonic

module, or the remote television inspection camera,


1} The track protects the vessel seal surface from accidental

damage.

137

2) The track extends a maximum of 88. 9 mm above the

flange surface and thus has minimum interference with refueling.

3) The fixture can be utilized to position a television camera

or other device during refueling operations,

4) The circumferential rotation and the vertical movement

can be used to manipulate inspection devices. The circumferen-

tial position can bo displayed on a recorder and the position can

1 be determined within ±0. 25mm, The general horizontal position

can be determined within ±0, 3mm. In addition, from a given

point, fine horizontal position can be determined. This

position is recordable and accurate within ¿0. 5mm.

5} The vertical movement can be used to position inspection

devices. Accuracy of position is ±0 .3 mm,

All movements of the positioner are adjustable from the control

panel with position being indicated by meters or counters. Adjust-

ments include both speed and direction.

VII. "PRY RUN"; AT OSKARSHAMN REACTOR SITE

During the latter part of January 1969 the Universal Positioning Fix-

ture and ultrasonic module being fabricated by SwRl were checked out and

the accuracy of positioning determined. After this checkout, these,items

were to be crated and shipped to Oskarshamn, The remote television camera

system was not completed at this time but was scheduled to be checked out

as an individual unit during March, crated, and shipped. The devices being

fabricated by TRC were scheduled to be completed and checked out in suit-

138

able mock-ups for proper operation and accuracy of position by May. After

this checkout, these iteras were to be crated and shipped.

It is planned to have a full-scale 1(dry run" of the entire system at

Oskarshamn in August 1969* at which time the compatibility and interface

of the TRC and SwRI equipment will be established. Any problems arising

can be corrected before the "fingerprint" is run on the reactor pressure

vessel. At the time of the "dry run" the reactor vessel will be in place and

some of the reactor internals, including the moderator tank and thermal

shield, will be installed.!;

VIH. INSPECTION PLANj

The detailed inspection technique for the "fingerprint" has been com-£

pleted. An inspection technique has been engineered for each item of the

in-service inspection program» As an example, the following is the general

ultrasonic technique for the reactor vessel shell:

1) The inspection would utilise three transducers, as shown

schematically in Figure 11. Transducer No. 1 will be operated in

the pulse-echo mode, controlled by instrument No. 1, and the display

gated to read the region being inspected and with the output gated on

an amplitude basis- to an X~Y recorder. Transducer No. 2 will be

operated as a receiver,' in effect being a pitch-arid-catch from trans-

ducer No. 1. This transducer will be controlled by instrument No. 2

with the inspection region gated on an amplitude basis and the output

recorded. Transducer No. 3 will be a longitudinal-wave transducer

controlled by instrument No, 3 with the output gated on an amplitude

basis and also recorded.139

2} The philosophy behind the above inspection technique is that

the pulse-echo and pitch-and-catch techniques should be able to locate

defects of significant siae anywhere in the reactor wall. The excep-

tion to this is when the ultrasonic energy might be blocked by a

lamination in the walJ of the reactor pressure vessel. Such a

lamination could be determined by the longitudinal-wave inspection.

3} The data acquisition system is designed to record all of the

inspection information generated by the three instruments, plus having

a recording of the information above a predetermined level and appear-

ing in the gated region of the three ultrasonic instruments.

There are several methods of recording jail the information

available during an ultrasonic inspection, including data on position,

transducer angles, rotation, signal intensity, etc. Some of these

methods are stop-motion photography, magnetic tape recording, and

digital acquisition on punch tape or a computer. At this time, it is

contemplated that the data, will be acquired via stop-motion photography

together with X-Y recording. Although this photographic method re-

quires relatively cumbersome data retrieval techniques, the photo-

graphic record is permanent and will last /or the designed life of the

reactor pressure vessel. The other methods of data acquisition are

still under study and if proven by the time of the Oskarshamn

"fingerprint, " such methods will be considered for data acquisition

purposes.

140

APPENDIX A

PERIODIC INSPECTION PROGRAM

CLOSURE HEAD

1. Circumferential Joints

Inspection Method:

Freqxiency of Inspection:

Fingerprint Required:

2. JLon RJtudi n al Join t s

Inspection Method:

Frequency of Inspection:


3. I^d Nogzlgj Inconel

Inspection Method:



Main Stud s^

Inspection Method:



Automatic ultrasonic from outsideMagnetic particle

2 years

Yes

Manual ultrasonic from outsideMagnetic particle

Zyears

No

Dye pénétrant from outsideVisual inspection from inside

2 years

No

Semi-automatic ultrasonic fromboth ends

Magnetic particle or dye pénétrantwhen required

1/4 of studs each year

Yes

14L

Mnin Stud Washers

Inspection Method:



6. Sealing Su rf ace s

Inspection Method:



VESSEL

7. ijbon gitudi nal Flan g e J pints,,

Inspection Method:


Fingerprint Required;

8. Flange -to - Ve s s el J oint

Inspection Method:



9. Longitudinal,Shell Joints

Inspection Method:



Dye pénétrant

Î/4 of washers each year

No

Visual inspectionDye pénétrant

1 year

Ho

Automatic ultrasonic from insidevessel

2 years

Yes


2 years

Yes

Automatic uïtrasonic from insidevessel

10 years

Yes

142

10. Plate and Circumferential Joints in Core Area

11.

12.

13.

14.

15.

Inspection Method:



8 Steam Outlet Nozzles

Inspection Method:



8 Circulating Water Nozzles

Inspection Method:


10 years

Yes

Automatic ultrasonic from insidenozzle

4 nozales every two years

Yes

Automatic ultrasonic from insidenozzle

Frequency of Inspection: 4 nozzles every two years

Fingerprint Required: Yes

Bottom Head, including Noz^zl e s

Inspection Method: Leak detection

Frequency of Inspection Continuous

Selected Vessel Wall Surfaces

Inspection Method:

Frequency of Inspection


Reacto r Inter nais

Inspection Method:


Remote, visual inspection by TVsystem

1 year

Yes

Remote, visual inspection by TVsystem

Certain parts each year plusgeneral surveillance

143

Cone receptacle forcentering instrumenton inner wall of annulas

Module on radius arm whichswings out from center tube

Dimensionforeshortenedfor illustration

Forcedrecirculation nozzle

Motor for lateralmovement

FIGURE 1. FORCED- U R G I R C U LATTONNOZZLE INSPECTION ntfV

144

Shaft to be fixed to thetilting mechanism

Rotatable (180 degrees) waterproofbox containing aervo-motors,potentiometers and clutches

Transducerturnable* 30 degrees

Gears for angulationof the transducer

FIGURE 2, FORCED-RECIRCULATION NOZZLE INSPECTION DEVICE.

C5

Reactor veasel wall

Positioning fixturema at mounting plate

Traniducer withcomplete angulation forinspecting two plañen

S.ngle tranaducerior inspectingon« plane

Steam outlet nozzle

FIGURE 3. STEAM OUTLET NOZZLEINSPECTION DEVICE.

Positioning fixtu-

Nos/ie inapectiorunit attached topositioningmast

Reactor vessel

Stub shaft with bearing attachedto flange and rot ai able insidenozzle

Transducer wheel withvariableangle probe

Weld lines

FIGURE 4, CLOSURE HEAD INSPECTION DEVICE.

147

FIGURE 5. STUD INSPECTION D E V I C E

148

Elevating winch

Rotating platform

Wheel track - — _/

Vesselflange —

Horizontal translationplatform

Mast

Reactor•<. essel

Re-actor vessel

Ladder

Ultrasonici nspection

U - h e a d -

Standoff wheels bear on innersurface of vessel shell-

FIGURE 6. VESSEL SHELL INSPECTION DEVICE.

FIGURE 7. VESSEL, SHELL INSPECTION DEVICE.

150

Pô sitiotuTjg fixture

.Brake and positioninggimbal for 1 V mast

Rotateblc TV rameravntn mirror in annulua

- Coaxialconductor

-Tubular support

Rotatable section-

Manuaiîyadjustablehght source

7

FIGURE 8, REMOTE TELEVISION SYSTEM.

Mast-Rot atable bridge

Elevating mechanism —iwith, horizontal travel I

•Reactor vessel-

FIGURE 9. UNIVERSAL POSITIONINGFIXTURE.

152

ence

FIGURE 10. UNIVERSAL POSITIONING FIXTURE.

Vessel shell

7X ¿—Module"^Transducer positions

Note: Transducer signals are input to twoX-Y recorders.

FIGURE 11. SCHEMATIC OF ULTRASONIC INSPECTIONOF REACTOR VESSEL SHELL.

154

NONDESTRUCTIVE TESTING OF IRRADIATED FUEL ELEMENTS IN THE U.S.A.*



ABSTRACT

Many nondestructive testing techniques are being used to examine fuel ele-ments after irradiation. Penetrating radiation techniques include both neutronand x radiography. Ultrasonic techniques are used to inspect bonding and todetect flaws in cladding; eddy-current techniques are used to measure spacingbetween components, measure thickness, and detect flaws. A variety of mechanical

/

devices are used to measure physical dimensions.

RESUME

De î"i<v.ù>roui.;o,i iocVsnloner ¿c co.itrGlo nou Cor, tructivcv. üo^vlc^ûjji xi'oilif.ocs poor or-uoir-c" ?.e¡_ fl^í.ic^tc cío corólniMi/iolairryxHo.lion, jjoo Iochjj.lc.uci3 to radiât. i.o,ÎP p¿.:í'uranU; COM--

-'rií; h le. j'ois Ir, radiographie ¿ax aicuti-o^b ül aa^ i-ci.¿'0i)¿, X.De3 c fx ' lm j rjae» u't.traoonioixT; coi-J; ub i l í róc r j pour iHc/>üc¡"or loojo-j.iD.s cl pour dvHocïOi- les ùéfî-xitfc ' dann le £j£'i;irH/> : Oc?, tocîi:/i-~qxioo •.• u.' ") in- -it jos co'arunls ce j''(jao.ivûlfc pon.clhcnt la j-oí^rc ô&o¿:iato:ico» oriro Icn conpo.scuiou ai i£..i c;uo lr c.6tcri'i..aulo^ do,3épr.âsf'Curs et la di-leo Lion ûo,-:, d<? t'î lits. j; ivovr> uiir^oi i.ix'o i.iooa-niqv.cr- r.orrir. uti .LiBÓy ¡^oxir lx nomrc. ûcr,. (;iîieanioji« \",jy£jj quao.

INTRODUCTION

Complète knowledge of the behavior of production and experimental nuclearfuel elements under operating conditions requires examination after service.Nondestructive testing (NDT) techniques play an increasingly important role in


155

the examination process to allow observation of assembly details, to detectinternal flaws, and to assure optimum results from the destructive examinations.Many of the techniques used on. irradiated fuel elements are similar to thoseused on nonradioaetive specimens except for the requirement that the examina-tion be performed remotely. Satphasis will be given to those aspects that arepeculiar to fuel elements. Some of fche techniques, such as dimensional mensura-tion, although not considered by many to be TOT, do, indeed, fit the generaldefinition. At some localities, entire facilities are being constructed forthe examination of irradiated fuel elements, with substantial consideration beinggiven to providing capability to perform NDT. This paper is intended to providetypical but not exhaustive examples of the interest in TOT for the examination ofirradiated fuel elements. Much of the discussion will be organised around thevarious methods.

NEW FACILITIES

Anticipating the very large number of fuel elements that will be used andsubsequently examined, several new facilities have been designed to accommodatethe large work load.1"3 For example, a new Fuel Examination Facilityhas been designed to provide experimenters at the Fast Flux Test FacilityRichland, Washington, with a reliable means of performing prompt nondestructiveexaminations of items irz-adiated in the Past Test Reactor ÍFTR). among the capa-bilities intended for the FEM arer (l) the interim nondestructive examinationduring a single reactor outage of selected irradiated fuel elements; (2) interimdisassembly, nondestructive examination, and reassembly during one or morecycles; (3) disassembly and nondestructive examination of fuels whose irradiationis complete. The NDT techniques include visual inspection, photography, dimen-sional measurements (including interpin spacing), neutron radiography, gammaautoradiography, gamma scanning, leak testing, arid bond testing (using eddy

156

currents). A small dedicated reactor is proposed as the neutron source for theradiography. A divergent bes.m colliraator will te used, with a variable sourceaperture to allow selection of resolution and speed according to the examina-tion needs.

A portable, shielded, gas tight enclosure •' was developed for the examinationof uncleanedj irradiated Fermi reactor fuel assemblies with provision for theelements to be returned directly to the reactor after examination. A precisionload cell attached to the support cable allows measurement of weight with anaccuracy better than ±0,5$. All external surfaces can be visually examined.Selected intei-nal surfaces are examined with a specially designed periscope.Optical micrometers measure the bow and width with an accuracy of ±0.002 in.Although not included originally, the system permits adding equipment for gammaattenuation with an external cobalt source, neutron radiography, ana ganrnascanning.

PENETRATING RADIATION METHODS

Many of the examinations performed on irradiated fuel elements use pene-trating radiation despite the high levels of radiation associated with suchspecimens. Special techniques have been devised to overcome the difficultiesintroduced by the radiation and to take advantage of the ability to form animage of the internal structure of opaque materials. The techniques includethe use of x~ and gamma rays, alpha parti cíes, and neutrons. In the recentpast, considerable attention has been given to neutron radiography. Gammaspectrometric methods are common to most examination facilities, but detailswill not be provided in this paper.

Neutron Radiography

Thermal neutron beams are used routinely to examine irradiated reactor fuelspecimens.* A primary cause for the significant interest in this mode of imaging

157

is the fact that the transfer method of detection5 can be used independent of aradioactive background. In this method a latent radioactive image is produced in.a thin metallic foil of material such as indium or dysprosium. The image is inturn transferred to a photographic emulsion by autoradiograpblc techniques.Both isotopes and .reactors are used as sources of neutrons for radiography. Afew of the many facilities will be cited in this paper.

Experimental fuel capsules from the Experimental Breeder Reactor-II (EBR-Il)Biay be radiographed in a hot cell with an isotopic neutron source6 at the IdahoFalls, Idaho facility of the Argonne National Laboratory. The neutron source isa radioactive l24Sb gamma source in a 20-cm-radius beryllium cylinder surrounded"by 30 era of beryllium oxide. A coliimated beam of neutrons with an intensity of3 x 105 neutrons <ya~2 sec"1 can be obtained from a 3000-Ci source. Radiographiecontrast sensitivity of 2$ and penetrameter sensitivity of 5% can be obtained.The useful neutron beam covers an area 3 x 17 in. For radiographie exposure thedysprosium transfer foil is moved into position inside an exposure bonnet adja-cent to the specimen. Neutrons penetrate the bonnet for foil activat-ion through0.06-in.-thick aluminum, and the dysprosium is not exposed to the cell atmosphere.The source container is mounted on tracks for adjustment of the source-to-bonnetdistance,

General Electric uses its Nuclear Test Reactor (WTE) for neutron radiography.7

A horizontal beam emerges from the core through a hole in a 5-ft-thick wall ofthe reactor building into a stationary receiving cask. The central cavity ofthe cask is open at the top to accept radioactive objects from transfer casks.Transfer plates (5 X 15 in.) of indium and dysprosium are used in the radiography.Track-etch imaging is also used experimentally. Experimental fuel assemblies areroutinely examined by neutron radiography with only a 24-hr delay between removalfroa and reinsertion into the General Electric Test Reactor ÍGETR). Fuel-pelletinterfaces, cracks, and voids due to fuel melting are revealed.

158

A high-intensity neutron radiographie facility0 installed in the Oak RidgeResearch Reactor (ORR) is used to provide pre- and postirradiation examinationof fuel capsules as well as interim examination of those capsules being irra-diated in the ORE. The high-intensity (1.7 x 10s neutrons cm"2 sec"1) thermalneutron flux at the object plane allows the use of L/D radios {source-to-specimendistance divided by the size of the aperture through which the neutrons enter theconical collitnator) as large as 1000:1. Exposure times as short as 1 ffiin aretypical. Among the detected conditions within fuel elements are pellet cracking,void formation, and fuel-pin bowing.

Neutron radiography will, also be used for fast-reactor fuels.9 Preiï-radiationradiographs provide standard references for comparison with interim and postirra-diation radiographs. Ueutron radiography has been shown to be capable of detectinghydriding in the fuel cladding and determining proper assembly and placement ofsprings, insulators, spacers, and thermocouples within the fuel assembly. Of per-haps greater importance is the examination of the fuel itself. Preirradiationexaiaination ¡shows the placement and spacing of fuel pellets as well as densitiesand concentrations in powdered fuel. After irradiation, neutron radiographyallows the detection of pellet cracks, void formation, fuel redistribution, andswelling. Cladding failure can be determined and in some cases predicted byinterim radiographs.

X-Ray Radiography

X-ray techniques for radiography have also proven useful, for the examina-tion of radioaetj-ve fuel elements. Obviously precautions must be taken to mini-mize the exposure of the x-ray film to the unwanted radiation. One suchapplication used a betatron to radiograph irradiated fuel elements.10 Theradioactive element was contained within a shielded cask until the actual radio-graphic exposure was ready to begin; then it was raised into a thin-walled con-tainer intended to prevent parbiculate contamination. A typical setup employed

159

a target-to-object distance of 3 ft with a similar object-to-film distance.The separation of the object from the film reduced the fogging due to theradioactivity; the small focal spot of the "betatron permitted the enlargement ofthe image with no significant loss in sharpness - Assemblies with activities upto 500 Ci have been usefully radiographed. Radiographs of plutonium alloy fuelrevealed frothing at the top of the fuel, gas bubbles in the solidified fuel,and thinning of the capsule wall. Other effects have been fuel cracking andswelling and breakage of the mechanical assembly.

Lower energy x-ray machines have also been installed in hot cells to allowradiography of fuel specimens. Such a system11 was installed in the Transuranium(TRU) facility at the Oak Ridge National Laboratory. Mechanical fixtures weredesigned and fabricated to hold the 150-kV x-ray tube, the specimen, and thefilm container in proper alignment. The film container that was transferred intothe cell for radiography was heavily shielded with uranium to prevent exposureof the film except during actual radiographie exposure. The mechanism causedportions on a strip film to be successively indexed into a window position forthe radiographie exposure and then reindexed into the circular shielding uponconclusion of the exposure. Acceptable radiographs have been obtained despite aradiation field from the speciraen of greater than 10,000 R/hr.

Autoradiography

Autoradiography is used routinely on nuclear ceramic fuel cross-sectionsduring postirradiâtion examination.I2 The autoradiographs show plutonium dis-tribution in Plutonium-bearing fuels and fission product distribution in all fuels.Photographic emulsions in direct contact with the specimen record the beta andgamma radiation from fission products; a film such as cellulose nitrate recordsthe presence of alpha-emitters. Etching of the cellulose nitrate in sodiumhydroxide produces a visual image from the alpha-produced pattern.

160

ULTRASONIC METHODS

Ultrasonic techniques have been used for the postoperation examination offuel elements. The details are similar to those used, on nonradioactive speci-mens, although the performance may be more difficult or require more mechaniza-tion. Except for accumulated damage to i,n~cell components, the method isinsensitive to radiation. Ceramic piezoelectric transducer elements such aslead zirconate-titanate have been shown to have high resistance to radiationdamage.

Many of the ultrasonic techniques were intended to detect noribonding in, fuelplates and pins. Resonance, pulse-echo, and through-transmission techniques haveall been used depending upon the configuration of the specimen and requirementsof the test. Among the simplest was a resonance technique13 on flat fuel plateswith elementary mechanical guides and manipulator movement of the probe. Othersimple systems have been fabricated and inserted temporarily into hot cells to•perform short-term examinations. For example, to detect noribonds in an irra-diated annular fuel tube1 ultrasound was directed through the wall of the tubu-lar element to reflect from the inner surface of the tube 1&0° away from, theportion being examined. Monitoring of the amplitude of reflection alloweddetection of nonbonds with this single-crystal, through-transmission, pulse-echo technique. An inexpensive lead screw provided both linear and rotarymotion to the specimen as it was examined. Nonbond areas larger than 1/16-in.in diameter were continuously recorded on a facsimile recorder using eiectro-sensitive paper.

tore complex systems have been assembled and installed for long-term opera-tion to mechanically scan fuels, plates., and pans. For example,15 equipmentwas installed under 12 ft of water to inspect a cylindrical fuel element forquality of bond, to measure cladding thickness, and to detect flaws in the core.

161

Ultrasonic equipment was Installed in cells for detecting defects in thecladding of irradiated fuel rods.16 Cracks as small as 1/8 in. long with depthsof 5—10$ of the wall thickness were detectable. Undoubtedly many other "undocu-mented applications of ultrasonics have been made to solve specific short-termproblems- Since many mechanically operated ultrasonic techniques are In effect"remotely" operated, the transfer of the technique Into a hot cell Is relativelystraightforward with minimal effect caused by the radiation.

EDDY-CUKRENT METHODS

As with ultrasonics, eddy-current techniques can be applied with minimalconcern for high intensity background radiation. The principal differencebetween hot and cold laboratory performance is the mechanization used for theremote operation. Pulsed eddy-current techniques have been used to detectcracks in fuel element cladding after service and before disassembly.

Eddy-current probes have also been used beneficially to measure thespacing in coolant channels17 between fuel plates as part of the postoperationexamination. For example., for a fuel element for the High Flux Isotope Reactor(HFIR) a special probe was fabricated to measure the channel between theinvolute-shaped plates.16 The eddy-current coil was ma.de with material havinga low thermal coefficient of resistance to minimise effects of the fuel elementtemperature (as hot as about 200°F}, The range of the probe was from 0.035 to0.065 in. with a sensitivity of 0.001 in. The cylindrical element was placedon a rotating table for indexixïg selected coolant channels under the probe.The probe was driven along the channel by a chain-and-pulley system to whichthe upper part (handle) of the 30-in.-long probe was attached.

Thicknesses of aluminum cladding over uranium fuel cores and of anodizedcoatings of aluminum and of corros3on film on fuel samples have all beenmeasured with simple eddy-current techniques and commercial Instrumentation.19

The probes were handled with hot cell, manipulators.162

PHYSICAL MEASUREMENTS\

Measurements of physical dimensions, density, and similar attributes areusually needed to assess the amount and direction of fuel element growth, shrinkage,or distortion. Many techniques for viewing and measuring of linear dimensions,density, electrical resistivity, and surface features have "been devised.^° Onlya few will "be cited in this paper. *

Modified machinists micrometers, tool-maker microscopes, shadowgraphs, dialindicators, and linear variable differential transformers all find extensive usefor measuring length, width, thickness, diameter, bow. or other dimensions ofirradiated fuel elements.2 -1

At the Oak Ridge National Laboratory, modified lathe beds are used to pro-/

vide stable and precise positioning of specimens for dimensional measurement.22

Opposing dial indicators mounted on the cross-feed of the lathe measure flat-ness, bow, diameter, and thickness. To facilitate the continuous recording ofdimensions, a pneumatic system was linked to the dial indicator to generate a\signal related to the position of the stem of the dial indicator. A comparisonof the pneumatic system with electronic! systems that could be used reveals thatthe pneumatic system is one-fifth as expensive, less noisy in the radioactiveenvironment, and simpler to operate and maintain. On the other hand, thepneumatic system has slower response and decreased accuracy. However, theattainable accuracy of ±0.0002 in. is adequate for many applications.

A technique and associated equipment23 were developed to determine accu-rately the density of irradiated fuel specimens suspected of swelling. Thecommon method of weighing in both air and water is difficult to apply with con-fidence because the convection currents from the hot specimen affect themeasurement of weight. The weighing error has been largely overcome by theintroduction of a magnetically actuated horizontal stirrer at the bottom of thewater bath. Results have been excellent. For example, when a simulated speci-men weighing 130 g was heated with 78 W, the apparent weight dropped 64 mg.

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After application of the stirring, the error was reduced, to ±5 mg and thedensity error to less than ±0.05$. Care must be taken to avoid overstirringthat would increase the apparent weight.

A vacuum pycnometer was developed to measure "balk volume of fuel pelletsin a hot cell.24 Mercury was chosen as the sample-enclosing liquid since itdoes not react with ror wet the samples "being measured. With its high surfacetension, the mercury does nob penetrate the cracks and pores as readily asmost other liquids. At atmospheric pressure, flaws less than 0.00047 in. widewill not he penetrated; therefore, the measured value of volume is that boundby the exposed surface of the sample and the area over cracks and pores lessthan 0.00047 in. wide. Volumes up to 2.5 cm3 are measured with an accuracybetter than 0.001 cm3- Porosity of pellets and volume of powders have beenMeasured with gas and mercury porosimeters,

FUTURE HEEDS

Eeactor fuel elements will soon be producing an increasing amount ofelectrical power. Required examinations for a portion of these elements callfor higher speed and more efficiency from both examination techniques anddata handling. Pâture applications should see increased usage of neutron.radiography with small dedicated reactors or, more likely, with isotopicsources. Ultrasonic and eddy-current techniques will be used more widelyin hot cells. Laser beam technology including holography and interferometryare being developed to improve the speed and accuracy of dimensional measurements.Increasing essphasis will "be placed on examination in the reactor, monitoringdynamic changes due to active reactor conditions and variations.25

SUMMARY

The use of nondestructive testing techniques to examine irradiated fuel ele-ments is increasing. Hew facilities are being designed with specific provision

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to allow the efficient application of nondestructive testing at both interimstages during the life of the fuel element and after the element has completedits performance. Neutron radiography is being used at an increasing rate becaxise5t can image opaque objects without cor cera for the high level of radiation emittedfrom the specimen. Special techniques with both high- and low-energy x rays arealso being applied to the examination of radioactive fuel elements - Ultrasonictechniques have been applied to many special examination problems, includingdetection of nonbond in clad structures and cracks in fuel cladding. Eddy-currenttechniques are used to measure spacing between components and thickness of clad-ding or coatings and detect defects in cladding- Micrometers, dial indicators,and other mechanical measuring devices have been modified for use in hot cells.Advanced but simple techniques and equipment have been devrloped to measure den-sity and volume of fuel materials. Increased future needs will require higherspeeds, 5-mproved data handling, and,probably application in the reactor.

REFERENCES

1. N. J. Swanson et al., "Preliminary Design of the Hot Fuel ExaminationFacility (HFEF)," Proceedings of thi 17th Conference on Remote^SystemsTechnology, American Nucl r Society. Hinsdale,'111., 1969} pp. 131-14-0,

2. C. L. Boyd, D. J. Meyers, and E. B. Ramsey, "The FFÎF Fuel ExaminationFacility," Proceedings of the 16th Gonfjg j J ote ^ Systems Technology,American Nuclear Society, Hinsdale, 111., 1969, pp. 3~l8.

3. J. G- Duffy et al., "A Portable Alpha-Gamma Hot Cell for NondestructiveExamination of Irradiated Fermi Subassemblies," Proceedings of the 15thConference on Remote Systems Technology, American Euclear Society, Hinsdale,111., 1967, pp. 21-27.

4. H. Berger and W. N. Beck, "Neutron Radiographie Inspection of RadioactiveIrradiated Reactor Fuel Specimens," Kucl. Sci. Eng. 15, 411-414 (1963).

5. H. Berger, Neutrón Radiography, Elsevier, Amsterdam, 1965.6. D. C. Cutforth and V. G. Aquino, "Neutron Radiography in the EBR-II Fuel

Cycle Facility Using an Isotopic Heutron Source," Proceedings jof the, 15thConference on Remote Systemsntechnology, American Nuclear Society, Hinsdale,111., 1967, ppT 97-98.

Ï65

7. J. L. Hoyt, C- E. Porter, and C. D. Wilkinson, "Neutron Radiography Appli-cations at General Electric, " ibid., pp. 109—110.

8. B. E. Foster, S. D. Snyder, V. A. DeCarlo, and R. W. McClung, Developmentand .Operation of a High- Intensity^ High-gegolutlon Neutron. RadiographyFacility, OFNL-4738, in preparation.

9. J. J. Haskins, J- F. Jaklevick, and C. D. Wilkinson., "Applications ofNeutron Radiography in Fast Reactor fuel Development, " Proeeedings of '_ thel_6th_jC!or American Nuclear Society,Hinsdale, III., 1969, pp. 216-221.

10. B. E. Elliott and J. F. Torbert, "Radiographie Inspection of IrradiatedFuel Elements Using the 22-MeV Betatron/' ibid., pp. 223-227.

11. R- W. McClung, "Nondestructive Monitoring for Optimization of PerformanceTests, " paper presented at the ASTM Symposium on Testing for Prediction ofSfoterial Performance and Structures and Components , Atlantic City, New Jersey,July 29-July 1, 1971.

12. ¥. J. Gruber, "Autoradiography of Irradiated Nuclear Ceramic Fuels," Pro-ceedings of the 17th Conference ... . e tg gy emg ecbnolo r, AmericanNuclear Society, Hinsdale, 111., 1969," pp. 21-25.

13. R. W. McClung ana D. A. Douglas, "Nondestructive Testing of IrradiatedMaterials in the United States," pp. 179-202 in High Activity HotLaboratories Working Methods , Vol. 1, Proceedings of an InternationalSymposium Organised by MSA and EURATOM, McGraw-Kill, New York, 1965.

14. K. V. Cook and R. W. McClung, Oak Ridge National Laboratory, unpublishedwork.

15. D. 0. Hunter, "Ultrasonic Testing of Irradiated NPR Fuels," Paper No.CP 63-715, presented at the Institute of Electrical and ElectronicEngineers Electronuclear Conference, Riehland, Wash., April 29-30, 1963.

16. T. G. Lambert, "Ultrasonic Inspection of Irradiated Fuel Rods," Proceedingsof the .lltti Conference on Hot JLaboratovles and Equipment , American NuclearSociety, Hinsdale" 1117, 1963, pp. 255-265.

17- G. V. Dodd, "Design and Construction of Eddy-Current Coolant-Channel SpacingProbes," Microtecnic (Lausanne ) 18, 286-289 and 369-371 (1964).

18. W. B- Parsley, "Equipment Used for Examining, Dismantling and Re-assembly ofa HFIR Fuel Element, " ^ ^ ^Technology, American Nuclear Society, Hinsdale, 111., 1969, pp. 209-214.

19. W. C. Francis et al., "Nondestructive Testing in Test-Reactor Operation,"pp. 331—347 in Nondestructive Testing in Nuclear Technology, Vol II,International Atomic Energy Agency, Vienna, 1965,

20. F. L, Brown, V. F. Marphy, and E. H. Stearns, "Remote Metrology and Exami-nation Techniques on Irradiated Fuel Materials as Practised in the UnitedStates," pp. 245-265 in High Activity Hot Laboratories Working Methods,vol. 1, Proceedings of an International Symposium organized by ENEA andEURATOM, MsGraw-Hill, New York, 1965.

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21. B. J. Koprowski and P. L. Brown, "Examination Techniques for High BurnupU-Pu ïuel Elements for Fast-Breeder Reactors," Proceedings of the_15thConference on Remote Systems Technology American Nuclear Society, Hinsdale,111., 1967, pp. 63-67.

22. T, L. Cnandler and W. B- Parsley, "Apparatus for Dimensional Measurementsin a Hot Cell Utilizing Dial Indicators and Pneumatic Readout," Proceedingsof the 17th Conference on Remote § t __ echnology-? American Nuclear Society,Hinsdale, 111., 1969, ~pp'.' lya1 !"; '

23. A. C. Titus, "Weighing' Irradiated Fuel Specimens in Water," Proceedings ofthe 14th Conference on Remote Systems Technology, American Nuclear SocietyjHinsdale, 111., 1966, pp. 263-264.

24. 3. E. Disrauke, "A Vacuum Pycnometer Using Mercury with Plunger Displacementfor Hot Cell Use," Proceedings of the j.7th Conference on Remote SystemsTechnology, American Nuclear Society, Hinsdale, 111., 1969, pp. 37-39. "

25. E. A. Evans, J. E. Hanson, and G. W. Cunninghan?., "Trends in IrradiatedFuels and Materials Examination," Proceedings of the 16th Conference onRemote Systems Technology, American Huclear Society, Hinsdale, 111., 1969,pp. 215-217.

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ULTRASONIC IN-SERYICS IIISPSCTIOI? OF REACTOR PRgSSUKB .V]EjSgEL5_

H.J. MeyerFed. Rep. Germany

ABSTRACT

In view of the safety requirements for nuclear reactor pressurevessels, nondestructive inspections are to be carried out not onlyduring manufacture but also during the whole operational life of areactor. Ultrasonic inspection systems and manipulation equipmentare described which permit economical inspection of reactor components,at the eame time avoiding subjective evaluation and manual probeguidance. The inspection system and manipulation are ; ranged so thatwhen modern electronic components are employed two-dimensional flawsin particular (e.g. cracks) can be traced with a high degree ofdependability.

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Ultrasonic in-service Inspection of reactor pressure vesselsH.-J. Meyer Moschinenfobrik Aygshurg-Nüinberg AG (MAN), Werk NOr.iber;

1. luBei den mit fossilen Brennstoffen betriebenen Kraftwerkengehört die periodische Revision von druckführenden Kom-ponenten zu den seif Jahrzehnten bewährten und selbstver-ständlichen Einrichtungen Durch visuelle Inspektion und vor-wiegend manuellen Einsatz der zur Verfügung stehendenzerstörungsfreien Prüimoglichkeiten wird weitgehend ver-hindert, daß unvorhergesehene Stillstandszeiten durch Schä-den auftreten und daß Gesundheit und Leber* vom Bedie-nungspersonal Gefahren ausgesetzt sind.Bei Kernkraftwerken ist eine vergleichbare periodischeRevision nicht durchführbar. Dies liegt nicht nur an derbesonderen Form der Energieerzeugung durch den Reaktor-prozeß und die damit verbundene konstruktive Auslegung,sondern vorwiegend an der radioaktiven Kontamination, dieDruckbehälter und PrimärkressiauFsystsme annehmen und dievisuelle Inspektion und manuelle Prüfung weitgehend un-möglich machen. Die Kenntnis von diesen Schwierigkeiten,der Wunsch nach hoher Verfügbarkeit von Kernkraftwerkenund die Forderung nach Ausschaltung jeglichen Risikos beimBetrieb führte schon bei Beginn der Reaktortechnik zu außer-ordentlich hohen Anforderungen bei Konstruktion, Auswahlund Prüfung der Werkstoffe und bei den Fertigungs- undFunktionskontrollen.Seit Ende 1967 wurden in der Bundesrepublik Deutschbnd,bedingt durch die Erteilung von Baugenehmigungen fürGroßkernkraftwerke und die beginnende Planung einer Viel-zahl werterer KernkraHanlagen, die Forderungen nach volu-metrischen PröfmögÜchkeiten und Erfassung von nichtzu-gängüchen Reakforkomponenten bei Wiederholungsprüfun-gen gestellt [1,2]. Im dritten Atomprogramm der Bundes-republik Deutschland wird der Forschung for ReaMorsicher-heif besondere Bedeutung zugemessen. In diesem Zusam-menhang erhielt die MAN von dem Bundesministenum fürwissenschaftliche Forschung den Auftrag zur Entwicklung vonzerstörungsfreien Prüfverfahren zur Fehlersuche in dickwan-digen Behältern. In Zusammenarbeit mit der Firma Dr. J, u.H. Krautkrämer, Köln, begann die Entwicklungsarbeit Ende1968. Schon bald nach Beginn dieser Arbeiten ergaben sichakute Forderungen nach Wiederholungsprüfeinrichtungenfür im Sau und im Betrieb befindliche Kernkraftwerke. Esmuöte versucht werden, optimale Lösungen auch für Anlagenzu finden, bei deren Konstruktion noch keinerlei Wteder-holungspröfvorstellungen bestanden.

2. Schadenshypothesen für den ReaktorbsiriebZerstörungsfreie Prüfverfahren können am wirksamsten ein-gesetzt werden, wenn bei der Auswahl der Prüfmethodenund -verfahren die Art der möglichen oder bei Fertigung undBetrieb zu erwartenden Fehler bekannt ist.An Reaktordruckbehältern und Primärkreisiäufen sind beiden Wiederholungsprüfungen grundsätzlich andere Fehler•anzunehmen, als die Mehrzahl der Fehlererscheinungsforwertim Vormatertal oder bei der Behälterfertigung ausmacht.Hierdurch wird von vornherein, allerdings abhängig vonphysikalischen Möglichkeiten, eine bestimmte Prüfmethodikerforderlich.Während des Reaktorbelriebes ist praktisch nur mit Fehlernin Form von Rissen zu rechnen. Wenn auch Lage, Ort und

1. IntroductionIn power stations operating with fossil fuels the periodicinspection of pressure-bearing components has become oneof the accepted and well-proven faculties for a number ofdecades now. By means of visual inspection? and the mainlymanual employment of available non-destructive restingequipment, it is ensured la a great extent that unforeseenshut-down periods due to faults can be avoided and thehealth and safety of operating personnel protected fromhazards.in nuclear power stations a similar periodical inspection isnot possible. This is noi only due to the special form ofenergy production by the nuclear reactor process and itsassociated design, but mainly due to the radioactive con-dition of pressure vessels and primary circuit systems whichmoke visual inspection and manual inspection impossible toa great extent. The knowledge of these difficultfos, the desirefor a higher degree of availability of nuclear power stationsand the demand for exclusion of all risks during operationled to very high standards being laid down for the selectionand testing of materials and for the manufacturing and func-tional tests durir-.g design and construction.Since the end of 1967, in view of the granting of approvalfor large nuclear power stations in the Federal Republic ofGermany and the initial planning of a number of furthernuclear power stations, the demands have been made forvolumetric examination and coverage of non-accessiblereactor components by in-servke inspections [1r2{. In theThird Nuclear Programme for the Federal Republic of Ger-many particular importance was attached to the researchwork on reactor safety. In this connection, MAN received anorder from the Federal Ministry for Scientific Research todevelop a non-destructive inspection process for the deter-mination of flaws in thick-walled vessels. The developmentwork was started at the end of 1968 in conjunction withMessrs. Dr. I and H. Kraufkramer, Köln.Shortly after the start of this work, acute demands arose forin-service inspection equipment for nuclear power stationsboth under construction and already in service, An attemptmust be made to find optimum solutions also for those plantswhere no consideration was given to in-service inspection atthe time of their design and construction.

2. Fault hypothesis for reactor serviceNon-destructive inspection can be most effectively employedif during the se-'ection of the inspection methods and proces-ses the type of defect possible or to be expected duringmanufacture and operation is already knowi-On nucfear reactor pressure vessels and primary circuits,other defects are to be assumed in principle during inserviceinspections than the majority of defects in the basic materialor during vessel production, in view of this it is necessaryfrom the very start, although dependent upon the physicalpossibilities, to use a systematic approach in developinginspection procedures.During reactor service, practically only defects in the formof cracks are to be expected. Even though position, locationand orientation of the cracks can be different, there is still

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Orientierung der Risse verschieden sein können, so besteh»doch die Möglichkeit durch Spannungsanalysen und Span-nungsmessungen bei der Drückprobe, durch rechnerische Be-anspruchungsermittlungen und Modelluntersuchungen dieStellen zu ermitteln, an denen, wenn überhaupt, während derSetriebszeit eines Reaktors Schädigungen auftreten können,übereinstimmend werden von allen Autoren (z. B. J3j), diesich mit den möglichen Fehlererscheinungen an Druckbehäl-tern und Primärkreisläufen beschäftigen, drei hauptsächlicheEntstehungsmechanismen von Schäden angeführt; Spröd-faruch, »low cycle fatigue« und Spannungsrißkorrosion.Die Sprödbruchana/yse verlangt die Kenntnis der örtlichenBeanspruchung und der übergangstemperatur de» betreffen-den Werkstoffes, Dies gut- besonders für Schweißnähte,SchweißnahtöbergcJnge, Stutzenausschnitte, Flanschanschlüs-se usw. Eigenspannunger» von der Fertigung und Span-nungen durch unterschiedliche Temperaturverteilungen sindebenfalls zu berücksichtigen. Je sorgfältiger ein Behälter inMaßhaltigkeit {z.B. gedrehte Schösse] und Wärmebehand-lung gefertigt wurde, desto genauer wird man die Stellenangeben können, die für Wiederholungsprüfungen interes-sant sind, wodurch wiederum der Prüfumfang beschränktwerden kann. .Der Schadensmechanismus beim »/ow cyde fafi'gue« ist weit-gehend von der Fahrweise eines Reaktors abhängig. So kannz. B. in der Umgebung von Speisewasseretntritisstutzen, andenen hohe Temperaturunterschiede in Abhängigkeit vonder Fahrweise möglich sind, mit höheren Beanspruchungs-wechseln, verursacht durch Temperaturunterschiede, gerech-net werden. Bruchmechaniküberlegungen können heute einsehr genaues Bild von dem Zusammenhang zwischen Fehl-steile, örtlicher Spannung und Rißforrschritt geben.Schäden durch Spannungsri'ßfcorrosion können als die wahr-scheinlichste Form von Schäden an Druckbehälter« ange-nommen werden. Die meisten der bis heute bekannt gewor-denen Schäden sind auf diesen Schadensmechanismuszurückzuführen, wobei besonders die austersitischen Plattie-rungen (z. B. on den Srutzenkanten) oder andere austeni-tische Komponenten des Reaktorsystems betroffen, waren.Es ist bis heute jedoch noch kein Fa)t bekannt, bei dem einRiß in der Plattierung auf das ferritisch-perlitische Grund-material übergegriffen hat und in diesem für die Festigkeitdes Druckfaehälters verantwortlichen Werkstoff bereits fort-geschritten ist.Im Zusammenhang mit dem Wunsch nach Wiederholungs-prüfeinrichtungert is! es wichtig zu wissen, an welchen Stel-len eines Druckbehälters erhöhte Beanspruchungen vorliegenund auf welche Zonen die Bemühungen um periodische Über-prüfungen konzentriert werden sollen.Fig. l zeigt die typischen KonsJruktionsformen sines Siedß-wasser- und eines Druckwasserreaktors für etwa die gleicheLeistung (600MW). Neben der unterschiedlichen Wanddickeist es besonders die Siurzenanordnung, die beide Behältervoneinander unterscheidet. Die Kreise in Fig. l deuten an,an welchen Stellen mit besonderen Beanspruchungen zurechnen ist und an welchen Zonen aufgrund der metallurgi-schen Gegebenheiten eine Nachprüfung auf RiQfreiheit er-wünscht oder besonders erwünscht ist.Wenn auch die Reihenfolge der Notwendigkeit von Wieder-holungsprüfunger» auf die Fehlerwahrseheinlichkeit obge-sfimmr werden sollte, so erscheint es doch zweckmäßig, be?der Konstruktion und Entwicklung von Prüfeinricntungen undVerfahren auch die Stellen am Reaktor zu berücksichtigen,die keine beanspruchungsmäßige oder metallurgische Son-derstellung einnehmen, wenn diese Steifen ohne großen Ma-

a possibility, by stress analysis and stress measurements du-ring the hydraulic rest, to determine by means of cafcufatorystress investigations and model investigations, those pointsat which, if at all, defects can occur in a reactor duringservice.All authors (e.g. J3j) who have dealt with the possible defectphenomena on pressure vessels and in primary circuits havelisted three main origins of defects, these being brittieness,low cycle fatigue and stress corrosion.The brittfeness analysis necessitates knowledge of rhe localstressing and the transition temperature of the relevant ma-terial, This applies in particular for weld seams, weld seamtransition, nozzle cut-outs, flange connections etc. Inherentstresses from manufacture and stresses due to differing tem-perature distributions also have to be allowed for. The morecarefully a vessel is manufactured dimensionalfy (e.g. ma-chined courses) and the heat treatment carried out, thepoints which are of interest for in-service inspection can bespecified all ihe more accurately so that in turn (he scope ofinspection cart be limited.The low-cycle fatigue defect mechanism depends to a greatextent upon the mode of operation of a reactor. For ex-ample, in the area of feed water inlet nozzles where hightemperature differences are possible dependent upon themode of operation, higher stresses are to be expected dueto these temperature differences. Considerations in connec-tion with fracture mechanics con today give a very accuratepicture of the association of defect point, focal stressing andcrack development.Defects due to stress corr ;ion can be assumed to be themosf probable form of defects in rhe pressure vessels. Mostof the defects which have become known to date were aresult of this defect mechanism when in particular the aus-tenitic cladding {e.g. at the nozzle edges) or other austeniticcomponents of the reactor system were involved.

BWR

Fig. 1-. PriniipsdinUtbilder von Druckbehältem fOf Druckwasserreaktor(PWR) und Siedewasserreaktor {BWR) mit gWdier leisttina (600 MW);PrafbereicheFig. 1: Srftemotic drawings of -wessure vessels for pressurized water reac-tor {PWR} and boiling water reactor (BWR) of equal outpvtj («00 MW);inspection areas

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nipuiaiions- und Zeitaufwand mit erfaßt werden können.Hierdurch wird die Gewähr gegeben, doß auch an Zufall--steilen zumindest noch dem Ergebnis stichpcobenwetser Prü-fung im Laufe der Reoktorbetriebszeit kein« Veränderungen«ingetrefen sind.

3. Auswahl zerstörungsfreier PrüfverfahrenDie zerstörungsfreien Prüfverfahren können aHgernein inzwei Gruppen unterteilt werden: Verfahren, die Schadenoder Veränderungen an den Baufeiloberflächen aufzeigen(hierzu gehören auch optische oder elektronisch-optischeMethoden) und Verfahren, die zusätzlich auch innere Fehlerin Konsfruktionsteilen zur Anzeige bringen.Bei den Wiederholungsprüfungen schränken der Mangel anZugänglichkeif und die ionisierende Strahlung die Einsaiz-möglichkeitert sehr stark ein. Visuelle Verfahren durch op-tische oder elektronisch-optische Methoden setzen voraus,daß die w betrachtenden Oberflächen frei von Ablagerun-gen sind, die eventuelle Risse verdecken könnten.Die Erfahrungen bei den ersten bereits durchgeführten Wie-derholungsprüfungen haben gezeigt, daß Endoskope undFernsehen besonders bei der Untersuchung von Brenneie-ment-Hüürohren von großem Nuizen waren- Das Erkennenvon Rissen an den Plattierungen der Reaktorwond oder -stui-zen stößt jedoch schon auf erhebliche Schwierigkeiten. Ma-gneüsche und Penefrrerprüfverfahren fallen, abgesehen vonEinsotzmögfichkeifen am Deckel, bei der Druckbehälterwand-prüfung praktisch aus. Wie alle optischen Verfahren, so sieilfauch das in Japan mit Erfolg bei Wiederholungsprüfungeneingesetzte PofenHalsonden-Veifahren [4j nur eine Qber-flächenprüfung dar, während für die Wiederholungsprüfun-gen die voäumetrische Prüfung verlangt wird. Da für volu-meirische Untersuchungen aufgrund der vorhandenen Radio-aktivität auch alle Dwrchstrohlungsmethoden ausfallen unddie passiven Uitraschallprüfiriethoden durch Ausnutzung der»Schau-Emission« beim Rißfortschreiten [5] noch ersi cm An-fang der Entwicklung stehen, ist die aktive Ultraschallprü-fung die einzige heute bereits anwendbare Methode, inter-essante Prüfkonzept'onen und eine Anzahl bereits gebauterund eingesetzter Wiederholungsprüfeinrichiungen in ver-schiedenen Ländern bestätigen den Vorrang dieser Prüftech-nik für volurnetrische Untersuchungen [<>~9J.

4. UlirrocHdipritftedhntk4.1. A //gemeinesin Deutschtand wurde 1966 erstmals eine RkhHmie für dieUltraschallprüfung von Schweißnähten vorgelegt, die eineSystematisierung der Prüfungsdurchführung,, eine Reprodu-zierbarkeit der Prüfergebni$se und eine Objektivierung derBeurteikmg erlaubte [10J.Die Wiederholungsprüfung an Reaktorwandungen kann auchan Reakforsieilen, wo keine Längs-, Rund- oder Stutzennähteliegen, weitgehend mit der Prüfung von Stumpfschweißnäh-ten gleichgesetzt werden. Für die Wiederholungsprüfungenergeben sich jedoch im Vergleich mit den Ultraschallprüfun-gen bei der Hersteilung von Druckbehäftern zwei WesentlicheUnterschiede. Der erste liegt in Fefcferart und -orienf/erung.Wie in Abschnitt 2 aufgezeigt, sind durch den Reaktorbetrieb,wenn überhaupt, nur Risse oder Trennungen zu erwarten,d. h. zweidimensionat ausgebildete Reflektoren, die denSpannungs- und Beonspruchungsverhältnissen entsprechendsenkrecht zur Oberfläche orientiert verlaufen. Der zweiteUnterschied zur Hersrellungsprüfung liegt in der Gröös derFehlstellen. Nachdem bei Wiederholungsprüfungen praktischkeine'Ausbesserungen (abgesehen vor» Reparaturen an ent-

To date, however, no case is known >n which a crock in thecladding has gone over to the ferritic periiiic base materialand continued in this material which is responsible for thestrength of the pressure vessel.In conjunction with the wish to hove in-service inspections,it is important to know at which points of a pressure vesselincreased stresses exist and in which zones the efforts toprovide periodical inspections should be concentrated.Fig. 1 shows the typical designs of a boiiing water reactorand a pressurised water reactor for about the saute outpul(600 MW). Apart from the differing waN thickness, the ar-rangement of nozzles in particular provides the differencebetween the two vessels.

The circles <n Fig. 1 indicate at which points special stressesare to be expected or in which zones a subsequent inspectionfo<- freedom of cracks is desired or particularly desired inview of (he metallurgical factors.Even though the priorities of in-service inspections should becoordinated with Ihe probability of defects, it appears ad-visable, during design and development of testing equip-ment and processes, to include those points on the reactorwhich are not critical in respect of stress or metallurgy ifinspection of these points can be provided without onyappreciable extra manipulation ond time. This would pro-vide an assurance, Qt least on a random basis, that nochanges hove occurred at non-critical points during thereactor life.

3. Selection of non-destructive inspection processes

The non-destructive tesiing methods can be roughly dividedinto fwo groups: methods which indicaie the defects ordeficiencies on the component surfaces {this also includesoptical or electronic optical methods) and methods whichalso indicate internals defects in constructional compcnenfs.During in-service inspections the possibilities of employmentare greatly restricted due 5o inaccessibility and ionizingradiation. Visual processes by optical or electronic opticalmethods can oniy be applied on the understanding that thesurfaces to be inspected are free of deposits which couldcover possible cracks.The experience with the first in-service inspection alreadycarried oui has shown that endoscopes and close-circuittelevision were of important use particularly during the in-spection of fuel element shroud tubes. The recognition ofcracks in the cladding of 'he reactor «rat! or nozzles, how-ever, meets up with considerable difficulties. Magnetic andpenetrant inspection methods can practically be ruied outfor pressure vessel wall inspection opart from the possibilityof use on a closure head. As in the case of ail optical meth-ods, the electric resistance probe method [4] employed suc-cessfully in Japan for in-service inspections couid ortiy pro-vide surface inspection while volumetric inspection is de-manded for inservice inspections, in view of the foct thatfor volumetric inspections ell radiogrophic methods areexcluded due fo the existing radioactivity and the passiveultrasonic inspection methods using sound emission pro-duced by crack propagation [5] are only of the beginning ofrheir development, the active ultrasonic inspection methodis the only one which can be applied today. Interesting in-spection concepts and a number of in-service inspectionequipment already constructed and employed in variouscountries confirm the importance of this inspection tech-nique for volumetric inspections (6-9).

172

-i,,-, löiiftn, wie z.B. dem Reaktordecksl) durchgeführt'.-,rü<5n könne ,, interessieren hier Fehler erst von einerGröße cb, die zwar noch weif unter der durch Brychmecha-pik ermirtelfen kritischen Rißgröße liegt, {edoch einen deut-lichen Abstand von den Fehlerdimensionen hat, die bsi derHersteiiung gerade noch belassen worden sind. Dieser Ab-stand ist notwendig, um Prufdatenerfassung und -auswer-•furg zu vereinfachen und dadurch Obersichtücher zu gestal-ten. Es muß jeaoch sichergestellt sein, daß von einer be-s!irr>iyilen Größe ab aach unter den erheblich schwierigerenBedingungen einer Wiederholungsprüfung (fernbedienteManipulation, Piattierungseirtfluß, Zeitbeschränkung) dieseFehler reproduzierbar aufgefunden und dem ryaptitritivor»Datenvergieich zugeführt werden können.

So entstand die Vorstellung vom 10-mm-Durchmesser-Ersatz-Kreisscheibenfehler senkrecht zur Oberfläche als die Größeeiner RefiexionssteUe, die von den Prüfeirtrichtungen nochaufgefunden werden muß. För Fehler in odar unfer derPlottierung, deren Auffindung besondere Schwierigkeifenbereiten, sollte eine Länge von 20mm und eine maximaleTiefe von 5 mm (elliptische Form) mit den Mitteln der Wie-derholungsprüfe'mridYtungen erkennbar sein. Man ist sichdarüber klar, daß bei bereits ausgeführten Anlagen In Ab-hängigkeit von Plaffierungsqualität und Zugänglichkeit auchdieses Maß wahrscheinlich noch nicht als die untere Grenzeangesetzt werden kann.

Fig. 2 zeigt die schaltgeometrischen Verhältnisse, wie siebei der volumeinschen Prüfung dicker Wandungen und einerFehlerorientierung senkrecht zur Oberfläche vorliegen. Beiden Prüfkopfstellvngen a bis f ergeben sich unterschiedlicheSchallwege von Prüfkopf zur Reflexionsstelle und zum Prüf-kopf zurück, (rt Position a wird z.B. ein gleich großer Fehlerwegen der kürzerer» Laufzeit auf dem Bildschirm eine er-heblich höhere Anzeige ergeben als in Position e. Bei glei-cher flächiger Trennung v/erden Anrisse an den Oberflächendurch den sogenannten »Kcmteneffekt« bei gleicher Geräte-aussteuerung größere ReRextonsanzeigert erzeugen als Feh-ler im innern des zu prüfenden Querschnittes. Dieser Effektist ouf die gleiche Laufzeit aller Anteile eines Schaf'böndels(EinschaHwinket 45°) an einer Kante oder einem Oberflä-chenriß zurückzuführen. Ist zusätzlich noch eine ausfemfischePlattierung zu durchschauen {siehe auch Abschnitt 6), sozeigen sich für Fehterflächen gleicher Größe im Querschnittweitere erhebliche Scrtallsrhwächungsunterschiede.

Fig. 2: Ullraschollgeometrie an dicke» Wandungen; Kanteneffekr UM) Tan-ds rntechnikFig. 2: Ultrasonic geometry on thick walls; corner effect and tandemtechnique

4. Ultrasonic inspection technique4.1, Genera/

In Germany a code for ultrasonic inspection of welds wastabled in 1966, which permits a systematization of inspectionprocedure, reproducibiliry of the inspection results and ob-jective evaluation [10],Even at points where no longitudinal, peripheral or nozzlewelds exist in-service inspection of reactor walls can also belooked upon as closely akin to the inspection of butt welds.However, there are two mojor differences for inservice in-spection compared with the ultrasonic inspections during themanufacture of pressure vessels. The first, the type of defecfand orientation. Due to reactor service, if at all, as shownin Section 2, only cracks or separations are to be expected,i.e., fwo-dimensionai reflectors, which run perpendicular tothe surface according to the stress and strain conditions. Thesecond difference to manufacturing inspection is in the sizeof the defects. Due to the fact thai with in-service inspectionspractically no repairs can be carried out {apart from repairson removable components, e.g., the reactor cover} the de-fects of 'nterest here are only from a size upwards which iswell below the critical crack size determined by fracturemechanics but has a clear difference from the defect dimen-sions which were just allowable during manufacture. Thisdifference is necessary to simplify and therefore elucidateinspection data processing and evaluation. It has to be as-certained, however, that from a certain size onwards thesedefects can be reproduced even under the considerably moredifficult conditions of inservice inspection (remote controlledmanipulation, influence of cladding, time Jimitation) andused for (he quantitative data comparison.This resulted in the idea of 10 mm diameter equivalent disc de-fects perpendicular to the surface as the size of a reflectionpoint which would have to be traced by the inspectionequipment. In the case of defects in or under the cladding,the tracing of which involves special difficulties, a length of20mm and o maximum depth of 5 mm (elliptic shape) are tobe recognisable with the in-service inspection equipmentfacilities. It is folly appreciated that in the plants alreadybuilt, dependent upon the cladding quality and accessibility,even this dimension can probably not be employed as theiower limit,Fig. 2 shows the ultrasonic geometric conditions existingduring volumetric inspection of thick walls and a defectorientation perpendicular to the surface. The probe posi-tions a to f give differing beam paths from the probe to thereflection poini and back to the probe, in position a, forexample, a defect of a certain size would result in a con-siderably greater indication on the screen due to the shortertravelling time than in position e. With the same 2-dirnen-sional separation, cracks on the surfaces cause larger reflec-tion indications with the same equipment amplitude thandefects at the inside of the cross section to be inspected dueto the so-called "corner effect". This effect is due to the sametravelling time of all parts of a beam bundle [45° angle ofincidence) on a corner or a surface crack. In addition, anoustenitic cladding hos to be passed through {see also Sec-tion 6), so that much more considerable attenuation differ-ences are indicated in the cross section for defect surfacesof the same size.As con also be seen from fig. 2, o considerably longer travel (ime (fac-tor 2,06) results when using a flatter angle of incidence [a.g., 70°) com-pared to a 45° probe, v/hicK opart from a iower test sensmviiy of the 70°probe is also duo to the larger beam bundle diometar on the reflectionsurface and results in a detrimental effect on the flow location detcrmino-

173

Wie aw Fig. ? ersichtlich, ergibt sich weiterhin bei der Verwendungfacher Einschailw'inke! (z. 8. 70°) gegenüber einem 45°-Kopf eine erheb-lich (ärgere lagfieir (Faktor jtßfy, was iahen einer geringeren Prüf-empfind!; ch'«ei l des 70°~Kopfes. wagen d«s größerem SctKi>'ibtodefdurch-messers am Reflektor auch eine Verschlechterung der Fehlerlogc-nfoestirn-mynfj bedingt. Selbsr bei einer Schaüsfrah!- und Laufwegfconstellcition, wiein fig, I (70"~Strah! direkt, 45°-Tandsm-Umweg über Röckseife), spricht dosLaufzeitverhäifnis 1,6 zu 1 immer noch zugunsten dar 45"-Tandem-Methode(»pifd» and catch«), um z. B, einen Reflektor, der 20C mir-, unter der Biech-Oberfläche liegt, mit 70" anzuschnallen, ist ein laufweg von 2 • 59 cm -.1,18 m zurückzulegen.

4.2, Vielkoptmethocle (»Tatzelwvrm«-System}Die Köpfe a, b' und rJ in Fig. 1 zeigen den zweifacher» Vor-feil der Taridemmethode. Die Winkelspiegelung der senk-recht zur Oberfläche orientierten Fehler läßt in den senden-den Prüfkopf nur einen sehr kleinen Sdiaüanrei! zurückkeh-ren. Je glatfer eine Rißfläche, desto geringer ist der vomSendeprüfkopf wieder empfangene Schalfanteil. In der ge-zeigten Konstellation arbeiten b' und c' ob Sender, b" undc" als Empfänger, wobei [edes Kopfpaor Trennungen ineinem bestimmten Tiefenbereich zu erfassen hoi. Prüfkopfo, der Trennungen an der gegenüberliegenden Oberflächeond dicht unier der Oberfläche auffinden soll, braucht wegender eng zusammen i legenden Tandemstrahlengänge nichtgeteilt zu werden und kann als Sender und Empfänger nachder üblichen Einkopfmsthode arbeiten. Um einen dickwan-digen Querschrwt lückenlos zu prüfen, muß in Abhängig-keit von der Wctnddicke eine Anzahl von Prüfkopfpaarenund zwei Einzelköpfe artgeordnet werden, wobei jedes Paareinen Tief an abschnitt und die Einzelköpfe die OberflächenZu prüfen haben.Der zweite Vorteil des Tandemsystems mit mehreren Kopf-Paaren und zwei Etnzelköpfen liegt darin, daß jedes Prüf-kopfpaar for einen bestimmten Tiefenbereich in der Emp-findlichkeit eingestellt werden kann (gleiche Anseigen beigleich großer Trennung) und daß praktisch keine Loufzetf-unterschtede vorhanden sind, wodurch auf dem Bildschirmeine enge Monitorbiende genügt und Platz för weitereMortiforbienden, z. 8. zur Ankopplungskontrolle, bleibt.Bei dem Einkopfsystem müSte in jeder Position des Prüfkop-fes die Empfmdüchkeitseinsteilung verändert werden, wennüber den ganzen Querschnitt VOR einer bestimmten Ver-gieichsfehjergröße on der Monitor ansprechen soll. Auf dieseVerhältnisse wurde bereits 1963 hingewiesen [11]-Bei der MAN-Kroutkrcurter-Konzepfton für U!traschaH-Wie-derfiolungsprüfeinrichtungen wurde versucht, die aufgezeig-ten scfiaügeometrsschen Verhältnisse mit den Forderungender Kernkraftwerksbetreiber nach Wirtschaftlichkeit (Schnel-ligkeit bei größtmöglicher rVüfvoiumenerfossung] und denAnforderungen der Stcherhsits- und Überwachungsbehörden(Reproduzierbarkeit, genaue Fehiergrößen und -lagabestim-mung) in Einklang zu bringen. Das System mußte danach fol-gende Forderungen erfüllen;o) Verwendung «ine? SchcUstrabigeorftetrie, die ftacMg orientierte F«h'srsenkrecht zur Siechaberfläche aufzufinden jn der Loge is*.b) Verwendung von so vielen Präfköpfen wie erforderlich, um boi einemelektrcrüschan UmsehaHen aller Köpfe ohne Veränderung dor Kopfpostfioneinen bestimmten Querschnitt IGOproserttig volumferrisch erfassen zu kön-nen (Auslegung richtet lieh nach der Forderung, zwischen Scriolldruck-maxiinum und -rninimurrt im individuellen Prürcjuerscbmtf keine größereDifferenz als & dB zu hoben).c) Auslegung der Pröfko.afsystema in der Weise, dorö gleiche Reflektorenin verschiedenen prüfquerschnittsebanen tnöhef oder weiser von den indi-viduellen Piüfköpfen entfernt) die gleiche Anieigehöhe auf dem Bildschirmhaben. Hierdurch ergehen sich Erleichterungen bei der Auswertung,

d) Einsatz von Xonlcktpiütköpfen (mit- Wasserspaltkcpp'uftg oder in Touch-technik)' zur Vermeidung von Ooppeibrechung, Schot!energisverlus*en undvyediseliden Geometrie« hei unebenen Oberflächen.

iicii. Evon \vith an ultrasonic bfeam and path constellation as shown inFig. 2 (/If beam direct, 45° tanderr. detour via back face) the travs! limeratio of 1,6 to '. is stitl in favour of ihe 45° roncasm maihcd (pifcH andcatch). Por example, in order to beam onto a reflection surface located290 mm bolow the metal surface with 70 deg, o irnvel distance of2 X 59 cm - 1,18 m has So be covered.

4.1. My/ft'-profae meihod ("Tatzelwurm" system)The probes ar b' and c' in Fig. 2 show the double advantageof the tandem method. The angle beaming onto the defectlocated perpendicular to the surface only allows a verysmall portion of the ultrasonic beam to return io fhe trans-mitting probe. The smoother the crack surface is, the smalleris the portion of the ultrasonic beam received by {he trans-mitting probe, in ihe arrangement shov/n b' and c' operateas transmitters, with b" and c" as s^eceivers, each probe pairbeing intended to indicate separations in a certain depthrange. Probe a, which is intended to trace separations onthe opposite surface and closeiy below the surface, does nothave to be divided in view of the closely arranged tandembeam paths and can be employed as transmitter and re-ceiver according fo the usual single probe method. In orderto provide full coverage of a thick wall cross section, anumber of duo! probes and two single probes have Jo beemployed, depending upon the wai! thickness, when eachdual probe is to inspect a depth stratum and the singleprobes the surfaces.The second advantage of the tandem system with a numberof dual probes end two single probes is in the fact thai eachdual probe can be adjusted in its sensitivity for 'a cerfamdepth range (uniform indications with equally icrge separa-tion) and practically no travel ?*ne differences exist so thaton the screen o narrow monitor mask is sufficient and spaceremains for further monitor masks, e.g., for probe-to-specimen contact checks.With the sing!« probe system the sensitivity adjustment hasto be varied in each position of the probe if the monitor isfo react from a certain comparison defect size over the ful lcross section. These conditions were afready referred in1963111].In the MAN-Kraufkrämar conception for ultrasonic in-serviceinspection equipment an attempt has been made to coordi-nate She indicated ultrasonic geometry conditions wiih therequirements of the nuclear power station operators inrespect of economy (rapidity with maximum possible inspec-tion volume coverage) and the requirements of the safetyand supervisory authorities (reproducibiiity, exact defectsizes and location deierminationJ.The system should there-fore fulfill fhe following requirements:a) Application of on uitrcsonic beam geometry which is capable of detect-

ing the 2-dimensioriai c'sfect perpendicular to the metai surface.b) Use of os many probes as necessary so that with an electronic diangs-

over oil probes cover o certain cross section 1008/* volumetriceliy vitK-out altering the probe position, (iciyaul according to ihc requirement ofno greater difference thon 6 dB between sound pressure maximum andminimum in the individual fest cross sections).

c) Layojt of the probe system in ci moiner 10 thai «qutvaierit reflections invarious inspection cross section planes {closer lo or furtttor away frornthe individuell probes) have ths same indication height on the screen.This results in simplification of evaluation,

dj Employment of contact probss (with water probe-to-speciroen cortlac*or the immefsed method) to avoid double redaction, energy losses andvarying geometries on uneven surfaces.

el Emptoymeri? of digital ultrasonic dato read-out or recording equipmentwhich permits thö iranslation of ths analog records without great dif-ficulty for fvj tuie use in computers in digital values.

jj Details of ihe position values in iengih or angle-coded voiues.3) Provision of the possibility of addressing certain positions ond reflec-

tion points by slow probe ^ovement to investigate the dynamic indica-tion behaviour.

174

e) Einsatz von digitaler Ultrascholldatenausgabe oder von Registrierein-richtungen, die ohne große Schwierigkeiten die Oberleitung der Analog-aufzeichnungen för spätere Verwendung in Rechnern in Digitalwerte er-tauben.f) Angabe der Positionswerte in längen- oder winkelkodierten Werten.g) Schaffung der Möglichkeit, bestimmte Positionen anzufahren und Re-flexionsstellen durch langsame PrüfkoptVerschiebung in ihrem »dynamischen«Anzeigeverhalten zu untersuchen.Eig. 3 zeigt in sdiematischer Darstellung das System der Ultra-schall-Vielkopf-Prüfmethode (»Tatzelwurm«-System) am Bei-spiel einer 225mm dicken Wandung. Neben den erforder-lichen Winkelschwingern (45°) zur Impuls-Reflexion- oderTandem-Prüfung ist jeder individuelle Prüfkopf mit einemSchwinger zur NormaleinschaJIung ausgerüstet, um vor derEinschaltung des betreffenden Kopfes zur Fehlerprüfung denAnkopplungszustand kontrollieren und über einen elektroni-schen Ankopplungsausgleich die erforderliche Schallenergiedosieren zu können.In Fig. 4 ist eine Vielkopf-Prüfeinheit dargestellt, die für Er-probungen an einem plattierten Reaktorwandungsabschnittvon 230mm Dicke mit künstlichen Fehlern angefertigt wordenwar (Außenprüfung mit Wasserspalt-Ankopplung).

Fig. 4: Ultraschall-Vielkopf-Prüfeinheit (»Tatzelwurm«) für die Prüfung vonaußen an einer 230 mm dicken Reaktorwand (Wasserspaltankopplung) mitFahrwerkFig. 4: Ultrasonic multi-probe device ("Tatzelwurm") for the external scan-ning of 230mm thick reactor wall (water-gap coupling) with drive unit

4.3. Scha/fsysfem, PrüfdatenaufZeichnung und -VerarbeitungFig. 5 erläutert das Prüf- und Registrierprinzip der »Tatzel-wurm«-Prüfung. Die in einem handelsüblichen Ultraschallge-rät erzeugten Impulse gelangen über den elektronischen Um-schalter zu den einzelnen Prüfköpfen. Die Steuerung des Um-schalters kann über Programmkarten erfolgen oder auchüber andere geeignete elektronische Schalteinrichtungen, dieder Anzahl der durchzuschaltenden Prüfköpfe und dem vor-bestimmten Programm angepaßt sind.Die normalerweise notwendigen Kabellängen bedingen denEinsatz von Vorverstärkern für jeden individuellen Prüfkopf,wobei die Vorverstärker direkt am Prüfkopf in einem Vor-verstärkerkasten untergebracht werden müssen. Der Kastendient gleichzeitig als Prüf köpf halter und Zwischenglied zwi-schen Manipulator und Prüfköpfen. Es hat sich als zweck-mäßig erwiesen, in diesen Kasten weitere UJtraschallprüf-köpfe als Normalschwinger einzubauen, die durch Messungder Wasserstrecken bis zu Wandungen oder Hindernissenbeim Abfahren (z. B. zum thermischen Schild) die Lagebe-stimmung der Prüfkopfeinheit erlauben.Wie Fig. 5 zeigt, gelangen die Ankopplungskontroll- undPrüfimpulse über die Vorverstärker auf Monitorsysteme, dieim Fall der Ankopplungskontrolle einen elektronischen An-

Fig.3 shows a schematic arrangement of the system forultrasonic multi-probe inspection methods (Tatzelwurmsystem) on a 225 mm thick wall as an example. Apart fromthe requisite angle crystals (45°) for impulse reflection ortandem inspection, each individual probe is equipped witha crystal for normal beaming in order to check the relevantprobe for faults in the coupling condition before switchingon and also to permit adjustment of the requisite acousticenergy via an electronic probe-to-specimen coupling bal-ance.

n n

: «—Hnh-in «4

ft*.k | a • i j < r

^f^ir^i i^~^.i i~ •U i~

lusteniloche PliltiiranglostenKlceliMIng

Fig. 3: Schematische Darstellung einer Vielkopf-PrQfeinheit (System MAN-Krautkromer)Fig. 3: Schematic diagram of the multi-probe scanning device (MAN-Krautkrämer system)

In Fig. 4 a multi-probe test unit is shown which was manu-factured for inspections on a clad reactor wall section of230mm thickness with artificial defects (external inspectionwith water probe-to-specimen coupling).

4.3. Connection system, inspection data recording and pro-cessing

Fig. 5 explains the inspection and recording principle of the"Tatzelwurm" inspection. The impulses produced by a stand-ard ultrasonic inspection unit pass via the electronic selec-tor switch to the individual probes. Control of the selectorswitch can be provided via programme cards or via othersuitable electronic switching devices which are adapted to

Fig. S: Systemskizze för das Prüf- und Registrierprinzip bei der Verwen-dung von »Tatzelwurim-PrüfköpfenFig. S: Schematic diagram of test and recording principle using the "Tat-zelwurm" probe

175

kopplungsausgleich auslösen oder die Reflexionen von Prüf-impulsen diskriminieren und auf die Schreib- und Registrier-geräte weitergeben. Trotz der hohen Durchschattfolge (bis zulOmal pro Sekunde) wird hierdurch gewährleistet, daß An-kopplungsschwankungen im Bereich von 36 dB ausgeglichenund die Prüf impulse jeweils in einem genau dosierten Energie-niveau in die Wand eingeleitet werden. Erst bei Überschrei-tung der Ausgleichskapazität wird an das Registriersystemgemeldet, daß die Ankopplung an dieser Stelle und für einenbestimmten Prüfquerschnitt ausgefallen ist.Obgleich eine Analogregistrierung auf einem Vielkanal-X, Y-Schreibermöglich ist, hat sich bei Vorversuchen und in der Prüfpraxis die U.nter-teilung der maximal möglichen Bildschirmhöhe von 24 dB in vier 6-dB-Stufen bewährt. Durch einen Analog-Digital-Umsetzer werden am Monitordie Binärwerte 0, l, 2, 4 und 8 ausgegeben, wobei diese Schritte jeweilseinem Verhältnis 1:2 der Größe eines Reflektors entsprechen. Das Prüf-und Registriersystem kann nun z. B. so ausgelegt und mit Hilfe von künst-lichen Kreisscheiben-Reflektoren senkrecht zur Oberfläche so geeicht wer-den, daß eine Fläche von 75 mm2 (entspricht einer Sacklochbohrung paral-lel zur Oberfläche von etwa 10 mm Durchmesser] eine Anzeige von V» derBildschirmhöhe ergibt; 150 mm! entsprechen dann '/s, 300 mm' tlt und600 mm2 Vs der Bildschirmhöhe.Bei den bisher durchgeführten Wiederholungsprüfungen wurde ein elektro-statischer Schreiber mit 8 Registrierkanälen und 10 Ereigniskanälen ein-gesetzt, gleichzeitig erhielt ein Magnetspeichergerät über ein Interface diegleichen Code-Eingaben, digital-Positionsangaben und binär-kodiertenWerte von den Prüfungsergebnissen wie der elektrostatische Schreiber(Fig. 6).

Fig. 6: Elektronische Ausrüstung für Manipulatorensteuerung, Positionie-rung, Ultraschallprüfung und RegistrierungFig. 6: Electronic equipment for manipulator control, positioning, ultra-sonic inspection and recording

Fig. 7 zeigt als Beispiel die vereinfachte Darstellung der Aufschreibungdes elektrostatischen Schreibers. In Kanal 1 wird die Position der Prüf-kopfeinheit, in Kanal 2, 3 und 4 das Ergebnis der Prüfung in verschiedenenPrüfzonen aufgezeichnet. Für jede Prüfzone (Tiefenlage, z. B. 1 bis 5 inFig. 5) ist ein Kanal erforderlich.Ergeben sich bei der Prüfung keine registrierwürdigen Anzeigen, so läuftder Schreiber kontinuierlich mit einer langsamen Vorschubgeschwindigkeit(stand-by speed); im selben Augenblick, wo sich Anzeigen der Registrier-stufe 1 (75 mm* Ersatzfehlerfläche) ergeben, wird der schnelle Vorschubeingeschaltet, der solange eingeschaltet bleibt, bis die Anzeige verschwin-det. Das Beispiel in Fig. 7 zeigt, daß bei Position 16 ein Fehler in Regi-strierstufe l im Prüfkanal 2 erscheint [A 1). Die FehlergröBe wächst bis aufStufe 3 an, auch im Nachbarkanal 3 wird zwischen Position 20 und 24 eineStufe-1-Anzeige erhalten. Eine weitere Fehleranzeige ergibt sich zwischenPosition 64 und 72 mit großer Anzeige (Stufe 4) in Kanal 3, wobei derReflektor auch bis zur Stufe 2 in Kanal 4 hineinreicht (A 2).Können Ankopplungsschwankungen nicht mehr automatisch korrigiert wer-den, wird in dem entsprechenden Kanal eine Markierung gegeben, die zu-sammen mit der Positionsanzeige (54 bis 70) die genaue Ortsbestimmungerlaubt (A3).Die Digital-Positionsanzeigen können entweder mit Reibrad (Beispiel Fig. 5),kraftschlOssig am Antrieb oder winkelkodiert erhalten werden.

suit the number of probes to be employed and the pre-pro-grammed system.The cable length normally required necessitates the employ-ment of pre-amplifiers for each individual probe, these pre-amplifiers having to be arranged direct on the probe in apre-amplifier box. This box serves simultaneously as theprobe mounting and intermediate link between manipulatorand probes. It has proved advantageous to include in thisbox further ultrasonic probes as normal crystals which per-mit location determination of the probe unit by measure-ment of the travel path in water to the walls or obstaclesduring traversing (e.g., to the thermal shield).As shown in Fig. 5, the probe-to-specimen coupling checkand inspection impulses pass via the pre-amplifiers to themonitoring system which in the case of probe-to-specimencoupling checks initiate an electronic coupling balance ordiscriminate the reflections from inspection impulses andtransmit them to the recording and registering equipment.Despite the high rate of switching (up to 10 times per second)this ensures that coupling variations within the range of36 dB are balanced and the inspection impulses are trans-mitted to the wall in each case with an exactly controlledenergy level. Only when the balancing capacity is exceededan alarm is given at the recording system that the couplingat this point and for a certain inspection cross section hasfailed.Although analog recording on a multi-channel X, Y-recorder is possible,it has been found in preliminary tests and in inspection practice that thedivison of the maximum possible screen height of 24 dB in four 6-dBstages is advantageous. From an analog-digital converter the binary out-puts 0, 1, 2, 4 and 8 are given at the monitor, these steps each beingequivalent to a 1 :2 ratio of the size of a reflector. The inspection andrecording system can now, for example, be arranged and calibrated withthe help of artifical circular reflectors perpendicular to the surface sothat an area of 75 mm2 (equivalent to a blind hole, parallel to the sur-face, of appr. 10mm dia.) gives an indication of Vi» of the screen height,150mm2 then equivalent to Vs, 300mm2 '/• and 600mm2 Vs of the screenheight.During the in-service inspections carried out until now an electrostaticrecorder with 8 recording channels and 10 event channels was employedand at the same time a magnetic storage unit received via an interface

Start fai PqfcrancMnM:^Recant duit Mag (tart*! :

Inkn mental« PosffloiurejltttiiIneremtb! position record!»!

_FeMergri)l«in|istit«ng «talgt la 4 Stuft»Jhfect na iwwtini h 4 tteps

PgsHtal

,. EnatzMMkkt*EpfaMMMmi

Fig. 7: Ausschnitt von einem Registrierbeispiel, mit einem »Inkremental«-Schreiber aufgezeichnetFig. 7; Part of a sample recording produced with an incremental recorder

176

Einzelheiten über die Prufort (Längs Querfehlerprufungj, Prufort (RundnoMLangsnoht Stu'zennaht) eingeschol ete Prufkdpfe usw können mit Hilfevon 10 Einieischaltern ouf die 10 Ereigmskonale gegeben werden (d hJ'" «s 1024 verschiedene Möglichkeiten der Kennzeichnung) Sie erscheinenm einer Kombination von schwarz geschriebenen Spuren out dem Ereigniskctnal

44 rVufkopf-BauJeosferwysfem für universelle Emsafzmog/ich-keilen

Wahrend fig 4 einen »Tatzelwurm« mit starr angeordnetenEmzelprufkopfen zeigt, der nur für eine bestimmte Wanddicke eingesetzt werden kann, ist m Fig 8 eine neueie Konzeption zu erkennen, bei der die Cinzelpruf köpfe durch Biaftfedern ongelenkt sind und Lage und Anzahl der Emzeipruf-kopfe m Abhängigkeit von der zu prüfenden Wanddicke undder Prufzonenunterteilung verändert werden können Mitdem m Fig 8 gezeigten Kopf, der an einem mit Elektro-magneten angesetzten Schienensystem vertikal und auf demAusleger horizontal verfahren werden kann, wurden bereitsbei der Herstellung eines Druckbehalters Ultraschallprüfun-gen on Langsnohten vorgenommen Es wird überhaupt dieTendenz sem, schon bei den Herstellungsprufungen weitge-hend dasselbe Prufsystem (z B die Tondemmethode) einzu-setzen, dos spater ouch bei Wiederholungsprüfungen ongewendet wird Durch die Blattfederanlenkung konn außerdemdie Hohe des »Tatzelwurmes« sehr niedrig gehalten werden,was bei der Prüfung in Spotten zwischen Außenoberflocheund Isolierung oder z B bei Prüfungen von Nahten hinterdem thermischen Schild Voraussetzung ist

Pig 8 »Tolzetwurm* i flacher Bauweise mil Ölottfeoeran'ertkung £ uzeip ufkopfe n Pos lion uno Anzahl veiande'lichF<g 8 Talzetwi-rm of flof design w th leaf spnng attachment ndi* dualP'obes voriobe n pos lion ana quoni ty

AT einem »Totzelwuimi. können ouch gleichzeitig Kopfe angesetzt weidendie »n einer um 90 vei setzten ft chfung einscho '«n W ii man 2 B >n. oüfgesetzten Stufen ohne Prufkopfumbou die Ncht des aufgesetzten Stut/e^sund die Rohrteitungsonschlußnaht untersuchen so können olle hie 'ur e'forderlichen Kopfe n einem >Tatrelwurm« ongebiocht wt'den is a^diF'9 «l

5. Manipulation der Prufkopfemheilen5 ? AllgemeinesAus der Unterschiedlichkeit von Reaktortypen ergibt sich dieNotwendigkeit, verschiedene Konzepte für Manipulierenrichtungen zu entwickeln, die den Eigenheiten der Reaktordruckbehalter und ihrer Umgebung Rechnung tragen DasZiel, mit Standordprufemnchtungen für Typengruppen wieDruckwasserreaktoren, Siedewasserreaktoren und gasgekuhtte Reaktoren auszukommen, fordert eine prüfgerechte Gestaltung der Behalter und bei Reaktoren einer bestimmtenGruppe ähnliche Zugonglichkeit und ßewegungsmoglichkeitfür Manipulatoren Diese Forderung ist für bereits konzipier

the same code inputs Oig tctl position details and bmory coded vc ues ofthe inspection result« os «he electrostatic leco'der (Pig 4)Pig 7 shows a* an example the <, rfipufieti »epie^entotion üt »eco rtmg bythe electrostatic recorder In channel 1 the position of the probe on tin channels 3 3 and 4 the 'esu ts of 'he nspection m venous inspectionJfjnei ore reco-deaOne channel is necessary for each inspection zone (depth location e g1 lo 5 in Pig 5)If dur ng the inspection no indications worth lecording appear the recorder runs continuously with o slow speed (stand by speed! At the somemoment when indication», of recording sioge 1 75mm' equ»volent detectareas} result the fast speed is switched on und remains switched on untilthe indication vanished The example m pig 7 shows that ot position16 o defect n record ng s»agtt } *• sconn ng channel ? appeo's iA i Thedefect size ncreases up to stoge 3 and also irr |he neighbouring channel3 o stoge 1 indication is received between position 20 ond 24 A furlhetdefect irdrcahon s shown between pos lion 64 one! 72 w In large ndication (stoge 4) n chonne 3 whs-eby the reflector olso extends a» for asstage 2 n channel 4 (A 2)If the coupling vor ations conr«.' be further corrected automatically omarking is mode m the relevant channel which logelhei with 'he positionindico'or i4 to 70) permits accurate location deterrr motion JA 3*The digital postponing naitahons can be obtained either with a factionwheel (example Pig 3) positively on the drive or angle codedOeta 's on the method of nspection Mong 'gdinoi Ironsverse defect nspection/ inspection location (pertpheroi weld longitudinal wela 10^2 twe d E* coe$ switched on etc can be obtained w>tn the he p &1 10ndividuaf switches on )0 event channels ,i e 21" ~ '024 d fterent pos

sibifities of ideilificalionj They appeo' n a ccmbmotion &' black letoroed traces on the event cha-ine1

4 4 Probe bui/dmg-fa/ock sysfcm for universo/ empfoympnfWhereas Fig 4 shows a Totzelwurm ' with ngidiy orrangectindividual probes, which can only be used for o certain wailthickness. Fig 8 shows a more recent conception to which theindividual probes ore attached by leaf springs ond the locotion and number of these individual probes con be vonedaccording to the wall thickness for inspection arid the nspection zone divisions With the probe shown m csg 8,which con be (ravelled vertically on o roil system attachedby elec'ro-magnets ond horizontally on a boom, ulttasoruc•nspections on longitudinal seams were tarried out alreadydurmg manufacture of o pressure vessel There wit! no doubtbe a Tendency already durmg the manufacturing nspeclions, to extensively employ the same inspection system(eg the tondem method) which vull be usea later ogamduring in serv.ce snspec'ions By means of the leaf springattachment arrangement, »he he<gnt of the Tarzeiwurm canbe kept very low which s a prerequisite for the inspect onscornea cut in gaps between outer surface and msu'a'ion or'or example duntg inspections ot seams &eh "d the heofshieldA Totze»*urm ^.cn s mu 'on<sOi,sly h jve prooes *• tea which beom n odi'echon o'Up* 90f It tor examp e the we d starr c-f on otlcchea lojrl«jnd trve piping con CI.UQII seem ore ^c be ^spfccteö w 'ftou! reo ont^rtrtof the pioue ail the «qu s t« pro&es tor be f lieo (o c. To'ie w^'m 'etalso F 9 ij

5. Manipulation of the probe unit5 I GeneralDue fo 'he differences ,r~ reactor types it is necessary todevelop voriOub ioeoa for T>ampuia4iori equipment whichsuit the ind viojai requirements of *he reocfoi p'essu-e vesse! ond ts surrounds The target of sionoord inspect onequipment for type g'Oups such as piessunsed water reactors, boi1 ng water reactors ond gas cooled reactors neteisitotes a vessel ctes gn 'o suit inspection requirements onü m• he case of reactors of o cerfoin group simlcr cccessib i hyand freedom of movement (or moripu'afors This reqoi'ement s usually 'iof fulfilled m plants already des gried sothat extensive compromises rs the inspection possibility orcomplicated special ot occessory equ pment will hove lo beaccepted

177

te Anlagen meist nicht erfüll}, so daß weitgehend Kompro-misse in der Prüfmöglichkeit oder kompltzierte Sondet- oderZusatzeinrichtungen in Kauf genommen werden müssen.

5.2. SpmnenmasfkonzepfDiese Konstruktion wurde von der MAN entwickelt und sollnoch 1970 erstmals m einem 600-M.W-Kernkraftwerk mitDruckwasserreaktor für die Basismessungen eingesetzt wer-den. Ahnlich wie das schon fast klassische Drehbrückenkon-zept [7,12] verlangt diese Manipulierung einen ausgeräum-ten Druckbehälter.Der Manipulator (Fig. 9) eignet sich zur Prüfung an nicht ab-gedeckten Innenflächen und von Halbkugelboden sowie vonStutzen- und Stutzenanschlußnähten. Er wurde speziell fürden Einsatz in ausgeräumten Druckwasserreaktoren konzi-piert. Er besteht aus einem von einer feststehenden Brückegetragenen, zentral in den geöffneten Behälter ragendenMast mit einem spinnenbeirsförmigen Ausleger am unterenEnde für die Prüfung der Behälterwandung einschließlichKugelboden und mit radialen Auslegern für die Prüfung derStutzen. Auf Höhe der Brücke, die im Trockenen liegen sollte,sind alle Antriebe für die Bewegungen untergebracht, diemit Seilzügen bzw. mit Ritzel und Zahnrad für die Mastdre-hung übertragen werden, tnkrementale Impulsgeber dienenzur Positionsbestimmung.Der Spinnenmanipulator mit dem Drehmasf durfte wegenseiner leichten Anpaßfähigkeit an verschiedene Behälter-größen, wegen der baukastenförmigen Auslegung der ein-zelnen Komponenten und seiner Vielseitigkeit hinsichtlichPrüfmöglichkeiten dem Drehbrückenkonzept überlegen sein

5.3. Dreftgesfef/konzepfDer Drehgestellmanipulator eignet sich zur Prüfung an nichtdurch Hindernisse (Stutzen) unterbrochene Behälteraußers-flächen {Fig. 10). Ein typisches Anwendungsbeispiel sindSchwerwasser-Druckwasserreaktorbehälfer, bei denen oineinnenprüfung praktisch nicht möglich ist. Für die Bewegungdes Manipulators ist ein Ringspalt zwischen Reaktorbehälterund Wärmeisolierung erforderlich, im Ringraum ist ein Dreh-gestell um den Behälter herum verfahrbctr angeordnet. Aufeinem leiterförmigen Ausleger des Drehgestelles bewegt sichferngesteuert ein Prüfwagen. Duich Bewegung des Drehge-steiles und des Prüfwagens lassen sich in horizontaler undvertikaler Richtung alle Punkte der freien Behalterobeiflache

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Fig. 10- Konzept der Drehgestellprufung [AuÖenprüfung}

Fig 10- Rotary frame inspection concept (external inspection)

Prüfkopltär atfgecelzte StubenProbe For attached g

Fig 9 Konzept des Spmnenmaniputators mit feststehender Brocke

Fig. 9 Spider manipulator concept with stationery bridge

5.2. Spider masf designThis design was developed by MAN and is to be employedfor the first time in the Stade 600 MW nuclear power stationfor the basic measurements in 1970. By this manipulation acleared pressure vessel is required as it is the case with thealmost classical rotary bridge design [7,12].The manipulator (Fig. 9) is suitable for inspection on uncov-ered inner surfaces and half-spherical ends as well as noz-zles and nozzle weld seams. It was designed specially foremployment in cleared pressurised water reactors. It com-prises a fixed bridge carrying a centrally arranged mastextending into the open vessel with a spider leg at the lowerend for inspection of the vessel walls including the sphericalend and also radial stays for inspection of the nozzles Atth? level of the bridge, which should be in a dry area, alldrive motions are located which transmit movement viacables or with pinions and gear wheel for mast rotationIncremental impulse transmitters serve for position deter-mination.The spider manipulator with the rotating mast is probablyan improvement on the rotary bridge design in view of theformer's better ability for adaptation to various vessel sizes,due to the building block system for the individual com-ponents and its versatility in respect of inspection possi-bilities.5.3. Rotating frame designThe rotating frame manipulator is suitable for inspection onvessel outer surfaces which are not interrupted by obstacles(nozzles) {Fig. 10). A typical example of application areheavy water pressurised water reactor vessels where internalinspection ii practically impossible. For movement of themanipulator a gap between the reactor vessel and the ther-mal insulation is necessary In the annular space a revolv-ing frame moving around the vessel is provided. On a laddershaped boom of the frame the inspection carriage Travelswith remote control. By movement of the frame and theinspection carriage all points of the exposed vessel surfacecan be reached in horizontal and vertical directions with the

178

mit Hilfe der Positionseinrichtung gezielt anfahren oder inautomatischem Ablauf bestreichen. Das Drehgestell mud fürdauernd installiert werden, für das Einbringen des Prüfwa-gens ist ein Schacht vorzusehen.

5\4. FünrungsschienenfconzepfFür die Außenprüfung von Siedewasserreaktorbehältern, de-ren Einbauten schwer entfernbar sind, wurde von der MANeine Führungsschienenmanipulation entwickelt und erprobt,die erstmals im Frühjahr 1971 an einem 600-MW-Reakfor fürdie Basismessungen eingesetzt werden soll. Hierbei dienendie in Einzelstücken zusammensetzbaren Schienen als Fahr-bahnträger für einen senkrecht, außen am Behälter verfahr-baren Motorschlitten, der an horizontalen Auslegern in Um-fangsrichtung verfahrbare Prüfkopfsysteme trägt (Fig. 8). Aufdiese Weise läßt sich ein größerer Flächenbereich um diesenkrechten Führungsschienen herum zur Prüfung bestrei-chen. Die Führungsschienen werden auf fest in Höhe des Be-hälterbodens (Standzarge) montierte Zentrierkonsolen abge-stützt und über ihre Länge nach der Montage für die Prüfungdurch Elektromagnete gehalten. Die Positionsmessung fürHöhe und Umfangslage erfolgt über inkrementale Impuls-geber. Der notwendige Spalt beträgt 200 mm.Für die erste Prüfung an einem Siedewasserreaktor nach die-sem Konzept, bei dessen Konstruktion noch keine Wieder-holungsprüfung vorgesehen war, können trotz der stark ver-setzten Lage der Stutzen 10 Mantellinien abgefahren werden,wodurch ein beträchtlicher Teil der Behälterwandung erfaß-bar ist.Bei der Konzeption wurde besonderer Werf darauf gelegt, an möglichstvielen und kritischen Stutzen die 12-Uhr- und 6-Uhr-Positionen der Stutzen-radien auf Anrisse in und unter der Plattierung untersuchen zu können.

5.5. Verwendung der Lodemosch/ne zur ManipulationUm bei bereits in Betrieb befindlichen Kernkraftwerken, beidenen noch keine Wiederholungsprüfungen vorgesehen wa-ren, zumindest partiell an kritischen Stellen prüfen zu kön-nen, wurden Überlegungen angestellt, die Lademaschine fürdie Aufnahme spezieller Prüfmanipulatoreri zu verwenden.So war die Aufgabe gestellt, für einen 300-MW-Druckwasser-reaktor Manipulatoren zu konstruieren, die in die Greifer-vorrichtung des Lademaschinenmastes kraftschlüssig einklink-bar sein mußten und mit denen die Anschluß-Rundnaht,Flansch-Zylinderwand, die Schweißnähte der vier aufgesetz-ten Stutzen und die Anschlußnähte geprüft werden konnten.Die beiden für die Prüfung verwendeten Manipulatoren er-

help of positioning equipment or covered by an automaticprogramming system. The frame has to be a permanentfixture while a shaft has to be provided for installation ofthe inspection carriage.

5.4. Guide rail designMAN developed and tested a guide rail manipulation sys-tem for the external inspection of boiling water reactor ves-sels where the internals are difficult to remove. It is to beemployed for the first time in spring of 1971 for the basicmeasurements on a 600-MW reactor. In this design the railsmade up of individual sections guide a travelling motor-driven carriage running vertically on the outside of the ves-sel, this carriage carrying the travelling probe system onhorizontal arms in the peripheral direction (Fig. 8). In thismanner a large area around the vertical guide rails can becovered for inspection. The guide rails will be supported oncentering brackets securely mounted at the level of the ves-sel end and held over their length for the inspection byelectromagnets after erection. The position measurementsfor height and peripheral location will be via incrementalimpulse transmitters. The requisite gap will be 200 mm.For the first inspection on a bailing water reactor accordingto this arrangement, where the design did not provide forin-service inspections, ten shell lines can be covered despitethe considerably offset location of the nozzles and thereforea considerable portion of the vessel walls can be inspected.In the design particular value was attached to being able to inspect asmany as possible and critical nozzles in the 12-hours and 6-hours posi-tions of the nozzle radii for cracks in and under the cladding.

5.5. Use of the refueling machine {or manipulationIn order to be able to« at least inspect critical points innuclear power stations already in service where no provisionhad been made for in-service inspection, consideration wasgiven to using the refueling machine to mount special in-spection manipulators.This resulted in the requirement to design manipulators fora 300-MW pressurised water reactor so that they were ableto be positively attached to the grab device of the refuelingmachine mast and permit inspection of the peripheral weldconnecting flange and cylinder wall, the welds of the fourfitted nozzles and the attachment welds. The two manipu-lators used for the inspection were provided with attachment

Fig. 11: Rundnofirmanipulotor mit »Talzelwurm«-Prüfkopf bei der Montageund Einstellung an einem Vergleichskörper mit künstlichen FehlernFig. 11: Circumferential weld manipulator with "Tatzelwurm" probe du-ring assembly and adjustment on a reference block with artiflcol defects

Fig. 12: Manipulation mit der Lademaschine an der oberen Rundnaht undan Stutzenndhten eines DruckwasserreaktorsFig. 12: Manipulation with the refueling machine on the upper circum-ferential weld and on nozzle welds of a pressurized water reactor

179

hielten Anschlußstücke, die den oberen Stirnplatten der Brenn-elementbündel entsprachen und die so nach Absetzen imBrenneJementbecken von dem Greifer des Lademaschinen-mastes aufgenommen und zu den Prüfungen durch dieSchleuse in den Reaktorbehälter gefahren werden konnten(Fig. 11).Wie auch aus Fig. 12 zu entnehmen ist, mußte für diese Prü-fung eine große Anzahl von Schwierigkeiten überwundenwerden. Neben den Schwierigkeiten der Manipulation unterStrahlung und der Unsicherheit über die Genauigkeit vonZwischenpositionen der lademaschinen-Masthöhe mußtendie Prüfköpfe für die Querfehlerprüfung der oberen Rund-naht hinter das thermische Schild gebracht werden. Es warenbeim Umfangsabfahren aufgeschweißte Klötzchen für In-strumentierungen zu berücksichtigen (von denen keine ge-nauen Positionsmaße vorlagen!), und der Umfangsbereichwurde von Konsolen für öberwachungsproben unterbrochen.Für die Stutzenprüfung war die OberflächenbeschafTenheitnur ungenau bekannt. Aus letztgenanntem Grund wurde dieInnenoberfläche aller vier Stutzen vor dem Einfahren desStutzenmanipulators mit einer Fernsehkamera1 inspiziert, dieam umgebauten Rundnahtmanipulator (Fig. 11) befestigtworden war.Fig. 13 zeigt den auf dem Brennelementgitter im Brennele-mentbecken abgesetzten Rundnahtmanipulator und auf demGrund des Beckens die Vergleichskörper mit künstlichen Feh-lern, an denen die Manipulation erprobt und die Ultraschall-prüfkopfsysteme eingeeicht wurden. In Fig. 14 ist der Stutzen-manipulator zu erkennen, kurz bevor er zur vollvolumetri-schen Prüfung der aufgesetzten Stutzennaht und der Rohr-anschlußnaht in einen Stutzen eingefahren wurde.

Bei allen Manipulationen unter diesen erschwerten Bedin-gungen haben sich berührungslose Annäherungsindikatorenbewährt, sog. »Pulsoren«, an deren einer Stirnseite ein hoch-frequentes magnetisches Feld austritt, dem bei Annäherungan Metall Energie entzogen wird, wodurch die Schwingampli-tude verkleinert und ein Schaltsignal ausgelöst wird. DieseAnnäherung wurde für größere Abstände auf grünen, fürkleinere Abstände auf roten Lampen amManipulator-Steuer-schrank und in einem sog. »Stutzenmanipulations-Simulator«angezeigt, wodurch gefährliche Berührungen vermieden wer-den konnten.

Fig. 14: Stutzenmanipulator mit Prüfkopf im Reaktordruckbehälter vor demEinfahren in einen Stutzen

Fig. 14: Nozzle manipulator with probe in reactor pressure vessel beforeinsertion in a nozzle

pieces of the same design as the upper end plates of fuelelement bundles, and after being positioned in the fuel ele-ment basin they could be picked up by the grab on therefueling machine mast and travelled through the lock intothe reactor vessel for the inspections (Fig. 11).As can be seen from Fig. 12, a large number of difficultieshad to be overcome for this inspection. Apart from the dif-ficulties in manipulation in radioactive contaminated areasand the uncertainty about the accuracy of the intermediateposition indications of the refueling machine mast height,the probes for the transverse defect inspection of the uppercircumferential weld had to be brought behind the thermalshield. During travelling around the circumference allow-ance had to be made for wefded-on bosses for instrumenta-tion {for which no exact location dimensions were available)and the circurmferential area was interrupted by bracketsfor supervisory specimens. For the inspection of the nozzlesthe surface condition was not accurately known. In view ofthe latter reason the internal surface of all four nozzleswas inspected with the help of a television camera1 beforeinserting the nozzle manipulator, the television camera beingmounted on the modified circumferential weld manipulator(Fig. 11).Fig. 13 shows the circumferential weld manipulators posi-tioned on the fuel element grid in the fuel element basin andat the bottom of the basin the reference block with artifical

Fig. 13: Vorversüche im Absetzbecken ohne Wasserfüllung: Stutzenmanipu-lator in den Stutzenvergleichskörper eingefahren. Auf dem Brennelemenf-gestell abgesetzter Rundnahtmanipulator

Fig. 13: Preliminary tests in basin without water filling: nozzle manipulatorinserted in the nozzle reference block. Circumferential weld manipulatorset down on the fuel element frame

defects on which the manipulation was practised and theultrasonic probe system calibrated. In Fig. 14 the nozzlemanipulator can be seen shortly before being travelled intoa nozzle for full volumetric inspection of the attached nozzleweld and the pipe attachment weld.During all manipulations under these difficult conditions thenon-contacting approach monitors proved valuable, the so-

1 Die Fernsehuntersuchungen erfolgten in Zusammenarbeit mit der AllianzVersicherungs-AG, München.

1 The television inspections were carried out in collaboration with theAllianz Insurance Company, München.

ISO

Ultraschall-, Registrier und Mantputationssysteme haben sich bei diesenschwierigen Prüfungen sehr gji bewährt. Die noch anfänglichen Kalkula-tionen fuv die Prüfung im DructcbehSIter mit vier Tosen angeseilte Zeitkonnte aufgrund einer intensiven Vorerprobung auf weniger als die Hälftereduziert werden. Ober Einzelneren der Prüfung wird zu einem spätersnZeitpunkt berichte! werden.Ultraschallprüfköpfe, Vorverstärker und Annöherungsindikatoren siid beider Manipulation starker Gammastrahlung ausgesetzt. Wie sorgfältigeUntersuchungen ergäben, hat diese Strahlenbelastung keine Verminderungder FunktionstöaStiglceit zur Folge.

6. EinfluS cki HaftierungDurch die Piaftierung wird weilgehend die Empfindlichkeitder Ultraschallprüfung an dicken Wandungen beeinflußt.Wie umfangreiche Untersuchungen gezeigt haben, ergibt -sich der Empfinditchkeitsveriust durch die Außenoberflächeder Plattierung, die Gefügeausbildung (Kornorientierung undKorngröße) und durch die Kontur der Aufschmelzzone zwi-schen Grundstoff und Piattierung.Neben der Empfindlichkeitsverringerung durch ungünstigeAusbildung der Außenoberfläche bewirken Weilen und Ab-sätze in der Plottierungsoberftäche Winkeiabweichungen beider Einschallung, die die Prüfempfindlichkeit weiter herab-setzen oder in .Extremfaltert zu falschen Fehlergrößen- undFehlerJagenbestimmungen führen können.Durch die unterschiedliche Brechung beim Obergang vonPlexiglas zu Stahl und von Wasser zu Sfahl wirken sich Win-kelunterschiede der Oberfläche bei der Koniaktmethode ge-ringer aus als bei der Prüfung mit Wasservorlauf. Dies giltbesonders auch für den Unterschied Oberflächen- zu zwangs-geführter Prüf köpf manipulation. Bei der Kontaktmethodebewirkt z. B. eine Ofaerfiächenneigung von 6,5°, bei der Was-servorlaufstreckenmethode von nur 2° eine Reduzierung derBildscbirmanzeige auf 50 % beim Anschauen einer Sackladvbohrung mit 10 mm Durchmesser.Die umfangreichen Versuche haben gezeigt, daß die Band-plattierung einen geringeren Streueinfluß auf Ultraschallausübt als die Handplattierung mit Elektroden. Mit zuneh-mender Platfierungsdicke erhöht sich die Schwächung, wobeimit einem mittleren Schwächungskoeffaiertten von etwa1(500 • J0~s dB/mm gerechnet werden muß.!n Abhängigkeit von Schweißverfahren, Plattierungswerksloffund Grundmatenaioberfläche können bei den einzelnenDruckbehu«".,.--Herstellern In Hinsicht auf Uitraschall-Prüf-empfindltchkeif unterschiedliche Verhältnisse vorliegen. Es istdeshalb außerordentlich wichtig, für die Vorversuche undGeräteeinstellungen bei Basismessungen und Wiederholungs-prüfungen Vergleichsstücke zur Verfügung zu hoben, dieeine Bestimmung des jeweiligen Einflusses erlauben.Um bei Wiederholungsprüfungen die Ultraschallprüfung mitder erforderlichen Empfindlichkeit und Reproduzierbarkeitdurchführen zu können, müssen in Zukunft bei der Hersteilungvon Druckbehätfern bestimmte Bedingungen, die inzwischendurch die Versuche erarbeitet worden sind, eingeholtenwerden.

7. SchraubenboixenprüfungDie Schraubenbotzen zur Deckelbefestigung unterliegen wiedie Druckbehälterwandungen dem periodischen Uberpru-fungszwang. Durch Konstruktion von Speziaiprüfköpfen, diemit einer auf die zu prüfenden Bolzen aufgesetzten Manipu-lationsgiocke in die Bolzenbohrung eingeführt und dort mitHilfe eines Rastersystems die kritischen Bereiche abfahrenkönnen, ist eine schnelle und vollständige Prüfung des Ge-windes und des Dehnsdhaffes an den interessierenden Stellenmöglich.

called "pulsors", at one end of which a high frequency mag-netio field is emitted from which energy is extracted on ap-proaching metal so that the amplitude is reduced and asignal tripped. These approach positions were indicated forlarger distances by green lamps and for smaller distances byred icmps on the manipulator control panel and in a so-called "nozzle manipulation simulator", so that dangerouscontacting couid be avoided.The ultrasonic, recording and manipulation, systems have proved to bevsry effective for 'hese difficult msp«ciions. The period of four daysoriginally calculated for inspection in tho pressure vessel was able lo bereduced to less than half this time thanks to intensiv« preliminary trials.Details on }he inspection wilt be reported on later.Ultrasonic inspection probes, pre-ampiifiers and approach monitor« areexposed to infens« gamma-radiation when in service. In very earful testsit nets been proved that this radiation load has no effect on the perfor-mance.

6. Influence of claddingThe sensitivity of ultrasonic inspection on thick wails is ex-tensively influenced by the cladding. As extensive investiga-tions have shown, the loss of sensitivity results due to theexternal surface of the cladding, the grain structure (grainorientation and grain size) and due to the contour of thefusion zone between base metal and cladding.Apart from the reduction in sensitivity due to unfavourableshape of the external surface, waviness and steps in thecladding surface result »n angle deviations during the beam-ing which further reduce the inspection sensitivity or in ex-treme cases lead to faulty defect size and defect locationdetermination.Due to the differing refraction on transmission from Perspexto steel and water to steel, angular differences on the sur-face have less influence during the contact method than du-ring the inspection with a water travel path. This applies inparticular also for the difference between surface guidedand positively guided probe manipulation. With the contactmethod, for example, a surface inclination of 6,5° and withthe water travel path method, only 2° result in a reductionof the screen indication to 5Q*A> when beaming a 10mmdiameter blind hob.The" extensive tests have shown that the band cladding hasa lower scatter influence on ultrasonic waves than the man-ual cladding with electrodes. As the cladding thickness in-creases, the attenuation also increases and a mean atten-uation coefficient of about 1600 X NT" dB/mm has Jo beexpected.Depending upon the welding process, clodding material andbase material surface, differing conditions can exist with theindividual pressure vessel manufacturers in respect of theultrasonic inspection sensitivity. It is therefore exceptionallyimportant to have reference blocks available for preliminarytests and equipment adjustments for basic measurementsand in-service inspections to permit determination of therelevant influences,In order to be able to carry out the ultrasonic inspection withthe requisite sensitivity and reproducibiiity during in-serviceinspections, it is important in future that the manufacturersof pressure vessels comply with certain conditions that havebeen laid down in the meantime on the basis of the tests.

7. Bolt inspectionThe bolts for cover fastening also have to be inspectedperiodically as in the case of pressure vessel walls. Due tothe construction of special probes which are inserted in thehole through the bolt to be tested with the help of a manip-ulation beli and the possibility of covering the critical areas

181

Da die Prufung on den Bolzen rm eingebauten Zustand durchzufuhren istund hierbei meistens starke Strahlung vorliegt, wurde durch moglichst emtache Manipulation und Verstandigung zwischen Pruíer una Bitdschirmbeobachter tm drahtlosen Gegensprechverkehr in Form eines Check Systemse*n Minimum on Zeitaufwand erreichf So betrug z B die durchschmttïichePrufzeit fur emen kleineren Bolzon eines 240 MW Siedewasserreaktors12 mm (Fig 15), fur groflere Bolzen eines 300 MW Druckwasserreaktors mitschwierigerer Hondhabung durch die uber den Botzen sitzende isolationshoube wurden 16 rnm benotigf

(Emgegangcn am 20 10 1970)

Lileralur References|1] Wengler J Techn Uberwachung 8, 423 (1967)|2] Franzen L f Techn Oberwachung Í, 424 (1967)(3) Wyl.e, R D Technical Reporl Series No 81 Wien IAEO 1968, S 43¡4) Yamoguchi, T , u a Inspection Technique by Electric Resistance Probe

Method on a Nuclear Reactor Vessel First International Conferenceon Pressure Vessel Technology New York ASME 1969, Teil 2, S 75

[5] Weidemann W Kontinuierhche Oberwochung der RiBausbreilung inDruckbehaltern literalursludam und knfiscne Auswertung von in denUSA gewonnenen Orgebmssen 1 Bencht fur dos BMwF FrankfurtBatieile Institut 1969, RS 31

|6) Wyhe, R D Nuclear Engineering and Design S, 117 (1968)¡7) Ahlberg, G C loutzenheiser und O Sondberg Periodic Inspection

of Oskarshamnverket Reactor Vessel First International Conferenceon Pressure Vessel Technology New York ASME 1969, Teil 2 S 80

|8] Suguri S , u o Postoperation Inspection of JPDR Pressure Vessel in1968 First International Conference on Pressure Vessei TechnofogyNew York ASME 1969, Tei! 2, S 76

[9] Gross L B, vnd C R Johnson In Service inspection of NuclearReactor Vessels using on automated ultrasonic method MaterialsEvaluation July 1970, Volume XXVIII Seite 162

[10] Trumpfheller R Schweifien und Schneiden 18, 268 (1966)[llj deSterke,A Automation in nondestructive testing ofweids 6thWorld

Petroleum Congress 1963 Section VIII, paper 15[12] Gasparmi, R A Nobili und F Verona Technical Reports Series No

81 Wien IAEO 1969 S 94

with the help of o grid system, fost and complete inspectionof the thread and the elasticated shank at the points of interest has been made possible

Fig 15 Uftroschafipfufung von Schraubenbolzenan einem geoffneten Siodewasser DruckbehoMer

Fig 15 Ultrasonic inspection of flange bolts m an opened boiling waterreac or

As the inspection of the bolts is carntd out m thetr fitted condition andusually a h gh radiation toad exists, a minimum time requirement wasafta ned by having the simplest possible manipulation and communicationbetween Ihb tester and -screen pb^erver in the form of a check systemusing lodio transmission apd lectption ror example the average mspecI on time for a small bolt of a 240 MW boiling water reactor was 12 miñutes (Fig IS] and for larger bolts in a 300 MW pressurised water reactorwith difficult handling due }o the insulation hood above the bolts 16 mmules were necessary

182

TOT FACILITIES .APPLIED TO ZR BASE ALLOY CAN FABRICATION

Lars L-jungberg an,d J'drgen Wiklund

SWEDEN

The basis for specifications on NDT of fuel element canningis discussed together with the philosophy of rejection. Thisimplies general demands on the manufacturing process. Pro-perties considered in a specification for NDT are dimensionsand mechanical defects, A review of general types of defectsand the testing techniques in use is £,iven, Inducing e.g»ultrasonics, eddy current and mechanical sensing. The calibr-tion metnods and the need for testing the transducers areconsidered to be important factors»

Lars LJun¿berg (1), Jorgen Wi.clund (?)

The basic for a specification on fuel element canning should bethe expaeted behaviour of the canning during noriral service andduring probable abnormal incidents. Also the aimed life of thefuel is important. VI i thin this outer frair.e it is in principlepossible to optinize testing technically and economically,

Considering these statements a sound rejection basis should bereal defects, and not the amplitude of a signal from a standard

1) Al- A SEA-ATO:-:, VSsterAs.. Sweo.en2) THE SAMDVIK STEEL WCHK3, Sandviiken, SvvOQcn

183

defect. A pre-requlsite for this cora rol philosophy is that theamount of rejectable tufoinr is low compared to the total amountof tubing,

As an example of the difference between the two rejectionphiliscphies a shallow scratch r,iark with an adjacent ridge ofmetal could be mentioned. This ridge is known to reflect soundby ultrasonic testing, and give a large signal of the same kindas froin & heavy crack. Usually a light polish is enough to removethis type of defect, and the tube is good for use,

On the other hand, if the same testing is done with sound fromonly one direction and it hits a slanting crack, from its back-side, part of the sound may riot be reflected back, and the signalregistered makes you underestimate the depth of the crack. Frychecking defects this fact will soon be revealed. I.t is theneasy to realise that testing from two opposite directions willtake care of the problem.

The soundest basis for producing; good Zircaloy tuidng forcanning purposes is to run the production in a way -chat, producesfew defects, rauhsr than to remove defect tubing by control,First of all this gives you the possibility to investigate thedefect signals you get, secondly the cost to handle a lot ofmaterial that will be rejected as high, and third a lot ofacceptable tubir.g from a batch that has had a high rejectionrate contains K coiaprirativoly high nunbcr of tubes with defects—close to but below the rejection limifcT The effect this will 2.See

have on the probability for a breakdown in the reactor is obvious.

The set of defect types found in a lot of tubes is differentbetween the different manufacturers. This is natural, as thedefects produced are dependent upon thefabrication route, the fabrication equipment and the tubespecification. Sometimes a not well-thoughtout specification onone property can force the manufacturer into operations whichcause problems on other properties. Problems of this kind arebest avoided by a good knowledge on the effect of different

184

types of defects and properties, together with good contactbetween the tube and fuel manufacturers. The fuel designermust have a picture that is as complete as possible on. theeffect of propertj es and âefçctsi:on the life of the fuel. Atechnical and economical optimization must take care of what isa risk and what is acceptable.

In setting up a specification for non-destructive testing,the properties which are considered are dimensions and mechanicaldefects.

Por well-known reasons, the tolerances on the inner and outerdiameters are relatively tight. In addition there may be ademand on wall-thickness. The stress-temperature situation in thecanning in the reactor also requires that the excentrieity isbelow a given value. However the requirements on the three first-mentioned properties often are so tight ^hat they automáticallytake care of the excentricity requircr-eny. In that case "Destine J&*for excentricity is unnecessary.

usually a specification also contains a requirement on straight-ness. To avoid turn-out it is important to keep an even spacingbetween the fuel rods in all levels of the reactor core. Itmight also be beneficial to avoid extra stresses in the spacersand in the canning.

The most dangerous type of defect is cracks. Their main effectsare as stress raisers and, if they are sharp, ao crack nuclei.The effect is dependant upon the size, mainly the depth of thecrack. For practical reasons cracks below a specified depthmust be accepted. However it must be realJz.ed that a great numberof cracks below the specification limit present an increasedprobability for break-down, specially following unexpectedreactor excursions.

Another type of defect that occurs in tutes from all or r.osttube manufacturers is pits on the inside surface of the tube.Calculations show that a pit creates a zone of extra stresses

185

and strains, at the rim of the pit. The amount of extra stressarid strain is dependant upon the depth and width of the pit.It is possible that pits from this reason can take part in thetype of hydride attack known as "sun-burst". We feel itis important to test for this type of defect.

A good non-destructive testing specification should also takecare of unexpected types of defects. A never or rarely occurringdefect in Zircaloy tubing is a crack parallell to the surface of thetube. This crack will not be found by the methods usually appliedto detect cracks. However it can be found in ultra-conic wall-thickness measurement. The effect of this type of defect on theperformance in the reactor is not well-known, but it could beabout the same as for a pit.

Another unexpected defect is a large inclusion. This type ofdefect usually deflects sound in the same way as a crack, andshows up as signals on the channels both for longitudinal andtransversal cracks. The main hazard with inclusions is aspotential spots for localized corrosion.

In testing canning tubes several method variations have beenused. Today the testing for defects by ultrasonic methods isperformed \\rith four transducers, two each for longitudinal andtransverse defccts/T This technique is well established and wildetect most of the previous mentioned defect types. One excep-tion is smooth shallow pits which compared to other defects donot present a good reflecting surface because of geometricreasons. This type of defect produces very weak if any indicationsBy introducing a nie? sure ment of the sonic attenuation :in thetube wall one can detect even this defect type by ultrasonicinspection. The technique itself is well-known but the appli-cation to canning tubes is relatively new»

The testing is performed by different transducers mounted infixtures which allow independent and accurate positioning,The tube surface is scanned along a helical line with a pitch

186

determined by a demand for a certain degree of ovex'lapping.The system is calibrated against standard defects with depthand length agreed upon by both parties. This is.the only wayof compara ne sensitivity between different equipment andadjustments. It is important to notice that this is not con-tradictory to the previous discussed rejection philosophywhich was merely the principle hew to evaluate registeredindications.

Another very important question is whether the desired over-lapping does exist or not. For example a not quite perfectbonding between the crystal and the lens material in the trans-ducer can give rise to a non-uniform sound field/which means J y&J.¿sthat son;e sections of the tube will be tested with a decreasedsensitivity. Transducer defects of that nature will not showup by normal calibration against standard defects. They haveto be revealed in a separate transducer test where the soundfield is mapped.

A testing technique which rrore and more is being used is theeddy current technique. Sometimes it is a very good complementto ultrasonic testing because of its good ability to detectspecial types of defects such as the pits mentioned above. Themoderate demand for good cleaning of ühe tube surface and thevery high testing speed are also valuable advantages. The maindraw-back is the sensitivity to variations in irrelevant para-meters. It has however, been employed in the process controlwhere it functions well in maintaining a constant quality levelthereby reducing the number of rejects in the final inspectionof finished tubing.

One of the dimensional measurements, via 11 thickness, is normallyperformed during the ultra-sonic testing as it employs the ultra-sonic resonance method.

Several attempts have been made especially with the impuls-echotechnique but with no remarkable results. Difficulties in main-taining the necessary accurate geometry in the sonic system have

187

adversely effected the measuring accuracy. Due to the inherentstrong nonlineariuy in the resonance method the calibration isvery important and the accuracy obtained depends to a great extenton how vieil the calibration points have been chooseiy. Furthermore /it is important to have a sampling rate sufficiently high tt> ensurmeasurements taken on the extremes. Otherwise the sampling ratewill" liir.it the testing speed.

When special requirements on excentricity are included in thespecification it is quite possible to extract this informationfrom the wall thickness measurement.

Air gauges as well as electromechanical gauges are used inmeasuring diameters. The air gauge is inexpensive and accuratebut slow and it relies on taking a mean value over relatively-large section of the circumference. The electromechanical gaugeon the other hand is more expensive but also very fast and muchmore flexible. The measurement of OD is very straight forward,ID on tiie other hand is more difficult due to the minor spaceavailable inside the tube* However, gauge systems for thispurpose have been designed with various degrees of flexibility,one ID-gauge can for example simply by changing measuringstylii be used over a diameter range of several millimeters.The electromechanical gauge also measures a more true djameterin the sense that it measures the distance between two shapedpoint contacts with the tube surface and hence it can show verylocal variations of the diameter. This is essential not onlyin order to guarantee the diameter of the finished tube butalso when being used as a tool for process control.

The testing for straightness, from an instrumental point ofview, is less developed. It is often performed simply by puttingthe tube on a flat table surface and observing the space bet-ween the tube and the s\arface. The hang-down due to the weight cfthe tube itself is often considerable and if riot treated withcare a source of big errors. A better system is obtained if thetube is placed on supports and- rotated. The deflection ismeasured by an micrometer gauge -which gives a more reliable

188

~T '-* Jmeasure of the straightness¿ The distance between the supports ''«c<? y,should be the specified, e.g. the same as between the spacersin the fuel element.

Careful NOT or canning tubes is very expensive due .to theadvanced technique required to ineet the augend for safety intube function.. Efforts will therefore be put on topics likefaster testing and more efficient utilization of testingfacilities. In the last case much work already has been per-f-orrred in autotr.?.tic tube handling in the testing lines andprobably more will be done on e.g. automatic data processing.

Finally it is evident that in writing a test specificationone must carefully consider the relative merits and costsof the different testing techniques to achieve at the optirr.unsconditions that produce the desired safety level for the tubingin its final function in the operating fuel element.

189

Number Defect distribution

Reject limit

Percentreject

Defect size

Uncertainty atultra sonic inspection

Defect size

Pig. 1. Due to the uncertainty at the ultrasonic inspection thesorting of tubes with defects close to reject limit will be hazardous.The heavy broken line indicates the defect distribution in a productionlot with few defects in the uncertain area while the fine broken lineindicates the result of a bad defect distribution (heavy line) and theinspection uncertainty.

/Wt max

Fig..2. Relationship between inner and outer diameter, wallthickness and exceritricity.

190

Transducer arrangement

Fig. 3» Transducer arrangement for ultrasonic testing and wallthickness measurement.

Pig. 4. Sound beamprofiles from transducertests.Vert, scale: 2dB/div.

191

Calibration of wall thickness measuringequipment (resonance method)

Measuredwall thick-ness

Possible real calibrationCorrect calibration

Calibration point

Actual wall thickness

Pig. 5. The diagram shows the effect on the accuracy when not usingthe proper scale or when the oscillator is not perfect adjusted. Theextrapolation error grows fast and if calibration points and tolerancelimits not coincide it can give rise to considerable sorting errors.

Measurement of straightness

v77//// Y7

Fig. 6. Measurement of straightness by rotating a tube on supportsand measuring the deflection.

192

THE NOM-DESTRUCTIVE TESTING OF FUEL ELEMENTS AND THEIR COMPONENTSAS APPLIED BY THE BELGIAN MANUFACTURING INDUSTRIES. REVIEH OF METHODS

R.DEKNOCK* ; N.MOSTIH** ; d. GERARD***

AbstractThis paper deals v/ith the situation In Belgium with respect to thenon-destructive inspection used by the manufacturers of fuel pinsand fuel assemblies.The pins are, typically» stacks of oxide pellets enclosed in metaltubes with welded closures at each end. The present paper givesan enumeration of the non-destructive tests and measurements appliedto the basic material and the fuel pin before and after assembly»

I. IntroductionThe representative manufacturing industries of nuclear reactor fuelelements and fuel assemblies in Belgium are M,M.N. (uranium-enrichedfuel elements) and BEL60NUCLEAIRE (Plutonium-enriched fuel elements).Each of these factories possesses its own control service.The aim of S.C.K./C.E.N. is to support the Belgian industry carryingout basic research either for fabrication and for control.

jl. General Philosophy; of^non-destructive testing

As in any industry, the overriding purpose,of non-destructive controlsduring the pre-production and production' stages, is to ensure that

S.C.K./C.E.N., 200, Boeretang, B-2400 MOL (Belgium)** BELGONUCLEAIRE> B-2400 MOL (Belgium)*** METALLURGIE ET MECANIQUE NUCLEAIRES (M.H.N.)» 12,Europa!aan,

B-2480 DESSEL (Belgium)

193

each component has achieved a certain level of quality, so that theywill perform satisfactorily over the period of service and under themost stringent conditions. This calls for an inspection at the variousstages during component fabrication. Any variation that will impairservice performance constitutes a defect in the context of the qualitycontrol. The permissible level of these variations depend not only onthe specifications but also on the manuractonng economics.There are three reasonably distinct stages in the fabrication of fuel-element assemblies :- acceptance and categorising of materials and components needed forfabrication of fuel-element assemblies (tubes, rods, end-plugs,springs, fissile material)

- fabrication of semi-finished and finished components (pellets» fuelpins and assemblies)

- Checking the integrity of finished products (fuel pins and assemblies)At each of these manufacturing stages, particular non-destructive

techniques are required to ensure the maintenance of quality andconformity in production components. Looking forward., they can alsoprovide data to assist in further design and development.These techniques enable also statistical interpretations in caseswhere 100 % non-destructive control is not needed to garantee the levelof confidence written in the specifications.

The general philosophy given in II demonstrates the importance ofthe controls performed at the acceptance stage. The results obtained atthat stage are necessary for the fabrication and the characterizationof the final product. Some of the characteristics are inspected uniquelyby the supplier and delivered by certificate. The characteristics of primeimportance however are controlled in co-operation with the supplier ofthe material

194

Tests dijHnfabri cat ton

Here, process control and monitoring is applied as to ensure thatno structural variations or defects of one sort or another are introducedat the various stages during fabrication,

The same philosophy is applied as in II.1. Tests are performedon the final product to ensure the level of confidence written inthe specifications. Some characteristics of the final product howeverare verified only by checking the validity of the'process control,taking into account the certificates obtained at the acceptance stage.Tests at this stage are performed following an agreed control planand if required in the presence of a client representative

The methods of non-destructive testing and measurement as appliedby the manufacturing industries M.M.N. and BELGONUCLEAIRE will beconsidered under headings, corresponding to the three distinct stagesin the fabrication of fuel element assemblies.

^material and components

Fissionable material : U and PuNon-fissionable material : Stainless steel and Zircaloy 2 and 4Components : rods for end-plugs, end-plugs, pellets, springs, tubes,

tubes with welded closure at one end, hoiddown-devices,plates...

Applied test methods : see table 1.

195

a

Components : pellets of uranium oxidepellets of mixed uranium and plutonium oxide

Applied test methods : see table 2

III.3. Testsj[pj £hec:_kijiig J:he

Finished products : fuel pins and assembliesApplied test methods ; see table 3.

196

Table I

and components

Types Characteristics Test Methods3 ReferencesGeneral

Dimensional

Aspect

FineStructure

enumerationidentification of lotindexationweighingprocedure of fabricatorqualificationamount of Uamount of Puamount of fissionablematerialexternal and internallenghtdiameterthicknessboringbowpositionwidthdepthaligmentperpendicularityangledensityquality of threadcolourfine surfacecleannessoutern defects

- longitudinal andtransversal defects- inclusions

verification of productcertificates and reportsaccountabilitybalance

(8)(9)

(7)

gaugemicrometermicroscopespecial fixtureprojectorbalanceultrasonics

U)(2)

- visualcomparison with standard- surface finished meter- microscope- autoclave- ultrasonic pulse-echo method

- eddy-currents

197

Table I (continued)

Integrity of welds shrinkage holespenetrationporosityfissuresinclusionsleaks

- X~ray- leak testing

_1

This table represents a listing of the main characteristics and a listingof the main methods. No direct concordance has to be seen between each itemsof both columns.

198

Table Ií

Tests and measurements during fabrication

Types

Dimensional

Aspect

Enrichment

Characteristics*lengthdiameterdensityshapedish volumeshoulder widthweightfine surfacecoloursurface cracks-chips-pitscleannessamount of Pu and Ühomogeneity Pu and Udistribution Pu and U

Test methods*

gaugemicrometermicroscopebalancespecial equipment for ;diameter and density

- visualcomparison with standards- surface finished meter

- spectro -i - a- balance- calculation

References

(4) (5)

See table I.

199

Table IIITests for checking the Integrity of finished products

TypesGeneral

Dimensional

Aspect

Integrity ofwelds

-Radiography

-tightness

Loading

Characteristicsenumerationidentificationindexationprocedure of fabricationqualificationweightexternal lenghtdiameter of welded zonebowpositioningaligmentcolourfinished surfacecleannessdecontaminationexternal defectsexternal defects welds

shrinkage holespenetrationporos i tyfissuresinternal inclusionsmicro-leaksmacro-leaks

weightdistribution of PuaUdensity distributionposition of springsposition of pellets

Test Methods*verification of productcertificates and reportsbalance

gaugemicrometer

Visualcomparison with standardmicroscope^-detectorsurface finished meter

- X-ray

leak testpression testultrasonicsbalance/-detectorneutron-detectorneutrographyn.y-coïncidence detectorX-ray

References

{10)

See table I.200

REFERENCES

(1) P. Li botte, H.Inniger* J.M.Leblanc (BELGONUCLEAIRE}"Contrôles non destructifs de barreaux combustibles enrichis auplutonium"Extrait de Non-Destructive Testing in Nuclear Technologie.Symposium - Bucarest 1965' Vol 1.

(2) P.Libotte (BELGONUCLEAIRE)"Contrôles non destructifs de barreaux combustibles enrichis auplutonium"Editions de I1Institut Belge de la Soudure - Bruxelles 1965

(3) J. Wolper (M.M.N.)"La détection des fuites" Extrait de la Revue de la Soudure -Lastijdschrift n° 4/62.

(4) N. MOSTIN et al (BELGONUCLEAIRE}"Equipement et enregistrement automatique des données en provenanced'une machine de contrôle des pastilles".8ELSONÜCLEAIRE 1969

(5) BELGONUCLEAIRE Control Procedures (Commercial use only) :"Mercury density of pellets and powders : BELGONUCLEAIRE 1968 -600.00/351/n/050 M

(6) 8ELSONUCLEAIRE Control Procedure (Commercial use only)¡"Helium LeakTest"BELGONUCLEAIRE 1968 - 600.00/351/n/115 M

(7) N. Mostin (BELGONUCLEAIRE)Code relatif au contrôle de matière fissileBELGONUCLEAIRE - 01520/355/n/011 M 1969

(8) BELGONUCLEAIRE Control Procedure (Commercial use only)"Réception and expedition of materials"

201

(9) BELGONUCLEAIRE Control Procedure (Commercial use only)"General terms for expedition reception of fissile materials"BELSONUCLEAIRE 500.00/757/ni/060 M - NM/LMP

(10) J. Gérard ( M . M . N . )"Le Contrôle automatique des soudures par ultrasons"Communication à la tribune de l ' institut Belge de la soudure 1969

(11) BELGONUCLEAIRE (note interne)"Unité de mesure de la contamination résiduelle sur les élémentsde combustibles decontamines" BELGONUCLEAIRE 1965

(12) 0. Gérard et L. Van Hove (M.M.N.)"Détermination de la teneur en uranium dans les alliages à based'aluminium" Bucarest 1965

(13) N.Mostin - E.Vanden Bemden (BELGONUCLEAIRE)"Contrôle du plutonium dans les ateliers de BELGONUCLEAIRE"BN 7106-01-540.00/060/n069)

(14) G.Verstappen, R.Deknock (S.C.K./C.E.N.)"Eddy-Current Testing of Thin-Nailed Cladding Tubes"Bucarest 1965

(15) R.Deknock (S.C.K./C.E.N.)"Werkwijze voor het kontroleren van een object en inrichting voornet uitvoeren van deze werkwijze". Patent Pending.

202

Nondestructive Testing Procedures duringjPjBCbr i cat ion o;f^ Steel_ Près sure Vessels

R. tt-umpfhellerFed ."Re p. of Germany

ABSTRACT

Though nondestructive testing everywhere is considered asvery important for the safety of nuclear power plants, theHDT-ciethods used in various countries and 'the acceptancerules differ in a relatively wide x»ange. This paper mainlydeals with the differences between USA and Germany. Por theultrasonic inspection of plates and forgings nearly the sametechniques are used in both countries but the sensitivitycalibration and with it the acceptance rules are different.For the inspection of weld seams in Germany the ultrasonicexamination is regarded as a much more reliable method thanthe radiographie inspection» Therefore in many cases theradiographie examination is not applied if the ultrasonicinspection is carried out in a sufficiently perfect way asa "complete ultrasonic inspection". According to ASME CodeSection III the radiographie iixspection dominates.

RESUMEBien que dans tous les pays, l'essai non destructif estconsidéré córame très important pour la sécurité des centralesthermiques nucléaires, les procéd-Ss appliqués et les règlementsd'admissibilité diffèrent sensiblement suivant les pays.La pró.sente étude porte essentiellement sur los différencesaux USA et er¿ Álleroa^ne. Pour l'ensai aux ultra-nono de tô3.ei>et de pièces forgées, on applique, dans les deux pays, desméthodes quasi identiques, toutefois le calibrage de sensibilitéet, par conséquent, l'admissibilité» sont différents. Pour lecontrôle des joints de soudure, on considère en Allern/me que

203

les procédés aux ultra-sons sont beaucoup p.lu« sûrs que lesméthodes radiographiques. C'est pourquoi, dans de nombreux cas,l'examen radiographique n'est pas appliqué» pour autant quel'essai aux ultra-sons est effectué d'une façon suffisammentcomplète et consiste en un "essai total aux ultra-sons"•Pjjr contre, d'après la AGME Code Section III, c'est 1' exainenradiographions qui a la prépondoran

1 » Poa^-evordNondestructive Testing of reactor pressure vesselsis being considered as very important for the safetyof nuclear power plants in all countries» But thereare different points of view in electing in NDTmethods, in specifying the details of the proceduresand in judging the results. Ve found the greatestdifference between the procedures specified in theASME Code Section III (t) and the methods used inGermany. In Germany the conditions for NDTexaminations of reactor pressure vessels are stillmostly fixed individually for single objects at thepresent time. But the cooperation of inspectingauthorities guarantees a unified point of view. Thework on standards for the NDT examinations of reactorpressure vessels is far advanced but some special factsinvolved with the large wall thicknesses and similardesign details of reactor pressure vessels for powerplant units with i.200 Mégawatts caused subsequentalterations* Further supplements became desirable

204

afbe3~ we bad. found smaî 3 cracks j.u llie parent material"bom- a vn the au?tonifc:Lc veJ clod cladding, ThereforeGerinaji ¿«tandardft deal ing vith HOT examinations ofreactor prç>&su:re vessels during fabrication, will notbe edited before the next year. In most of Europeanon. other countries NDT conditions are based onASME Code Section III". As far as German manufactureand Goi'Gian inspecting autboi'itics arc concernedspecifications as used for German pJants are fulfilledin addition.

Swedish specifications seem to be similar to theGerman ones,

2 . Hojoclc <:st ru ct i c Jîx 0 m:¡ i T . 3 o.n _ of 4

2 . 1 Examina Lion ojfIn most countries plates are to be examined by usingthe ultrasonic method vlth a normal sti-aight; beamsearch. But according to AS>iE Code ai 3 platos with 2 in.nominal thickness and less used for piping, pumps andvalves shall be examined by the angle beam methodexcept the examination shall be performed over 100 percent of one major plate surface. All platen for vceoej sand all plates greater than 2 jn. thickness shall beexamined by the straight beam method.

ASH£ Code and German specifications demand that 100per cent of oie major p3 ato surface shall be coveredby moving: the search unit in parallel over Lapping

205

paths, Platens ordered in the heat-treated conditionshall bo examined following heat treatment.

According to ASME Codo defects which cannot be en-compassed within a circlo whose diameter is 3 in. or1/2 of the plate thickness, whichever 1st greater,are inacceptable in plates greaterthan 2. in. thickness.Two or more smaller defects sh-all be unacceptable unies:separated by a minimum distance equal to the gx^eatestdiameter of the larger defect or unless they may becollectively encompassed by bhe circle whoso diameteris 3 in. or 1/2 of the thickness.

German specification accept 10 defects ( 5 as avex*age}extended parallel to the major surface up to 1.000 mm

per m^ on heavy section plates in general. In plateswhich are to be manufactured to parts of the shellstressed in normal directions to the major surfacedefects are only admitted with an extent up to 100 mni^.Plates with larger defects or a greater number ofdefects are not acceptable, "m special cases suchplates might be accepted if the inspecting authoritywould agre e.

2.2 Examinationof'Forcings Provided for the Voa^sel She'UAll forgings for reactor pressure vessels ar*e to beexamined by the ultrasonic arid by a magnetic particleor a liquid pénétrant method in all countries as faras we know. In Germany the surface roughness of theareas under inspection must not exceed the average

206

valuó of lO^t». According to ASME Code Section I.I.Ïthese examinations shall be clone in tho as-furnishedcondition» The entire volume oí' bhe forging is to beexamined by the ultiasonic pulse echo method. Thomaximum possible volume is to be examined after finalheat treatment excluding post weld heat treatment.

In alJ countries which we could get informations fromthe ultrasonic examination shall be performed by thestraight beam technique fx'om two directionsapproximately at right angles. Hollow forcings shall beexamined fVom one of the circuraferenbjal surfaces andfrom one or both of the surfaces normal to their axes.Disk forcings shall be examined from one or all of thoflat side s} arid from the circumf erential surface.

In addition ASME Code Section III demands ringforcings and other hollov forging& to be examinedusing angle beam technique, unless xvall thickness orgeometric config-urafcioii of the forcings makes ari£l ebeam examination impract Loable. TJUC angle beamexamination shall be performed in the circumferentialdirection. In Germany the necessity of using the anglebeam technique as still being in discussioxi. Nozzlerdngs and areas along weld ectgjfcs mostly arc examinedby angle beam technique jn circumferential direction ancparallel to tho axe.

According1 to ASMJ5 Code Section III the ultrasonicinstrument for straighc beam e;cani Lnation shall be set

207

so that the first back reflection is 75-90 per coat

of full screen height when, the seax-ch unit is placed

on an indication-free area of the forging. One or more

reflectors which produce indication?; aocornp3,nied by a

complete loss of back reflection not associated with or

attributable to the geometric configuration shall be

inacceptable, while the straight beam search unit must

not exceed one square inch in area. Complete loss in

back reflection is assumed when the back reflection

falls below j per cent of full screen height. It has

to be reported for information, if forcings are

containing one or more indications with amplitudes

exceeding the resulting bacY reflection and if forgingB

are containing one or more discontinuities which

produce travelling indications while the back reflection

i R reduced.

The acceptance standards and the conditions for the resulta

to be reported do not consider that the height of an echo

amplitude depends on the distinct! very much. The following

example:

A reflector, the extension of which exceeds? a little one square

inch may produce an indication lower than that of the resulting

back reflection, if ihe distance from the search unit is large

enough- This reflection would not have to be reported.

The same reflector will produce

an indication accompanied by a complete loss of back

reflection, if it is situated near the scanning surface

208

which is opposite to that one from which the forginghad been examined before, The sarae reflector even didnot need to be reported in the first case and had to berejected, in the second case.

On. this reason. German specifications arc using circular

reflectors in shape of flat bottom holes as reference

reflectors for flaw distances largor than the near zone

length of the beam. Flaws shall be recorded, if their

echo amplitudes reach or exceed the echo amplitude from.

a cii"cular reflector in shape of 8 rnrn diameter flat

bottom hole at the corresponding path length. Altering

attenuation has to be taken into account. Areas where

the attenuation of a 2-mHz-ultrasonic propagation is

exceeding1 0,03 decibels per millimeter ai-e to be

regarded as flaw areas. The p-izee of mall flawr. are

estimated by comparison with a circular flat bottom hole reflector

and the sises of large flaw» "by usiny the half-value or the

aero-value method. The rules of acceptance are similar to

those for the platee

The anfj'le beam examination required by ASME Code

Section I J.I, shall be calibrated to obtain a signal

amplitude of 75-90 % full screen height from a square

notch of a depth equal to the lesser of 3/8 in. or

3 per ,eent of,,the nominal section thickness and a

length $>f -ïpproximately 1 lin- Indications whose

amplitude exceeds hO per cent of the calibration notch

amplitude are to î>e reported. One or more reflectors

209

which produce indications exceeding in amplitude the

indication from the calibration notch are unacceptable.

Distance amplitude correction methods for the angle beam

technique are not provided for forcings an the ASME Code.

as far an German specifications dea_ with an^le beam examination

of forgings until now they use the same calibration methods

and similar acceptance conditions which are described for the

examination of welds.

2.3 Examination .of Bqlt£'tj ffuts .andThe difference between the calibration methods ofASMS Code Section ÎII and those of the known continentalEuropean specifications are of similar nature as thedifferences in examining forgingc are.

All bolts, nuts and studs shall be examined by magneticparticle or liquid pénétrant methods after threading oron the materials stock at approximately the finisheddiameter.

ASMS CQdie SecUon III

In addition all bolts and studs greater than 2 in.

nominal bolt size shall be ultrasonically examined

prior of threading by straight beam method in radial

scanning with a frequency of 2,25 niHz and a search unit

not to exceed one square inch area. Any discontinuity

which causes an indication in excess of 20 per cent of

the height of the first back reflection or any

discontinuity which prevents the production of a first

back reflection of 50 per cent of the calibration am-

plitude is not acceptable.

210

In addition all bolts ar?d studs over 4 in* nominal sizea longitudinal scanning (in axe direction) shall boperformed, A circular reflector in the shape o.f a 3/8xnch diameter flat bottom hole is the leferoncc fjav.Discontinuities whose indications cxeocd that oneproduced by the fJat bottom hole with regard to thedistance correction are not acceptable.

Usual Gerinaii Spc ci Cri cat ionsIn addition a!3. bolts, nuts and studs shall be ultra-sonica!3y examined prior to threading by straight boanimethod in radial and axri al ¿.canning and by &ng;le beaiamethod in axial scanning. Indications corresponding- tocircular reflectors in shape of flat bottons holes w5.th6 uim diameter in x^adial scanning and '4 nun diameter inaxial scanning shall be recorded. Flaw sizes extending-10 mm in radial arid k mm in axial direction aro un-acceptable.

Ex aininat i on »Lf ...._P -.PL.C- The methods for nondestructive testing of tubular partsof the reactor pressure vessels seem to be very siuilarin. the various countries. Ultrasonic examinations usethe angle beam method scanning in cii~cumferontialdirection. Other directions are applied on pipes withgreater diameters and wall thicknesses. The standarddefects are grooves wa tlj a depth of 5 peí" cent ofthe wall thickness; the minimum depth is about O,1 mm.Indicabaons exceeding: thobc of the inside or outsidegrooves are unacceptable in general.

21Í

Depending cm the type of steel other methods are usedadditionally, for instance: magnetic pax-bide, liquidpénétrant or eddy current inspection.

3 « Rxarrti nation of \j olds

3 , 1 J o in t Jgd ifce. Tgf.i'ii l -ffAll joint edges of voids and all other areas of theparent metal surface where weld material will beattached shall be examined by the magnetic particle orthe liquid pénétrant method before welding. In Germanythe avera.ge roughness of these surfaces must not exceeda value of 10 ¿Wm.

3 . 2

3 « 2 . 1 Compara son be t ween ASME fio do and German Spe c if i cat ions¥hcreas accor-ding to ASME Code Section JIT all weldedjoints of categories A and B shall be fully radiographedand. this method seems to be the favourite one of theASME Code, most of European countries, in particularGermany, prefer the ultx~ason.ic examination,

3.2.2 Gertnari Spec i f 1 c ¿\ t i on sIn Germany the butt welds with wall thicknesses belov

40 mm usually shall bo examined both ultrasonically andradiographically. In addition the surfaces are to beinspected by magnetic particle method or, if notpossible, by liquid pénétrant method, after averageroughness had been . rediiced by grinding to a maximumvalue of "SO /frin. Radiographs can be useful as additional

212

method controlling1 the ultrasonic testing personnel,especially at wall thj cknesscs bclov ^U mm, because

raphs, gave the best results in this thicknessand they are no I too expensive on 3 ow thicknesses

At vail thicknesses above ¿¡u «mi in general the ultra-sonic inspection .is the onJ y examination method besidethe magnetic particle or the liquid pénétrant tes u, irthe ultrasonic inspection may be regarded as a completeexamination. Tor the completeness of ultrasonic testingthe Relieving conçût i on?, must be fulfilled;

Every volume element of the vc3.d and the parent metalzone with a wic5t3i between 10 and 20 mm ( spending onthe wall thickness) on each, side of the weld shallbe examined in angle beam technique with twodifferent angles betveen the inaán beam and ^he nortna.1to the surface. In the areas noax" the surfaces one oíthese angles must bo in the range of 3î>~35 ° withregar-d to the normal dir* ction to the xiearest surface,The second angle shall ensure that the angle deviation

between the main beam and the normal to a defectorientated perpeiidicuJ ar to the surface vill be assmall as possible.

2. Sc..armin D.i r e c t i on¥ith regard to the tangential plane of the scanning-surface care must be taken to all side angles foi-détection of any possible defect orientation and asa conclusion of this ail scanning directions must be

213

used. The scanning directions for detection oflongitudinal and transverse defects are especiallyimpox^tant. For defects not orientated in longitudinalrespective transverse directions it is sufficient tosviwel the probe laterally while it is moved forwardsand backwards. At electroslag welds and velds ofsimilar types it is necessary to use an additionaltesting direction with a side angle of 5 ° to detectdiagonal defects. Each scanning direction is to beused with regard to both directions to the weld.

These conditions involve that in most casos ofgreater vail thicknesses the examination must becarried out from both surfaces-of the weld and fromfour scanning areas of the parent material on bothsides of the weld and on both surfaces (figures 1, 2and 3).

3. Tandem TechniqueWhen the wall thickness is 100 mm and more the tandem

technique shall be used additionally to find smooth

longitudinal defects orientated perpendicular to thesurface. Scanning with tandem set shall be done fromone. of .the four pai^ent material surface areas.

4. -CQXl4jLtion of th et S annin Sur f ac e sAll contact surfaces including the weld surface roust befree from i-idges and valleys which could have a harmfulinfluence on contact of probes or allow the probes torock or cause that the beam angle is not fixed. Toavoid this the surface has to be pi*epared.

214

Undercuts, scales, veld spatter and similar surfaceirregularities must bo removed so that the probes canbe moved with uniform contact. The average surfaceroughness of the scanning surfaces may not exceed

If all scanning directions according to the mentionedconditions cannot be used at all or not satisfactorily for'instance due to conditions of the cladding or geometricalreasons or if the evaluation at some scanning directions dueto the cladding is considerably complicated, the examination isto be considered as not completed.ultrasonic testing through cladding has no influence on thecompleteness of the testing, if comparative examinations canpx*ove that the sensitivity requirements according to thespecification are fulfilled.

If it is not possible to fulfil all conditions for thecompleteness of the ultrasonic examination the veldshall be radiographed additionally. But. -essential, partsof the ultrasonic programme must be performed In any caseexcept austenitic velds. For material thicknesses of morethan 4OO mm, radiographie examination is not applicablewith satisfactory result. Therefore const fuel-ions withmaterial thicknesses of more than 40O mm and oonstructions ..of less than 4OO mm being impossible to be radiographicallyexamined satisfactorily be cause of e. g, the fací: that Uiereis not an acceptable radiation source available, nat.sl: hodesigned in such a way that a completo ulti'asonie tcis possible to be performed.

215

German specifications prefer the ultrasonic Inspection asthe most reliable method if the conditions oí' completenessare nearly fulfilled. A lot of experience had shown Uiafcradiographie method did not detect all cracks. Such resultsare to be expected even with best radiographie conditions,for instance the use of high effective lineal* acceleratoras source, high fine grain films without buck, .screen andwith tantal as front screen.The extent of the ultrasonic examination work to be donoby the operator makes it desirable to develop automaticmethods. The successful efforts in the fMd of in-servjceinspection justify to expect a less expensive testing byautomatic methods* But at the present moment the sensi-tivity of the ultrasonic examination during fabrication must behigher than that of the automatic methods used.for in-serviceinspection. ohe reason is that relatively mall fabrication defectsalso must be detected.

3.3 Methods for Testing Nozzle Welds and Fillet Veldsas well as Temporary yelds in Germany ______As far as the material (no austenitic weld material) admit«ultrasonic inspections, in Germany these welds use to bebested by the same methods as the butt welds. Owing to thedifficulties caused by the shape of some nozzle weld desi&nsthe German specifications use to regard an examination to becomplete even if there are only three equivalent scanning areasinstead of four ones..

Pi U et welds with a throat larger than 10 ran» sha.11 t>eultrasonically tested, especially the parent material benoal-.U

216

the attachment surface in addition to the magnetic particleex amin a t i on.

Auatenitic nozzle velds are examined as well as poss.ibi.fj.Liquid pénétrant tests are performed on the final surfaceand so I'm- as possible in intermediato state» of Uie wo i clini:process before. If the shape of the weld admits an u) trnson.í cinspection in straight beam technique (longitudinal waves)the weld will be examined in this way additionally. An u.l r.ra-sonic inspection in the angle beam technique mostly niui*t b«*I imitated on the parent material beside the -weld tmd thetransition zone* Radiographie examination shall, be performedin all cases of possibility.

« t State of Production at the Examination^Procedures In GorimtnyThe examination of welds shall be performed before as ve.I ias after the hydrostatic pressure test.

Weld joints being either heat»treated or stress-re!ievedthe. ultrasonic examination to be carried out before thehydrostatic pressure test has to be performed 'after tbisheat-treatment according to the method mentioned fon Toro.Mostly an additional ultrasonic inspection had been ¡nr.deJust after the first or after the last heat treatment beforethe hydrostatic pressure test.Por the testing after hydrostatic pressure test the extent of

: .. ' C

scanning may be reduced*It is sufficient to scan from one surfsico or

the pressure vessel in two scanning directions with onepr*obe thatprobe. The angle/must be of such a type/at testing of the

217

areas close to the surfaces the angle between the main beamand the normal to the surface is less than J>3 ° and moro than^ . Two main scanning directions are to he u ¿tod atdetection of longitudinal and transverse defects, mid :« f anelectroslag weld is to be examined, at detection or diagonaldefects too.If the last circumferential weld joints of large* pressurevessels are site welded, the intended ultrasonic fce.si.jn/.rafter the hydrostatic pressure test may also foe carried outbefore the hydrostatic pressure best, but after i.lie Lostheat-treatment according to the reduced method when thecomplete ultrasonic testing already has been made after anearlier heat-treatment. In this case, however, it isnecessary to make a random check after the hydrostaticpressure test. The spots chosen shall be representative.withregard to the stresses at the pressure test. If thereare anypeak stresses received at the measuring of tension, regard,must be paid hereto. Butt welds and nozzle welds which cannot be ultrasonically tested must be radiographs cal l.y examinedafter the last heat-treatment. Welds being both u3 trasonicai J.yand radiographically tested can be radiograph!cal.1 yexamined at an earlier time and ppssibly also before thecladding.

*î. 5 Sensitiveness Calibration for Ultrasonic ExaminationAccording to ASME Code, Section III, the sensitivity ofanfjle beam technique shall be calibrated by using »cylindrical reference reflector the diameter of which isdepending on the wall thickness. Distance-amp]itude

218

corj.-ect.ion curves shall be constructed by utiJ i'/s ui{>; !;!ioheights of echo-amplitudes from the cylindrical baM.ccalibration hole. The attenuation shall be -regarded bytransfer controls. Then the top points oí' the hole-echo-amplitude» are to be- joined to -the reference curvo. Al.lindications which produce a response greater than £0 per centof this reference level shall be investigated to the extent,that the operator can determine the shape, indentlty andlocation of all such reflectors and evaluate them in t.t»rra«of the acceptance standards» But ASME Code Section TIT doc.snot contain the criteriafi>r the determination or kitei.dcntj ty.

In Germany the ultrasonic operators are not believed to be experiencedenough to determine the identity of flaws by means of ultrasonicexamination only. Therefore they are not to try to determine thenature of an indicated flav as far as the estimated extentsand the frequency of flaws are low. Large de foci» or n f*reatnumber of defects make a repair necessary if it is notpossible to recognise a harmless cause of the indications byother methods» by a radiography for instance.

.Tn Germany the sensitiveness calibration shall be dono i¡»U»o following ways

All echos not depending on geometi^y from the tested «rea.shall be recorded as flaw echoes, when their amplitudes atnominal material thicknesses up to 2O mm at thecorresponding path length reach or exceed the echoamplitude from circular reflectors in shape or .'¿nun d:io.flat bottom holes and at nominal material 'thicknesses*

219

above 2O mm the echo amplitude from circular- reflecto!in shape of 3 mm dia. flat bottom holes. For defocts nearthe surface - located max 5 nun below surface - a .squaregroove of 1 mm depth and 20 nun length shall be us«d. asrefemce reflector* All echos not depending on goomet.ryshall be recorded as flaw oehos, when tjhf.ii' amp I i tildesreach or exceed a level which is l> decibels, tower thanthat of the groove with the corresponding path.The level value for recording can also be stated throughother testing defects of any type if .it Is quito surethat no higher value can be obtained.The level values shall be reduced with 6 decibels, whenthe values for the extent and the frequency of flaws arelarger than those being acceptable without qualifiedretesting.

Transfer correction has to be carried out.

For the examination performed by the tandem techniquea circular reference reflector in the shape of a IMac

i

bottom hole with 10 mm seems to be sufficient. Echoamplitudes reaching or exceaiing a level which in 1:,<decibels lower than the height of the coi'respondim» «>choof the 1O mm diameter circular reflector wi th regard tothe distance and the attenuation are to be recorded.

3.6 Acceptance StandardsAccox*ding to ASMS Code Section III discontinuitiesindicated by ultrasonic inspection are inacceptable » I"the signal amplitude exceeds the reference level and fcliolength of these descontinuities exceed values dependía;.;

220

on the vail thickness but being: 1imitated by 'jt/-k lu.

Discontinuities with lover indications but heights-

above 20 per cent of the reverence level shall bf>

investigated as mentioned.

In Germany the reference level according to ASME Code Section III

is regarded as too higgi on great wall thicknesses. Furthermore the

opinion dominates that the present technique does not enable the

operator sufficiently to recognize the nature of the indicated

defects only by ultrasonic methods. The height of the echo

amplitude depends more on the shape and the deviation of

the discontinuities and on the stress

conditions than on the sizes of those discontinuities.

Therefore German specifications require that relatively

small indications are to be recorded. The acceptance

depends on length, depth and frequency of d«J"ects. Tf

defects can clearly be recognised as slag inc.Uisiona ¿'or

instance by X-Ray- inspect ion the accepted size.'? of

defects might be larger as ASME Codo Section TIT

demands.

Anatenitic Weld Cladding

The cladding shall be examined by liquid pénétrant and

ultrasonic method. To get indicated the small cracks in the

parent material zone under the cladding by ultrasonic testing

an angle beam technique with longitudinal waves is used.

Owing to the different dilatation coefficients this zone

is under pressure when the temperature is low (;,:0 ° c,)

and under stress when temperature is hi#h (* -. ;.-O ° < - ) .

221

The clad surface must be smoothly ground for this in-spection on under-cladding»cracks; this examinationusually is carried out as a random check.

The possibility of examining the parent material uitro-sonically through the welded clad material depends onthe welding method* Shield arc -welding- (with wj re)causes more difficulties in ultrasonic inspections fchansubmerged arc welding with tape.

Acoustic EmissionIn Germany acoustic emission as an examinât".ion methodis still being in the investigation state and not beingused in practical inspection.

Parry and Robinson describe the practical performanceof the examination of pressure vessels by this method ".!''].

References:

£*1 / ASME Boiler and Pressure Vessel Code Section it'llRules for Construction of Nuclear VesseJ s,Edition 1971P« l>» Parry and D. L. RobinsonIncipient Failure Detection by Acoustic àaiss.ionA Development and Status Report, August 1'-.M>,Idaho Nuclear CorporationIdaho JPal1s, Idaho

222

Fig. 1Completeness requirements oí"ultrasonic examination of veJdsRequired infractions angles

Fig. 2Completeness requirements ofultrasonic examination oí' w«*J.<1í«Required scanning direction?;

fa-fa~ -d5~7 7s Vv S

Fig.Completeness x'equii* ementaultrasonic examination oí'longitudinal, transverse anddiagonal defects

Receiver Emitter

X Li„...

Fig, kCompleteness requirements ofultrasonic examination oí' weldsTandem technique

223

CONTROLE DES GAINES EN ACIER INOXYDABLEET ALLIAGES DE ZIRCONIUM POUR LES ELEMENTS COMBUSTIBLES

__ par A.C.PROT

Astracts : The non destructive testing of canning tubes for nuclear reactorfuel elements ', > pevior; >~<i by \ssing two mains techniques :ultrasonics and eddy cuirents.

This paper shows the various aspects of ultrasonic nondestructive testing :

- choice of the testing method~ working frequency- transducer(s) type~ electronic- standards flaws~ influence of the metallurgical structure- incidence of specifications.

Eddy currents are faced with difficulties related to theirprinciple :

- number of partr.eters involved- difficulty in extracting them from the raw signal- choice of the apparatus- matching of trausducere (coils, probes .,.)- influence of the working frequency-* limite of the various possible techniques.

1 - INTRODUCTION

Chacun sait que diverses méthodes sont envisageables pour lecontrôle de gaines d'éléments combustibles.

Toutefois, le choix se restreint de manière significative sil'on considère à la foi^s^a^géométrie des tubes de gainage (diamètre, épaisseu»n particulier), et les spécifications propres à l'énergie nucléaire.

225

Deux méthodes sont réellement exploitées, seules, ou encombinaison : les ultrasons et les courants de IVoucault.

Nous allons exposer dans ce qui suit les problèmes relatifs àchacune de ces méthodes et les solutions quo nous avons tenté d'y apporter.

2 ~ RESUME

Le contrôle non destructif des gaines de réacteurs fait appelà deux techniques : les ultrasons, les courants de Foucault,

On montre que le contrôle par ultrasons présente divers aspects- choix de la méthode- fréquence de travail- type de traducteur (s)- électronique- machines de contrôles«• tubes étalons- influence de In structure- interprétation des spécifications

Les contrôles par courants de Foucault se heurtent à desdifficultés de principe :

- multiplicité des paramètres« difficulté de les extraire du signal- choix d'un Appareillage- bonne adaptation des transducteurs- influence de la fréquence de travail« limites des divers procédés envisageables.

3 - LE CONTROLE FAR ULTR_ASOM DES TUBES DE GAINAGE EN ACIF.R INOXYDABLE ETALLIAGES DE ZIRCONIUM

,3-1 Cho ix de 1 a me t ho de~*~™ """"r~ r7Si l'on considère les dimensions de^éubeè à contrôler (diamètre

de 5 à 15 nun, épaisseurs de 0,2 à 1,5 ran) seule une méthode par écho226

est envisageable. On peut imaginer de travailler avec un ou plusieurstraducteurs (méthode Grabendorfer par exemple). Pour des raisons desimplicitéf mais aussi de sensibilité, noas avons retenu la méthode hun traducteur, émetteur-récepteur. D'autre part, seule une méthode parimmersion pouvait être envisagée pour un contrôle automatique.

3-2 Choix de 1 a fréquence

Si l'on se réfère à ce que l'on trouve dans la littérature concernaitla relation entre défaut à déceler et longueur d'onde, on est viceamené à penser que le contrôle est très difficile. Aussi, toujours dansun but de supplication, avons nous travaillé avec des appareils ducommerce dans «ne gamme de fréquences allant de 2 à 10 MHz (exception-ncllesnent 20 MHz). On verra plus loin l'importance de ce choix.

3~3 Types de

Les premiers essais ont rapidement montré l'influence descaractéristiques des traducteurs sur la qualité du contrôle. Dans unpremier stade, noua avons utilisé des traducteurs plans, avec despantillcs piézoélectriques circulaires ou rectangulaires, associéesà des dispositifs de caches destinés à réduire l'importance deséchos de surface. Les premiers essais réalisés avec des traducteursfocalisants avaient été décevants.

A cette occasion, des études systématiques des champsultrasonores des traducteurs ont été entreprises. Elles ont permis debien situer l'importance des données théoriques relatives à leursperformances (étude manuelle puis automat iqui des champs de traducteurs,calcul sur ordinateur des champs de traducteurs rectangulaires).

Par la suite, une meilleure connaissance des traducteursfocalisants (foyer linéaire ou ponctuel) a conduit à généraliserleur usage.

Certaines difficultés peuvent cependant se manifester. Il estalors bon de penser au mode de propagation ultrasonore mis en jeu dansle contrôle (ondes du type ondes de Lamb, ou ondes transversales).

227

3-4 Choix de 1 '63ectrgnjique

Le matériel utilisé est généralement du matériel du coratnerce ;dans ce cas} les ondes mises en jeu sont proches des ondes àc Lambque l'on peut engendrer dans les tôles minces, et font intervenir leproduit fréquence x épaisseur, si la fréquence est faible (2 à 6 MHz).Au-delà, il est possible de travailler en ondes transversales.

La méconnaissance de cet aspect a conduit certains cubistes à desérieux déboires dans l'utilisation {Jes ultrasons.

Pour certains problèmes nue électronique spéciale a étédéveloppée, pennettant l'utilisation de traducteurs haute fréquence(15 à 20 MHz) pour un contrôle en ondes transversales. Des résultatsseront présentés dans un paragraphe suivant.

De même, les caractéristiques de 1"impulsion d'excitation(impulsion brève ou train d'impulsions sinusoïdales) sont un facteurtrès important de la sensibilité du contrôle, en particulier dans lecas des ondes de Lamb.

3-5 Importance des machinesde^contrôle

La simple comparaison des dimensions des tubes et de celles destraducteurs classiques, montre que la position relative de ces diverséléments doit être constante tout au long du contrôle, ce qui poseun certain nombre de problèmes mécaniques : un '-.ube est en effetimparfait : ovalisation, flèche ; sa mise en rotation à grande vitessepour un contrôle rapide est délicate.-C'est pourquoi le C.E.A. adéveloppé divers prototypes de machines et finalement s'est arrêtésur deux types :

- dans un cas, la cuve de contrôle, étanche, remplie d'eaudégazée» est précédée d'une chambre de prémouillage des tubes, destinéeà éviter la formation de petites bulles de gaz pouvant interférer avecle contrôle. L'exploration hélicoïdale du tube est assurée grâce àdeux poupées transmettant au tube un mouvement de rotation (jusqu'à3000 t/tnin), et de translation (jusqu'à 4 mm/tour). Fig 1 et 2.

228

- dans l'autre cas» la cuve est alimentée en permanence par unréservoir d'eau additionnée d'un agent mouillant. Translation etrotation sont assurées par un. jeu de galets inclinés réglables. Uneautre communication décrira en détails les contrôles effectués surcette machine, plus particulièrement dans le cas des gaines de réacteursà neutrons rapides,

Dans les deux cas, toutes les précautions sont prévues pour quele centrage des tubes au droit des traducteurs, soit assuré dans lesmeilleures conditions.

3-6

Cette opération, indispensable, est destinée à assurer au contrôleun certain niveau de sensibilité, à vérifier que cette sensibilitén'évolue pas au cours du contrôle, et est identique pour des défautssitués à la surface externe (013 defects) ou à la surface interne(ID defects).

Il est bon de signaler à ce propos que la vitesse relativementlente des ultrasons dans les zircaloy, a permis de mettre au pointune méthode originale qui grâce à l'utilisation d'un seul traducteurpermet d'enregistrer sur deux voies séparées les défauts externes etinternes. Cette méthode a été développée à partir d'une observationsimple i dans certaines conditions d'incidence du faisceau ultrasonore,il est possible de créer une onde du type de Lamb, qui se trouve trèsrapidement amortie à l'extérieur par l'eau dt couplage, alors quel'intérieur, en contact avec l'air, conduit à \m amortissement beaucoupmoins intense. Une telle procédure de contrôle présente un grandIntérêt pendant la phase de mise au point de fabrication des tubes,

Les défauts standards sont généralement usinés par électro-érosion(spark erosion process), parfois par poinçonnage. Ils sont réaliséssuivant les deux directions principales de contrôles : sens longitudinalet sens transversal. Un écueil doit être éviter : confondre lesdimensions de ces défauts avec celles des plus petits défauts réelsdécelables. En effet la "détectabilité" d'un défaut est liée, non

229

seulement à sa géométrie, ( longueur } largeur, épaisseur), mais à saposition (orientation), à son état de surface} à sa forme (défaut plan,volumique), à son impédance acoustique relative par rapport à lamatrice dans lequel il se trouve (inclusion, fissure), à la fréquenceultrasonore (longueur d'onde), au type d'oncîes mis en jeu (transversalesou de Lavnb) à la nature du faisceau ultrasonore, (ondes planes, ondessphériques ou cylindriques pour les traducteurs focalisés) et enfinà la structure des matériaux considérés, en particulier la grosseur degrains.

met urc _

On constate en effet très rapidement, l'influence très grande decette structure en particulier de la grosseur de grains. Des étudessystématiques ont été faites jx>ur l'acier inoxydable, et, pour unefréquence déterminée, l'influence du grain sur îc rapport signal/bruit aété mise en évidence sans ambiguïté (toutes choses égales par ailleursc'est-à-dire : rafime nuance, même géométrie, même état de surface etc ).

3-8 ^Interprétation des spécifications

II s'agit là, croyons nous, d'un point extrêmement important.

En effets la plupart des spécifications stipulent que l'on doitpouvoir déceler un défaut étalon déterminé, dont les dimensions sontgénéralement précisées (par exemple : profondeur égale à 10 "À de e,longueur égale à n x e).

En fait, le paramètre permettant de déceler le défaut est ce quel'on peut appeler sa "surface apparente", c'est-à-dire la surfaceprésentée normalement au faisceau ultrasonore, corrigée d'un facteur deforme, d'état de surface, d'impédance acoustique relative ..; On conçoitdonc que pour un défaut donné, l'importance de l'indicationultrasonore dépende étroitement de la dimension relative du défaut etdu faisceau ultrasonore. Ainsi, un défaut dont la longueur est plusfaible que la lax'geur du faisceau, peut-il donner lieu à des signaux

230

récurrents d'amplitude identique, proportionnels à la surface apparentedu défaut. Le même, vu avec un traducteur à faisceau plus étroit peutdonner lieu au même nombre de signaux récurrents : mais ils serontalors proportionnels à une surface apparente dont l'une des dimensionsest la profondeur du défauts et l'autre la largeur du faisceau. Sur leplan tía la sensibilité, le se ond contrôle sera donc plus sévère quele premier. Fig. 3

On voit donc, comment uno spécification peut conduire, suivantle procédé de contrôle utilisé, à des qualités de produits totalementdifférentes.

Il y a là semble-t-il, une question importante qui touche lescontrôles de Lubes en général» et n'est pas propre aux tubes de gainesnucléaires.

Cet aspect est très bien illustré par l'examen des enregistrementseffectuées sur une de nos machines prototypes, où la détection dedéfauts étalons poinçonnés aussi petits que i. - 0,3 ram

p « U /J

a pu être effectuée avec un rapport signal/bruit très favorable. On apu noter au passage l'excellente reproductibilité permise par l'utili-sation d'une mécanique de' qualité. Fig. 4

"*9 Cône l \ï s i <xn_s

Actuellement le contrôle par ultrasons >>s tubes dans une gatamed'épaisseurs et de diamètres correspondant aux tubes de gaines, est unproblème qui peut êtie considéré coiame résolu, Ce contrôle présenteun certain nombre d'avantages :

- grande sensibilité, en particulier aux défauts du type criques,fissures qui sont les plus dangereux.

- les cadences permises par les machines de contrôle modernes sontconvenables. D'autres postes dans la fabrication des tubes de gainagesont plus lents. Tl est toutefois certain que tous les progrès réalisésdans ce sens seront les bienvenus.

231

L'inconvénient de cette leélhode est essentiellement de nécessiterun personnel bien au <courant des techniques ultrasonores. L'exploitationpeut, âans une certaine mesure, être automatisée (comptage d'impulsionspar exemple), niais un tel contrôle nécessite toujours l'intervention d'unspécialiste, (voir la communication relative aux gaines de réadtcurs àneutrons rapides). ' '

D'autre part, s'il est très sensible à des défauts bien orientés,il laisse des doutes sur certains types de défauts et c'est pourquoiil est préférable de l'associer à un contrôle par courants de Foucault,

4 - MESURES DES EPAISSEURS PAR ULTRASONS

Le procédé utilisé jusqu'à maintenant faisait appel a unappareillage ultrasonore du type "Vidigage" fonctionnant sur le principede la résonance. C'était à l'époque du lancement de nos études et de nosfabrications, le seul qui permette d'envisager le contrôle d'épaisseurs del'ordre de 0,2 à 0,3 nun avec une précision de l'ordre de 1% . La possibilitéd'effectuer ce contrôle en inmersión, en parallèle avec le contrôle dedéfauts, a permis de généraliser son emploi. Actuellement les appareillagesfonctionnant en échos d'impulsions permettent d'accroître les cadences decontrôle (le taux de récurrence des impulsions est très supérieur à lafréquence de modulation des appareil, à résonance.)

Ce procédé étant classique nous nous bornerons à ces quelquesremarques.

La mesure des épaisseurs de gaine en zircaloy et alliage deairconium a donné lieu dans le cas du réacteur EL 4, à un développementintéressant. En effet» dans ce cas les tubes sont corruguéSj après contrôlede santé par ultrasons, le profil étant une sinusoïde d'amplitude crête del'ordre de 0,2 à 0,3 rara et de pas 2,5 à 3 ïam. Après cet usinage, laprofondeur en fond de corrugation doit rester dans certaines limites. Or,les tolérances admises sur le tube lisse sont telles que des rebuts

232

iaiportants sont apparus lors des premières corrugations. Une machined'usinage (Fig. 5) a été mise au point, elle comporte les éléments suivants :

*• un tour à usiner classique«• une poupée de guidage comportant trois organes de mesure :v 2 sondes fcnjrnissarU nn paramètre de position de lit surface

externe du tube par rapport au bftti dans une sectioiai dáterminée, 1 traducteur uîtrasonore de mesure de l'épaisseur dans une

section déterminée, 1 calculateur PDP 8 qui traite les informations issues des

mesures procedentes et qui grâce à un convertisseur digitalanalogique, commande la position de l'outil (par 1*inter-médiaire d'un vérin hydraulique) de telle sorte qu'il enlèvejuste la matière nécessaire pour que la cote terminale soitdans les tolérances fixées. Il est relativement aisé demontrer que» dans ces conditions, la machine est amortie trèsrapidement grâce à la diminution très importante dés rebutsde fabrication,

II s'agit là d'une jnéthode intéressante dont l'originalité résideen ce que la surface prise pour référence lors de l'usinage est la surfaceinterne de la pièce (dans ce cas le tube) et non plus sa surface externe.

5 - CONTROLE DES GAINES EN ACIER INOXYDABLE ET ALLIACES DE ZIRCONIUM. .PARCOURANTS. DE FQUCAULT

En préambule nous voudrions signaler que les études relatives auxcourants de Foucault ont démarré fort tard à Saclay. De ce fait, cetteméthode n'a pu ôtre utilisée dès le début des fabrications d'élémentscombustibles.

5-1 L'une des difficultés majeures du contrôle par courants de Foucaultréside dans la multiplie i té despar^niètrej; actifs :- paramètres propres au matériau : dimensions, conductibilité électrique,perméabilité magnétique»

233 t-

- paramètres propres au mode d'élaboration : structure métallurgique,présence de contraintes résiduelles» déformations locales, dans lamesure où ces paramètres modifient les précédents,

- paramètres liés h la technique de contrôle î fréquence» entrefer oucoefficient de remplissage (cas des tubes et des bobines encerclantes)sforme et dimension des détecteurs, vitesse d'exploration des placesà contrôler, température.

On conçoit que l'analyse de signaux obtenus lors d'un telcontrôle soit délicate.

On sait d'outre part que la plupart ties appareils du commerceexploitent ces signaux en analysant leur amplitude et leur phaserelative,

Une étude faite sur ordinateur nous a montré que l'analyse dessignaux n'était possible pour un matériau et une géométrie déterminés, quepour une fréquence bien déterminée : cette fréquence permet uneséparation optimale de phase des divers signaux d'origine différente, etconduit donc à leur discrimination aisée.

~2 Extraction dos paramètres

Un appareillage, utilisant ce principe a été développé. Il permetles contrôles entre 300 Hz et 300 KHz, toutes les fréquencesintermédiaires pouvant être obtenues, -Cet appareil, commercialisé, permetd'obtenir des résultats de ce type ;

- séparation sans ambiguïté des défauts do surface interne etexterne. Possibilité de les déceler avec la taSme sensibilité (réglagede gain des voies d'enregiscrement)

- analyse de la nature du défaut : coup, fissure, variationdxroensionnelle brutale etc ...

En travaillant en absolu et non plus en différentiel, il estpossible de mettre en évidence :

234

•» les variations dimensicmneîles- les paramètres non localisés modifiant la conductibilité

électrique (modification des structures métallurgiques par exemple).

Un. second appareillage a également été développé. Réalisant entemps réel une analyse en fréquence dee signaux recueil1! i s, il permetd'effectuer une discrimination sélective» particulièrement clc£> défautsgéométriques. L'étude systématique de lots de tubes et des défautsqui leur sont associés permet de fixer des seuils et d'effectuer untri-automatique. Contrairement à l'appareil précédent, constituéessentiellement par un pont alternatif, ce dernier comporte outre unappareil du commerce, un dispositif comportant deux bobines, partiesintégrantes de deux oscillateurs couplés. Le signal obtenu correspondà «ne variation de couplage, introduite par les défauts de la pièceà contrôler. Une autre communication décrit plus en détails les étudesrelatives à ce dernier appareil.

5-3 Adaptât ion des traducteurs

Dans le cas des tubes, cette adaptation est fonction de lanature et de la dimension des défauts à déceler. La sensibilité pourun type de défauts determinó, est très étroitement liée à la géométriedes bobines utilisées qu'il s'agisse de bobines encerclantes pour lecontrôle par l'extérieur ou de sondes pour le contrôle par l'intérieur.Des études ont été poursuivies dans ce domaine.

5-4 Influence de la fréquence de trayail

Comme il a été dit précédemment, seule une fréquence déterminéepermet la discrimination aisée des différents types de défauts. Pour îecontrôle des gaines en acier inoxydable, le premier appareillage citén'est utilisable que dans une certaine gamme de diamètres, Si lediamètre est trop faible, et si l'épaisseur est faible relativementau diamètre, cette discrimination optimale n'est pas obtenu (de tnCme,ei le diamètre est trop grand, l'épaisseur trop importante).

Toutefois l'analyse des signaux peut encore être possible, maisse présente moins aisément.

235

5-5 ¡.imite du

Hous venons de voir que dans la cas des Lubes de gaine en acierInoxydable, une certaine limitation est. donnée par la fréquence detravail. Cette .limitation est propre aux dispositifs utilisant unealimentation en courant alternatif sinusoïdal des détecteurs.

On sait que d'autres procédés existent, en particulier ceux faisantappel aux champs electi-omagnetiques pulses.. Leur domaine d'applicationcouvre la plupart des tubes destinés aux éléments combustibles desréacteurs à neutrons rapides. Des études sont actuellement en coursà Saclay.

236

A - amplitude du signaicaractéristique duproduit Ixp

Traducteur

A

A = Amplitude du signalcaractéristique de îaprofondeur p

FIG 3

239

1. .QQ.Í/I !

1 !

, i1

i ! ií . i . . . . . , :

í : i

VR»«OOí/mn0,2 mm.

i

T

FíG 4 Enregistrements détalons -» Reproductibiitté

G, BOULANGER *JP. DUPA YET *A. SAMUEL *B. SPRIET *A. STOSSEL *

A B S T R A C T

In thé course of the développement of fuel elements for nuclear reactors,carefull examinations of canning tubes were performed on pins for the various reac~tor files.

Numerous and various non destructive testings were carried out on thou-sands of seamless or welded tubes, drawn or laminated, either in stainless steel orin zirconium alloys. The experience gained allows us to compare methods and to drawconlcusions on their respectives performances. This leads to the definition of anentirely automatic control system.

Three testing arc used î multiple frequency eddy current testing, ultra-sonics, metrological testing, on two separate benches :

- an eddy-current bench used for a first soundness testing, associatedwith a tnetrological system vich enables countinuous measurement of mean diameterand ovalisation to be carried out.

~ an ultrasonic bench used for additional defect detection and a simul-taneous wall thickness measurement.»

Feeding of the machines and selection of the tubes are automaticallyachieved (sorting in various categories according to the tubas quality). All theinformations collected during these controls tor each tubes are recorded on punchedcards, using a data collecting system.

*DMBCN/DDKC/SDECn -

243

Dans le cadre des études s'ur les elements combustibles deréacteur, nous avança, depuis quelques années,, effectué des examens ap-profondis sur les tubes de gainage des aiguilles de différentes filière

Des contrôlée; non destructifs nombreux et variés ont. £tcexpérimentas sur des milliers de tubes lisses, avec DU sans soudure,étirés ou laminés, en acier inoxydable ou en alliage de zirconium.L'expérience acquise nous a permis de comparer IBS méthodes et de tirerdes conclusions sur leurs performances respectives. Mous avons abouti dla définition d'une chains de contrôle entièrement automatique.

Trois méthodes de contrôle sont utilisées {courants de Foucault è plusieurs fréquences, ultra-sons, métrologie), réparties sur 2postes de travail -ï

- un banc courants de Toucault pour un 1er contrôle ce SÔ'Tté associé à un ensemble de métrologie permettant la masure on continudu diamètre moyen et de 1'ovaliaotion.

- un banc ultra-sons pour une détection supplémentaire desdéfauts couplée avec uns mesure simultanée de l'épaisseur.

L'approvisionnemfcrst et la sélection sont automatiques surces installations (cl^sseirent simultané en plusieurs catégorie;-» su•**.•<!-T-la qualité des tubes), foutes les informations recueillies au cours dp c-'contrôles sont rassemblées pour connue tube sur cartes perforées à l'rajd*un système de centralisation dus données.

1 - INTRODUCTION

La première décision à prendre devant un problème de con-trôle non destructif ecfc le choix de la ou des méthodes à"mettre enoeuvre. Ce choix doit tenir compte d'un certain nombre de facteursen particulier î . .

- de la géométrie des pièces à contrôler~ de leur nombre- du niveau de qualité estimé nécessaire.

Pour la chaîne que nous allons décrire, il s'agissait decontrôler systématiquement 40 000 tubaa en acier inoxydable de6,5 mm de diamètre et de 2 m de longueur, l'épaisseur étant de

244

0,45 rrtrn. Ces tubes sont destines au gainage dos éléments combusti-•bles du réacteur à neutrons rapides Phénix. Il est bien évident quecet ensemble de mesure et de contrôle peut fître utilisé pouc d'autresapplications que la filière à neutrons rapides, en particulier pourle contrôle des tubes de gainage en alliage de zirconium, de la fi-lière à eau légère, et d'une manière plus large dans tous les css oùl'an s'intéresse à des tubes dont les dimensions sont comprises entre5 et 20 mm de diamètre pour une longueur maximale de 5 m.

En ce qui concerne IBS éléments combustibles de réacteur,la gaine est soumise à diverses sollicitations telles que la pressiondes gaz de fission, les gradients thermiques, le gonflement des pas-tilles, le bombardement neutronique,' etc . . . Afin de pouvoir garantirune durée de vie de l'aiguilla aussi longue que possible, il estnécessaire de contrôler finement les tubes pour éliminer ceux quiprésentent des défauts géométriques ou métallurgiques susceptibles

leur rupture prématurée en pile.

Parmi les méthodes développées au Département de Dévelop-pement des Eléments Combustibles à Cadarache, notre choix s'est por-té sur deux méthodes particulièrement bien adaptées à la géométriedu tube et facilement automatisables : les ultra-sons et les courantsde Foucault.

L'expérience montre, en effet, l'inturët tant techniquequ*économique de l'utilisation systématique de ces deux méthodesde contrôle qui s'avèrent beaucoup plus complémentaires que concur-rentes. Chacune a son domaine propre de sensibilité et leur emploicombiné permet d'éliminer la quasi totalité des tubes généralementclassés douteux et de diminuer globalement les taux de rebut parune meilleure connaissance de la "santé"du tuba.

245

2 - DESCRIPTION DE LA CHAINE DE CONTROLE

Mous décrivons dans ce chapitre les principales caractéris-tiques des méthodes et des appareillages mis en oeuvre pour le con™rtrôle non destructif des tubes de gainage.

Ces contrôles sont destinés à surveiller en permanence lafabrication des tubes en révélant la présence éventuelle de défauts

»de santé, et à rassembler pour chaque tube examiné un certain notnbrede données intéressantes le caractérisant (mesures métroJ cagiquesde l'épaisseur et du diamètre extérieur, mesures de résistivité re-présentatives de l'étafc structural}.

Trois méthodes de contrôle sont utilisées (courants.deFouc3u.lt à plusieurs fréquences, ultra-sons, métrologie), répartiessur deux postes de travail :

- Un banc courants de Foucault pour un premier contrôle desanté associé à" un ensemble de métrologie permettant la mesure encontinu du diamètre moyen et de 1'oval teafcion.

— Un banc ultra-sons pour uns détection supplémentaire desdéfauts couplée avec une mesure simultanée de 1Tépaisseur.

Sur- ces deux installations, l'approvisionnement et la sé-lection sont automatiques (classement en plusieurs catégories suivantla qualité des tubes).

Toutes les informations recueillies au cours de ces examenssont rassemblées pour chaque tuba sur imprimante ou sur cartes perfo-rées au moyen d'un système de centralisation des données.

246

2.1 — Banc -S,o>iy .n ti£ g jgiuciiiauuj.J:i (fig. 1 )

Le principe de la méthode de contrôle par courantsde Faucault est basé sur l'exploitation simultanée de plusieurs fre-quences de courants induits dans le tube par une bobine du type en-cerclant. C'est pourquoi l'appareillage a été conçu pour testersystématiquement chaque tube à 3 fréquences : tu kHz - 100 kHz -5DO kHz environ.

Ces conditions impliquent donc :

- un double passage du tube à travers les bobines~ un système de traitement qui centralise les résul-tats et les compare afin de donner un jugement surla qualité des tubes et de les classer en diffé-rentes catégories.

1) Description_de 1 ' installation

- un banc de défilement (fig. 2)II comprend :. un approvisionnement des tubes en nappes sur

.plan incliné les dirigeant vers l'axe de contrôle.

. un dispositif de translation aller et retourentrainant le tube à travers les bobines placées dans le bloc dedétection.

. un système de déchargement qui amène le 'tube 5se placer dans un des 3 tiroirs d'un meuble de stockage des tubesaprès contrôle.

- Un bloc de détection placé sur le banc et cornportar(fig. 3)

247

. pour les courants de Foucault ;1 bobine Forster 10 kHz et 100 kHz1 bobine Dudu 500 kHz environ

. 1 bobine poux mesure de résistivité

, pour la métrologie du diamètre extérieur î4 capteurs de déplacement a induction avec pal-peurs mécaniques.

- Appareillage électronique (fig. 4}

On distingua. des appareils de mesure dont les éléments im-portants sont ï

1 tiroir Forster 100 kHz1 tiroir Forster 10 kHz1 tiroir Dudu 500 kHz (1)1 tiroir de mrStrologie à 2 sorties j dx&tnnètre

moyen et ovalisation1 tiroir de mesure de résistivité

. un système de sélection automatique comprenant :

1 module analysant les signaux provenant des 3canaux courants de Foucault (10 kHz -100 kHzSOD kHz). C'est l'analyse de la forme dessignaux (2)

{1} Brevet 1.470.386 du 11.1.1966 - additif PV 107.736 25.5,1967(2) Brevet EN 70.31.801 du 1.09.1970. -•

1 modulo à seuils définissant plusieurs niveauxde signaux comparativement à des étalons (cou-rants de Foucault, métrologie).

248

1 centralisateur à circuits logiques comparanttoutes les informations courants de Foucaultet métrologie ; ce centralisateur est program-mé de façon à classer les tubes en 3 catégo-ries (suivant leur niveau de signaux) corres-pondant aux 3 tiroirs du meuble de stockagedu banc,

1 dispositif de réglage commandant l'accès dutube dans un des 3 tiroirs de stockage.

- Traitement des informations (fig. 4)

Les informations peuvent être recueillies sousforme analogique ou digitale (sorties binaires)

'. informations analogiques : courbes d'enregistre-ment

î diamètre extérieur moyen•* { ovalisation ou diamètre absolu

résistivité résistivite relative

DuduContrôle desanté

Forster 10 kHz (phase Q-phase 90Forster 100 kHz( " 0- » 90'

La mesure de diamètre est faite par quatre cap-teurs à 90° l'un de l'autre, le diamètre moyen est obtenu en faisant1g demi-somme de 2 diamètres perpendiculaires et l'ovalisation enfaisant la différence. La phase 0 du Forster correspond aux défautsmétallurgiques, la phase 90° aux défauts dimensionnels.

249

informations digitalisées : sorties binaires surimprimante nu perforatrice

métrologiemoyenne du diamètre ext. moyendiamètre extérieur maximaldiamètre extérieur minimal

résistiuité] résistivité relative moyennei rêsistivité maximale: rêsistivité minimale

contrôles desanté

sélection C.d.F (bon ou mauvais)numéro du tiroir de stockage

Le fonctionnement de l'installation est entièrementautomatique. La seule opération manuelle consiste à placer le tubesur le plan incliné et à vider les tiroirs du meuble de stockage.

2) Description du •fonctionnement

- Numérotation éventuelle- Approvisionnement~ Contrôle

ALLER

RETOUR

'Voie I

10 'kHz^ a 90°

100, kHz(f « 90°

Voie II

10 kHz9=0°

100 kHzIf = 0°

Voie III

500 kHz

rêsistivité

Voie IV

0 moyan

ovalisation

250

- Ejection du tube et stockage dans un tiroir dumeuble, suivant sélection automatique

— En même temps impression des résultats.

La cadence de contrôle est de 10D'tubes de,2 mpar heure.

2.2 - Banc _ ult x a. - s g n s (fig, 5}

La méthode de contrôle par ultra-sons est basée surla détection de la variation d'impédance acoustique provoquée dans -lemilieu par un déT-ut. Pour que.l'onde ultrasonore explore la totalityde la gaine, celle-ci est animée d'un mouvement hélicoïdal relatifpar rapport aux traducteurs. Ces traducteurs, au nombre de quatre,sont positionnés par paire pour s'affranchir de l'orientation dudéfaut. Un cinquième traducteur permet de mesurer en continu l'épais-seur de la gaine.

1) Description de l'installation

On distingue quatre parties principales dans lebanc de contrôle :

~ le banc de défilement- la cuve- l'électronique de contrôle~ le traitement des informations

- Banc de défilement (fig. 6}Le mouvement hélicoïdal est assuré par des sé-

ries de 3 galets moteurs inclinables (10 séries en amont de la cuvede contrôle, 10 en aval). Les gaines défilent à l'unité. Elles sontpositionnées avant contrôle sur un plan incliné et amenées automati-quement sur les galets par rotors crantés à impulsions, en amont dsla cuve de contrôle. Le galet supérieur (galet de maintien) est aJcrseffacé.

251

L1entraînement est assuré par galets caoutchoucou plastique selon la position sur la chaine, et CBS galets sontentraînés par friction par une soûle ligne d'arbre (tous les galetsporteurs sonfc moteurs). Le réglage de la vitesse ds rotation s'ob-tient par moteur à courant continu, et celui de la vitesse d1avancepar inclinaison simultanée de tous les galets au moyen d'un systèmede leviers et de secteurs crantés. La précision du pas est définieà mieux qu*1/lQe de mm, L'éjection automatique se fait, à partir d'unsignal de commande venant de l'électronique, dans des bacs diffé-rents selon cjue les gaines sont réputées bonnes ou mauvaises.

- Cuve de contrôle (1) (fig. T)

Le contrôle a lieu par immersion, il faut donc pré-voir une cuve permettant d'assurer le couplage acoustique entre lestraducteurs et la gaine. Nous utilisons une cuvs fuyarde à 3 compar-timents ; le compartiment central est le compartiment de contrôle,les compartiments latéraux, ceux d'évacuation et de récupération del'*eau de couplage. La figure 8 schématise le circuit d'eau réalisé.

Le bac à niveau constant en tampon entre le bacde rétention et la cuve de contrôle, permet d'encaisser.les pertur-bations introduites par la pompe et d'obtenir un écoulement laminairedu compartiment de contrôle dans les compartiments latéraux „" On peutainsi conserver tous les avantages d'un système de cuvs du type"étanche" (absence de turbulence dans la cuve) sans en récolter lesinconvénients (paliers tournants, liaison mécanique entre la cuve etle système d'entraînement et linison entre les tubes eux-mêmes}.

- Electronique de contrôle

La partie électronique de l'installation est com-merciale et se divise en deux parties, l'une pour la détection desdéfauts, l'autre pour la mesure en continu de l'épaisseur.

(1) Brevet EN 70.16042 du 30.04,1970252

Détection des défauts

Cette detection est assurée par quatre tiroirsémetteurs récepteurs dont la fréquence est variable en continu entre0,5 et 15 MHz destinés à l*attaque des traducteurs.

Quatre sélecteurs bi~canaux leurs sont associésautorisant d'une part un enregistrement en continu des quatre voieset d'autre part une sélection automatique pour tout écho dépassantun seuil préréglé,

L*automatisme de sélection est complète parl'adjonction de compteurs qui comptabilisent les impulsions reçuespar les quatre traducteurs à deux niveaux. Pour chacun de ces niveauxon présélectionne le nombre maximal d'impulsions admissibles avantd'enclencher le râlais commandant la sélection.

Mesure de l'épaisseur

Elle eat faite par la méthode de la résonanceultrasonore avec un appareillage "Vidigage Branson". Un tiroir desortie réalisé par le service d'Electronique du Centre de Cadaracheautorise une impression automatique des résultats sur imprimante ouperforatrice. Le tube hors tolérance est rejeté dans un casier specia.

- Traitement des informations (fig. 9)

, Informations analogiques î courbes d'enregistre-ment

métrologie | mesure épaisseur suivant pas{ hélicoïdalréponse des 2 traducteurs longitu-

contrôle de dinauxsanté réponse des 2 traducteurs trans-

versaux(1 courbe par traducteur)

253

Informations digitalisées î sorties binaires surimprimante ou perforatrice

épaisseur moyenneépaisseur maximale

,, , . épaisseur minimalemétraloqxe p , ,y excentrement moyenexeentî-Giisnt maximalexcentrement minimalnombre d'impulsions émises par les4 traducteurs de contrôle (par tube

Contrôle desanté

nombre d'impulsions reçues par les4 traducteurs dans deux seuils pré-réglés.| sélection (bon ou mauvais, hors cot

2) Description du fonctionnement

- Approvisionnement- Contrôle (santé et épaisseur) (enregistrements

éventuels)- Ejection du tube et stockage d'ans un des 3 casiers

(bon - mauvais - hors cote)- Impression des résultatsLa cadence du banc est de l'ordre de 40 tubes/h.

3 ~ TRAITEMENT DCS INFORMATIONS

Pour uns meilleure connaissance du contrôle et de la fabri-cation, seuls IES résultats statistiques nous intéressent, C'sst àdire qu'il n'est pas indispensable de personnaliser les tubes maissimplement les lots de livraison. A partir des sorties binaires, ileat possible de perforer directement des jeux de cartes nous donnantpar lot les variations des divers paramètres ; rîous pouvons suivre decette façon :

254

- l'évolution des taux de rebut C.d.F~ l'évolution des taux de rsbut US- l'histogramme du diamètre extérieur moyen- l'histogramme de 1*ovaiisation moyenne- l'histogramme de l'épaisseur moyenne- l'histogramme de l'excentremcnt moyen- l'histogramme de la résistivité relative moyenne.

Par contre la nécessité, pour les études dos propriétéssous irradiation des matériaux de gninage, de caractériser chaquegaina à l'aide de toutes les informations recueillies au cours descontrôles, nous oblige è personnaliser les tubes et à conserver cenuméro, IL s'agit donc de regrouper sur une même carte perforée tousles renseignements suivants :

- numéro <3s lot- numéro de tube- sélection C.d.F- 'sélection US- moyenne du diamètre extérieur moyen- ovalisation moyenne- diamètre maximal- diamètre minimal- épaisseur moyenne- excentrementsmoyen , maximal, minimal- épaisseur maximale- épaissaur minimale- résistivité relative moyenne- variation de résistivité

Le regroupement des informations en provenance de deuxinstallations différentes implique une numérotation des tubes auto-risant une lecture automatique sur chaque banc. Cette numérotationdoit être indélébile, mais ne doit pas enlever de métal (risqued'affaiblissement du tube).

255

{fig. 10}

Elle est obtenue sous forme d'anneaux par micro-sa-blage à l'aide de billes de verre de très faible diamètre (quelquesmicrons). Le marquage est effectué sous forme codée, 'le code étantun tout ou rien. Ces deux états logiques sont obtenus par variationdu coefficient de réflexion optique entre un anneau sablé et un an-neau non sablé.

- un anneau sablé donne 0- un anneau non sablé donne 1

Le rangement successif des éléments du code sousforme d'anneaux se fait au pas de 3 mm, l'épaisseur d'un anneau étantde 1 mm.

Deux anneaux supplémentaires situés à l'extrémitéde la numérotation autorisent une lecture à la voice en assurant unbon centrage du tube par rapport aux têtes de lecture.

Dons le cas d'un numéro à marquer de 6 chiffrera, par«xemple, c'est è dire 24 caractères binaires, l'installation se com-pose :

- d*une machine à sabler- d * uns enceinte de sablage- d'une buse de sablage à 26 fentes de projection

(24 buses pour les caractères et 2 buses pour lecentrage)

Le niveau Q, c'est a dire partie sablée, est obtenupar projection pendant 5 s environ de In poudre de vorrs sur le tubequi fait alors une rotation. ,

Le niveau 1, c'est à dire- partie non sablée, estobtenu par déviation dans la buse correspondante du jet de poudre deverre par dfâ l'air comprimé.

256

Le numéro en code binaire est ainsi marqué sous for-me 0 ou 1 par projection de poudre ou déviation de ce jet à partird'une logique-pneumatique suivant las informations données parolepupitre de commande.

3.2 - Lecture

Elle se fait par cellules photo-électriques. Unelentille collecte la lumière issue d'une source lumineuse et la di-rige sur un faisceau de fibres optiques. Ce faisceau est divisé, enreprenant l'exemple, précédent d'un- numéro do 6 chiffres, -en 26 fais-ceaux de 1 ram de diamètre, chacun de ces faisceaux élémentaire éclai-rant un anneau sablé ou non.

26 autres faisceaux de fibres optiques sont associésaux précédents et reçoivent le pinceau lumineux aprè-s-.une réflexionsur le tube. Ces faisceaux sont dirigés sur une photo-diode associéeà un ampli opérationnel dont le seuil de déclenchement est réglable.L'anneau non sablé entraîne le déclenchement du niveau logique.Bans le cas où l'anneau est sablé, la diffraction du faisceau esttelle que la lumière reçue par la photo-diode est insuffisante pourdéclencher le niveau logique,

La méthode est indélébile et non destructive puis-qu'il n'y a,en fait, qu'une simple modification de l*êtat de-surface.La rugosité induite est très faible (de l'ordre du micron) et nondécelable en métrologie ou par les moyens de contrôle non destructifles plus perfectionnés,

La machine de numérotation (1) a un cycle automati-que et peut traiter jusqu'à 75 tubes/heure.

(1) Prise de brevet en cours

257

lJ!l.!Lll ^ (fig.

A chaque banc de defilement sont associés une têtede lecture du numôro et un centralisateur de données qui regroupeet mémorise :

~ pour le banc Courants de Foucaultles valeurs de diamètre exté-rieur et valeurs de résistivi-l té.la sélection santé.

~ pour le banc ultra~sonsles valeurs d'épaisseurle nombre d'impulsions émiseset reçues.la sélection santé.

Ces centralisateurs à double mémoire attaquent paralternance une perforatrice de cartes. On perfore ensuite automati-quement une carte finale donnant,par tube, toutes les valeurs énu-mérées ci-dessus.

Ce système a l'avantage d'Stre extrêmement souplepuisque les deux installations (courants de Foucejult et ultra-sons)peuvent Être déconnectées ou utilisées séparément tout en gardant laperforation autorîB tique des résultats.

CONCLUSION

Cette chaîne de contrôle non destructif est en fonctionne-ment continu depuis juillet 1970 au Département de Développement desEléments Combustibles à Cadarache. Tout en gardant une grande souples-se de fonctionnement, son automatisation poussde et la possibilitéde perforation automatique des résultats sur cartes autorisent descadencée de contrôle très importantes sans diminution ,de la sensibi-lité dos appareillages ni du nombre et de la qualité des informationsrecueillies.

258

FIGURE 1

BANC DE CONTROLE PAR COURANTS DE FOUCAULI

TETE DE CONTROt £

,APPROViSIQNNilVtENT

DEFILEMENT

TIROIR Di SELECTION

259

FIGURE 2

VUE GÉNÉRALE260

FIGURE 3

Approvisionnement

APPROVISIONNEMENT des TUBES

et

T T E d e CONTRLE261

FIGURE A

BANC DE CONTROLE PAR LA METHODE DES COURANTS DE FOUCAULT

-BIOC DEDEIECIIDN

MESURE

OEfECÏOGRAPHEIOKHZ & IOOKHZ

DETECTEUR ACOURANTS OE

FOUCAULT500 KHZ

METROLOGIE

RESISTIVITE

TRAITEMENT DES INFORMATIONS

ANALYSEUR SEUILS

SEUILS

CENTRALISATION

LOGIQUE

ACQUISITION

NITRES

ENHEGISIREtlft.

IMPRIMANT

SEIECJION

SYNOPTIQUE262

FfcURE

BANC DE CONTROLE PAR ULTRA-SONS

-EJECTION ET SELECTION

TRADUCTEUR D'EPAISSEUR.TRADUCTEURS TRANSVERSAUX

APPROVISIONNEMENT

TRADUCTEURS LONOITUDINAUX

DEFILEMENT

263

FIGURE

INSTALLATION de CONTROLE par ULTRA-SONS264

FIGURE

CUVE de CONTRÔLE265

FIGURE

CUVE POUR CQNTflOlE AUTOMATIQUE PAR UlTRft SONS 0£ TUBES ET BARRES

266

FIGURE 9

BANC DE CONTROLE PAR ULTRA-SONS

MMIMIt

Détection

O'BPAISSEUH

1-1.

IH,

LrlNftll UDINAUX EmissionRéception

Mesura

Mesured'épaisseur

Seuilsdétection

ComptageImpulsions

SYNOPTIQUE

267

FIGURE 10

INSTALLATION de NUMEROTATION AUTOMATIQUE

TETE deNUMEROTATION

S F S G U R E 11

Bane

courants de Foucauff

Numéro | ResistSvite

?Diamètre extérieur Selection

ultra - sons

Impulsions

Epaisseuru

Selection

CENTRALISATEUR 1

î c a r t ecourants de Fsucauit

diamètre extérieurrésistivîte

CENTRALISATEUR 2

I car teu l t r a - sans

épaisseur

TRÂSTEMEMÏ

I ear te

courants de Foucawit

diamètre extérieur

resistivîté

ultra - sens

épaisseur

269

COffTROLE PAR ULTRASOÎTS DES TUBESPS FORGE 33U REACTEUR EL 4

A. Prot

FRANCE

A B S T R A C T

This paper reviews the various testings performed on the aircaloy 2 pressure tubesfor EL.4 Reactor.

Contact as well as immersion (local, total) ultrasonic testing wereconsidered. Wall thickness mesureraent's were made using the standard resonanceultrasonic testing.

Problems related to inhoroogeneitees of the ultrasonic beams and tosurface finish of the tubes were also taken int;o account.

Les tttbeB de force du réacteur Sï» 4 ont été réalisésen Zircaloy 2. Les dimensions de ces tubos étaient lessuivantes t

Longueur 4 ,?7Q mDiamètre î07 mmEpaisseur "5 mm

L 'é tude de ce contrôle est la première oui ait étéconfiée aux laboratoires de Saclay dnas le domaine du contrôlade tubes.

Plusieurs méthodes ont été envisagées soit par letubiste, soit vsr 1^ C « B . A .

271

en utilisant dea traducteurs d'angle,contrôle en ondes transversales des défauts longitudinaux.Compte tenu dos vitesses à envisager, les traducteurs étaientmontés sur un patin plastique alimenté en eau de couplage.

Plusieurs disposition? ont été étudiées s- un traducteur émetteur-récepteur- deux traducteurs érnetteurs-récentaurs situés dans

une itiêcie section droite, à une distance correspondant à unnombre entier de rebonds, et avoc une incidence synétrioue tdans ce cas* l'existence d'un ocno fixe sur l'écran del'apuareil, correspondant à la réception de l'émission del'un des traducteurs par son vis-à-vis, permet de s'assurerque le couplage est constant*

Pour obtenir des incidences correctes dans leZircatoy 2, oh la vitesse du son est relativement faible,il a fallu utiliser deg traducteurs d'angle équipés d'unesemelle en plastique spécial (trolitul), telle que lavitesse du son dans cette semelle soit compatible avec lesangles recherches dans le sircaloy ?»

Cetce méthode par contact s'est révélée difficileà mettre au point ; des éehos-parasates provenant del'écoulement de l'eau de couplage gênaient en permanence3e contrôle.

- M JJt ho e_ .!>ar i ,?n .p.r.ion.. to ta IBCette méthode ^itiliB^e our des tronçons de t\xbes s'est

révélée tout de suite très supérieure. En l'abf?ence detraductexirs focalisés, différents types de collimateurs ontét«* ufciliséfi, qui tous ont montré que le champ des traducteursplans étaient très

272

3 -Combinant les deux techniques (utilisation de

traducteurs d'angles standards, montés, sur une glissièreen plexiglass permettant d faire varier leur incidence, etd'une petite chambre d'immersion serai étanche), c'est laméthode qui a été retenue par le fabricant de tubes, l'avantagede cette méthode réside principalement dans la suppressiond'une très grande duve d'immersion, d'où facilité accruede manutention et gain de temps relativement important*

4 -Ces défauts réalisés par electro érosion à la surface

externe et à la surface interne d'un tronçon de même géométrieet de mènes caractéristinues métallurgioues, avaient lesdimensions suivantes j

rainures : L « 3 mm p - 0,1 mmtrous t 0-0, 5 nm p = 0,5 mmSeuls les défauts longitudinaux ont été recherchés.

5 - M e su r e des, é p ai s s eurs.Cette mesure initialement prévue avec des moyens

mécaniques, a finalement été réalisée par ultrasons à l'aided'un appareil "Vidif«feM fonctionnant swr le nrincipe dela résonance «

r»e contrôle de tubes de force en Zircaloy 2 pourle réacteur EL 4 s'est révélé relativement aisé.

L'influence de l'état de surface sur la détection depetits défauts a ét^ mine on évidence» Certains tubes ont dû£tre polis à la bande pour rendre possible le contrôle enimmersion locale»

273

IAEA PANEL OK NOT FOR REACTOR CORE COMPONENTS AND PRESSURE VESSELS,Vienna 29 November ~ 3 December 1971

Some aspects ronradiggrap1iic and ultrasonic inspection ofbutt_ weldsin s teel rear tar P££ssure vessels

S. Dahn, J. Osterberg; Tekniska lo'ntgencentralen AB, Stockholm

AB3ÏSACTSThe advantages and disadvantages of radiographie and ultrasonic inspectionof buttwelds -within the thickness range of 150-200 mm. are discussed.'She opinion of TRC is that using both methods to 100 % will result in thebest inspection but under certain conditions only ultrasonic examinationcould be accepted.At radiographie inspection the visibility of volumetric defects is veryfood if an adequate technique is used,The visibility of tvo-dimensional defects is shown in diagrams. Stronglyeoiapressed two-diaansicnal defects e.g. a crack where the one facet sur-face together with the other facet surface forms a labyrinth could be un-deteetable.At ultrasonic inspection soiae types of volumetric defects are bad reflectors,The detectability regarding two-dimensional defects is theoretically verygood but an adequate technique must be applied and certain conditions mustbe considered.An ultrasonic inspection specificati xx recently worked out in Sweden forultrasonic inspection of weIda in reactor pressure vessels is briefly, pre-sented, fiecoauaended reference blocks with flat bottom holes for sensiti-vity setting are shovn in sketches. The specification presupposes thatthe character of the defects can be aetermined. The acceptance standardis partly based on fracture mechanic calculations. Permitted length ofelongated volumetric defects such as slag lines is different for differenttypes of weltts and also depending on the depth from the surface to the de-fect.

The general advantages and disadvantages of radiographie and ultrasonicinspection of welds within the thickness range of 150-300 mm have beendiscussed in several papers and are probably well known. It is a factthat the opinion is split about the reliability of the two methods.

275

The 1971 issue of the ASME Code» Section III, requires only radiographieexamination of welds while in Europe tnore coafi-íence is put in the ultra-sonic inspection as required coapletnentary method or the only inspectionmethod.Based on experiences from inspection work during manufacturing of fourreactor pressure vessels, the Swedish inspection and testing organizationTelcniska Rontgencentralen AB, (TRC) consider the best inspection is touse both methods as they are supplementing each other. Swedish codesrequire radiographie inspection but exemptions have in soree cases beengiven by Swedish Authorities to use only ultrasonic inspection. It isalso the opinion of TRC that under certain circumstances radiographieexamination could be partly or completely deleted but that the ultrasonicinspection must be required,TRC believes that all weld defects and specially the more severe twodimensionaltypes of flaws as cracks„ lack of fusion and incomplete penetration can be'detected with certainty if an adequate technique is applied and the operatoris sufficiently experienced.When the advantages and disadvantages of radiographie and ultrasonic inspectionare closer evaluateds the most interesting question will probably be the abilityof the two methods to detect twodimensional defects. Several cases are reportedwhere such defects have been detected by ultrasonics but not by radiography,but¡ there are also cases xjhere the reversed condition has been observed andthis paper will mainly deal with the ability of the two methods to detecttwodimensional defects.

Radiographie inspectionIn the following the visibility of volumetric defects and narrow slots isdealt with. For these types of defects it has been possible to make theore-tical calculations and to verify them with experiments which have been madearound the world. The location of the defects in the object is chosen insuch a way that they are situated near the surface which is facing the sourceof radiation, as the poorest visibility of the defects is obtained here.In industrial radiography there are about a dozen variables which effect thesensibility of the examination. In this study the variables are chosen insuch a way that they conform with the radiographie technique which is describedin Swedish Standard (SÏS 11 41 01).

276

X'olumetric defectsBy using the law of radiation absorbtion and the gradient of films an expressioncan be put up which shows the change in film density that appears when thereis an air-filled cylindrical cavity in the object. (1)In fig. 1 both the theoretical and experimental values (2) of the leastvisible cavity diameter are plotted ^s a function of material thickness.As -can be seen, the visibility is very good and can from stress-considerationbe. regarded as satisfactory.Twodiíaensi o nal defectsThis kind of defects can have a depth-to-width ratio that exceeds 100:1 andtherefore considerably differs from the previous volumetric defects.It is possible to define a crack by stating its width, depth and direction inrelation to the X-ray beam. A natural crack has, of course, not a constantwidth but varies from zero up to a certain value. However, when making calcula"tions and experiments, the crack is generally allowed to be represented by aslot with constant width (fig. 2),The least detectable defect of this type can be divided into two groups:a) defects parallel to the radiation beam (fig. 2)b) defects that form an angle with the radiation beam (fig. 3).Narrow slots parallel to the radiation beamIn the same way as for cylindrical defects an expression (1), (3) can be putup, which shows the change in film density caused by the slot. It isinteresting that the visibility is direct proportional to the cross-section,area of the slot ( bxd in fig. 2).If in the same way as before a standarized examination technique is used, itis possible to calculate the least detectable cross-section area. This hasbeen done and is plotted in fig, 4 as a function of material thickness (5).The figure shows that at 25 mm material thickness the cross-section area hasto be 0,01 mm (e.g. if the width is 0,05 mm, the depth has to be 0,2 mm), at150 mm material thickness the cross-section has to be 0,11 mm and at 300 mm atleast 0,25mm. Except calculated points there are in the figure plotted someexperimental results (6).Narrow slots that form an angle with the radiation beamIt is possible to expand the previous calculation (1) to be valid even for thiskind of defects, if it is noticed that the change in amount of radiation,

277

caused by the defect» is not depending on the angle, but only the width ofthe image will be changed.In figure 5 the experimental curve (lj 7) is drawn, which shows the leastdetectable slot width as a function of slot angle.Of special interest for this paper are angles between 3-5 , as they are themost common joint angle in thick-walled butt welds. Fig. 5 shows that thevisibility of slots is not noticeable detoriated, until the slot angle exceeds5°.Some studies have also been made on natural cracks (7), Here it is noticedthat though the crack was on the bolder of visibility at 0° angle, it did notdisappear until the angle had increased to 8 .Aspects on \'isibilitya) Defects have been dealt with as twoditnensional (width and depth) but ofa/course they have also'certain length. For those defects, dealt with here

which are on the verge of visibility, they ought to have a length of about10 wra to be detected. The reason for this is that at a normal distancefor filia-viewing the eye can at one fixation receive an iraage length,which is about 10 urn. This means that the probability of detection isproportional to the defect length up to 10 son.

b) A natural crack does not propagate down through the material at a constantangle in the same way as a slot. This varying angle gives rise to thefollowing two effects due to partly the macro angle and partly the microangles depending on the facet surfaces of the crack:1. It increases the possibility that some parts of the crack

lie in such a direction that it will be detected.2. It decreases the possibility that the entire length is

detectable. The last point is not a critical one, as anobject containing a crack will be regarded as not acceptable.

c) The type of crack, where due to shrinking stress the facet surfaces arecompressed» might not be detected as the width approaches zero.

Volumetric flawsSpherical pores are very small ultrasonic reflectors, which means that the echo

278

amplitude can be below the stated value for recording. For welds in heavythicknesses, porosity is however seldom a problem and pores are not harmfulwith regard to the strength. The above mentioned can also be applied for smallslag inclusions.Slag lines on the contrary is a common type of weld defect. Side wall slaglinescan usually be indicated with certainty while slaglines between weld beads oftenare unfavourable reflectors. For elongated slag inclusions in line with shortinterruptions, the possibilities are limited to ultrasonically decide if thedefect is continuous or riot. The possibilities to determine short interruptionsdecreases with increased beam path length, If the interruption is shorter than5-15 ran» the defect will probably be interpreted as being continuous.In ultrasonic testing specifications, the limit for reportable flaws is oftenstated in sxich a way that recording of very small defects is not required.For volumetric flaxes it should in such cases be stated that volumetric flaxes belowthe reportable level shall be considered if the flaws appear systematically.Twodimensional flawsWith angle probes for generally used frequency 2 MHz, twodimensional gas filled

—6flaws with a gap width of 10 mm will reflect about 50% of the sound energy.If the flaw is filled with oxide or a combination of oxide and gas, the gapwidth must be somewhat larger. Theoretically thus the detectability by ultra-sonic is very good. Experience has, however» proved that twodimensional flawsin some cases have not been detected probably depending on some of the followingreasons.The testing technique might have been unsatisfactory or the geometricalconditions have not allowed application of an adequate technique. Anotherreason can be that the flaws even after one stress relief operation canhave been so strongly compressed that the flaw has been transparent for theultrasonic beam. Regard to this should be paid by performing the ultrasonictesting also after final relief and/or after hydrotest.For a twodimensional flaw the deviation between the direction of the ultra-sonic beam and the normal to the surface of the flaw must be as small aspossible. In this respect, a crack is usually a good reflector due to itsfacet surface. On the other hand, flaws having a smooth surface can act as amirror which reflects away the main part of the ultrasonic beam in such away that no or only a very low echo is obtained. .Regard should be paid tothis by stating most suitable and sufficient numbers of scanning directions.

279

In welds where the groove angle is small, flaws orientated perpendicularto the surface can occur. The safest way to detect such flaws is to usetandem technique,

Inspection technique;With the comprehensive erection programs-ï for nuclear power plants in Swedenas background much work is now laid down to NOT questions regarding nuclearcomponents. Especially the ultrasonic inspection is of great interest anda number of procedure specifications have been worked out for ultrasonic exami-nation of both base material of various kinds and welded joints.In connection to the questions dealt with in this paper, the specificationfor ultrasonic examination of butt welds in a reactor pressure vessel is theraost interesting part of the specifications.The provisions for this specification were:a) In principle all types of weld defects should be detectable but

especially the twodiraensional flaws should be detectable in sucha way that the real defect depth could be determined.

b) The testing sensitivity should be based on the reflection from a4 ram dia flat bottom hole.

c) The character of the flaws should be determined.d) The acceptance standard for permitted length of volumetric

elongated flaws should be different for different types ofwelds.

According to the specification, testings shall be performed at three differentoccasions;1) After the first stress reflief the manufacturer's operators shall

perform a complete testing with 11 scanning directions as well astesting with tandem technique witnessed by the Inspection Agency,

2) After final stress relief the Inspection Agency shall perform asecond examination with 11 scanning directions. Testing withtandem technique need not to be repeated.'

3) After hydro test certain selected welds shall finally be examined•with a reduced number of scanning directions,

One of the reasons for the requirement on testing after final stress reliefand hydro test is to ensure detection of possible compressed defects. Further-

280

more the testing shall as far as possible guarantee that no unacceptable defectswill be detected at a later performed preservice mapping with special testingequipments. It is also an advantage that testings are performed both by themanufacturer and by the Inspection Agency.

\

In the specification certain requirements are stated on the qualification ofthe operators as well as requirements on the ultrasonic apparatus and the probesto be used. The surface finish of the scanning areas is stated and the weldreinforcement shall be such that no disturbing echos occur or the reinforce-ment shall be ground flat with the parent material. Before testing with angleprobes the base material besides the weld shall be examined with regard tolaminar flaws. Testing through cladded surface is accepted under certainconditions except for welds within the core area.For calibration of the ultrasonic apparatus and check of probes the IIW calibrationblock shall be used. For setting of testing sensitivity three different referenceblocks with flat bottom holes shall be used. The reference blocks shall be madeof same or similar material as the vessel and have a thickness approximatelyequal to the thickest part of the vessel wall. The surface finish of the blocksshall be equivalent to that of the areas adjacent to the welds in the vessel.Figure 6 shows in principle such a reference block which has a number of 4 mmdia flat bottom holes which fit to the different probes to be used.Figure 7 shows a similar block having one surface cladded for sensitivitysetting at testing through clad surface.With these blocks it is not necessary to use AVG-diagrams for determinationof flaw sizes.Figure 8 shows the reference block which is to be used at testing by tandemtechnique. The flat bottom holes are here as big as dia 10 mm, while thetandem testing is intended to be a supplementary examination with regard topossible larger defects undetected by the single probe testing.At single probe testing the sensitivity shall for each probe type be adjustedto 100% echo amplitude from a 4 v~* dia flat bottom hole located at a depthequal to the thickness of the weld to be tested. To this value it shall beadded min 6 dB for compensation of variations in sound transmission duringscanning. When a flaw is found, the sensitivity shall be adjusted so thatthe indication from the 4 mm dia flat bottom hole in the reference block at anequivalent depth to that of the flaw is 100% screen height, (Référence level)At this sensitivity the flaw shall be evaluated.

281

Figure 9 shows the scanning directions to be used for longitudinal and cir-cumferential joints at single probe testing.Figure 10 shows the scanning directions for nogzle-to-shell welds.For defects over a stated size, the character of the flaw shall be determined.Regarding the possibility to do this with sufficient accuracy the opinions aresplit, Determination of flaw character is with no doubt the most difficultpart of the examination and only a very experienced operator can do this.The operator must be at least able to separate twodimensional defects fromvolumetric defects as the acceptance standard does not permit twodimensionalflaws over that size, where the character can be determined. The results froma great number of excavations of defects have proved that an experienced opera-tor can achieve an almost surprising accuracy.Some uncertainty, however, has to be taken into account and especially combinedflaws, e.g. side wall slagline in combination with lack of fusion, are hard tointerprete.At tandem testing two 45 angle probes mounted in a fixture shall be movedalong the weld for detection of longitudinal flaws. The cross section of theweld shall be divided into areas within which the sensitivity taust not havegreater deviation than 6 dB and each such area shall be tested. Areas adjacentto the root and face side of the weld need not to be tested»Acceptance standardIn the acceptance standard there is stated that lack of penetration, lack offusion and cracks with such size that determination of flaws character ispossible, are not permitted.Regarding elongated volumetric flaws such as slag lines, the maximum flawsize is corresponding to 100% echo amplitude at reference level and a lengthof 25, 50 respectively 100 mm. The lengths permitted depend on the types ofwelded joints where the flaws occur, for example longitudinal or circumferen-tial weld, radiated or irradiated weld and the depth to the flaw from thesurface. In general no flaws with a ïër.gth over 25 nan are permitted withinthe area froai inside surface and to 20 mm depth.For volumetric elongated flaws adjacent to each other a minimum distanceseparating them is given for flaws in line and for flaws at different depths.If the distance is less, the flaws shall be considered as one flaw with alength equal to the sum of the flaws.

282

As a general quality requirement the maximum sum of the lengths of defectswithin 1 m weld is stated.All flaws giving an echo amplitude over 25% at the reference level shall bereported. Other testing data to be reported are also stated. After completion,of the vessel a drawing over the vessel shall be submitted which shows thestart points and the "recording direction" for the various ultrasonic reports.This drawing shall make it possible to easily identify not repaired flaws atinservice inspection.

Discussion regarding acceptance standard for slag linesIn reactor pressure vessels, delivered or under manufacturing for use inSweden the requirement regarding slag lines has been either max. 25 mm lengthor according to the ASME Code max. 3/4", It is questionable if the strengtho£ a weld will be increased by repair of for example a 35 ram long slaglinelocated at a depth of 75-100 mm, If a 25 turn long slag line is acceptable atan unfavourable location, for example close to the inner surface in a radiatedlongitudinal weld joint, it should with regard to the stresses and the materialproperties be possible to accept longer flaws in for example an unradiatedcircumferential weld. The above mentioned was the reason for an acceptancestandard in the specification which in some types of welds permits 50 or 100 mmlong slag lines.

At ultrasonic examination the length of a flaw can be detected with sufficientaccuracy but the determination of flaw character is somewhat unsafe. A 25 mmlong flaw misinterpreted as a slag line cannot be expected to have a flaw heightof more than a few mm, even if it in reality is a crack, A 100 ram long flawmisinterpreted as a slag line can be a crack with a considerable height. Forthis reason also the tandem technique testing was required and a fracturemechanic calculation has been performed, where the flaws have been consideredas cracks having a length of 25, 50 or 100 mm and a certain height. Thefracture mechanic calculations showed that even if cracks with these lengthsshould be misinterpreted as slag lines, there is a "safety factor" of about8 to the critical crack size even if a conservative calculation is applied.When also radiographie inspection of the welds is performed the same acceptancestandard for approval of the radiograph must be valid as for the ultrasonicexamination. As it is not possible to determine at which depth a flaw on theradiograph is situated, the radiograph interpreter in some cases must have

the ultrasonic testing report to be able to interprete the radiographs.

283

The specification for ultrasonic examination of welds in a reactor pressurevessel which has been presented here is a preliminary issue. It has beendistributed to authorities, inspection organizations and other people inSweden which are involved in questions concerning NDT of reactor pressurevessels. The intention is to revise the specification» when the involvedparties have given their coBvments, so it can be accepted and used by theparties.

284

Diameter, cf cavity (mm)

X - Experimental points0 = Calculated points

Material thickness

BUB

20 40 60 80 100 120 140 160 180 200 220 240 260 280 300Fig, 1 Least detectable diameter of cavity as a function of material

thickness.

Slot

TIT....

Source

.1d

-Mb if-

tm ...«•«,..„«. H.H.II i. .,i.,» .«•••• i •••x^»in. • .n.iu....-*,....,...^, •

Fig. 2 Slotformed defectfilm

Source

Slot

film

Fig. 3 Slot that forms an anglewith the radiation beaia

Cross-section area(mm2)

0,20

0,10

0,05

0,02

'0,002

X « Calculated points0 « Experimental points - (slot)o <= Experimental point - (crack)

Material thickness

nun-t———tor

20 40 60 80 100 120 140 160 180 200 220 240 260 280 300Fig, 4 Least detectable cross-section area of slot as a function

of material thickness.

0,05 0,15

Fig. 5 Least detectable slot width asa function of angle between slotand radiation beam (experimental).

286

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Literature References

1. C.G. Pollitt, ÏUL, Durant A.R.D.E. Memo (MX) 19/61C,G. Pollitt Brit. J.N.D.T, 4 (3) 1962 71-81

2, R. BertholdAtlas der ZerstSrungsfreien Prufverfahren (Barth, Leipzig, 1938)Anon. Brit. Weld, J, 5.244, Í958

3. R, HalmshawPhysics of ind. radiology (He rood, 1966)

4. R. HalmshawReport fr. IIW, Comm VA (Doc. VA-71-65/OE)

5, R. Halrcshaw, Brit. Weld. J. 6, 456-461, 1959W.H. Laws, Weld & Metal Fabr. 28, 357-361, 1960R. R*th, IIW Corom. VA, Doc. VA~55~64/OFA, Cottell, Brit, Eng. & Boiler Ins. Co. Report 4, 71-87» 1962

6. G«L. BeckerLecture at ASNT, 1969 National Fall Conference, Oct. 14, 1969

?. J.W. Dutli and G. TenneyNon-destr. Test, 12 (2) 13, 1959

NONDESTRUCTIVE TESTINGOP COATED PARTICLE FUELS

J. HOLLÏDAYM. S. T. PRICE

U. K.OECD HIGH TEMPERATURE EBACTOE PROJECT (DHAGOl)

INTRODUCTORY FOTSThe European Nuclear Energy Agency (MEA) of the Organisation

for Economic Co-operation and Development was invited by the Inter~national Atomic Inergy Agency to present a co-ordinated paper onthe Non-Destructive Testing of Coated Particle Fuel at the IAEA.Panel on íkm-JDestructive Testing of Reactor Core Components andPressure Vessels, This invitation was passed to the OBOB High.Temperature Reactor Project (Dragon) one of the joint undertakingssponsored by MEA.MPRODTJCTIOH

Progress in Materials Technology is critically dependent upona small number of factors and ranking high is the availability ofadequate Materials Testing methods, The advance of coated particlefuel within the last decade from an interesting idea to productionon the tonne scale has been made possible? inter aiia? by theparallel development of suitable testing méthode.

The basis of the coated particle concept is the sub-divisionof the m;clear fuel into smell spheroidal particles each coated withits own primary containment.

In the High Temperature Gas-Cooled Reactor (HTB) the fuel isin the form of oxide or carbide microsvlaere? 100 - 800 u m in -diametercoated with layers of pyrolytio carbon or pyrolytic carbon andsilicon carbide and dispersed in a graphite matrix. The graphitematrix fuel body can be loaded into the reactor in a graphitestructure such as machined hexagonal graphite blocks 11> 2, 3y 4 3»Alternatively in the AVE [5] and THTE concept [6] the fuel elementis the 60 ma diameter graphite matrix fuel body which forms apebble bed reactor core»

Whilst all th'e UPE designa employ coated particle fuel, it isonly one of several possibla fuels for the Gas Cooled Fast Reactor(GOER) [7], fhe difference between coated paa^ticle fuel for theGCFR and the BTR lies in the requirement, for the fast reactor, to

293

have a low .moderating carbon, content and a nigh fuel density» Onedesign of partióle envisaged for the GCPR [8] therefore has thefollowing parameters s-

Fuel Kernel 20 - 30$ Pu/ü oxideKernel density 85$ theoretical0/M ratio ~1 .95Kernel liiameter 850 JÍÍBCoating ThicknessInner pyrocarbon layer 45 Sealing pyrocarbon layer 5 {¿mSilicon carbide outer layer 100 ¡am ..

Heavy metal density in fuel space 1 «9 g/cmAlthough work on GGPR coated particle fuel is proceeding in

Austria, Belgium, the Federal Republic of Germany? the Netherlands,Sweden and the United Kingdom } the effort involved is far lesa thanis committed to HTfi fuels» The testing techniques are, however, sosimilar that, although this paper concentrates on HTR fuel, theremarks are likely to be equally applicable to GGFR particulatefuels*

Work on fffR fuels is being carried out in at least the followingcountries*—Austria, Belgium, Canada, Denmark, Federal Republic of Germany, Srance,Italy, Japan, the ííether lands , Sweden, United Kingdom, United Statesof America-

In RTR fuels, the prime object of testing must be to giveconfidence on thermal and nuclear performance whilst at the aarnetime giving assurance on the release of fission products. In a600 MW(e) HTH the population of coated particles, each of which canbe considered as a "canned element" is ~10' » With such a largepopulation a small but finite probability of a "defective" particleexists. This means that in normal operation a minute but predictablerelease will occur and is in contract with the customary situationmth other reactor types where* an &fcaential3.y aero fission productrelease is unpredictably interrupted by large releases from failedcans»

2, li Ap O g.jPTO S , .SKBLBOyiOK OP TBST8Manufacturing processes of coated particle fuels are described

in detail elsewhere (9), The block diagram (Figure 1) illustratesa typical sequence of operations, commencing at the raw materialstage and ending with a completed fuel element. îïon<-deetruotivetests do not feature much at the raw material stage but are soon inevidence as the manufacturing processes proceed*201 SiBv.jf led a a j i s . Q u n s t a s d t arameters

For, the purpose of thin paper and to establish somecontinuity from raw material to finished product, most of thedescriptive data, unless otherwise stated, is based on DragonI*roject Quality Control Group testing techniques and procedures,

294

Assuming that low enriched coated particle fuels are beingmanufactured for a commercial High Temperature Reactor asimplified form of the production route would, therefore, "beas follows with the key works underlined«-

Sphere forming

Reduction to UO,

sintering - KjRgglgICoating - Porous Layer/Seal

Inner High DensityIsotropic PyrocarbonSilicon Carbide LayerOuter High DensityIsotropic Layer

COATED PARTICLESiOvercoating with resinated

matrix powder4-Consolidation into COMPACTS

The above layout is divided into 4 separate stages of manufactureand to give some idea concerning the Values of the parameters tobe measured in terms of pre—irradiation testing of the fuel sometypical values have been chosen and applied to each stage toillustrate the nature of the tests.

JüSjLJfek ír Jl are purchased according to the currentpurchasing specification which for fissile uranium-containingmaterial governs powder properties, impurities and enrichmentin U-235.

Prom this raw material Sinterjd JCernels. should be as follows:Enrichment 3-8$ U-235

-- Porosity 2Q%Kernel Diameter 800}j.m

The Coated. JParticlGS should be coated with pyrocarbon andsilicon carbide layers as follows s~Porous layerSealing layerInner HDI layerSilicon Carbide layerOuter HDI layor (?yC)

pyrolyticcarbon

35 MW

30 ¡.tin

35 fjn

35 &

55 tin

Total coating layerThickness 190 jamTotal Coated particleDiameter 1,180 j.«n

295

2.2

The fuelled Compacts are expected to have a heavy metalloading of 0.8 gll/omi"'" of~"fu e 1 space and the average density ofthe graphite .matrix between particles should be~1»7 g/cmP^

The number of broken particles per compact in terms offraction failed at the start of irradiation should be notgreater than 1 x

The definition used here of a . . .one in which fuel so tested, provided that it passes the testmay be loaded into the reactor for irradiation. Any otherteat, which even if it does not actually destroy the fuel maycontaminate or put its performance at risk and thus will not"b© loaded into the reactor s is described as a destructive test,The principle features of the fuel described in each stage inpara» 2>1 above? which must be measured are indicated belowand the non-destructive tacts aro underlined, The verificationof raw materials must include

Chemical analysis to determine impurity levels.Isotropic abundance and oxygen to uranium ratio (for U02powder).Particle size and

All of these tests are normally expected to be performed by theraw materials supplier»Verification of kernels must include

Check on oxygen/uranium ratio and certain impurities*Measurement of porosity» This can also bo evaluatedby non-destructive method but with less accuracy»

of dla;netor .Verification of coated particles must include

Measurement jajTjuidiyjüranium/U~2.3 5 content measurement.Study of structure (ceraraography) «

Measurement of silicon carbide layer density.Measurement of anisotropy of the structural pyrocarboixlayers»Examination of coated particles to check the coatingintegrity presence of flaws, diffusion of kernel intocpatin'gSj etc»

296

Measur cinen_t^ cf i:L9ontaming frión of t outer .Verification of compacts includes

jjfoecj pja fjígIfeasurement. _Qftubeavg;;iin;e'i;al loadings,,Determination of "free" uranium or failed partióle fraction in. the compacts - At the moment this is a destructive test,but it io hoped that the non-destructive method referred toin a later section of this report will be in use in the nearfuture .

At all stages other tests of a non-destructive nature occur such asshape -eparation? and. sizing. These are regarded as process

- controls j and the validity of such process controls is verifiedfrom time to time by quality controls which could be eitherdestructive or non-destructive»

2«3 Qh.oice.._.pf. ..tea .m.gffiThe selection of test methods for the characterisation of

coated particle fuel cannot be discussed without reference tosampling techniques.

dealing with the testing of coated particle fuels itis important to bear in mind that usually very small samples areselected to represent very large populations and thus it isessential that the basis of the selection shall be statisticallyorientated [ 10, 11 7 12],

Even when the sample is statistically satisfactory, it isincumbent on the Quality Control organisation to ensure thatthe tests .selected are the best possible, give the maximuminformation, are economic, and if possible, non-destructive andrapid. In the Dragon Project and elsewhere considerable thoughthas been given to tho choice of testing methods with regard totheir automation wherever this is economic and practical* Aboveall a tost must provide useful information and contribute towardsQuality Assurance thus having the dual role of assisting productionto maintain high quality and acting as a safeguard against inferiorfuel being loaded into the Reactor.

HOH-BESTRUCTIVE TESTING METHODS . APPLIED J?0 COATEE PARTICLE FUEL3«1 Ra,w.L Mater iale (Enriched Uranium-Containing- Powders, Kex-nel Binders,

Eesinated Graphite Matrix Powders)Bormally verification of raw materials manufactured to meet the

requirements of a given purchasing specification is the responsibilityof the supplier, but as the raw materials are expensive and some ofthe imporatant parameters such as (J-235 enrichment, particle size,specific surface area? [13? 14j can be measured on a small samplerepresenting a large consignment it is worth while verifying by acheck test.

29?

Graphite matrix powders are normally obtained in theresinated state from supplying manufacturers. It is assumedthat tests such as the particle siae distribution, truedensity and impurity element content have been determined onthe unresinated powder, that similar proof tests (includingthermal neutron capture croi>s~section) have been executed onsample unfuelled "bodies and that these properties are withinspecification»

3.2 Kernels3.2*1

Quality Control of manufactirred sintered kernelscannot he performed on full sise batches and it istherefore necessary to estimate the measured parametersfrom a sample taken from the batch, by raeaas of standardstatistical relationships, the "Confidence IntervalLimit" in which the true value of the batch lies, isthen calculated»

The sample under test must be representative ofthe batch.

3.2.2 j5agffiliK ._Eguifimegi (Spinning Riff 1er)The sampling equipment used is illustrated in Fig. 2.

A standard funnel, into which the batch to be sampled ispoured, feeds a hole drilled in the centre of a smellrotating disc. The off— centre output of the hole ispresented over a small collector at each turn whenrotating the disc. Tho sise of the collector is calculatedto take out -¿- of tha hatch. The remaining 7/8 oí' thehatch are collected into cons held in position by loadedsprings, so that the system is completely enclosed andspillage of particles is prevented (see Pig. 3)»

3.2.3This information is essentially useful as a process

control» Minor variations in diameter between batchesare self-compensating in that the ratio

kernel diameterremains approximately constant with a constant mass inputto the coating stage* There are two non-destructivemethods available which can he applied to kernels* Thediameter can also "be measured after coating by X-raymicroradiography as referred to later.

This method developed "by Belgonuoleaire SA andLefevre (Dragon Project) is illustrated in Pig. 4» Itis hased on measuring the integrated length of 200

298

pax-tides carefully aligned in to a single row by meansof a "V" .shaped slot» The 200 particles are selectedusing G plate counter' drilled with 200 holes which actas cups to retain the particles poured on to the plate.

The accuracy in measuring loads to an absoluteerror of ¿ 1 ¡am. A correction factor must be applieddepending on the shape ana size homogeneity of thekernels. In fact this nicthod is "biased giving asmaller -value than the true nuan due to the non-alignment of the centres as is illustrated it) Fig. 5*

A correction factor must therefore "be applied anaexamples are given "below s-

Shape and SiseHomogeneity

Correction Factor

Very Good

0

Good

«.*

Poor

+7.*

[16|At OSGAE Seibersdorf a sophisticated particle

diameter measurement technique suitable for kernels orcoated fxiel particles has been developed by Dr. P« Kossand his colleagues [16] * This method can be used forprocess or quality control.

The apparatus at Seiborsdorf consists of an opticalsystem through which the particles at the kernel stageare dispersed, falling through a colliraated beam oflight which is obscured as the particle passes» Theinterruption to the light beam is transferred fron anoptical effect by the photo-diode to an electronic pulse,This pulse has a form, or shape, which is detected by apulse form or, which is colectivo in that, if two particlespassing are touching, a non-spherical pulse forn is seen?and is not transmitted by the pulse former. Thereforeonly single particles produce the forra accepted by thepulse former which transuite this pulse to a multi channelanalyser which performs analogue to digital conversion,data storage, and display»

This data in turn 3 s passed to a plotter and/orprint out system. The measurement made by the apparatusis the diameter of the shape passing through the slit.Hence, if samples are extracted during each coatingstage, the difference in diameters measured provides thethickness of each layer deposited» The maximum rate oftesting is about 500 particles per second. To achievereliable statistics, a sariple should consist of at least

299

6000 kernels (or particles). This method can check kernelTotal coating thickness or coated partióle diameters, and,'by subtract i on, total >x:.;< -1 d; a <et< :-, FTOÜÍ data obtainedfrom this test,distribution curvee can he plotted and muchvaluable information obtained concerning variations withinkernel or coated particle "batches as well as batch to batchvariation.

Coated FueltParticles3.3.1 galling., anji ^

The sampling procedure for coated particles by spinnerriffler technique is exactly the same of previously describedfor sintered kernels in Section 3«2.2»

The very large number of items manufactured leads tothe adoption of a sampling scheme. Quality Control measure-ments are therefore performed on samples of batches underteat.

Apart from measurement errors a spread of results willbe found when measuring différant samples taken from thesame batch, inherent in the statistical nature of randomsampling. It is therefore necessary to evaluate the truevalue of the batch from the estimâtes measured on thesample, using standard statistical techniques for determiningConfidence Interval limits [10, 11, 12],

It will be appreciated that some of the parameters ofthe coating, such as coat thickness and density, are variableswhilst other, such as broken particles, inclusions areattributes since they ^an only assume discrete values withinthe batch,

3 • 3 » 2 Metrqlpgy of Coated Particíes by Radiographie Techniques3»3.2.1 Bad o apJbj- Tojghn? gups

X-ray projection microscopy has been shown toprovide a means for psrfoming routine metrology ofcoated particles which has distinct advantages overceramographic methods» These advantages are*

(i) The method is «.on-destructive, samplesdo not need mounting or sectioning.

(ii) fhe proceas may be rapid, e.g. 40 rainfrom receipt of sample to initialpublication of results.

(iii) The measurements may be more accuratelyrelated to the equatorial plane of theparticle than is possible with ceramography»

In addition to raetrology of particles andcoatings, radiography using projection X-ray microscopes

has enabled rapid inspection of coated particles300

to assess a variety of ether characteristics«These include particle shape, contamination ofparticle surfaces toy kernel materials, diffusionof kernel material into the coatings, the presenceof múltiple kernels and.» with, certain reservations,the presence of cracks and other defects in thecoatings.

3.3»2.2 Pesoription of ApparatusTwo PXM instruments are used for this workj

both operate on the same principle [17] •An electron beam produced "by a directly heated

filament is accelerated "by applying a negativepotential between the electron gun and the columnof the microscope, which is at earth potential.The beam current is controlled by applying a variableMas voltage te a ¥ehnelt control grid (bias cup) »A pair of electron lenses form a reduced image ofthe point cathode onto a foil anode (Fig» 6), Thetarget thus formed is of the transmission type andmay be changed to give radiation of differentcharacteristic wavelengths provided the necessaryaccelerating potential is within the range availablefron the equipments1 power supply. Although theoverall dianetsr of the X-ray source produced maybe as little as 0.1 p.m under opcinrom conditions,the need for speed and reliability of themicroscope during constant use as process andquality control instruments, reduce the resolution.to less than 2 (jira.

The X-ray bube is continuously pumped to ~provide n.working pressure of less than 5 3t ÎQtorr.

Soth instruments are assembled, vertically»The earlier instruisent ? the XM30 (Pig. ?)» utiliseson electron beam pointing vertically upwards andfocused onto a target which also forms the vacuum,window. The source-sample-plate geometry isdetermined by a series of spacers assembled on theupper surface of the objective lens.

The later instrument, tho EM?5 (Fig. 8) [18],uses the components from a small electron microscope,the beam points vertically downwards, the targetsare remotely interchangeable and the sample andphotographic plate assembly is in a vacuum chamber.

3.3.2.3 MethodFollowing earlier experiments 09 3? aluminium

targets have been used for most work on coatedparticles. For multiple coatings, including anintermediate layer of silicon carbide, an

301

accelerating potential of 20 fcV is used»Although for maximum resolution targets of1 inioron t-Mokness and less woxild be required,thicker targets aro used as they are raoreresistant to accidental "burn-out. They alsoprovide a certain degree of tnonochrornatisationas they are thick compared with penetration ofthe electron beam at the potentials used, Furtherimprovement in image contrast la gained byselecting filter materials and thicknesses; Thefilters are usually incorporated in the sampleholders *

The magnification obtained from the promotion.usually approximately 23 is selected to suit themeasuring technique while minimising the effect ofgrain in the recording plates» An exposure timeof 30 a to 3 minutes according to the sample isrequired for the Ilford Contrasty Special Lanternplates normally used. Where coatings having widelydifferent mass absorption coefficients, such ascarbon and silicon carbide, surround the sameparticle,* well defined layer boundaries can onlybe obtained at the expense of structural definitionwithin the layers» Hence radiographie conditionsfor the production of plates for subsequent metrologyare 'essentially a compromise.

The method for measuring X-ray images of particlesuses a NIXDN hand operated profile projector modifiedby Audebeau and Hedderly (Dragon Project) for semi-automatic operation.

Í^-S» 9)A. hair line cross wire engraved on the under-

side of a vertical Perspex atrip is displaced hori-zontally across the face of the projector screen "bya remotely controlled carriage sliding along a linearpotentiometer» A digital voltmeter measuring thevoltage between origin and slider is connected to aprinter and/or puncher unit providing either a dataprint-out or a data punched tape. Simple calibrationmade before starting any set of measurements permitsthe computer, fed with the punched tape, to giveactual metrology values of the sample under inspection.

A photograph of the equipment is shown in Fig» 10Sample Size

Normally a sample of 100 particles is taken froma batch by riffling. The particles are placed on thesample holder and radiographed as described above»

302

The images of the 30 partiólos that are nearestto the imago of the standard reference steel ball arethon measured as described below,Modo i of t Qger a t_i on

The plate under examination is placed ontothe slide table of the profile projector lay theopex»ator0 Particle iríiages to be measured are thencentred onto the scroon by vernier adjustment» Thecross-wire is next positioned to be aligned withthe successive edges of the particle. After makingeach coincidence the operator presses a button tosample the electrical equivalent to the cross-wireposition vhich is then processed.

Tho design was made in such a way that theoperator cannot simultané ou sly move the croas-wire and sample the position.Calibrât ion

A standard steel ball is adhesively bonded tothe aluniniusi sample holder. This 1 mrs diametersteel ball is then neasured with each particlesample and the result used as a calibration factor.

3»3«2«5 Err or s r ancj . Jjjun i ta i ; longRsolution Q

The resolution of the microscope is limited bythe size of the X-ray source. It cannot be "betterthan the diameter of the source, and this isdetermined by the focal length of the electron opticalsystem, the depth of penetration of the electronsinto the target^ and to a certain extent the targetthickness» The particular equipments used whenoperated at 20 kV produce an X— ray source 1 to 2 (imin diameter and so features separated by less than2 jjm cannot be distinguished from each other on thephotographic plate»Bis tort ion

The projection of objects not on the X-ray beamaxis produces a cosine error in the magnification ofthe image. This error distorts tho image in a radialdirection from the beam axis, whereas, the magnificationnormal to this radius is tho came as for the on-axieobject. Tho result is an egg shaped image of asphere. Tho error is not noticeable in the contrezone of the image "fields used? but may be up to 3$in its periphery.

303

Setting of Pr ojo et i on Mn^ni_f icat ionAs the oan.plos sre placed on a plane sample

holder the true raagnif i cation will vary as theequatorial piano of the samples will be displacedaccording to tîxo overall -liameter of the particle*The maximum error arising from thi's effect for thegeometry employed where the particle diameters donot vary by more than 20$ of the standard diameterwill "be <1 »Measurement of the

With an X-ray image which is sufficientlycontrasting and well defined and with an opticalmagnification of X100 antl an X~ray magnification of-"X2 it is possible to measure f and to reproduceneasurements to an aeexiracy of better than ¿ 2 micronsrelated to the relative position of the partiólewithin the sample»

of _Using a sample of 30 part i olea grouped around

the axis of the beam, the error of measurement ofany feature does not exceed ¿ microns or 15& of thetotal measurement whichever is the greater. Thisdétermines the accuracy of coating thickness measure-ments to Hh 3 pía maximum and of kernel and particlediameters to '-¿10 ^m» In practice experience teachesthat the maximum error has never exceeded ¿l m forcoating thickness»

3»3«2.6 Inspectiqq .of .It must be appreciated that coated

particle fuel is now produced to high standards andthat the figures in this section are illustrativeonly of defects and not of normal quality.

The images of the whole 100 particles sampled arecarefully examined to count whenever necessary partialespresenting one or moro of the following defects sDefective kernel (Fig, !1)Multiple kernel (Fig. 12)Múltiplo kernels (Pig. 13}Inclusion of kernel material into any layer (Pig« 14)Diffusion of kernel material into the inner PyCcoating layer (Fig, 15)

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Mitssiag coating layer (Fig, 16)Kissing silicon carbide layer (Fig. 17}Gross variation of thickncso of the same coatinglayer (JPig. 18)The general aspect of the sample regarding the shapeof the images is also recorded. Good, medium, badshapes are related in the four photographs reproducedin Figure 19»

3.3.3 Surface ContaminationTho measurement of surface contaminate on by uranium

occurring in particle "batches in required to ensure that only ,-an acceptably low fraction of Free Uranium, typically *-1 x 10or loss of the total uranium content of the batch is presentprior to the consolidation of particles as fuel compacts»Measurement of the free uranium content of fuel compacts isdioeusscd later.

Apparent surface contamination is measured either bystandard radionotric techniques "based upon the 4.6 $eV r,emitter of the IT-234 isotope or by a chenical leaching methodusing nitirc acid. The term apparent surface cantaninationis used in the senoe that from theoretical calculations {20]contamination lying within 17 jam of the outer pyrocarbon surfaceis detectable by a-counting. In the caso of the acid leachingmethod used it is considered that cnly uranium lying virtuallyon the surface is detected.

The a~counting method is i rapid non-destructive techniquebut cannot be used as n. method for detecting broken or damagedparticles unless gross failure has occurred and exposed uraniumis within range of theo¿ detector» The method is thereforeregarded as s, measure of apparent surface contamination only.

The nitric acid leaching nethod involves chenical analysisto determine uranium levels removed from the surface of particlesand again provides a measure of apparent surface contaminationfor batches having nc broken particles present. $owever? ifbroken particles are prosont within a batch, they are detectedby the acid leach method assura ing the dogree of breakage permitsthe nitric acid bo leach ou I uranium present in the kernels ofthe damaged particles. The acid leach method is thereforeregarded as & method of measuring apparent surface contaminationand free uranium content and servos as a general integriby testwhich can be applied to the whole batch.

Only the a-counting method in discussed in greaterdetail (see Section 3.3«3»1)•

305

3.3,3.1 Surface. ContfliBiiqat^

The method used is based on standard radiometiy[21 j 22]. The particular apparatus used by theDragon Project is completely automatic. A "blockdiagram and phot graph of the apparatus aro shown inFigures 20 ana 21 .

In earlier reports [19 & 20] a method of relatingalpha ooturrc rates obtained on roonolajer samples ofparticles to the total uranium content (defined asCore Visible ratio) was described, based upon thepreparation of particle raon-olayere apeciall-y dopedwith niicrograinffie quantities of uranyl nitrate of asimilar enrichment to the type of particles beingmeasured. This procedure has now been discontinueddue to difficulties encountered with the dispersionand measurement of very low quantities of uranium(less than 5 Me uranium por gramme of particles)required to simulate current contamination levólefound in low enriched particle batohss,

A simplified procedure has now been adoptedbased upon the oourst rates obtained from raonolayersamples and those obtained from a flat source,prepared by evaporating known' quantities of uranylnitrate solution onto a 20 mm diameter steel disc.The standard "source" its prepared from a uranylnitrate solution of the some uranium dioxide powderbatch previously used for kernel manufacture andtherefore minor variations in the isotopic compositionof a nominal enrichment are automatically compensated.Similarly the flat disc source is a permanent "standard"and daily variations in counter efficiencies and back-ground rates aro again compensated.

Normally flat disc sources having quantities of5 Hg total uranium of the appropriate enrichment areused. An empirical factor has been derived fromalpha counting measurements and acid leaching datawhich correlates count ratos obtained on monolayersamples expressed as counts per gramme of particlesper kilosecondj with count rates obtained from thedisc source expressed as counts per kilosecond.

The following expression is then used to calculatesurface contamination levels in terms of core visibleratioss

VTen™ *A,TTO FRES DSAHIUM OK SURFACECORE VISIBLE RATIO „ ~ÇoTAL UEAïïIUM CÔmwT

whereFREE URANIUM OK SUBPACE « - . P „ Q ng U/g particles

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and... .:A « counts per gramme por kilosecond of sample ) corrected

} for back-is « counts por kilosecond for STAÎIDAH3) source ) groundP « factor (normally ~1.3)Q » quantity of uranium on STANDARD source.

3.3.4 DensityThe density of coated particles has to "be ascertained in

order to determine the coatod particle vplume loading, finalmatrix density, ©tc,> in the fuel "body manufactured frora theparticle "batch tested.

.The following method is preferred as it is non-destructive.Mctfroa

The method applied to a randomly drawn sample from the "batchunder examination uses the formulas

vA* where. W a weight of the sample (g)V « volume of the sample (cm )

Normally two samples of 50 g re taken from the batch byriffling and successively measured» The msan, range andconfidence interval limit are thon calculated.

The volume is measured using a coranerical comparisonpycnometer, BECKMST Model 930 (Figures 22 «and 23). Themethod consists of comparing the volume of gas in-" anidentical measuring cylinder containing the sample byequalising the pressure in "both cylinders. The volumeof the sample is given directly by a dial mechanically • .connected with, the positioning of the piston of thomeasuring cylinders»Calibration

A check calibration is performed once a day by makingmeasurements with the two test balls supplied with theequipment. Whenever necessary, re calibration is made byfollowing the operating instructions.Accurac °£ fr Method

The method has been checked and the practical reproducibility .of measurement was such that the standard deviation over 10 ,different measurements of the samo sample is equal to 0.015 g/om .

The accuracy of the method is therefore, for differentdegrees of confidence, as follows *

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Bcgree of Confidence (#)

Accuracy ($)

90

0.22

95

0.2T

99

0.38,

99.9

0.5?

The only non-destructive examination under this headis the micro-examination of whole kernels and/or coatedffcel particles.

3«4 FUel BodiesIn the case of the prismatic HTR (eg the Dragon, Peach Bottom and

Port St Vrain reactors) fuel bodies can be defined as fuel bearingsub-assemblies, part of a larger assembly of fuel rods and fuel elements.»Por the AVR and THTR reactors, the fuel.body (a sphere) is the completedfuel element. The non-destructive tests described are based on thosecurrently in use and being developed for the prismatic HTR fuel compactand are virtually identical for spherical fuel bodies»3«4ol description of the jpragpn Fuel Compact

Although the various geometrical forms of fuel body compriseshort solid "teledial" cylinder, long cylindrical fuel containerand hollow cylinder, it is the latter which is discussed in detailhere» in the case of the Dragon Reactor, this hollow cylinder hasthe following mean dimensions: 40,0 mm long,, 35-30 ram outsidediameter, having a bore of 22»95 turn» It is made from a knownvolume or weight of overcoated fuel particles pressed in apre-heated die forming the well known annular compact and which issubsequently heat treated«

3,4*2 BriefDescriptionof the! AV1R/THTR Fuel ElementThe AVR/THTR fuel elements are manufactured by NUKEM GmbH,

West Germany and consist of a quantity of U(Th)CL coated fuelparticles in the centre of a graphite sphere the shell of which isunfuelledo Each sphere is 60 mm in diameter»

3.5 Routinefn-Destructive Tests3.5.1 Peterminatipn ofV?235 Content by Gamma Spectrometry

The application of gamma-spectrometry to the determination ofisotopic content of fuel compacts has been reported elsewhere

In this application the equipment is used solely to comparethe emission of gamma rays at the 184 fceV U-235 photopeak from theend regions of a single compact, and between a sample and a standardcompact.

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ApparatusThe counting assembly used is a stabilised single channel

gamma- spectrometer assembled from Harwell 2000 series modules.A block diagram of the system is shown in Pig. 24» ......• . The detector consists of a 45 ram x 50 mm NalCTh} crystal andphotoraultiplier in which is incorporated a small-, standard source/

• (22)scintil ator assembly . The reference pulses from this sourceare us4tî"tô stabilise the spectrum by modifying thé EHT to compensatefor changes in gain and temperature. The output from a single channelpulse height selector is counted by a six decade sealer, controller bya timing unit.

The counting time is maintained such that the total count oneach sample is greater than 8 x 104 in order to give a relative errorsmaller than 1.5% for the ratio at the 3 <T level. Takinginto account the margins of error in the determination of the U-235content in the standard samples by chemical analysis, the practicalaccuracy of the gammaspectrometric method for the determinationof the uranium content In compacts is better than -

The standards are selected from the first production batch anda pair of compacts, judged to be of average and equal loading andexhibiting minimum inhomogeneity are selected. These are retainedand designated the counting standard and the chemical standard»

The total uranium content of the chemical standard is thenaccurately- determined by chemical analysis and the U-235 contentcalculated from isotopic analysis carried out at the raw material

The U-235 content (A) of the sample compact is given by:» B trA * . K

whereB » the total count from the sample compact, andC * the total count from the counting standard

rK » T . U-235 content of chemical standard

whereC and D are the mean total count rates from the countingstandard and chemical standards respectively, accuratelydetermined at the time of selection.

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3.5.2 DeteCTCáiuMir

The measurements of count rate resulting from the déterminationof 0-235 content described in Section 5.1 are used to determinequantitatively the «end to end* variation in compact homogeneity tosupplement the qualitative impression gained front the radiographietechniques later described. By comparing the U-235 activities of thetop and bottom of the compacts, a coefficient of heterogeneity iscalculated which indicates the quality of the U-235 dispersion in thegraphite matrix of the compact. Where the count rates from the topand bottom of the .compact are A and B respectively the coefficient ofheterogeneity is expressed as a heterogeneity factor HP where:

> *A. *" ..."

Typical values of HP for well dispersed fuel within, a compact,range from 0-5% calculated as. above.

3«5»3 uali.tatiye jtetcEmimtion of_ Fuel, JJispersigntechniques . . .Radiographie methods arc used to determine qualitatively the

homogeneity and distribution of heavy metal within and betweencompacts ":nd to provide information on the presence of. cracks, voids,inclusions and other def > r.ts«.'

/23)An adaptation of Derbyshire's slit scanniro techniquedeveloped at AERE Harwell was engineered so that the product ofone furnace loading., i e 45 fuel compacts, could be tested in oneexposure. A series of parallel slits are arranged about the focusof an X-ray tubo and the. fuel compacts are mounted on cylindricalcassettes containing film on a solid beam trap. These are arrangedso that the axis of the compacts is parallel with the slits (Fig 25 >•As the film is exposed to the Jt-ray source the cassettes, togetherwith the film and compaccs are rotated* Because the sample and thefilm are moved synchronously through the beam tho resolution of thesystem is limited to the slit width which is actually however smallerthan one particle diameter. The radiograph presents a developedview of the annulus (Fig 26), allowing the homogeneity of the fueldispersion to -'be assessed»

The "evaluation is reported using an arbitrarily chosen scale ofvalues» for which purpose a standard set of comparison radiographsis kept.

In addition to tho slit scanning apparatus described a largeindustrial radiographie assembly is used»

The system may be «sed for conventional radiography * or. inthis case more usefully, *or fluoroscopic viewing. A 550 mm square

* it can also be used for examination of fully assembled fuelelements to observe features such as thermocouple hot junctions.

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fluorescent screen is fitced together with a 125 nun image intensifier..A fault may be detected in this manner which, because of orientationmay not be fully distinguishable on the panoramic radiograph» Àconversion xmit is also available to enable panoramic radiographsto be made using this equipment, it is therefore possible to evaluatecompacts fully, although at a reduced rate, in the event of abreakdown of the normal panoramic radiographie unit»

3«5»4 The Distribution of | Heavy Metal •Loading in ¡a Fuelled ompactAxial and circumferential variations in the fuel density of the

fuel elements fabricated for prismatic High Temperature <3as-CooledReactors are important factors in the calculation of peaje fueltemperatures o These variations, may be due to changes :ir* packing inthe annular graphite matrix compacts» Since several compacts make upa complete fuel stack, there could be a considerable variation indensity from end to end»

Earlier work on the distribution of heavy metal graphitecontainers with double fuelled annuli loaded with "loosely-bonded11coated fuel particles, had demonstrated that the -scintillographymethod developed by Cotterell * 4) at the Non-Destructive TestingCentre at AERE Harwell, could be modified and used to obtainquantitative results for fuel compacts.

t of rTechniqueIt was necessary to establish that the technique should

a) estimate the changes in fuel density and record them along thelength of the fuel compact

b) be capable of dealing with fuel compacts on .a production scalec) have an accuracy of measurement compatible with reactor physics

and thermal analysis requirements.Gamma absorptiometry with a scintillographic display system assuggested by Cotterell C24) seemed offer khe j st solution forattaining the essentials listed above-, but certain importantmodifications were needed to achieve them. Basically, the equipmentconsists of a radioactive source on one side of the absorbingmaterial and a suitable detector and indicating apparatus on theother « In the application described, the indicating equipment isdivided into sections performing the quantitative and qualitativeaspects of the tests. A measure of the variation in the .fuel densityis given by the variation in the transmitted radiation, and» as such,can be measured.

4 .

The relationship between the intensity, of a well col lima ted beanof mono-energetic photons emergent from a thin absorber and theintensity of the incident beam is given by:

T » ! e'"* (24)o where u is the mass absorptioncoefficient of the absorber at theenergy of the incident photonx Is the mass of the absorber per

g j j unit area.

The fuel bodies to be tested consist of a mixture of severalelements» The mass attenuation coefficient of a mixture is the .average of the mass attenuation coefficients of the constituentelements weighted in proportion to their relative abundance by weight?

„ . (25) v.'hoire u is the mass attenuationVr * UaMa + VV» * etc a coefficient of the element

aM is the mass per unit area

•""•' a of the element a and so on.As variations in fuel density are linked with variations in the

abundance of other constituents it was decided to compare theattenuation due to the fuelled annulus with an identicallyconstructed standard containing sections of known fuel densities.

For earlier work on annular fuel bodies of relatively low fuelvolume loadings * (15-25%) it was decided to use a source havinga relatively long half life of mono-energetic gamma rays of.60 key (26). . ... ...

' • " - • ' • • • .. . •:;"... ' (25)'-' ' -Bearing in .mind the mass absorption, coefficients : .the onlyisotope giving sufficient photon flux in this energy region andhaving a sufficiently long half-life is americiur»-24i which may be

Í5*»considered to be a source of mono-energetic Mrays of 60 keV. ~Method

The source is mounted at the end of a rod so that it is possibleto manoeuvre the source into the open end of the rotating annulus»The tip of this rod is removable so that the source can be storedconveniently» The source collimator is formed by a hole in the wallof this renovable tip (see Fig» 2?)

The detector comprises a thallium activated sodium iodidecrystal optically coupled to a photoroultiplier» The crystal isshielded from external radiation apart from a small area exposedthrough a colliraator directed towards the collimated source.

For the series of experiments which are described in thisreport a crystal 3 rara thick having the same diameter as thephotoraultiplier was used.

Details of the source and detector units are shown in Fig« 27»Sensitivity and Accuracy

It was required that thefuel density should be measuredwithin 0»5-h throughout the length of the fuel element in incrementsof 0«2 in (**5 nun) since the coated particles in the fuel body werelikely tobeuptol.2nsn diameter and that the mean fuel loadingof that increment be measured to within an accuracy of-1%» 18 s

* t.he volume loading is the percentage of the fuel compact occupiedby the coated fuel particles»

312

are thus allowed for each count, the time being partly dictated bythe requirements of the scintillographic display systenu Thecounting'time was further reduced by 2 s by the requirements of theprint-out system.

To detect and measure a change in the count rate due to a changein the attenuation of the photon beam, the. change must be greaterthan the statistical variation in the count rate» ..Using a 35 md

•' 4As*-241 source a$d 2 mm colliroators, a count raté.of 2«5 x 10 cps andhence a total edunt of •*• 5 x 10 was attained without an attenuator

4 5and, 1 x 10 cps and 1.8 x 10 total count respectively, with thefuel compact in.position» Thé background counts in each case wereless than 0,1% of the total* The time constants of the electroniccircuits and detector are sufficiently short that, at these countratesj relative dead time corrections are unnecessaryo If it is .assumed that the fuel densit*os of the standards are known absolutely,then the fuel density measurements of .the individual increments willhave an error of 004%. .. • .

These count rates are compatible with those required for thescintillographic display. The factors contributing to the overallsensitivity of the display system are dealt with elsewhere, (24)Apparatus

The equipment consists basically pf a turn-table and lead screw,both driven by the same constant speed motor. A speed of onerevolution per second was chosen as a speed suitably, for therequirements of both the absorptiometer scan and the scintillographicdisplay (Fig. 28). The motor also, controls the timing of theabsorptioraeter sealer print-out system. The error in the resultingintegrating period is negligible compared with other statisticalerrors.

The collimators are pro-aligned, while the source and detectormay be positioned at any point on a diameter of the sample.Data Processing

A block diagram of the system is shown schematically in Pig.. 29.The scintillation counter is one of the special units mentionedearlier in this report. The pulses from the photomultiplier areamplified by a standard •Harwell» 2000 series amplifier chain, andpassed via a discriminator to one of the two processing systems*

A d.c. voltage level proportional to the pulse rate .receivedfrom the discriminator is generated by a ratemeter. This wasspecially constructed and has a very short integrating time (10 ms)to give an optimum resolution <24)„ This voltage level is passedto a quantizer consisting of a series of voltage discriminatorsand hence to the facsimile recorder resulting in a facsimilepresentation of six discrete grey tones which may be varied fromblack to white to enable the recording to be evaluated. Therecorded trace resembles a panoramic X~ray picture of the fuel annulus,and, as such, changes in density, the presence of cracks, fuel

313

separation, voids, etc», are easy to recognise. The quantizer isnormally adjusted so that the tones portray areas of low absorption asblack, Thus it is possible to show Changes in fuel densitydistribution as follows:i) Area of high density white to pale grey.ii> Areas of normal density medium grey .iii) Areas of low density dark grey .iv) Areas where the fuel is completely absent, eg voids and cracks

and the ends of the fuel sleeve are clearly defined as blackareas on a lighter background.The pulses from the discriminator are counted by a sealers

"She counting "time determined by the constant speed motor drivingthe turn-table is usually the time taken to scan 18 revolutions(5 ram) of the sample* A pulse Is generated by a cam controlledmicroswitch which initiates the printing sequence of the Harwell-Assembly 3017 Sealer Read-out equipment. During the printing •sequence the sealer is inhibited for 2s, tho count produced beingthe total count over the 16 s preceding the initiating pulse»Facilities are available to present this information on punched tape»compatible witsh a KDP~9 computer, and for larger scale fuel elanentproduction this arrangement is recommended»Results

The mean heavy metal loading, calculated from the absorptioraetryresults is presented. As the determinations: of fuel density arecomparisons with a standard, the overall accuracy of the method isgreatly dependent on the accuracy to which the density of thestandard is known and to the reproducibility of the geometry of thestandards and samples <. The observed exactness of the method hashowever been within the statistical accuracy of these parameters.The more recent work employing this technique has been the measurementof heavy metal loadings in solid fuel compacts of the «*teledial"type. Por this changed fuel geometry the testing method wasmodified. The Am~241 source was replaced by a Cs-137 source havinga major peak energy of 661 keV. The electrc~mechanical scanningequipment was basically as described above except that the sourcewas positioned near the circumference of the "teledial" at right

• angles to the horizontal axis of the rotating compact. Thefacsimile recorder was discarded for this test and quantitativedata was proœased by a direct link to a small computer. This test•was successfully applied to a fairly large production of <» 65000"teledial" compacts.General assessment of the method

The simplicity and low cost of the method must make it .attractive for use in other applications. Any .tubular, or annularfabrication may be inspected in this manner,! providing, the apertureavailable is large enough to accept the source, .and/or detector»It can also be applied to solid bodies.

314

The results have shown that the technique described isreliable and cheap to operate» It could be scaled-up very simplyto control quantity production of fuel elements, or othercylindrical bodies on which analysis of density distribution andflaws or defects is required, It has the advantage of printingresults as they are obtained, and a "pictorial" representation inthe form of the facsimilae recorder trace, which is easily '",reproducible is given at the same time. The accuracy of thedeterminations of the heavy metal loading in the Dragon teledialfuel bodies is ± 0,5% which is better than had been predicted.

3.5.5 •^^^„•^ ..e•^•^^ffi•^^--.9 ...-^fi,.;•^-^^• ,?a.rticlG faction inContacts '1J'rJ

•This i§ "by. far the most important test relating tothe quality control and assurance of HT3" fuels. Thedetectibn'of faileft ooated particles ensures that theamount of free uranium in the fresh loaded fuel can be assessedand its effect calculated. fho definition of a failedcoated particle is one which is either cracked or damagedor has incomplete coating layers such that uranium fromthe kernel has access to the reactor primary coolant, thuspermitting the release of undesirable quantities of fissionproducts into the reactor environment.

Unfortunately the routine tests at present employed tomeasure the failed particle fraction are destructive in <nature and consequently extremely expensive. Thorô are'»howsver, important development now taking place whicû it ishoped will materialise into a routine non-destructive testand two of these are very briefly doser ibod.3»5»5«^ Li t Irradiation and Gas Bolease Toohni ue

If a compact is irradiated for a very shortperiod at a low dose rate it will contain, say,about 0.5 ffiCi Xo-133. By subsequently heatingthe compact to over TOO'0°C in an inert carrier gas»the release of Xe-133 (for example) from bare fuelcan be measured.

This is a direct method which is potentiallyextremely useful as a calibration device for othermethods used to determina the failed particlefraction.

3»5»5»2 ladon Tracer Method f or i _ the MoasuremQnt of GagLeakae !Jn'iCTiad"'lÍ7H ei s «-—-——Earlier experiment s by Freck at CBGB Berkeley

Fuclear Laboratories (27) on the development ofequipment to monitor noble gas emission has resultedin a series of collaborative experiments betweenBerkeley ÎTuclear Laboratories and Dragon ProjectQuality Control Group to examine. the possibility ofapplying a gaseous monitoring technique suitablyadaptad to measure the fraction of failed ooatedparticles in a fuel body.

315

MethodThe original proposal (2?) was to incorporate

a trace quantity of Hadiuni-226 in the U0_ fuelkernel. Radium-226 decays by alpha emission totho noble gas Badon-222> which itself decays byalpha omission with a half -life of 3» 825 days*After coating and consolidât ion „ fuol "bodies or compactswould thon be tested in the apparatus shownschematically in Fig 30, where an inert as iscirculated over tho fuel compacts in a furnace.As the compacts are hcatod, Radon gas emanatingfrom the exposed uranium fraction would bo releasedand e<irried by tho inert gas into tho decay chamber.Solid daughter products of radon, which are themselvesalpha emitters, would be collected on a filter at thoexit of the decay chamber and subsequently counted bya detector positioned in the chamber» Tt wasenvisaged that such a test might bo convenientlycombined with the carbonising heat treatment stageor the final degassing stage as an on-line facility,in the fuol body manufacturing process.

Tho trace of radium required to be present intho fuel is very small. It has been calculated tobe 0«003 ppm Radium with respect to uranium. Thushandling difficulties arc no problem at any stageof manufacture or during or aftor th« tost.

The advantage's offered by tho proposed testmethod if successful, aro namely $(a) Tho method is completely non-do*! truct i ve,(b) Haden emission simulates and may correlate with

fission gas release during irradiation.(c) The method offers the possibility of up to a

inspection as an on—line facility.(d) The cost of materials and j&onitoring equipment

is relatively inexpensive.Validity of tho Method

To demonstrate the validity of thd method thefollowing four major points required experimentalverification?(1) It was necessary to show that radium could be

mixed with the uranium and retained in thokernel through all the stages of manufacturo.

(2) The distribution of radium in the fuol had tobe shown to be uniform, in particular the

316

distribution of radium from kernel to kernelhad to be proportional to the weight of ursniiaain thekernels.

(3) The radon from a failed particle in a, compacthad to "be released in a reasonable time at afeasible test temperature.

(4) It had to bo shown that the release of radonfrom intact particles, with negligible surfacecontamination, -.Tas very lov/»Finally, it would be necessary to correlato the

release of radon in the test with the release ofkrypton and xenon in a light irradiation experiment.Results

So far only the results from laboratory scale testsare available» These are insufficient in number topoint to firm conclusions on the future of this method.They do, however, indicate that tho method is likely toprove valid and indeed feasible for production testingon a large scale» The experiments completed so far usingfuel compacts having doped bare kernels mixed withcoated particles have shown that the radon releaseffleasured from an intact coated particle is negligiblecompared with the release from a single bare kernelin a compact»

Much remains to bo done, and work is now proceedingto set up the monitoring equipment on a fuel productionline. The installation will be attached to the fuelcompact final heat treatment, or degassing furnace (Fig31), the radon monitoring will take place during theheat treatment cycle of the compacts, and ideally theresults will be available before the end of the heattreatment process. It is too early to predict theoutcome of this development but present indications arefavourable enough to justify some optimism.

BSTAILS OF OTJT1SR ICT-SESTIlu'JTIVE TESTS USED BY THE -logoff FitOJEffP

Associated with the routine and future tests described in thisreport are various tests carried out on ancillary fuel and reactorcomponents. Some of these are worthy of mention such asi(i) The non-destructive testing of graphite fuel element components

by eddy current methods (28). (32).(ii) The non-destructive examination of heat exchanger tube coils by

an ultrasonic test associated with a fibre optic and introscopedevice »

(iii) The measurement of alpha-tracks on fuel kernels by autoradiography.

317

(iv) fhe use of x-ray promotion microscopy to assist in thocharacterisation of irradiated fuel particles, ie ooating thicknessmeasurement,

Othor applications may include the use of neutron radiography whichoould in certain circumstances be used as a /(jo-vice to measure orcalibrate heavy me tal 'loading in a fuel compact,

Tho post irradiation examination of lïTS'fuûi has not boon dealtwith in this report? boing considered to bo 'outside its scope } butthe gruaœa. spoctrora¡-- Cric scanning of parts ' or' whole fuel elements orfuel rods (fuelled -and unfuelled), forms a most important part ofpost irradiation examination in support 'of '£uol ..development.

5 comxLUSioHs AMD O(1) An , at tempt has been made to give a general, .picture of tho wholo

array of non-destructivo tests on un-irradiatod coated partiólofuels for HTRs, and to describe a, few of those in detail.

(2)" No attempt has boon made to describo in detail or to present thobasic principles of conventional httn-dostructivo testing. Thishas boen expertly done elsewhere (25? 29» 30 and 31).

(3) The detailed descriptions given f ó?-' sotap of the methods have, itis hoped? highlighted sorae of. the ay,s in: which the complexity of•tho nii rominiatxtrised coated part loi o"'fu&Í- has lod to innovationsin mf mothcds.

(4) The massive sub-division of tho fuel compacts into numy millionsof coated particles as components of -the fuel b';dy has made-statistical sampling in tho eharactcr'iskt ion of this fuel soimportant that only test data so obtained can be reliable,

Within the signatory countries of the Dragon Project there isconsiderable interohr-ngo of information on fbsting methods applied toHTR fuels. 'The authors arc indebted to a vb'ry large-number oforganisations working in this field, too many to make individualacknowledgments. 'They are particularly conscious of the assistancegiven i»...proparing this paper by Dr K G Knofeèèéiïn' (?îukom Oafch)Vv-'-Dr D V Brock (Central Bleotricity Generating Board, Berkeley lîuclearLaborfttor-ies)» Dr P Xoss (OSGAE, S -ibersdorf), Dr P. ÏÎ blowers (¿81 ,Harwell )•• and tho members of tho staff of tho Dragon Project QualityControl -Group.

318

H5FER15WCES[I] G. E. Lockett and S« B. Eosegood, "Engineering Priaciples of

High—Température Keactors31, Proc. Symp. Advanced end High-Teaperature Gas-Cooled Beactors, Jiflich 21 - 25 October 1968(IAEA ! Vienna 1969) p.155

[2] H, B. Steward, E. C. Bahlberg, W. ?. Goeddel, D. B. Trauger,P. B» Kasten and A. L, Lotts, "Utilization of the Thorium Cyclein the HTGR", Paper A/CQMF 49/P/837 presented at Fourth Inter-national Conférence on Peaceful Uses of Atomic Energy, Geneva,6 - 16 September 1971.

[3] B. D. Yaughan, G. M. Insch and J. B. Thorn, "Objectives andRealisation of the Heliuia Cooled High Temperature Reactor",Paper A/CONP 49/P.4ÔO presented at Fourth InternationalConference on Peaceful Uses of Atomic Energy, Geneva, 6 - 16September 1971,

[4] P» Tanguy, H, Loriers, B. Maillet, A« Buisson and J. Chaboseau,"Work done in France on High-feinperature Eeactors", PaperA/COKP 49/P/585 presented at Fourth International Conference onPeaceful Uses of Atomic Energy, Geneva, 6-16 September, 1971*

{5} !«• Autnfiller, K» G, Hackstein, M» Hrovat, E, Balthesen,B. Uebiaann, K, Ehlers and K. HSllig, "Fuel Bleraents for Hightempérature Reactors - Production Experience and IrradiationTesting in the Federal Bepublic of Germany", Paper A/COBF49/P/385 presented at Fourth International Conference onPeaceful uses of Atomic ifeergy, Geneva, 6-16 September 1971»

[6] V, Mattick, H* ¥, Mtíller, E. KrSmer and R. Scbulten, "TechnicRlStatus and Potential of the High Temperature Beactor Line inthe Federal Republic of Germany", Paper A/CONF 49/P/370presented at the Fourth International Conference on PeacefulUses of Atomic Energy, Geneva, 6 ~ 16 September, Î971»

[7] M, Dalledonne, E. Eisenaan, F. Thfealer and K. Wirtz, |fHighTemperature Gas Cooling for Fast Breeder", Proc, Syop.Advanced and Rigib.--Q?enjperati3xe Gas—Cooled Eeaotors, Jfilich,21 - 25 October 1968 (IAEA î Vienna 1969) p.345.

[8] C. P. Gratton, B. G, Sevan, A. T. Hooper and G. W. Horsley,ibid, p. 359.

[9] M. S* T. Price, J. E. C. Gough, G. W« Horsley, J. Brit,îïucl. Energy Soc., 5, (361 ) July 1966.

[10] M. C. Hatiella "Experimental StatisticsSBS Handbook 9", (1965)United States Department of Commerce.

[II] H. L. Billson and F. C. Leone "Statistics and ExperimentalDeaign", J. Wiley & Sons London»

[12] ASfM Manual on Quality Control of Materials, January 1951*Seventh Printing 1960,

319

[13] Perkin Elmer - Shell aodel Nc. 2123, "Sorptoiaeter InstructionManual".

£14} Brunauer ïtamett, Teller J. of Au. Ches» Soc* 60? 309-318 (1938)

[15) British Standard 1902 Pt, 1A 1966.[16] Private Communication Br* P» Koss, 0*S.G»A»E» Seibersdorf ,

Vienna, Austria.[17] v» B» Cosslett and W. C. îïixon (1951) Sature London 165-24[18] J. Pelsmaker and S. Anerlincks, Rev. of Scient. last,

32.7.828 (1961).[19] J. P* Conde, J. Holliday et al, "The application of non-

destructive techniques in the Quality Control uf fuel aaterielsfor the Dragon Reactor Experiment"., D. P. Report 326,

[20] M. 2, Maksimov, "Range-Energy relationships for some substances'Soviet Physics Sept. Vol. 37 10 Mo, 1 Jan. 1960»

Í2l3 P. Barr and a, Pointud, "A non-destructive method for thedeteraination of the Uraniun and Thorium contents of powdersand coated fuel partiólos", P.P. Report 194'

[22] 3>* Williams, G. P. Saclleng and J, Pickup "A gamna spectrumstabaliser with compensation for the effects of detectiontemperature variations" AERE R*4923.

[23] B* T. P. Derbyshire "A Multiple paKoraiaic apparatus forradiographie examination of IITH fuel sleeves" ASBB R.3855(1961).

C 243 K. Ootterell "Quantitative Estimation of the UraniumDistribution in MTR fuel plates by ganaaa ycintillography"ABRE

[253 N. D. T. Handbook Editod by H, 0, McMasters, Section. 27*40Ronald Press Company, Hew York 1959*

[26] P. G. Miller "Amer i slum -241 as a Photon source for gannaattenuation techniques1'* H.W» 39971*

[2?] B. ¥. Frock, C.B.G.B. Eritish Patent 1, ¿53,6363iaprovements in or relating to the manufacture of coatedparticle fuel for nuclear reactors, 20th April 1970»

[28] J. P. Conie and J, Holliday "Non-destructive testing of graphitefuel eleoent components for the Dragon Reactor Experiment", DPReport 576 June 1967.

[29] -T» Rooney, "The contact microradiographic inspection of coatednuclear fuel particles5'. Non-destructive testing October 1971»

320

[30] E. S. Sharpoj "JC-Kay microscopy applied to tîie examination ofcoated fuel particles" AERE R.4252.

C31] H. W. McLung et al? "The use of microradiogrophy comMned withmetallography for evaluation of coated particles" OENL 3577»[32] H» S. Sharpe, S. Aveyard and W. L. Hodgkinson, "A quantized

oddyfaz technique applied to tlie inspection of reactor gradegraphite", Brit. Journal of H.D.T. £, 80, (196?).

LIST OF FIGURES» 1 Manufacturing process schematic diagram,

Fig» 2 Spinning Siffler sampling equipment diagram.Fig, 3 Spinning Riffler photographFig, 4 V Slot end Counters.Fig, 5 Th0 effect of non-wiiforia particle size»Pig, £ X-fîay Projection Microscope Schematic diagram .Pig, 7 X,M»30 Projection microscope»Pig, 8 E.M.75 Projection microscope.Pig, 9 Automated measurement schematic diagram of Kikon projector.Pig. 10 The Nikon Projector.Pig. 11 Defective kernel.Pig. 12 Multiple kernels.Pig. 13 ífol tiple kernels.Pig, 14 Inclusion of kernel material into any layer.Pig. 15 Diffusion of kernel material into the inner PyC coating,Pig, 16 Missing- coating layer,Pig, 1? Missing silicon carbide layer.Pig, 18 Gross variation of thickness in the same coating layer,S» 19 Particle shapes.Pig. 20 Alpha-counter bock diagram.Pig. 21 Auto alpha counter,

321

Fig. 22 Beoionan Pyononeier»Pig» 23 Density determination "by air pycnometer.Pig» 24 Singlo channel gamma spectrometer»Fig» 25 Multiple slit technique.Fig. 26 TJOg compacts.Pig, 2? Source and detector.Pig. 28 Schematic diagras of scanning equipment»Fig. 29 Schesatic layout electronics,

. 30 Schematic diagram of radon detection apparatus*» 31 Schematic proposed radon release detection equipment^

322

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R. ROCHESKMT- Contre d'Etudes Nucléaires do Snelny

AbstractSince tho first use in Marcoulo in t<?59> application of

Prestressed Concrete Pressure Vessels (PCPV) for nuclear reactorhas been gained increased attention and Non Destructive Testingof PCPV has been developed.

From a general point of view, NUT problems in PCPV arevery different from those associated with metal pressure vessels.Normal behaviour of PCPV shows phenomenas like cracking whichare not expected in safe operation of metal vessels. Scale modelsare used in order to definite PCPV normal behaviour. In addition.,permanent instrumentation generally supplied with PCPV is largelyused for NDT purposes.

For concrete and prestressing tendons, there is not realNDF inspection, but only coiiventionnal destructive tests.Metallicliners, cooling circuit, penetrations and closures are examinedin the same way as motal vessels. Good water circulation andflow distribution in the cooling circuit are touted by meansof a radioactive liquid flow inside the circuit,

The whole PCPV testing takes place during proof testing.Following tests are conducted : leakage tests-Cracks patternrecording-overall deformation measuring- local strain monitoring-tendon load monitoring.

In service inspection is requiod by frenen regulations .Testing system used during proof testing performes this inspection,

In addition, indications are given on NDT inspection ofintegrated steam generators.

349

• ,-!• , La prom-lore utilisation .d*un caisson on béton* ,î • ' * . ' * '

précontraint comme recipient sous pression dans un réac-tour nucléaire remonte au réacteur G 2 construit à tfeurcouloqui fut couple au réseau électrique en 1°-59« Uopuis cette.réalisation, les caissons en béton précontraint ont béné-ficié d'un intérêt général en tant qu'enceinte sous près-sion pour les réacteurs nucléaires refroidis par g&zt

En Franco , ils ont ©té utilisés pour les réac-teurs à uranium naturel {jraphito-fjnfc (UNCfG) « Dans ce docu-ment» on tente do présenter 1' experience acquise au cours* * *de cet emploi,

En matière de control o non destructif1, loscaissons en béton précontraint présentent dos carac,térie-. tiques, particulières qui lu» distinguent nettement desrécipients SOUK pression métalliques. En part Jcul.ior,cor-. tains phénomènes tcl&..<iuc 1* apparition de fissurosj>ttoi-.vont être considérés comme* uti comportomont normal doscaissons c«n béton préoo;itraJ lit , alors qu'ils soi'it réputéscomme dangereux dans le cas do récipients métalliques.J)fautrc part, les casais préalables ?i la constructs on»cten particulier los essais sur maquettes poussé» Jusqufàla dos t ruction, sont bien plus développés quo pour lesrecipient*; métalliques »ot lour rôle dans la prévision ducomportement du cais.sun et dany 3 a définition do ce qu'estlo «comport «mont normal11 do l*«ppnroil o:H bien plus con-sider ablo.

. .l)fimo façon cém'i'uîo ,on se doit de sJ« contrôl.n des caissons en béton s 'appuie on général

sur dc« tecltn.1ques qui ne sont pas considérées tradition-npllomcnt COJUPIO doa méthodes de contrôle non destructif .

Apron cos considérations générales* nous al J on sexaminer les contrôles effectués sur los caissons construitsen 1-Vimce et rfur les é<{tiipemcnts qui Iou3* sont associesdans los réacteurs intégrés «

350

IMSTON ET CABLES PKpour oes» constituants, il n*y a pas de con-

trôle non destructif proprement dit.Les fils, tox^ons ot câblos,font l'objet tic

contrôles traditionnels sur échantillons prélevés sur desloto { 1 ' importance do ces lots est de l'ordre de 2O tonnes)Ces essais sont essentiellement dos essais <ie traction»

Le béton fait, lui aussi, au cours do la misettu point de sa composition, puis ou cours de eon élaborationl'objet d'esaain classiques sur échantillons, et en parti-culier d'essais de compression sur cylindres à 28 jourset h 90 jours.

PKAU TNTKKKK 1KJ CATSSQNLes tôles destinées à la peau interne du

caisson subissent un contrôle par ultrasons (en ondoslongitudinales ) suivant un quadrillage pour l'ensembledo la surface» et en continu sur les bords. Les défautsà rechercher sont principalement les dédoublurcs duesaux laminages d'inclusion

Les joints soudés intervenant dans l'étan-cîiéité do jta poau font l1 objet d'un contrôle radio ra-phiquo systématique. Pour les configurations particulièresoti co contrôle n'est pas suffisamment probant, il .estcomploté ou remplacé pur de» contrôles ultrasoniques ouwa{jnéto*>copiquo» , Un resmiufto ot une éprouve d'étanchéi—té à l'ammoniac sont effectués ensuite, surtout sur lesjoints spéciaux tels cme les fonds do piquage do tuyau-teries»

cincuTr yw. Kr&iQjnisfiRn\rr_ m i.\ PRAULéo tubes destinés à la confection du circuit

de refroidissement de la peau subissent, préalablementà leur mise en oeuvre, un examen visuel' et un contrôle"ultrnsonore *

351

Leur façonnage consiste on une mise en forme,en un raboutago par s'oudure et ou une fixation sur lapeau ( également par soudure). Le contrôle non destruc-tir, correspondant à cotte phase de la construction,8 'attache à vérifier :-la santé dos soudures de raboutago et en particulier leursqualités potentielles d'étanchéité—la bonne circulation do l'eau de refroidissement dans cestubes et en particulier a l'absence de rétrécissement «udroit des soudures .

Les soudures font l'objet d'un contrôle ou ra-diographique ou par particules magnétiques .

L'étanchéité est contrôlée file par file, lecircuit de refroidissement étant découpé en files indé-pendantes, isolables de l'extérieur ot on nombre sura-bondant. L1 épreuve d' étanchéité consiste on imo épreuvepneumatique avec détection des fuites éventuelles à labulle de , savon*

La bonne circulation de l.'oau dans lee tubesa fait 1* objet d'un contrôle par circulation d'un liquidecontenant un traceur radioactif. Cette épreuve permet dovérifier que 3 a 'vitesse <Jo circulation ost convcnnblo etqu'une répartition satisfaisant» do débit ontro lo» tubesest obtonuo. Cotte méthode de contrôle global o'eîst révé~3éc très efficace.

O_\S

Les caissons en béton précontraint comportentdo nombreuses ouvertures ou pénétrations, Cos pénétrationssont .limitées par les prolongements de la peau qui coni-portent des tubes, des pièces chaudronnées et des piècesforgées. Elles sont obturées par des tapes ( ou dos bou~ebon.-*) on goné rai fixées par des organes de boulormcrie(lacets, boulons ou goujons),

352

Leo pibceo prolongeant la 'peau font l'objetde contrôles analo&un&. à celui do la peau (contrôle US).Les soudures d*assemblage sont radiographiéoo. La bou-lonnerio fait l'objot do contrôles par ultrasons et par.rossuafje.

£>'une façon générale, le contrôle do toutes cospieces ect comparable à celui prescrit par la section IIIdu ASMB.B.P.V.Godo pour los récipients métalliques dodimensions comparables de la classe A.

CONTROLÏ2 DB I.* ENSEMBLE DU CAISSONLo contrôle est éventuellement basé sur-la com-

portement d« caisson lors do JEpreuve pneumatique. Cetteépreuve pneumatique, effectuée à une pression au moinségale à 1,1 fois la pression maximale do calcul, eilo-nièine supérieure ou. égale à la pression maximale en ser-vice, est imposée en France par le règlement défini par1.»Arrêté du 15 Juin 197O.

Il convient de noter qu'avant cette épreuve»la mise en précontrainte des câbles est la première miseen charge de l'appareil, et fournit une occasion do con-trôler lo. comportement du béton avant mise on pression *

Lo'but du contrôle non destructif ost de carac-tériser lo comportement du caisson en service. Co compor-tement alnai. d«'Pini o«fc comparé nu "comportomont normal**du caisson toi qu'il a été défini par les études et onparticulier par los «SROÏ.B sua* maquettes»

Les contrôles effectués nu coux'8 de cetteépreuve sont los suivants :— fissuration~ déformations jiuicroocopiques— allongement a dans los 20210 s le» plus chargées— oxtensométrio de certaines pièces métalliques— charge,de certains cables de précontrainte

353

fissuration fait l'objet 01*1111 contrôlevisuo.1 dont la but ost do drosaor une carte dos fissureséventuel] os tic façon à verifier ai cette fissuration entredans le comportement normal du cas «son .

Les d«formations macroscopiques sont dotor-nrënccs par des relevés topo£jraphiquo;ï fournissant les dé~placcinonte, par rapport à des nxos f i \&K\ • d * un* certain•nombre de repores fixés sur le caisson «

A l'intérieur même de la masise du béton» sont\noyés, lors de la construction, des témoins sonores aptos

à fournir l'allon^opent du béton à 1'emplacement et dansla direction dudit témoin, Lo nombre de ces appareils» de1*ordre de 500 sur les caissons pi-otofcypes, est réduitpour le.s caissons suivants .

A l'exception près dos réacteurs de hjarcotile,les caissons construits on France le sont suivant latechnique classique dos câbles injectés, c*OKt à. dire quoles câb.loa de précontrainte sont, après mise sous tension,noyés dans un coulis do mortier qui assure une liaisonétroite ontro lo câble et le béton et protège 3ee câb'joscontre les risques de la corrosion . Gopendtint certainecâbles ne sont pas injectés ot restent pax? conséquentlibres dans leur logement. Ils font ,bion entendu,l'objetd'une protection spéciale contre la corrosion.Il y a uno diicaino do câble» non injectés, iio «ont «ami»a 3 cura extrémité'K d ' aticra^o de dynamonu'troa capable» ctofournir à tout moment 3u traction du câblo.

ÎHSPKOTlON ION1 SÎ-7RV1 C.K

En Franco, 1'inspection en service du caissonoot imposée par la roulement. L'utilisateur doit établirun prqfjranwne d*inspection à cet offot. La périodicité deces contrôles no doit pas excéder 2 ans ot leo résultatsdoivent être communiqués à l'odnniiistrnti on compétente quipeut, le cas échéant, proscrire toutes dispositions utiles,

354

contrôler» comportent au moins le relevédes fiesuro» ot le relové dos indications dos dispositifsdo mesure à poste fixe, c'est à dire dos témoins sonoresot dos câbles do précontraintes a dynamomètres ,ainaid'ailleurs quo dos clinomètres fixés sur certains pointsdo lu paroi, A ces appareils, il faut ajouter les fchcrmo—couples noyés dans le béton qui permettent de drosser lescartes do températures. La connaissance do ces tempérâtureaest d'ailleurs indispensable à 1*interprétation correct©dos mesures d'allongement.

Ï5n outre, l'inspection compoi'te do nouvellesmesures de déformation effectuées comme lors do 1*éprouveinitiale par dos relevés topograplïiques.

Dans l'état actuel do l'expérience acquise,on cloj.t souligner quo dans les proms ers temps do la vi'odo la centrale, les inspections HC sont on fait effectuéesavec une périodicité plus réduite ( 6 mois à 1 an}- quecolle prescrite par In règlement.Ces inspections rappro-chées permettent d'obtenir la prouve qu'il n'y a pasd'évolution sensible du comportement du caisson au coursdo son utilisation.

ECIÎATsOBURS IXTKGPKSL'emploi do caissons en béton précontraint

dans lea réacteurs nucléaires o&t particulièrement valo-risé quand In totalité du circuit do rcfroidisoementprirani.ro est abritée par lo caisson. C'oat cotte technique*dite d<* i1.integration,qui ost noj'maloment utilisée avecîos caissons en béton précontraint. Dans cott'î disposition,les échan/tours (générateurs de vapeur sont pinces à l'in-térieur du caisson et dits tt échancours intégrés"«

ï)u fait do cet emplacement, très difficilementaccessible, lour entretien et leur réparation sont oxtrô-«!etuent difficiles. On co»t3>onoo cet inconvénient en utili-sant des échangours composés de nombreux élément» paral-lèles pouvant être isolés, mais également on recourant lo

355

plus largement possible à un contrôle non. destructif en.cours de fabrication, de manière à éliminer les élémentsdéfectueux . Co contrôle non destructif étant ândirecte-trtont imposé par l'utilisation du caisson on béton précon-traint, il peut être intéressant de donner quelques indi~cations .

Ces contrôles ont lieu à quatre niveaux— sur les tubes utilisés— sur la mise on oeuvre dos tubes— sur les panneaux c'est à dire sur les circuits élémen-taires isolables— surles échangeurs proprement dits, c'est à dire surles unités introduites dans le caisson .

Le eontx'ôlo des tubes est je lati veinent clas-sique, mais étendu : contrôle dimensionnel et visuel,épreuve sous pression, contrôle par ultrasons (apnaroil-lage rotatif à plusieurs têtes) et d'une façon plus par-tielle, contrôle par courants de Foucaalt. A cos con-«trôlos, il convient d'ajouter des essais destructifs suréchantillons ainsi que l'épreuve pneumatique destinée àvérifier* l'étauchéité. Lorsque la raise en oeuvre comportela fixation d*ailettes par soudage, les contrôles non dos*tructifs de la qualité de cotte soudure sont, diff ici 3 eset pf*u probant». On so tourne aloz*s plutôt vers le con-trôlw et 1 ' enregistrement des paramètres de soudag-c. Desessais destructifs sur prélèvement complètent le contrôle.

L*exécution des pan.noaux élémentaires com-porte le r About nf je par souci tir o do« éléntont^ de tubo , Uncontrôle de ces soudures (radiographie et ultrason») esteffectué, mais souvent difficile à interpréter » L'étan-chéité est contrôlée h l'hélium (localisation do fuiteséventuelle» pai" ammoniac). Knfîn los panneaux subiaeesitune épi'euve hydraulique au douUlo do la pression nominale.

L'asscjublose des panneaux en ochangour ne coia-portc pour le circuit qu'un, nombre limité do soudurea qui,quoiqu1intérieures au caisson, restent par la suite acces-sibles aj»rcs ai'rôt et dégonflufjc du réacteur. Ces soudu-res sont radio(jrap)iiées. Les echftnceurs font l'objet d'uncontrôle d'étunchéité à l'hélium sur le site »

356

TRINO NUCLEAR POWER STATION

IN-SERVICE MONITORING OF REACTORJNTERNALS

by

M. Calcagno D, F. Cioli {"), A. Gadola ("*), G, Possa (*

G. Vanoli (——),

A B S T R A C T

Trino Nuclear Power Station

In - Service Monitoring of Reactor Internals

The on-line surveillance program, which was set up with the

aim of monitoring after a repair .the structural integrity of the

Trino reactor internals, is presented.

Since the installation of sensor on irradiated structures inside the

reactor vessel was found unpractical, the surveillance was based

on techniques that use sensors placed outside the vessel.

These techniques utilize neutron noise, accelerometer signals^ and

main coolant pressure noise.

("} ENEL - Centro Progeítassione e Costruzione Impianti

Nucleari - Roma("") ENEL - Direzione Cosíruzioni Termiche e Nucleari -

Roma

("**) CISE - Milano

357

Surveillance des pièces internes du réacteur de la

Centrale nucléaire de Trino pendant son exploitation.

L'on présente le programme de surveillance pendant l'exploitation

qui a été envisagé dans le but de contrôler lrintégrité structurelle,

auprès une réparation, de s pièces internes du réacteur de Trino.

Puisque l'installation des sensors sur les structures irradiées à

l'intérieur de la curve du reacteur a été considérée insuffisament

pratique, la surveillance s'est basée sur des techniques qui utilisent

des sensors placés a l'extérieur de la curve.

Ces tecniques se servent du bruit neutronique, du signal des accé-

leromètres et du bruit de la pression du réfrigérant primaire.

0. INTRODUCTION

During the shut-down for the first refueling, the reactor

internals of the Trino Nuclear Power Plant were found damaged bycoolant-flow-induced vibration. After the repair, a surveillance

program was set up with the main objective of monitoring the struc-tural integrity of the reactor internals.

The purpose of this paper is to describe the surveillance program

and to present the more significant results so far obtained.The original design of the reactor internals, the damages of the

internal structures discovered during the first refueling shut-down,

and the repairs performed, are also briefly described to facilitate the

understanding of the surveillance program.

1. ORIGINAL DESIGN, DAMAGES AND REPAJRS OF THE REACTOR

INTERNALS

In the Trino reactor the core is supported and positioned by

the lower package of the reactor internals, which are shown in Fig. 1.

This package consists essentially of the barrel, the lower core plate,the control rod shroud tubes, and the lower casting.

358

The barrel is cylindrical and is divided in two sections connected

by a bolted joint. At the top, the upper section has a gusseted flangeresting on the vessel support ledge. A baffle structure, shaped to

accept the core, is built inside the lower section.The fuel assemblies rest on the lower core plate, which is con-

nected by bolts to the lower section of the barrel. The core plate is

stiffened by the control rod shroud tubes, which, at the tops are

bolted to this plate and, at the bot' om, are connected to the lowercasting.

In the original design the lower casting was connected to the

barrel by tie-rods. The core wieght was partially (about 50%) trans-

ferred to the barrel through the lower core plate; the remaining portionwas transferred by the shroud tubes to the lower casting and then to thebarrel by the tie-rods. After the repair, owing to the elimination of the

tie-rods, all the weight of the core is directly transferred to the barrel

through the core plate. The overall load is then transmitted to the vesselsupport ledge.

A cylindrical thermal shield, which now has been removed, was

placed between the barrel and the vessel in correspondance of the coreregion. The thermal shield was made up of three sectors pinned to-gether and was supported by lugs welded to the vessel walls.

The control rods are supported by the control rod drive mechanisms,which transfer their load to the reactor vessel head.

The primary coolant enters the reactor vessel through four inlet

nozzles, impinges on the barrel and flows downward through theannulws between the vessel and the barrel. The flow direction isreversed in the plenum at the bottom of the reactor vessel. Thenthe coolant flows upward through the core. The flow changes direc-tion again in the top plenum and, finally, the coolant leaves the vesselthrough the four outlet nozzles, which are located at the same elevation

as the inlet nozzles.

359

Because of the coolant-flow-induced forces.

- The pins connecting the thermal shield sectors failed causing

the sectors to oscillate in the coolant flow and knock against

the core barrel and vessel,- About one third of the bolts of the joint between the upper and

the lower sections of the barrel failed. Debris of the bolts

was found in the primary system and particularly in the steam

generator waterboxes which acted as strainers.- About two thirds of the tie rods connecting the lower casting

and the barrel failed.

- Other components were found damaged but they are not relevant

to this surveillance program.

The repairs of the above-mentioned failures consisted in thefollowing:

- The thermal shield was eliminated because of difficulties inrejoining the sectors into a solid assembly and in fastening

this assembly to the pressure vessel. To compensate forthe resulting increased vessel irradiation, the eight outermostfuel assemblies were removed.

- All the bolts of the barrel joint were replaced with highstrength

(Inconel X-750) bolts of improved design.- The tie-rods were eliminated because of design and installation

problems.. The tie-rods were replaced with bolts of high-strength ma-terial (316 SS cold worked) to anchor the core plate to the barrel.

In addition, the outermost bolts of the joints between the externalshroud tubes and the core support plate were replaced with bolts

of higher-strength material (316 SS cold worked). The replace-ment of these bolts was suggested by the higher load on these

joints consequent to the tie-rod elimination.

360

2' SURVEILLANCE PROGRAM ON REACTOR INTERNALS

Utmost care was exerted in conceiving the surveillance

program on the repaired reactor infernáis of the Trino station so as to

minimize, and possibly avoid, any interference with normal plant operation.

The installation of sensors on irradiated structures inside the

hostile environment of the pressure vessel to detect stress levels andvibration amplitudes was found unpractical. Therefore, the surveillance

program has been based on techniques that utilize sensors installed

outside the reactor vessel,Neutron noise analysis is used to perform the surveillance of

vertical and horizontal core oscillations.

Vertical core osculations are of concern since the eliminationof the tie-rods made the joint, between the core plate and the .barrel,rather flexible. During plant operation, the coolant flow pushes the coreupward and reduces the plate deflection originated by the core weight.Thus the pulsation of the coolant flow causes vertical core oscillations

which may impose an excessive alternating load on the bolts of the coreplate-barrel joint. Actually, the results obtained during the operational

testing after the repairs of the structures showed that these oscillationsare negligible. Neutron noise analysis is used to ascertain that thesituation remains unchanged during operation of the plant.

The coolant-flow induced forces also cause a pendular oscilla-tion of the entire lower package and thus a horizontal core oscillation.

The characteristics of this oscillation depend on the stiffness of the

entire structure; for instance, the stiffness will change if the integrityof one of the bolted joints affected by the repair is impaired. Thus,by monitoring the characteristics of the horizontal core osculation(i.e. frequency bandwidth, peak frequency and amplitude) it is possibleto perform,the in-service monitoring of the repaired reactor internals.This is the main basis of the Trino surveillance program by means ofthe neutron noise analysis technique.

A more general surveillance on the various components ofthe nuclear steam supply system, including the reactor internals,

361

is performed essentially by detecting abnormal acoustic noise, such

as that originated by anomalous knocking of romponenis or by impacts

of loose parts and debris transported in tbe coolant stream. For this

purpose accelerometers, installed on the components of the primary

system, are used.

Finally, the pressure noise oí the main coolant is investigated

in a research effort lo obtain additional information on the performance

of plant components and, possibily, also on the excitation forces

existing inside the primary system.

2.1 Surveillance by neutron noise analysis

The work on the neutron noise of the Trino reactor started

about four years ago'under the Research and Development Program

sponsored by the European Community/1 / The detailed results of

this work are presented in the Quarterly Progress Reports on the

above-mentioned R & D Program. Therefore, only the main resultswill be summarized here.

The routine measurements for the in-service surveillance

are performed by the Trino plant personnel and by CI SE.

The horizontal core osculations cause a variation in the thick-

ness of the water gap between the core and the pressure vessel

and thus a change in the neutron attenuation between She neutron

source and the neutron detectors positioned otifside the'pressure

vessel. ' This originates a fluctuation of the ion chamber currents,

which causes a resonance peek in the spectra of the neutron noise,This current fluctuation has been called ''attenuation noise'1.

To determine if a peak is originated by hoi izontal core

oscillations, two ion chambers positioned at 180° around the core

periphery can be used. In these two chambers, horizontal core

oscillations cause relative current variations, which are equal but

of opposite sign. Thus, in the peak frequency range, the noise

signals of these two chambers are in opposition of phase and have

an equal relative variation. Of course, other checks are required

362

to ascertain that a peak in the neutron noise spectra is originated

by horizontal core oscillations. For example, we check whetherthe peak is affected by changes in control rod position, power level,coolant temperature, etc.

The vertical core oscillations are detected by means of a

method originally suggested by V . Rajagopal of Westinghouse / 2/

A vertical core oscillation, due to the deflection of the lower coreplate, causes a relative movement between core and control rods,since the latter are supported directly by the pressure vessel head.This relative movement originates a core reactivity variation andthus a fluctuation of the neutron flux, which, in turn, originates a

resonance peak in the neutron noise spectra.The neutron flux variation, for a given amplitude of the

core plate oscillation, depends upon the control rod differentialworth, which is affected by the degree of the control rod insertion.The main criterion in associating a peak in the neutron noise spectrato vertical core osculations is to check whether the height of thepeak is dependent upon the degree of control rod insertion.

The neutron noise analysis not only allows the detectionof horizontal and vertical core oscillations but also an estimate of

the amplitude and damping coefficient of these oscillations.A calculation has been performed to assess ihe detection

sensitivity of the neutron noise analysis with respect to the various

physical mechanisms that may cause fluctuations of the neutron

detector signal. The results for the Trino reactor are showp inTable 1. Using these results, it is possible to determine whethera given phenomenon can affect the neutron noise spectra.

Fig. 2 shows typical curves of the Normalized Power SpectralDensity (NPSD) of the noise of four ion chambers at 100% core

power level and with the control group rods inserted by 5%. Two

peaks at 4.5 Hz and 5 ,5 Hz are evident. In the frequency range

of these peaks, the phase between the noise signals of two diamet-trically-opposite chambers is 180°.

363

The spectra are not influenced by the degree of control rod

insertion. This means that there is no detectable core movement

relative to the control rods and also no detectable movement of the

individual control rods relative to the core.

The neutron rsoise spectra appear to be affected mainly by

the circumferential position of the ion chambers. Fig. 3 shows the

peak values of the NPSD, at 100% core power level, for the two

resonances at 4.5 and 5.5 Hz versus the circumferential position of

the ion chambers.

The experimental points of the 4.5 Hz resonance are well

fitted by the sine-squared funtion. This is the behavior which can

be theoretically predicted by assuming that the 4,5 Hz peak is

originated by pendular oscillation of the reactor internals packagealong a given direction. Also the actual direction of the oscillation

is identified.

The above considerations appear to be applicable also to

the 5.5 Hz peak, but with more uncertainty since this peak is some-

what masked by the background noise. The direction of the corre-

sponding oscillation is approximately perpendicular to the oscilla-

tion that originates the 4.5 Hz peak.

Fig, 4 shows the phases of the cross spectra, at 100%

core p^wer level in the frequency ran^e associated with the 4,5

and 5.5 Hz peaks, between the noise signals of ion chambers

Ri -10 or RI-11 and those of the remaining chambers.

In the frequency range associated with the 4.5 Hz peak, as

shown in the upper part of the figure, the noise signals of all the

chambers located on one side are in phase, whereas they are in

opposition of phase in respect to the sigials of the chambers located

on the other side. This indicates that the osculation which originates

the peak occurs only along one defined direction.

The direction of the oscillation, identified by the phase

behavior, is in agreement with that indicated by the circtitnferential

364

behavior of the peak height (see Fig. 3). It should be noted that thetwo methods that identify the direction of the oscillation use independent

information.The lower part of the figure shows the phase relationship for

the 5 .5 Hz peak. As in the previous determination of the direction of

the oscillation, the uncertainties are higher.According to the above results., the actual movement of the

internals package, as detected by the neutron noise analysis, appearsto be a pendular motion resulting from two oscillations having differ-

ent frequencies and different orientations.The physical interpretation given for the 4,5 and 5 .5 Hz peaks

(i.e. pendular oscillations of the reactor internals package) is supported

also by the fact that the theoretically calculated natural frequency of thependular oscillation in water /3/ 5. 8 Hz. Furthermore, thependular oscillation interpretation is supported by quantitative calcula-tions on the amplitude of the horizontal core oscillations. This ampli-

tude can be evaluated from the area under the NPSD peak. The valueobtained by this method is in fair agreement with that experimentallyobtained by scratch gage measurements during operational testingof the repaired reactor internals.

The effect of the core power level on the peak frequencieswas investigated with the two chambers, R1-ÎO and RI-11, which arenot connected to the reactor protection system (Fig. 5). Chamber RI-10presents, at full power, only one large peak at a frequency of 4.5 Hz.

The frequency of this peak increases to about 5.5 Hz at40% power level,while the height decreases. Good reproducibility of the frequency

behavior with the power level variation has been observed.At full power, the spectrum of chamber RI-11 shows two

peaks. The peak at the lower frequency has the same behavior asthe peak of ion chamber RI-10. The peak at the higher frequency

is practically unaffected by the core power level. At about 30%power, these two peaks have the same frequency.

365

The frequency variations due to power changes cannot be

easily explained with the present theoretical models of the reactor

internal structures. More sophisticated models appear to be re-

quired. Since power reactors do not easily allow experimentation

inside cne pressure vessel, the neutron noise analysis technique

can be used to improve the understanding of the complex phenomena

which occur inside a reactor.

Fig. 6 shows three spectra of chamber RÏ-11 at about 30%

power. The upper spectrum was obtained in March 1967 when the

internals had already been damaged. The January 1970 spectrum

was obtained when the inlernals were repaired but the secondary

core support was provisional. The July 1970 spectrum was ob-

tained with the internals in the present arrangement.

The March 1967 spectrum presents a peak which has abandwidth of about 3 Hz, whereas tiie spectra after the repairs

present a narrower peak. This would indicate a higher damping

coefficient of the vibration in March 1967, At the same time, the

amplitude of the oscillation, which is proportional to the square

root of the area under the peak, was higner in March 1967. Both

the higher damping and the larger oscillation amplitude would be

indicative of a structure having a bolted joint near failure, which

was actually the case in March 1967.

In July 1970, the lower casting of the internals package

was heavier than in January 1970 by about 10%, due to the installa-

tion of the secondary core support. It is possible that this is the

cause of the shift of the peak frequency between the January and July

spectra.

These results appear to confirm the capability of the neutron

noise analysis technique to detect changes in the structural integrity

of the reactor internals,

The neutron noise analysis'technique is now used, on a

routine basis, as a part of the surveillance program which is under

368

way at the Trino plant. Neutron noise spectra are obtained each

week by the operating personnel using an on-line spectrum analyzer(SAÏCGR) and each month by C1SE using an off-line digital computer.

In the future we plan to use the Trino plant digital computerto perform a continuous on-line calculation of the neutron noisespectra , The final aim. of this calculation is the development of an

educated "nuclear ear", available to the reactor operator in the controlroom for long-term monitoring and incipient failure detection of corestructures and reactor internals.

2 • 2 Surveillance aFor the operational tests after the repair, thirteen mono-

directional accelerometers have been installed outside the primarysystem in various positions shown in Fig. 7. This instrumentation

is currently being utilized for the surveillance of the plant.The accelerometers are Guitón AXB 4909 C piezoceramic

transducers with a charge sensitivity of about 200 picocoulomb per

g of acceleration, with a maximum operating temperature of 600° F,

and a natural frequency around 10 kHa.Seven accelerometers are placed on the vessel head: two

groups of three accelerometers are oriented triaxially (vertical,radial and tangential) and one accelerometer vertically. These

accelerometers are bracket -mounted on instrumentation port columns.Four accelerometers are placed on the steam generators (one each),threaded into blocks welded to the 'manhole cover bolts. The tworemaining accelerometers are placed on loop A: one on the main coolantpump, mounted vertically on a socket fixture over a stud on the mainflange and the second horizontally on the cold leg.

The accelerometers are connected to charge amplifiers,located outside the reactor container, and then to voltage amplifiers(Dynamic s, 75 16 with offset control and s.witehable low-pass filter on

the output). The signals of the accelerometers can then be recordedon magnetic tape (tape recorder Sangarno 3500) or analyzed on line

367

(spectrum analyzer SAICOR 21 and 31) or recorded on a Visicorder

(Honeywell 1912} or visualized on an oscilloscope. This signal con-

ditioning instrumentation all provided by Westinghouse for the tests

after the repair, is located in the rear of the control room.

The analysis performed by Westinghouse on accelerometer

signals during the operational tests after the repair was mainly a

frequency analysis. The spectral power density plots pertaining to

each accelerometer and plant condition were considered "signatures"

of the plant itself., and unexpected variations of these "signatures1'

would have been regarded as possible indication of incipient plant

malfunctioning.

The results of this frequency analysis on accelerometer

signals can be briefly summarized as follows:

- The power density frequency spectrum, of the accelerometer

signals is not a continuous spectrum., but, being made up only

by components at discrete frequencies, it is a typical line

spectrum.

- The frequencies of these components are multiple of 24.6 Hz,

which is the rotation frequency of The main coolant recircula-

tion pumps.

- The individual amplitudes of these components vary from

accelerometer to accelerometer and change with plant

conditions. In general, the highest amplitudes occur at

multiples {6, 7, 14, 18, 28, 33, 34, and 35} of the rota-

tion frequency of the main coolant pumps. It has to be

recalled that 6 and 7 are the numbers of the mobile and

fixed blades of the recirculation pumps.

- The power spectral power density plots of accelerometer

signals corresponding to identical plant conditions and

taken at different times were often found appreciably

different. These variations have been attributed, in

368

some cases, to very small changes in the state of mechan-ical connection between the aceelerometer and primary

circuit.

- The rms of the accelerometer signals is practically all

comprised in the frequency range 100 - 1000 Hz.

On the basis of the above results it was concluded that the

power spectral density plots of the acceleronieter signals cannot beadopted as a "signature" to monitor structural changes in the reactor

internals.In the operation of the plant it was then decided to utilize the

accelerometers in order to detect abnormal acoustic noise originated

by anomalous knocking of components or by impacts of loose parts

and debris transported in the coolant stream. In fact, the acceler-ometers are very sensitive in the frequency range (200 - 2000 Hz)where the typical vibrations excited temporarily by impacts between

metallic parts occur. For this reason, the accelerometer signalshave been made audible in the control room by means of a high fidel-ity loudspeaker. In addition, the signals of some accelerometersare also recorded by means of a Visicorder.

This method of acoustic observation proved to be verysensitive many times. In particular, during a plant shutdown theprimary system was hammered in various positions (vessel head,steam generator manhole covers). The hammer strokes super-imposed the usual noise (main coolant pumps were running) andwere,clearly perceived in the control room, especially for theaccelerometers located near the hammer ing point. Another indica-tion of the sensitivity of the acoustic observation is obtained duringnormal power operation, by moving the control rod group by one step. All

the details of magnetic jack functioning are distinctly perceivedÍD, the controlroom, also for the accelerometers placed on the steam generators.

369

Routine surveillance by means of accelerometers consists

essentially in:

- audition of the signals of each aceeiercmeter, once every day;

-- recording of the signals of the accelerometers located on

the steam generators water boxes, once every week.

2.3 Surveillance by pressure transducers

The decision to install pressure transducers to detect main

coolant pressure noise in a wide frequency range was based on the

promising experimental indications obtained by OISE. According to

these indications, piezoelectric pressure transducers proved to be

sensitive to very small pressure waves transmitted through long-

piping.

The pressure transducers were also installed to gain ex-

perience on another surveillance technique, which is expected to

give useful information on the performance of plant components, in-

cluding the reactor internals, and on the excitation forces existing

inside the primary system. Besides, the pressure transducers

have a better sensitivity, in respect to the instrumentation chain of

the accelerometers, in the frequency range below 20 Hz where

reactor internals vibrations are expected to occur*".

The two pressure transducers areKistler type 701 H. Both

have been installed at the end of existii g instrumentation lines, one

connected to the hot leg of loop A, the other connected to the hot legof loop C. In both cases the overall instrumentation pipes (0 : 3/4")

are about 10m long, and have several 90° bends, a number of valves

interposed on the pressure path, and restrictions to smaller diameters

(3/8"). Experiments performed at CISE for this installation clearly

showed that instrumentation lines of this type would have not affected

the pressure noise at low frequency, being on the contrary good pres-

sure wave guides.

310

The pressure transducers are connected by a low-noisecable to the charge amplifiers (Kistler mod. 586), located in therear of the control room,

The analysis performed OB pressure transducer signals is

a conventional frequency analysis of the pressure noise. The results

obtained so far can be summarized as follows:- The power density spectra of toe pressure noise, taken at

different times, and even at 3nterxrals of several months,are quite reproducible. This fulfils the first requirement

of "signature" measurement: time stability.

- The two pressure transducers appear to perceive a similarbut not identical pressure noise, in fact, several differences

can be observed. They have to be considered as two earson the primary circuit located at a significant acoustic

distance.- In the frequency range up to 20 Hz the typical power density

spectrum of pressure noise is shown in Fig. 8 for full

power conditions. It should be recalled thai below 2 Hzthe plot is meaningless, owing to high-pass filtering. The

interesting features are: the peak at 4.5 Hz, the small peak

at 8.2 Hz due to the charging pump, the broad band peakaround 10 Hss, the peak at 3 6 . 4 Hz also attributable to the

charging pump.

- Power variations down to practically zero power do not sub-stantially affect the pressure noise, which is clearly origi-nated mainly by the pun»ps. However, some minor powereffects are observed: the most striking and still unexplainedeffect is the pressure peak at 4.5 Hz. The amplitude of the

pressure oscillation at this frequency is approximately pro-portional to the reactor power.

371

- Cross spectral analysis show a some correlation between

neutron and pressure noise, which is particularly significant

5 Hz, especially at the 4.5 Hz peak.

Incidentally ; hammering or the primary circuit, is distinctly

perceived also by the pressure transducers,

3. CONCLUSIVE REMARKS

The three techniques used in the surveillance program now

under way on the Trino reactor have two quite attractive features:

1 . They provide a continuous report on some important phenomena,

which occur inside the reactor vessel and primary system, without

perturbing normal reactor operation and by using standard detec-

tors located outside the primary system components.

2. They have a high sensitivity, which is typical of the noisetechnique .

In a first phase of the surveillance program, the so-called

"signature approach" is essentially used. This approach can be

better described by comparing ii with electrocardiography used inthe medical field to detect heart anomalies. A history of the patient

(in our case the reactor) is kept so taat when a defect shows up, the

anomalous graph can be compared with the normal one in order to

diagnose the problem without necessarily knowing the physical mech-

anisms which cause the departure from the normal graph.

However, significant anomalies in 3arge power reactors are,

hopefully, expected to occur quite seldom. Consequently s an empirical

correlation between reactor anomalies and associated changes in the

neutron noise spectra cannot be practically established as it was in the

case of electrocardiograms. Moreover, the above surveillance tech-

niques do not necessarily detect every significant reactor anomaly and,

on the other side, every departure from the reference condition does

not necessarily represent a reactor anomaly.

372

The "signature approach" may give only an alarm to the reactor

operator. Afterwards, it is difficult to establish whether the alarm isoriginated by an actually dangerous situation, or by a situation of

marginal concern or by a departure from some conditions of no

importance to reactor operation.In view of the foregoing, the "signature approach11 cannot be

considered fully satisfactory for the reactor operator. "We rather believe

that full acceptance of any surveillance technique will be possible only

when the physical mechanisms causing a departure from the reference

condition are well understood. This should be the main objective of

the research efforts on the surveillance techniques of the type used

at Trino.

Based on the results obtained at the Trino Nuclear Power

Plant, the surveillance techniques we have described appear to be

promising for in-service monitoring and incipient failure detection

of reactor internals and other plant components. However, the

validity and the usefulness of these techniques is not to be over-

rated before significant experience has been accrued.

REFERENCES

(1) M.Calcagno and F.Cioli, nIn~Service Monitoring of CoreStructures and Reactor Internals by Neutron Noise Mea-surements". ReportENEL-CPNC3.nl/D9.70, August1970.

(2) V. Rajagopal, D.Rawle, and G . C . Andognini, "NuclearNoise Measurements on Core Vertical Motion", ANSTransactions, Vol. 12, No. 2} 1969, ANS Winter Meeting.

(3) L.Lazzeri and O.Dini "Vibrations of a Pendular Structureof Circular Cross Section Inside a Concentric Pipe", ïnge-gneria Nucleare, anno X, No 4, 1969

373

T A B L E 1

DETECTION SENSITIVITY OF THE NEUTRON

NOISE SPECTRAL ANALYSIS AT FULL POWER

SOURCE OF THEFLUCTUATIONS

DETECTOR SIGNAL

REACTIVITY OSCIL-LATIONS

VERTICAL COREMOVEMENTS

HORIZONTAL COREMOVEMENTS

MAIN COOLANT AVER-AGE TEMPERATUREVARIATIONS

MAIN PRESSUREVARIATIONS

PEAK-TO-PEAKVALUES AT 5 Hz

0, 05%

0. 25 pern

20 mils

2 mils

0. 02°C

0. 1 kg/cm'

PEAK-TO-PEAKVALUE AT 20 Hz

0, 01%

0. 05 pcm

4 rails

0. 4 mils

0. 005°C

0. 02 kg/cm

The fluctuation is assumed to be a wide-band random oscillationsuch as that originated by a white excitation on a second ordersystem having a damping coefficient of 5%. The ratio between themaximum amplitude and the root mean square of the oscillationis assumed to be 3.

374

CONTROL ROD DRIVEMECHANISM

BARREL

BARREL JOINT

CONTROL RODABSORBER

THERMAL SHIELDCORE PLATECORE PLATE JOINTSHROUD TUBE JOINT

SHROUD TUBE

TIE RODS

LOWER CASTING

Fig. 1 REACTOR VESSEL INTERNALS OF THEd -*«>--»«»«»w»»s*w^

TR!NO PLANT.

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Abstract

The paper describes the non-destructive testing techniques applied toZr alloy and stainle&s stopl fuel cans» pellets and assemblies. The our-linoof post-irrndiatîon inspection facilities to be constructed for fast breederreactor fuels is introduced. Various NIT procedures during the fabrication ofpressure vessels are discussed and th«» comparison of radiop-raohic andultrasonic methods are given, Tho experience and the future possibility ofinservice inspection ar^ dealt with. The KHT problems of internals, especiallyultrasonic testing of aut-tenxtie staialesa steel veld metal arr presented.

1, In troduct i on

in Japan, or/net ruction is rapidly proceeding on the powerstations employ-', tig light-inter reactors. On Loe other hand,manufacture is ID progress of reactor-plant componente for exportto oversea QounJ.rlec; uhl lw cie\reloj;rpent work in proceeding on afact breeder reactor, an advancoa thermal reactor, and high-tetuperature gas reactor. In order to ensure the safety of thesepower reactors, it is essential! to carry out the nondestructivetesting or Inspection for any defects botu in the process ofmanufacture arc? during the rcac;tor operatJon. Without informationon the, sizes and location^ of aicb dc feo tc , the experts in fractureiaeohajiics are unable to give any ju'J^ement on th* intogrity ofstructural coMponeats,

383

In tnis report there ire described vie nondestructive et

and inspections cai^ried out on ti-o i'uoi elements, pressure vesselsand veesel internals of roacbore in vario as types anôi the futuredevelopments.

". Nondestructive l P F * i n % b of fu- 1 eler.eir* r

?.l» Zr baee illo r and rtainlcLf rteel o in?

Zr b-ti e al loy ^tnt. ar^ iced -"o- fuel ol-3doin t-c of Li^hi -waterreaotorc and in advanced faerr.il reactor {^-,0 m o v r M e d and boiling l

w^:er ooolea) oein^ ¿eveioped m Japan.

The teítin.¿r ind i n s p o c f i O n oT ;lidding hubot ir«.- >ht-n ' f t iJc both ir>

ttie minufanture of oans and the minufricture of r'uel oLcnen t r . The dimenFion.il

toleramef and illowable defect:, are ver/ b t r io t» therefore requiring the high-

level technique? of nondes +ru";tive tec i ings, . frtie following noTidectructivetettings are carnea out in the inspection of nladdin^ tuber:

a) Pinirh ^pne trance, including bhe surface rouannes i, Ir, viaual

"b} Meaeurementi of the inride Jiamctert. bv i±r gau¿j.n ,.c) Inrpeotion of the d e f e a t bt,r the uitn oni^ muthoHl .

d) Moj.&ureTiionl of the wii] thi o^nyt-r vfith îdi ?aê (ultnsonin method).

Stainless. stee3 oinr -*rc u£ cd in the nuoleir s h i p "Hutoa" arid j.lso f j,r t, breeder

reiotor. f'or the ¥»C. "%d su" , the came rethodL of nondo&truo' ive teriinf, asfor Zr alloy Ccxru were employed .

For fxf.t breeder reiotor, the add;/ curren f ie: tirv, it applied for defoots

inrpection, in ¿ddition to tho?e for Zr bare alio;/ ân" .

In ihe experimental i j s t breec^r êactor "Joyo", tnc o^

gathering cistern of norde. tractive I f ^ t i n ? reûltt ]i IP been developed, bo that

thtí in-core behaviour ind por t-irr iii ttion test result r" o -,n oe oorreLtied inthe fuel cladding tuber.

In thir c/ at era, tne u3 tr iconic -fester for 'iefcoif int.pe^ti ons the vid i gagefor wall-thioknerr meaf uremeni' ? the eddj-rarrent tetter for defects inspection

ana the electroni ï ni prometer for ou*" si le— diameter nearuremerit are arrangea

in perief. Glactdinii ¿a.nr are forced through the &erief for automiti^ moxsurenent ;

and tne acceptance or not «ire juagea from +he output eign-ilc produced.

384

The data then ai-c converted into "dlga-ljj ," to be stored inmagnetic2 » 2 . Manufacture of f a c t pellets

For the product lo^ of f u e » peJ lets !>) the "Joyo" » the automaticinspection machine has been developed ior ^'.doioatic iieasurenenteof the diameter, lergth, v/eifht and density of sintered pellet?.

As seen in Fig, 1 , the âchire in rê up of five stationsin a circular table. A pellet fer1 ir LO otntJon No. 1 is transferredto ono after another ¿station, by neaain of a rotatirg manipulator.Station No. 1 i £5 for pollen loading. In station No. 2, the diameterof a pellet is meaourccl I y eai ejectronic ¡aloróte tor; in stationHo, 3 the length by an electronic micrometer; an station So. 4 theweight by a high-precl c ion counter balancer; and finally in stationNo» 5, the pelle c is discharged, beJng collected in a specialtray» The pellote failing the teats aru automatically discarded.

The measured results in station Ko. 2 to 4 axe fed to digitalvolt meters, and thon recorded suitably .

The density of pellets can also be obtained from the measuredvalues in a computer.

The inspection speed of the machino is 180 pellets per hour.

2.5. Manufacture cf fuel assemblies

For the iuel pins produced, the end -plug u e lei IP inspected byX-ray radiogr-iphy, an-l the ioakê by a TÎ&SS ope -jtrome trie He-leak detector.

Fuel ase-e-nblacs for the "Joyo'' are of the wire trapping spacer type.The defects inspection of iUamlerr, steel wires ic made by the eddy-cur rentmetaod.

Por the wrapping of a fuel a?re<nbly, a hexagotial wrapper lube i& ueed,vihich is made from a round lube by cold drivcing through the dies»

Por the original round tube, defecre inopcotion is made by the ultrasoninethod ond the wall thicknosc in me-ieured by a vidiîge.

Tnen, for the hetagonal tube produced, the apparatus defacribed "belowhcic been developed for cumc/isionaj. irjrpeolion.

3y thi c apparatus, the following can be obtained; They are,

385

the outside faoe-to-fae^ dimension, the inside face-to-facedimension, straightness, and twist. It IB of a two-dimexisionuLmeasuring systeia. Twelve electron-microiaete-T heads are arranged,as shown in. Pig. 2, and they are moved along the hexagonal tubefrom one end to the other.

The outputs of analog signals frota ea-li micrometer arc recorded in recording

chart. The outputs are then processed in an Analog-Digital converter; andthe paper tapes are prepared. By computer processing the tapes,a list of the >?all thicknesses, face-to-face dimensions, rtraightnessand twist are printed out as output,

For the dimensional inspection of a fuel assembly, the followingmethod has been developed.

A fuel assembly is laid on the inspection Ublc,and subjected to three-dimensional measurement by a magnetic-scaling system. The measured values are automatically indicatedin digital and recorded by a printer at the same time. Paper tapesare then prepared from the records. Finally, by computer processingthe tapes, the following are given as output; the cross-sectionaloutside shape, face-to-faoe dimensions, len¿^th, atraightness, andtwist.

2.4. Post-irradiation inspection facilities

Fuel Monitoring Facility will be constructed close to theexperimental fast breeder reactor "Joyo" for the post irradiation nondestructiveinspection of fx;el pins and assemblies,

As shown in Fig. 3, the FMF consists of two large cells.Since Pu-bearing fuel elements are handled, they are of the

<x~Y type hot cells. The inride of concrete cellsare lined with stainless-steel plates to have «-tightness,.

In cell (l), fuel assemblies withadhesion of the sodium, are handled, and so it is in nitrogenatmosphere. Then, in cell (2), the fuel pins freed from sodiumby the washing are handled, and so the atmosphere is air.

A fuel assembly irradiated in the Joyo is introduced into thecell (l) through an underground passage. In this cell, thenondestructive testing of a fuel asuenibly arid its disassembly areraade. Then, ÍK the cell (/?)> the nondestructive inspectionof fuel pins is made.

386

The flow of inspection procedures in these cells is shown inFig. 4.

In cell (l), the visual inspection» dimensional measurementand the r-scanning to measure a burn-up distribution are made on» »a fuel assembly received from the Joyo. Subsequently to this, thefuel assembly is disassembled, to take out the fuel pins. Fuelpins are washed to remove sodium, and then transferred to thecell (2).

In cell (2), the following(nondestructive inspections are madeon the fuel pins; visual inspección f Q.£ .yie external appearance,measurement of the weight, outside diameter and length and straight-ness, the leakage, the r-scanning, and neutron radiography toobserve the pin Interior,

Studies are in progress at present on the nondestructivemeasurement of a fuel-pin absolute burn-up value by r-scanning.

Tíie FMP will also be advailable for the nondestructive testingsof other fuel assemblies irradiated in other reactors.

After the nondestructive inspection in the IMF, if "destructive"testing is further necessary, the fuel pin • is cut incell (2). The fuel portion is then sent to an Alpha-Gamma Facilityand the structural components such as cladding tube and wrappertube are sent to a Material Monitoring Facility.

Both the âGF and MMF are located in the Oarai Engineer-ing Center, together with the Fuel Monitoring Facility.

Operation of the PMP vail be started in 1974.

3. Nondestructive Testinge of Reactor Pressure Vessels

3.1. Nondestructive testings of reactor pressure vessels duringthe manufacture(l) The present state of nondestructive testingsThe nuclear power plants produced .to Japan are mostly light-

water reactors by introduction of the technology from the UnitedStates. Therefore, the nondestructive testings made on thepressure vessels and the acceptance standards for defects andflaws are in principle according to the ASME Section HI "NuclearReactor Vessels," However, the views are prevalent that,

38?

considering the great consequences of a reactor accident upon thenearby population, the regulations etc. in ASMS Section HI are toolenient, compared to those by the Ministry of International Tradeand Industry for thermal power plants and chemical plants.

In general, the purchasing inspections of vessel materialsand the inspections for vessel welds practiced in Japan are morestrict in both procedure and acceptance standard than are providedfor in ASME Section HI. And moreover, for the fabrication ofreactor-plant components, the pressure vessel in particular, thetechnology is highly advanced and the skill of workers also high,in Japan. The quality of reactor pressure vessels produced isthus superior, the reputation among users correspondingly high.

In the following there are described the nondestructive testings

and inspections carried out in Mitsubishi Heavy Industries Co.,which is a maker of PWR plants in Japan.

(2) Radiographie testingsPor the radiographie testing of steel plates (thickness over

100 mm), a linear accelerator (12 MeV) of Mitsubishi ElectricCorp. is used. The linear accelerator is very stable in operation,the operation ratio being nearly 100$; it is superior to similarmachines in other countries. The dose rate is 2000 H/min-m, andthe radiography of a steel plate, thickness /¡oomm ,for example,can be done in about 8 min. The sensitivity of the penetrameteris also high, about 0.5$. The radiographie inspection with theaccelerator is shown in Fig. 5«

Por seal weld between the tubes and tube sheet of a steamgenerator, X-ray radiography is also carried out, in addition tothe pénétrant testing. Since the procedure was used, the qualityof seal welds has been remarkably improved. The leakage at sealwelds is none, to an admiration by the users.

The acceptance standard in radiographie inspection is byJIS Z 3104. The JIS Z 5104, however, is too strict for porosity,so that slight defects have often to be repared. This occasionallyleads to the deterioration of the weld itself. It appears thereforethat the JIS Z 3104 needo to be revised appropriately.

(3) Ultrasonic testingsThe purchasing inspection carried out on steel plates is more

strict than in ASME Section HL. For instance, a Gakushin IEC type

388

standard test block (Fig. 6) is employed, in which a flat-bottomeddrilled hole of diameter 2,8 mm is made 150 mm from the contactsurface of a search unit. For the defect-detection sensitivity,the reflection wave-height from the drilled hole is set to theC.R.T. QQfo scale, for a plate of thickness over 80 mm. The per-missible size of a defect is a maximum of 2"0 (50 aim), whereasit is 5"0 in ASMS Section M.

For the forged materials, the Gakushin III type standard test blockis also used» If any indication exceeding the standard level of a reflectionfrom the 2.8-ram $ drilled hole after correction for the distance amplitudeis shown, the steel plate is rejected, ac unacceptable. Therefore, even ifpassed, the Inspection by MITI or other licenser, the materials are occasionallyrejected.

Por the inspection of the stainless-steel overlay cladding on the insideof a pressure vecsel, only pénétrant- testing is specified in ASME Section III.In order to examine the integrity of bonding between the "basse métal andcladding, however, the test ie. also made from the side of overlay claddingby the procedure in ASME Section V, The acceptance standard, in thic case,is far more strict than that by MITI; defects exceeding 3 mm x 25 mm arerejected a& unacceptable (in MITI» up to 3" 0 are acceptable). Consideringthe difference in ultrasonic penetration between the flat and valley portionsof an overlay cladding, reference elite are made, in a sensitivity teet specimen,as shown in Fig, 7-

I seeras then that the detection sensitivity as specified for thewelds in ASME Section III is too low. The defects detectedby radiographie inspection are readily observable by ultrasonictesting. In the case of ultrasonic inspection, however, the weldsare often accepted only because the reference level is notexceeded by a C .R .T , wave-height. In ASME Section HI, the weldsare rejected if a crack» incomplete penetration, or lack of fusionis detected. However, as also pointed out by R.J. Roehro, amember of the ASME, the distinction between plane flaw and slaginclusion is hardly possible in the present techniques ofultrasonic testing. It therefore seeras necessary to revise theASMS Section 2H in procedure and acceptance standard of theultrasonic testing, so as to meet the actual situation,

389

(4) Magnetic-particle testingIn ASME Section 2H, magnetic-particle testing is not specified

for the steel plates. For the purchasing inspection, hovrever,the whole face is tested by the prod method. Arc burning portionsare all removed by grinding. For the weld, testings at every 1/3thickness of the whole layers or the testing of a back chipped portion n

are carried out in order to ensure the nonex.istance of weldcracks.

(5) Pénétrant testingsÎPhough not specied in ASME Section 3H, pénétrant testing in

the whole face is carried out on all the stainless steel platesand Inconel materials. Then, for the weld between stainlesssteel plates and between stainless steel plate and Inconel, complete

inspection for a weld crack is made at each layer, at every 1/2",or at 1/3 thickness of the who^e layers, depending on the circumstance.

3.2. Radiographie Examination and Ultrasonic Examination

Ishikawajiraa-Harima Heavy Industries Co. , ltd (IHI) hasreceived orders of manufacturing of reactor pressure vessels forUSA and Sweden as well as for domestic nuclear power stations,and some of them have already be-n delivered from IHI1s YokohamaNo. 3 works.

The reactor pressure vessels for USA are fabricated andinspected in accordance with ASffiS Section Iff Class A vessel andthe Swed-î-h vessel is to meet Swedish own criteria and partlyconception of ASME Section 3ÏÏ is applied especially for designand materials» Domestic vessels have to comply with MITI Code(Ministry of International Trade and industry) which is consisted of both material,design and fabrication standard, and welding standard. 'The code if. basically similar

to A.SME Section III for nuclear power plant components and some additional and morestringent requirements are provided buned upon our own e^periencei; and understandings.

It ic caid that the Swedish concept for the -¡ode requirement is almo; t similarto the other Kuropean countries except countries adopting 4RME code.

accordingly we are in a position to be able to oomp-'ire these differentcriteria in U£A, Europe and Japan especially different application and standardof nondestructive examinations? for welding seams.

Table 1 showr- application of nondestructive examinations for typical

reactor pressure vessels which are made or being made by IHI. Pukushima Unit 1

390

and 2 vessels are for domestic use and Ringhals 1 and. Browns Ferry 2 are forSweden and USA respectively.

The most significant difference ¡between ASME and Swedish code is nondestructiveexaminations applied to the welding seams. ASME code requires only radiographieexamination for "both main butt weld joints and nozzle weld to the shell,

In addition, however, the ultrasonic examination was performed for theabove welding seams and CED (control rol drive) stub tube weld to a bottom headin accordance with the requirement of ASMS Section III 19&8 edition,

In the case of -vessels for USA, the results of the radiographie examinationprecede that of ultrasonic examination if a confliot exists between twoexaminât i on s.

In turn, no radiographie examination is required for all butt weldto be examined for Swedish vessel. Acceptance standard is basically statedin the specification, however, evaluation and judgement by authorizedinspectors shall be exercised for the decision»

Accordingly, it is requested that such anthoriaed inspectors shallhave knowledge of not only non-destructive inspection but also welding andmetallurgical engineerings» etc.

And in such case, this way has advantages that knowledgeable inspectorsmay determine the results of the examinations by evaluating several factorssuch as welding technique and several circumstances, etc.

As the early stages of the fabrication of the Swedish vessel,additional radiographie examination was performed for some weldingseams in order to get the idea of the difference of two methodsperformed by the fabricator.

Pig, 8 shows one example which contain several indications tomake comparison of two examinations, radiographie examination andultrasonic examination, and this indicates coincidence with someexceptions,

391

Table 1 APPLIED JTOÏî-DESÏRUCTIVB EXAMINATIONS FOR TYPICAL BWR KBAGTOR PRESSURE VESSELS

Plantof WeldedJoint Main Seam of Tassel Nozzle Weld CR.D Stub to Bottoü Head

Fukushiraa - Unit1 RPY (TEPCQ)

1.

2. ASME Code Section1965 Edition

1. RT + ÜT(Reference only)

2. ASME Code Section 121965 Edition+ MITI Code

1. 1/3 T progressive PT

2. ASMS Code Section HI1965 Edition+ MITI Code

CDKS

Fukushima - Unit2 ?J?V (TEPCO)

1. HT + UT (Longi +Shear)

2. ASKB Code Section HI1968 Editioni- MITI Code

1. RT + OT (LongiShear)

2. ASMS Code Section HI1968 Edition•f MITI Code

I Ringhals - Unit| 1 RPV (S¥3DISH STATE

BOARD)

BROVSTS PERRY - Unit2, 3 (TVA - USA)

1. UT (Longi 4- Shear ¥ave)2. ASEA-ATOM SPEC.

1. UT (Longi + Shear ¥ave)2. ASEA-ATOM SPEC.

1. RI + UT (Longi + Shear¥ave )

2. ASME Code Section m1965 Edition

1. RT + UT (Longi + Shearïave )

2. ASîyDB Code Section m1965 Edition

1. 1/3 T progressive PT+ UT (Longitudinal wave)+ RT (if unacceptabledefects detected byUT)

2. ASMS Code Section m1968 Edition + MITICode (No applicablecode for UT)

1. UT (Longi + Shear)2. ASEA-ATOM SPEC.

1, 1/3 T Progressive PT

2. ASME Code Section HI1965 Edition

* 1. ÎIDT procedures 2. Acceptance criteria

Applied procedures for the examinations are as follows;

Radiographieexamination : ASME Section III, 1968 edition.

Ultrasonicexamination : Method

Straight "beamAngle "beam with, angles of 45 an& 60

EquipmentKRAUT XRaEMER

Scanning sensitivityReflection from 2 mm diameter hole isto be 100$ screen height»

Acceptance cri fceriaRoot defects, laok of fusion, cracks andother notches, pores in accordance withI1W character blue, and 25 IBIS and longerslag inclusions shall be unacceptableindications.

As shown in Table 1, recent reactor pressure vessels bothfor USA and domestic use» ultrasonic examinations according toASME Section III are performed for the butt weld joints in additionto the radiographie examination. In most cases, it is very seldom

that indications or area accepted by radiographie examinationwere rejected by ultrasonic examination, and no crack-likeindications were discovered.

ultrasonic examination for Inconel GRD stub tube weld for FukushimaUnit 2 vessel has been performed as shown in Pig. 9 and one indicationwas removed and repaired. Size of the indication in the radiographie filmis shown in Pig. 9 and the data is compared with the data of ultrasonicexamination. The ultrasonic examination procedure is as follows»

Search Unit SMSB B4T separate typeReference hole 1,6 mm flat bottomMaximum permissible indication is 6» 5 KU» with

distance amplitude correction,

Either using radiographie or ultrasonic examination, whenexamined carefully and used proper acceptance criteria andprocedures, there will be no significant difference of thequality of the vessel in the practical point of view*

393

More important matter for "tíae production or for the workshop in regard to use whether radiographie examination orultrasonic examination, is the availability of the inspectionalpersonnel who is qualified and accustomed, with the particularexamination and third party inspectors who have sufficienttechnical capability not only for nondestructive examinationsbut general engineering knowledges for the reactor pressurevessels especially in the case of ultrasonic examination.3.3- Hondestractive testings in the inservice inspection

As an instance of the inservioe inspection for a reactorplant in Japan, the iare of the pressure vessel for Japan PowerL-emonstration Reactor, JAEHI, will be described. The JPDR, ofa BWR with output 12.5 MWe, is the first nuclear power plant inJapan; it started operation in 1963. In the first regularinspection in 1966, many fine cracks were detected in the overlayc.iadding on the interior of top head, nostly in the manuallywelded portion. Before continued usage of the pressure vessel,it was necessary to examine the overlay cladding for any cracksin other sections of the vessel, especially the positions offorced-circulation nozzles and the bottom head (manual overlaycladding). For this purpose, the techniques of surface exami-nation by a borescope and of the crack detection by a remotelyoperated resistance-probe method were developed.

In the obEorvatiorx of essel i iterxor surface by a remotely-operated borescopej an iodine lamp or an underwater light was usedto improve the illumination. Since the borescope lenses getcolored due to the high-level radiation, irradiated fuel andcontrol rods were removed in the inspecta on to minimize theradiation exposure; and the lenses were also exchanged. Then,to remove oxide film adhering on the interior surface, a surfacepolishing unit using a special brushing agent "Scatchbrite" byremote operation was developed. Por positioning a borescopeproperly, the remote control as shown in "Fig. 10 was contrived.

To measure the size and depth of cracks quantitatively, anondestructive instrument (a Sme- k gauge) utilizing the electricresistance method was developed in the cooperation of MitsubishiHeavy Industries Co, and J1ERI, As shown in Fig. 10, it is

394

based, on the principle that the resistance of an electric current,flowing transverse to the length of a crack in a conductingmaterial, is proportional to its depth. With this nethoa, theinterior surface around the position of a forced-circulationoutlet no Kale and the inside of vessel bottom head were examinedin x^68» As the result, several cracks were disclosed on theinterior surface of the west cablet nozzle, as shown in Fig, 11.ïhese cracks, however, were below 50 mm in length and about 2.5mm in depth. They had not reached to the base metal of pressurevessel, and so the continued usage of JPDR pressure vessel wasapproved by the concerned authorities. Similar inspections werecarried out again on the pressure vessel in 1969 and 1970; nogrowth of the cracks was observed. At the time of vesselinspections, the ultrasonic and the pénétrant observations werealso made, In the ultrasonic inspection of nozzle positions fromthe outside, where the radiation intensity was high, a personalexposure of 950 mR at; highest was encountered.

Por the inservice inspection of the pressure vessel of PWRpower plants, the development of a remotely-operated, automaticultrasonic inspection facility is in progress in MitsubishiHeavy Industries Co.

It is designed to "be capable of volumetric examination atall positions of PWR pressure vessel as specified in ASMESection XI. The schematic drawing of the facility being developedis shown in Fig. 12. She facility essentially consists of fiveseries of manipulators, a lead- airconate transducer beingattached to the end of each series} the search, units are remotelyhandled from the operation panel, The scheme will be testedshortly by mock-up.

395

4. Nondestructive tec-tings of the internal components

4.Î. Tîie present stats of nondestructive Sestings

Por the internal components of reactora, austenitic stainless steelsare mainly used, special materials such as ïnconel etc. in few cases.

The nondestructive test inga during the manufacture of reactor internalcomponents;, including their materials, are well established in various codesand regulations; the acceptance standards are also definite. Therefore» thereis not much problem.

In order to perform the testings both inexpensively and speedily, however,further studies are necessary in the future. Por the rolled and extrudedmaterials, ultrasonic and pénétrant testings and for the forced and oastmaterials, ultrasonic, radiographie and pénétrant testings can give theresults of fairly high reusability. Then for the welds, radiographie andalso ultrasonic and pénétrant testings are ueually used. To perform morerapidly and lees costly or at the construction site, ultrasonic inspectionis often substituted for the radiographie inspection; the eddy current methodis utilized; or in other caser, alternative nondestructive techniques ofconvenience are employed. It is however neceesary to investigate thereliability of each method.

What are more significant in the inspection of internal componen tsare the measurements of size and shape. A variety of the techniques areused at present, including optical, electrical» and special air micrometer,and three-dimensional measurement with a computer. The present state,however, is the constant search for any methods of measui^eroent of the exactsises and shapes.

The methods of nondestructive testings, readily availableduring the manufacture» become difficult to perform in the actualoperating reactor, oven during the shutdown, because of theintense radiation. Therefore, such inspections are mostlycarried out by optical or electrical visual observation and byultrasonic testing, including acoustic emission. For theacoustic emission method, various types will emerge in the future.Por the others, however, their maneuver» i.e. in the scanning,•will be an important subject to study.

396

In the following will be described the experimental testsperformed with internal components and the results.

4.2, Ultrasonic testing of austenitic stainless-steel(AISI 304) weld metal

Internal components are mostly made of austenitic stainlesssteels; and ultrasonic testing plays an important role in "boththe material and weld. For the austenitic stainless steel,however, the attenuation of ultrasonic waves is very considerable

^especially in the weld} making the method nearly impossible.ïo secure the safety of a reactor plant, urgent developmentwork is necessary in this respect,

Following are the experiment made for the pre-serviceinspection of a reactor plant and the results obtained.

There are various methods of ultrasonic testing for thewelds. For the shear wave method, assuming longitudinal andtransverse cracks in a weld bead, a test piece was preparedwhich had the two kinds of crack both on the outside and insideof pipe. Then, to obtain information on the characteristics ofweld metal as reference, for the straight beam method two testpieces ,each of raw material and deposited metal were prepared:their thickness were 1.5" (38 mm) with a circular flat-

bottomed hole. For the shear wave method, a probe of small sise9 aun x 9 nun, made of lead zirconate, was chosen, because oflarge attenuation in the case of rock crystal. The testerused was of the pulse echo type. Por the frequency, 2.25 MHis suitable both for the attenuation and sensitivity.Conditions are not so stringent in the straight beam methodas in the shear wave. But with emphasis on the sensitivity,a probe of zirconium-lead-titanate was used.

In the shear wave method, it is not easy to obtain the correlation"between defect shape and echo height and the correction for distance. Theyhave to be acquired at positions intermediate "between a half skip and one skip.

In the straight beam method, fairly good results are obtained (Figs. 13to 18). The difference in sensitivity between raw material and depositedmetal is as large as It dB for the 38 ¡rim; the difference in sound velocityis then even 430 ra/seo. (The austenitic- grain size of raw material is 4»)

397

In the shear wave method, the decay in the pr^obe is still appreciable.Efforts thus must "be made to develop the probes of improved material. Tofacilitate the testing operation, «otne means muot be thought of "by such as thesolution, heat treatment to the weld of aur-tenitic etainless otee].4.3. ïîondestructive testing of the abaorber rode

For the nondertructive inspection of stainless steel tubing for defects,ultrasonic and eddy current methods are available. Stainless steel tubingof outer diameter 4«p> rani and wall thickness 0. 4 rara are used for part of thecontrol rods in BHS plants. To establish the nondestructive techniques forthese tubing, studies have been made. In thi case, it is necessary to detectsufficiently the longitudinal "standard" defect LO u deep on the inner surfaceof a pipe.

The results of the studies are as follows:1) In the eddy current method, the ratio of defect detection sensitivity

on the inner and the outer surface is 1 : 2. In the ultrasonic method, however,no difference is observed in the eensiti^ity.

2) A defect of depth 6n u on the inner surface can be well detectedby the ultrasonic method; but it is not so by the eddy current method.

3) In general, the magnitude of a detection signal is not proportionalto the size of a defect in both the ultraaonic and the eddy current method.Por the "discontinuities", the value indicated is even about twice as lar¿ eas the actual size.

4) Noire Is caused by dimensional change and the variation in materialin the eddy current method; and by crookedness, variation in the grain sizeand surface roughness in the ultrasonic method. .

Standard defects can be detected by the ultrasonic method, but notthe size of discontinuities. The method ten-is to overestimate thecircumstance of defects, in actuality leading to unnecessarily hi^h qualityof the products. The criteria of acceptance may be Pet txv means of theabsence or presence of an indication signal equivalent to the signal heightfrom the standard éO u defect (a,ii the full scale).

eddy current method, on the other hand, if the signal analysis isimproved, can give better information of approximate location and kind ofthe defects, which are imposrible with the ultrasonic method. Moreover,owing to the rapidity of inspection and the ease of adjustment and maintenance,the raekho'l can be a highly economic one.

398

ACKtîOWLEDGEMMT

The author is deeply indebted to T. Ishida (Power Reactor and ¥uclearFuel Development Corporation), T. Yamaguohi (Mitsubishi Heavy Industries),M» Amano, A. Kurokawa (lehikawajima-Harima Heavy Industries, Ltd.) and R. Ishii(Tokyo Shibaura Electric Go») for the preparation of the present report.

.No.2 Station

No. 3Station

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JSo.l Station

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C onveyor

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Wo.l StationNo.2 StationNo.3 StationKo.4 Stationlío.5 Station

LoadingDiameter inspectionLength inspectionWeight inspectionUnloading

Fig. 1 Pellets automatic inspection machine

399

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0 A2,B2,C2,D2

Al,Bl,01,1)1

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400

OFFICE

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402

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Location of indication

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406

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407

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Pig. 12 Schematic drawing of the remotely-operatedultrasonic inspection facility inspection tool

Base metal Deposited metal

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0 20 inm*-Defect sise

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409

i yy^^

by

.G A MannU.K.

ABSTRACT

Nondestructive tests are described which are applied to the material fromwhich the cans are to be made, to intermediate stages of fabrication, and tothe finished cans» The ultrasonic method is in general use in the early andfinal stages of fabrication, while the eddy-current method ia also used inintermediate stages, as a complementary test to the ultrasonic one, or as afinal test on tuMng where for sosie reason the ultrasonic method is notconvenient. The ultrasonic test which is now conventional for metal fuelelement tubing is described»

RESUMELe mémoire décrit les contrôles nondcstructifs qui sont mises au point pourles matériaux destinés à la fabrication de gainage? pour les tubes au coursde la fabrication, et pour les gaines fabriquées» La méthode d'ultrasons estutilisée .avant et après la fabrication? la méthode de courants de Poucaultest utilisée pour les étapes intermédiaires? pour un contrôle complémentaire à laméthode d'ultrasonsj ou cornine contrôle finale pour le gainaga v¿uand la méthoded'ultrasons est peu convenable. On décrit la méthode d'ultrasons qui estappliquée actuellement pour le plupart de gainage métallique»

411

Son-destructive tecting is applied in the examination of the material fresa whichthe cans are to be made, to the finished cans, and depending on the particulardesign of canf at ono or moro intermediate stags in fabrication» The examinationin British practice is for flaws only. Other properties of materials eg elasticconstants, grain size, may Toe determined non~destructivôly but euch déterminationsaxe not called for in any current specification» ïhe preferred method for flawdetection is ultrasonics, with eddy-currents being considered e ooapleaiantary,auxiliary, technique,MaterialsCans are at present "being made of a variety of high chroî$ium~nickel alloys. Theelastic properties of these materials aro sufficiently alike for essentially similarultrasonic testing techniques to be used with appropriate adjustment of parameter»*Tests are carried out at various stages;Forged, rolled or extruded barExtruded or cole-reduced hollowsCold-reduced and drawn tubingMachined tubing(Any particular design of can may not include all these étages),Bars and HollowsThe ipose of testing bara and hollows is to reject material containing flaws suchas inclusions at an early stage» This ie desirable to avoid waete in processingmaterial vdiioh may subsequently have to bo scrapped, and also to remove flaws such asinclusions which will become harder to detect at lator stages when they have beendra*«m to * stringers1 of increased length and reduced diameter. Such extended flawsassy not be detectable at the later stages, and may not affect the mechanical integrityof the can, but may give rise to leak paths in the finished pin,

412

The ultrasonic test on the "bar or hollow may be made with, a transducer eitherin contact with the outer curved surface, or coupled by a water path. A typicalcalibration standard is maas using a sample of material identical in composition, anddiameter to the material tinder test, A hole is drilled parallel to the axis? in a"bar it is commonly at a depth below tho surface of 155« of the diameter of the bar, andthe diaaeter of the hole is 10$ of the diastoter of the barr while in a hollow it IBlocated midway in the wall ana its diameter is G»75 BUS* 2xt each case the hole is25 mm deep»

Ultrasonic equipment is used in the pulse-echo mods} it ie set up using thecalibration standard so that the amplitude of the returned signal i& at least half

tix tíascale deflection » With "bars, the standard is rotated so that th« standard, hole ison the side remote from the transducer. The specimen ie traversed with a pitch notgreater than half the diameter of the transducer, A signal greater than or equalto that fresa the standard is causo for rejection»

Material in the for» of thin-walled tubing (wall thickness of 2,5 jm or less) is0)tested ultrasonioally in the well-known way» The commonest form of equipment is one

*

in which the tube is traversed helically through a small tank containing water»Ultrasonic transducers £»o\sited in the tank are directed BO that pulses enter thotube wall and are refracted parallel to the surface of The tuts, ïhe arrangementis preferentially sensitive to flaws aligned with their major axis perpendicular tothe direction of propagation of tho pulse* It is therefore conmon practice to providetwo transducers, one of which directs pulses circumferentially round the tube, andhence is sensitive to flaws extending axially, while the other directs pulseslongitudinally, and is sensitive to flaws extending oircumferentially» The requirementsfor tho mechanical equipment are that the tube shall ba traversed with little play or

413

oé vibrationt and that the transducers shall 'be easily adjusted to precise posit icasand "be capable of being securely locked there while in use» Because flaws sa/be asymmetrical, ana reflect more energy when the pulse approaches freo on© directionthan Kh0n it approaches from the opposite direction, it is usual to ensure that eachpart of the tube is inspected twice, the pulses in the two inspections travelling inopposite directions» This may be done with, an equipment with two transducers » setmutually perpendicular as described above, by passing each tube through ta© equipmenttwice, reversing it between passes. Alternatively the tank aay house four -transducerswith two opposite~facing transducers in each direction* Trie saving in time resultingfresa this arrangement is of course accompanied by the added complication of theequipment, both in design and construction and in setting up»

ïhe mode of the elastic waves in the tube wall by which the ultrasonic pulsepropagates can not be simply defined, !he complexity of the situation - a thinplate curved cylindrically, with liquid at one surface and, usually» air at theether, with the propagating waves in a pulse with many frequency components - renderscalculation based on Lamb «ave propagation unfruitful» Nona©! practice is tooptimise the parameters of the transducer empirically» The nominal frequency isusually in the range 2-20 MHz, the choice being determined by a balance between

o »^increased resolution at higher frequencies and increase to -alteeaatiaa of signaland in noise fross scatter*

Physical setting up - the adjustment of the distance of the transducer fresathe tubes and the angular position - is governed by the need to inspect the wall ofthe tibe with uniform sensitivity» This g/ conveniently done by using a calibrationstandard of tubing with accurately sachined notches on it's outer and inner surfaces*The transducer and the gating arrangement of the eleotronic monitor are adjusted•until equal strengths of echos are returned from all calibration notches» Inaccuracyin setting usually results OR larger signals being received from f laws on the Outersurface of tubing, eo that the outer surface is mor© sensitively examined thaa theixmer«

414

Piezoelectric crystal transducers with conventional transmitting and receivingequipment are used» The pulse-echo mode is commonly employed, as the double probe("shadow") technique normally lacks sensitivity for email tubes. The lower limit ofdiameter of tube which can "be teeted with adequate sensitivity is determined by thedifficulty of transmitting' sufficient energy into the tubo. Por the smaller sisea oftube focussea transducers are usedf and tubing of diameters down to $ ÏÏB& c ^ ^®inspected.

Calibration standards are made by machining slots in a sample of the tubo undertest» Slots may be made by impressing a sharp tool on the surf ace f or more often byspark-ffiachining» A typical sensitivity of test IB represented by a Blot 1 HUB longand of a depth equal to 10$ of the wall thickness of the tube* A calibration standardwill normally carry at least four Slots, one longitudinal and one circumferentialon each surface.

To ensure that single pulses from electrical interference are not interpretedas signals from flaws, the pitch of the helix in tóiich the tube is traversed shouldbe small enough for a flaw to return repeated signals as it is traversed paat thetransducers. A ssinimvaa of three signals may be taken as • satisfactory in practice.For the speed of traverse of the tube through the équipaient to be conveniently fast,this implies a high rotational speed of the tubes which in tiara makes a high pulserepetition rat© necessary. Modern equipments now function at several thousand pulsesper second.

When an ultrasonic flaw detection system is set up, the limit of sensitivity is setby «noise». Discrete signals are obtained which normally appear to be randomlydistributed throughout the tube, and are derived from reflections of energy at grainboundaries in the metal» The interpretation of these signals quantitatively in termsof grain size is possible in principle but has not been achieved on any but a laboratoryscale. However, qualitative information onthe variation of grain eisss in a batch ofnominally similar material can be deduced if it occurs*

415

Edâ.y~curront •Lest ing

Eddy-current techniques, wiile well established in general tuba-teat ingt are ty: 1'o.callj-

considered to be insufficiently sensitive for most nuclear cladding* Under latora t-oi'y

conditions a high sensitivity may be obtained with earcj but the technique is not-

suitable for routine use eg for final inspection by a manufacturer. The technique

is of use however for examining small âiaaieter tubing (less thsai 5 HUH) ®%' tubing with

an unusually large grain sizc? or in rapid sorting of material in an inspection of

material other than, a final one»

0)

2» J Ryden Non-destructive testing of small diameter v stainless' steel fuel clad

tubing. USABC Report BîîWL-SÂ-2275 (March 1969)

3« A Prot Contrôle des gaines du reacteur FheniK par ultrasons - description

de la machine, I2me Colloque de Métallurgie, Saclay (June I96ír:)

416

TECHNIQUE FOR INSPECTION OPLIGHT-WATER HSLOTOR PRESSURE VESSELS

B. Watfcins, U.K.

Fot published by the I.A.E.A

417

THE POST-IRRADIATION INSPECTION OF FUELIN THE UK

byV* W. SLDRED

ABSTRACT

Non-destructive testing has been very important in the examination programmeon irradiated fuel elements in the IK. The techniques available include X-radio-graphy, visual inspection and photography (including peri-photography and photo-grammetry), metrology, gamma scanning, leak testing, eddy current testing andneutron radiography. The essential characteristics and purposes of those techni-ques are briefly described,

Topics that particularly require further development of non-destructivetesting techniques are listed.

INTRODUCTIONThe economics of nuclear power are very sensitive to the irradiation levels

and dwell times attainable by the fuel elements. The failure or unacceptabledistortion of only a small fraction of the many fuel elements or pins in a powerreactor could be a cause of severe operational difficulties and expense. Conditionsin a reactor and the changes that take place in the fnaterials under irradiation areso complex that it is not possible to predict the performance of large numbersof elements of a new type with complete confidence» For this reason it has beenthe policy in the UK to discharge carefully selected monitoring elements, at steadilyincreasing increments of irradiation level" and dwell time, for examination andassessment. This enables a comparison to be made with prediction and deleterioustjrends to be detected at an early stage. Any failures that occur are subjected toa particularly searching investigation so that appropriate action can be taken toprevent a recurrence. Prototype and experimental elements, involving novel designor material features, are also given special attention.

The application of this policy is well illustrated by experience with theBritish Magnox stations which have generated more than half the XA?orld*s nuclearelectricity. There are now, and will be for many years ahead, about 800,000uranium/magnox fuel elements in these reactors. The monitoring programme initiatedon the Calder reactors has been extended to include all the power stations of theCentral Electricity Generating Board and the South of Scotland Electricity Board.Since substantial financial returns can accrue from irradiating elements for longerthan their original design life, monitoring and optimisation are still continuing.A3.though a range of different failure mechanisms have been experienced the failurerates are now very low indeed, thanks largely to the action taken on the basis ofpost-irradiation examination. A similar examination prograwne is planned on elementsfrom the Advanced Gas-Cooled Reactors now under construction in the U«K«

*Dr. V.W. Eldred is a Research îfenager at R.D.L. Windscale.

419

Because the elements when da reharqeu aro intensely radioactive1, elaborateshielded facilities arc required ior their examination, which is therefore expen-sive. Iton-destructive testing techniques, which can be very convenient andcomparatively inexpensive to apply, hav<=> proved to be oí enormous value in thiswork. If, as is someta sacs the csoc, it is desired to return an element to the-reactor for a further period of irradiation, only r.on-dortructive roe t hod s ofinspection can be used. Undoubtedly there i ; still scope ior refâning and i_-xLe-nd.ingthe use of NOT in the examination cT irradiated Jucl elements and other reactoicomponents.

The purpose of this note is to describe briefly the uf>c that is? now ¡redo ofnon-destructive testing techniques at those establishments in the UK where fuelelements from power producing reactors are c-xainined._THE _UK_ SITES

Irradiated fuel elements from pov;er reactors are examined at four main ratosin the UK:

1. Windscale (Reactor Development Laboratory, ÜKAEA, Cumberland)The fuel elements and pins examined fall into four main categories:(i) Uraniura/Magnox fuel elements from the graphite-flîoderated and

CO - cooled reactora of the UKAEA, CEGB and SSEB.

(ii) Elements consisting of clusters of stainless—steel-clad pinscontaining UC>2 fuel from the graphite-moderated and CU2~cooledAdvanced Gas-Cooled Reactor at Windscale.

(iii) Elements consisting of clusters of Zircaloy-clad pins containingUC>2 fuel from the Steam Generating Heavy Water Reactor at Winfrithand from UKAEA irradiation experiments at Chalk River, Halden andBR3 (Belgium),

(iv) Elements from gas-cooled and water-cooled reactors overseas examined,as part of the world-wide service of post-irradiation examinationthat is now offered by the UKAEA on repayment, for foreign customers.

2« Dounreay (Experimental Research Establishment, UKAEA, Scotland)All examinations of fuel elements from the sodium-cooled Dounreay Fast

Reactor are carried out on site and facilities are now being set up forsiirdlar work on elements from the nearby Prototype Fast Reactor. The pinsconcerned consist of ceramic fuel in stainless-steel cans.

3* Winfrith (Atomic Energy Establishment, UKAEA, Dorset)Until recently the caves at Winfrith were used for the initial break-

down, inspection and testing of fuel elements from the Steam GeneratingHeavy Water Reactor at that site but that work is now being transferred toWindscale and the caves are being re-equipped for the examination of elementsfrom the Advanced Gas-Cooled Reactors on behalf of the Berkeley NuclearLaboratories of CEGB.

420

4» rfer}<:rel |facle jlgj oratorie£ (CEGB, Gloucestershire)Uraniunx/Magnox fuel elements from the Board's own reactors are examined

at Berkeley as part of the programme that is shared with thé Wind scaleLaboratories of the UKAEA*Samples are sometimes sent from the above laboratories to the Atomic Energy

Research Establishment at Harwell for the application of special tests and tech-niques not available locally*NO^-DESTRUCT/E TESTI&G

The majority of the following techniques are available at all the establish-ments although there are significant differences between sites in the dimensionsand activities of the fuel elements that can be tested.*

This technique* employed at all establishments, has probably been more usefulthan any other. It is generally used as shadowgraphy to give the size and shapeof the fuel but does sometimes reveal defects in the cladding. Various techniquesare employed to prevent fogging of the film by the intense gamma rays given offby the fuel element that is being radiographed.Visual Inspection and Photography

These are normally the first stages in any examination. Most frequentlyvisual inspection is by naked eye through the cave window or by per-iscope.

Stereo viewing .jand photggrapjiy are becoming more popular and there isparticular interest in p^hotoqrammetry for measuring the depths of surface defectsor the thickness of surface deposits.

Peri-photography has found some application at Wind scale as a convenientmethod of displaying in one picture all the surface features on the cylindricalsurface of a fuel element»

•Colour photography is roainly rewarding with w.ater —reactor fuel elements onwhich reddish deposits often accumulate.Metrology

While the dimensions of the fuel are often determined from radiographs, theexternal dimensions of .the fuel element or pin are generally determined by directcomparison with a calibrated scale (for coarse measurements) , by dial gauges ortransducers (of the capacitance or resistance types). Overall length measurementsare sometimes not required to an accuracy of better than 0«25 mm but diameters andprofiles have frequently to be determined to j; 0.05 mm.Gamma Scanning

The variation in gamma activity along a pin can provide a useful indicationof the distribution of power generation along the pin as well as revealing gaps

«See appendix for fuller details of techniques available at R«D.L, Windscale.

421

between fuel pellets and localised concentrations of certain gamma-emitting spsuch as caesium. Sometimes the method is calibrated to give an absolute d torr.in-ation of burn-up, but more often it is used merely to detect longitudinal variationsin the pin or to compare one pin with another.

Leak TestingA well-established leak detection technique is to pressurise the element

externally with a gas for long enough to enable sufficient; gas to reach the insideof the can through any leak that may be present. On xemoval of the element fromthe pressurising compartment it is either immersed in a liquid, so that the leakcan be detected by the stream of bubbles given o±f, or scanned by a suitabledetector (e.g. an infra-red gas analyser if nitrous oxide has been used) to locatethe escaping gas.

Other methods of leak detection and location are not strictly non-destructivesince they involve piecing the can at one point in order to evacuate it or pressurisewith a tracer gas (sucn as He or SF}»

Eddy current testingThis technique is proving very useful in detecting cracks, hydride concentrations

and wall thinning in Zircaloy-clad fuel pins from water-cooled reactors. The pinis traversed at a constant slow speed through the encircling twin sensing coils andthe amplitude and phase output of the detector is recorded on a chart synchronisedwith the pin movement so that the axial position oí" any defect giving a signalcan be located.Neutron Radiography

Neutron radiography is particularly valuable in detecting hydride concen-trations in pins that have developed leaks in water-cooled reactors and it canalso provide useful information on the conditions of the fuel.POTENTIAL FOR FUTURE DEVELOPMENT

In addition to the non-destructive examinations outlined above there is, ofcourse, a very large amount of destructive examination of fuel elements, particu-larly metallography. Since such destructive examinations are very time-consumingand expensive, there is considerable incentive to introduce new or 5_mproved NOTtechniques that can be substituted for them.

The most rewarding areas for development are likely to be:-(i) The very sensitive detection and location of fine cracks, voids and wall

thinning in cladding.(ii) The detection and location of cracks arid voids in ceramic fuel.Ciii) The identification (under xtfater or in air) of failed pins in a bundle

without the need to remove individual pins for testing.(iv) Very rapid methods of accurate mensuration to map comprehensively the

external dimensions of a pin.

November 1971.422

APPENDIX TRG(W)/VWE/EPG

Techniquesfuel pins at RDL, Windscale

Technique Object of work

1 . r - ^ - . ^ - - - - -(colour or black/white) of fuel clustersand single pins. 'Periphotography' canbe specified for lengths not exceeding100 cm. Pin lengths up to 40 an long canbe chemically decrudded.

2» Mensura.tion - on whole pins lengthmeasurements (+ 0.1 mm), and point dia.measurements C+ 0.02 mm) can be made.On pin lengths up to 90 cm profile anddiameter measurements Ç* 0.01 mm) can bemade at various positions round the pincircumference.

3. Gas Release - measurement of theamount of gas present and the free volumein the pin. Analysis of the gases present.In addition the gases released from crushedfuel can be analysed.4. keak Testing of pins from clusterswhich have given signals in reactor.Ca3 Testing of whole pins in a vacuum

chamber.(b) Leak location, using a tracer gas

(e.g. He or SF ) or whole or partpins.

5» Eddy current testing, of pin lengths.At present only 30-40 era pin lengths aretested, but the test could be extended towhole pins in future.6* V"—scanning Pin lengths up to 100 cmcan be examined for various "-emitters,depending on the decay time which haselapsed since discharge.

To record the condition of clustersor pins, inclttding any fretting dam-age or defects. The distributionof crud and flow patterns on the pinscan be studied. If the pin lengthsare decrudded then any areas ofthick oxide C 20 u.ra) can be detected.To determine the extent of lengthand diameter changes due to irra-diation. Pre-irradia'tion measure-ments are required. ; Measurementsdetermine the extent of circumfer-ential ridging, and ovality and canbulging, and are best done on pinsafter decrudding.To determine gas pressure in pinat end of life, and percentage offission gases released by fuel.It is also a check on whether thepin has failed.Test (a) determines whether the pinis leaking; the findings can beconfirmed by a gas release measure-ment Csee item 3). Test (b)determines the location of the leakso that it can be examined furtherby a macro or micro examination ofthe surface and by metallography.

This gives information on distri-bution of cracks and hydride patches',end supplements information obtainedby leak location on failed pins.To determine the rating distributionalong pins and any migration ofcertain fission products e.g. Cs.

423

Techniques Object of _work

7. RadiographY Pin lengths up to100 cm can be X-rayed in sections. Neu-tron radiography cannot be done at RDLbut can be done at AERE on lengths notexceeding 60 cm.

8. Isotopic analysis of fuel pallets

9. Crud deposition (a) on samples removed frora pins

by cellulose-acetate replicas(b) on composite sections through

crud and canï-fetallography and electron-probe microanalysis can be used to study the formand chemical composition of thesesamples.

10. Optical metallography of can and_fuela» transverse and longitudinalsections,

(a) Macro-examination e.g. X5-X20(b) Micro-examination e.g. XlGQ-XlOQO

11. Measurement of oxide thickness JDQcladding. Mean and maximum values areobtained.

12. Hydrogen (or deuterium) analysisof Zircaloy cladding

X-radiography can be used to examineirradiated rigs but is of less usefor examining water reactor fuelpins. Neutron radiography willdetect concentrations of hydridein Zircaloy, the cracking offeulpellets etc.This enables the burn-up and ratingto be determined at selected posi-tions along a pin» This informationcomplements that obtained by v-scanning»Method (a) allows many samples to betaken along a pin. It has thedisadvantage that not all the crudmay be removed from the area sampled,"so that it may be necessary to goto method Cb) to obtain a fullanalysis»

Macro-examination shows the crackpattern in the fuel, extent ofdimple filling Con longitudinalsections), and extent of coarseporosity and columnar grain growth.Cold can-fuel gaps can also beestimated. Micro-examination of thefuel reveals the extent and formof porosity, grain growth andmetallic fission products. Micro-examination of the can is used tostudy grain size, hydride distri-bution» oxide thickness and defectslocated by other means. End-weldsand wear pad regions are particularregions of interest.To obtain estimates of the rate ofoxidation many measurements arerequired» and it is necessary tohave archive samples for comparison.To determine the H Cor D) pick-upof the cladding as a result of ex-posure in the reactor* In conjunc-tion with oxide thickness measure-ments (11) the percentage pickup ofhydrogen from oxidation in steam orwater can be calculated»

424

Techniques Object of work

13» Mechanical to sti ng^ of clgdd iTest under stress alone, at temperaturesusually in the range 20-350°C:Ca) samples tested under hoop stress,

using uniform or strain concen-tration conditions»

(b) closed~end burst, tests on 30 cmlengths strain is bi-axiai withthe hoop stress 2X the axialstress»

(c) Creep burst tests, as (b) but at• much lower pressures and strainrates.

(d) tensile tests. Samples car» bespark-machined from the cladding»

14» Stresg corrosion testsThe cledding is subject to hoop strainat elevated temperatures in the presenceof the corrodent.15» Specialised ! •fcechnric|uesj(a) Transmission electron microscopy(b) Selected area electron diffraction(c) Electron probe micro-analysis (as

used for crud studies)(d) Scanning electron microscopy(e) A shielded X«ray diffractometer

and X~ray fluorescence analysisequipment is also available.

16» Autoradiography of fuel sections

Tests at reactor temperatures areusual, and archive specimens arerequired to assess changes due toirradiation,Tests can simulate strains causedby expansion of cracked fuel againstcladding.Parameters measured are the burst-ing stress,, total circumferentialelongation (TCE) and reduction ofwall thickness at fracture.

ditto

Not a widely used test on Zircaloybut gives properties in the axialdirection.To determine the susceptibilityof the cladding to stress-corrosionby fission products (e.g. iodine)

These are mainly electron-opticaltechniques which can be used tostudy interesting features foundby other methods, or for basicstudies of fuel/can behaviour.Samples must be small to minimiseradiation hazards» The scanningE«M» is particularly useful forstudying can surfaces and frac~tures.

The distribution of/3(S" activityacross a fuel section may be shown.This is not usually a routinerequirement.

425

OK NONDESTRUCTIVE EXAMINAT ION OF NUCIEARFUEL PELLETS IN THE U.S.A.*

, R. ¥. McClung

Metals and Ceramics Division, Oak Ridge National LaboratoryOak Kidge, Tennessee 37830

ABSTRACT

Several nondestructive evaluations are made of uranium- and plutonium-bearing fuels to assure conformance to specifications. These include visualexamination with comparison to physical standards and measurement of dimen-sions, density, and fuel content. In addition to these that are commonlyused, thermal techniques have "been studied for density measurement, gammaattenuation has been used for measurement of fuel concentration, and bothelectron microprobe and autoradiographic techniques have been used tomeasure plutonium distribution in fuel pellets containing mixtures ofplutonium and uranium oxides.

RFSUME

PlufJcur.s cxr.r.onr; r.on destructifs sont conduite tur JCK corùrae-tibies à uri.ne C ' xxrfùxivu' ci do pint-oniru afin au c'ast-iu-er c>\x'ils sontcor.i'orjViCrj ¿A»>: r-oóoií ¿raklor.rj. lin conjronneuu xia examen vjusuol avecco-Tiparaison BV.X ataní.arorj plij-oiquot; eb cica KOÍÍUI-OÜ c'.o r}v-.cn33.o:iSj de(lenoité et, clo teneur on co'.iLu«iiL\le. .,n i*Iv.;:, ûe co.s Mélhculos (;ui «onttt juíi U/,K-Í¿;C couraat, dc.s tccS'aicji'cr; th^rcanuof. ont oté ófcxKliéen por.rcîee iaecureu de dcncicd; .l!aot<5.iu-.;.i;ion oc,j rajour; ¿.cj-aua a ét<5 voilicácpo-iï' la 1ÚCÍ3V..CC de ?^a co.-ccvitrf'.lioii u.u. cos. Irak tibio et dec techniquoad'aukoradioóraphio r,s«ooi<5eB h 3'uti.lioni.ioii d'une Kievosoadu à tloc-tro"u:- ont kcrvi L, la laec-uro ac la répr.r «i oiou du piuî.o;ûuo ds.n^ ÜOÍL;pe.ftilles de cosituatibie contonant dco rr.ílac^oí; d 'oxyder, do plutoüiuáiet d'uranivua.


427

INTRODUCTION

Several nondestructive testing techniques are applied to the examinationof ceramic nuclear fuel pellets for reactor fuel elements to assure that thedesired properties have been obtained. In addition to those normally usedon uraaiumr-beariag pellets for thenaal reactors } new techniques are beingdeveloped for application to plutonium.- bearing pellets for breeder reactors.Because oí* the great number of pellets required for a reactor core loading,statistical sampling of the pellets rather than 100$ examination is frequentlyused for inany of the techniques.

YÏSUAL E

Visual observation of the pellets for surface appearance is a commontechnique to assure the absence of objectionable features such as cracks, chips,or other significant flaws. The reference standards are frequently pelletscontaining the largest acceptable discontinuities (e.g., chips from cornersor other surfaces). Some pellet designs require dished ends (concave) , andthese may also be observed visually.

SIMMS TORS

The size of the pellets ia important to assure proper fit within thecladding when they are assembled into a fuel pin. The gap between the outersurface of the pellet and the inner surface of the cladding is minimized toaid removal of heat from the fuel. On the other hand, a nominal gap isnecessary during assembly and to avoid excessive mechanical interactionbetween fuel and cladding during thermal cycles. Mechanical measurementsof the dimensions assure that the product meets specifications. Both manualand automatic gaging techniques are used.

428

DENSITY

The physical density of a pellet is indicative of the amount of fuelthat will be present within ils peripheral dimensions. This is significantnot only for the reactor loading "but also because properties such as thermalconductivity depend on the density. Several techniqxies are employed for mea-surement including both calculation from the weight and physical dimensionsand immersion -weighing- Measurement of the pellet volume with a mercurypycnometer and subsequent calculation of the density using the known mass ispresently the most accurate known method for determining the density of fuelpellets containing plutonium.

A recent development, a thermal transducer1to detect variations in heattransfer, has shown potential for measuring differences in physical densityJn ceramic bodies, because these differences affect thermal diffusion andconductance. In this method, the surface is heated -with a sheet of electri-cal resistant heater -material to which a short pulse of electrical energyis applied. A thermoluminescent coating on the sheet heater displays anylocalized changes in response as a function of the heat diffusing into thespecimen.

FUEL CONTENT

Although the special destructive techniques of analytical chemistry arerequired for accurate determination of the total amount of fuel presentwithin the pellets, a secondary measurement is usually mde of the gammaradiation emitted by the fuel material. In some instances this is done afterthe pellets have been placed in the cladding. The best practice is toassure the certification of the fuel column before it is inserted into thecladding. Gamma spectrometry allows isolation of the specific gaisma rayenergies that give good measures of the fuel isotope contents. Data of

429

value from such examinations include total fuel content, assurance of isotopicenrichment (originally established by mass spectrograph)? and.,- whereapplicable_, assurance that the desired variations (or uniformity) of loadinghave been obtained along the fuel pin.

Another method uses the atténuation by the fuel pin of radiation froman external source, such as 60Co, to determine genera,! or localized variationsin fuel content.2 The transmitted radiation is detected (e.g., by an Halcrystal) and the variation in fuel content is indicated by changes in theintensity of transmitted radiation,

FUEL HOMOGENEITY

Work on fast breeder reactors has involved fuel pellets that aremixtures of plutonium and uranium oxides. One approach for fabrication of

the pellets is tc blend the desired amounts of FaOg and UOa powders andthen ccBxpress the blend into a pellet. At least two techniques have beeninvestigated for the determination of homogeneity of the blend within thefinished pellet. The primary technique used an electron sdcroprobe to scanthe pellets for PuOg concentration, sizes of Pu02~enriched or depleted zones,and the distance between such sones.3 T'he microprobe data were used toestablish a mathematical model and figures of merit relating homogeneity ofthe mixed oxide fuel to characteristic responses under transient conditions.Standard pellets were made with different figures of merit and then alphaautoradiographed, Unknown pellets could then be alpha autoradiographed and -•compared visually with the results from the standard, pellets.

SUMMARY

Several nondestructive evaluations are made of uranium,- and plutonium-bearing fuels to assure eonforiaance to specifications. These include visual

430

examination with comparison to physical standards and measurement of dimen-sions, density,, and fuel content. In addition to these that are cosmonlyused, thermal techniques have been studied for density measurement, gammaattenuation has been used for measurement of fuel concentration, and boohelectron microprobe and autoradiograpMc techniques have been used to measureplutonium distribution in fuel pellets containing mixtures of plutonium anduranium oxides.

REFERENCES

1. D. E. Green, "High Speed Thermal Imge Transducer for Practical NDTApplications," Mater . STaluat ion gj|( 5 ) , 97-102 (May 1970).

2- B. E. Foster and S. D- Snyder, "Evaluation of Variables in the Measure-ment of Fuel Concentration Yariations in Nuclear Fuel Rods," Mater.Evaluation (2) , 27-32 (February 1968).

3. D. A. Stranik, H. G. Powers, and G. A. Last, gvaluation .ofHo igeneity in PuOg—UOg Fast Heactor Fuel by Scanning ElectronMicroprobe, WHAIî~FR-2Ô (October 197ÏÏJT

431

M) CONT3ÎOU5 NONWJÍSTHUCTIP AU COURS DE LA PABRjCATION DfíS ICKC15TNTKS

SOUS PRESSION 3ÍN ACTKil

SEMT- Centre d'Etudes NucU'airof, de Saclay

ABSTRACT

NOT methods for reactor pressure vessel may bevery different according to the use of the vessels-Experi-mental reactor vessels cannot be tested in the same wayas the nuclear power plants ones.

In France there i's not national code,but onlygeneral regulations. The manufacturer is responsible forpressure vessel safety and then, may use the constructioncode of his choice.

According to the current practice, radiographieexamination is performed on all wcldied joints. This ins-pection is no longer regarded as sufficient and ultrasonicexamination is used in addition in order to detect craksvich are not seen by radiography. It is believed that USinspection will be considered as the best method, •when re-productible i^ecordings can. be got. The advantages anddisadvantages of US and radiographie inspection are examinedin the first annex.

In Prance, US examination of austeiiitic weldshas been developed for the first french fast neutronsnuclear pJant {Phénix) .Long'3 tudinal *v-awos,foealised trans-ducers and automatic testing1 apparatus were used. Thismethod was very succesfuiland specified defects \vore easilydetected.Moro details on this problems are given in thesecond annex.

433

Acoustic emission xs a promising crack detectionmethod,but research, and development a.rc stall necessary,par~ticuliearly in order to overcome the ambiant noise perturba-tion.In France,research and development have been in progressfor two years and a commercially available device has leendeveloped.More explanations can be road in the third annex.

INTRODUCTTON

Du point de vuo des contrôles non destructifs, ileat commode do classer los enceintes soue pression enacier on deux groupes î-Les enceintes destinées aux réacteurs û& puissance uti-lisant l'eau connue fluide caloportour. Compte tenu du

développement actuel dos rcacteua-s de puissance refroidisà l'eau, ce groupo concerne dea enceintes sous pressionnombreuses et d'une grande importance industrielle. Toutesces enceintes sont de conception voisine .'-Los enceintes destinées aux réactour-a prototypes ou à desréo,ct<aux'3 de puissance à faiblo programme de construction.Ce groupe contient des enceintes sous pression extrêmementvariées qui posent en certains cas des problèmes très par-ticuliers de contrôle non destructif«

JSn Prance, où le programme de construction des réac-teurs de puissance refroidis par eau en est à son début,1 Experience acquise dans le contrôle non destructif desenceintes sous pression de réacteurs à eau peut paraîtreplus faible qu*il n'estdans d'autres pays. Cependant» uneexpérience notable a déjà pu être acquise dans ce domainepar la Société CRi£USOT~L01BK, fabricant connu qui a déjàréalisé un certain nombre de ces enceintes et do cuves de

434

rtSacteux'a PV.'R en particulier, tant pour des bOí>o.insfrançais (Chooz) que pour des hesodns ét x*íinftor«, D'autrepart, 1 o d éveloppeniont récent du programme français con-duit cette société à fabriquer un certain nombro d*en-ctïintes destinées à dos chaudières nucléaires du typePWR devant fournir1 une puissance électrique supérieureà 800 MW.

Lo controlo non destructif do ces enceintes faisantl'objet d'un développement important-en divers pays, nouslimiterons notre contribution en ce domaine à quelquespoints d'a-iitérêt » Cette contribution est essentiellementbasée sur l'expérience de la Société CREUSÛT-LOIRE àlaQ'Uôllo nous tenons à" exprimer nos remerciements pour sacollaboration .

Comme nous l'avons déjà signalé, les enceintes dusecond groupe ont posé dos problèmes de contrôle non des-tructif moins classiques et par conséquent techniquementintéressants. Parmi, ces enceintes,on se doit de citercelles destinées au réacteur à neutrons rapides Phénix,enceintes dont la plupart sont réalisées en acier austé-ndtique, matériau présentant certaines difficultés decontrôle .

1REGLEHENT ATTON FRANC Al S R CO î JES DE CONST UI iCTTOHLes enceinteñ sous pression sont en Prance soumises

àdeux règlements différents suivant qu'il s'agit ci"»en~eeintes contenant du gaz sous pression (loi du 28 Octo-bre 19^43 et décret du 18 Janvier 19'13} ou d'enceintescontenant des liquides cri ebullition ou surchauffés(décret du 2 Avril 1926) . Ces textes sont d'ailleurscomplétés par divers arrêtés prescrivant des niesu' jbcomplémentaires pour œrtains appareils ou explicitant labonne interprétation des textes de base , Cette reglo-mentation est par principe tros libérale et le construc-teur de l'enceinte est lui-même responsable de l'appli-cation du règlement » L'Autorité de tutelle, à savoir la

435

Direction doa. Ninon n fcout pouvoir pour surveiller 1'ap-plication du règlement-, ,mais n'est naturellement i>as res-ponsable do cctto application .

#.Ces règlements qui s'appliquent aux 'enceinte» deréacteur» nucléaires ne contiennent quo le» règles, fonda-mentales nécessaire?» à lu sóc mû té de l'appareil. Ilscomportent de ce fait certaines exigences quant <xu con-trôle et à 1 * inspection en service mais en termes généraux»Par exemple lo décrut du 2 avril 1926 prescrit que MA 1*ef-fet de roconnaître l'état de chaque appareil ...»,1'exploi-tant doit faire procéder à une visite complote tant à l'in-térieur qu*?i 3'extérieur .... sans que l'intervalle entredeux visites complote3 successive;* puisso être inférieurà 18 mois ,.« ",

En conséquence de l'esprit libéral du règlement, 11n'est pas pas imposé l'emploi d'un code quelconque. Leclioix des règles de l'Art est laissé à 1* initiative duconstructeur responsable. Dans la pratique courante,ceciconduit à utiliser de» codos dont l'audience est univer-selle /et en particulier, la section ÏII do l'MASMK Boilerand Proasure Vessel Code" , Mais il faut bien noter quece choix est fait sous la seule responsabilité du cons-

. tracteur.

CONTROLE DRS BXCB3XTRS SQJjS PRESSTON-•_.JRADTOCUAPI!1B KT ULTRASONSLa plus grande part dos cuves de réacteur» à eau

construites en Franco à co jour, étaient destinées à l'ex-portation. Le contrat correspondant imposait le respectde la section III de "ASM8.B.& PVC". Les cuves ont doncsubi un contrôle non destructif satisfaisant aux règlesprescrites par ce code.

Ces règles imposent un contrôle radio-graphique dotoutes les soudures. Ce contrôle a le ràérito de fournir

* Recueils 1331 et 133? (journal Officiel) fcur les Appareilsà Vapeur et les Appareils à Pression do gaz »

436

des documents matériel.*; ( photo») quo .l'on peut conserver,d'autre part, Joe défauts even lucio se présentent sur lesphotos par une indication donnant uno idée intuitivo dolour formo et do lo uro dimen lona »

Cependant ce type de controlo, au«si satisfaisantsoit-11, ne paraît p-is apte *». fournir une indication{suffisamment sûro, Ou pout on particulier lui rcprochox*do no pas etro très apte à détecter loa fissures qui sontparmi les défauts ICH p3 us dangereux quo peut comporterune enceinte sous pression. Si l'on veut un contrôle sûr til faut compléter 3e contrôle radi o^raphique par un autre •type de contrôle. Le contrôle ultrasonorc est tout natu-rellement appelé à jouer ce rôle puisqu'il est apte àdétecter les défaute du type fi G sure.- Los progrès cons-tants de cette technique ont tié^iù permis de réduire lagrande part qu'il luiy.se à l'habileté et à la compétencedo l'opérateur. On peut môme espérer que dans l'aveiiir,il sera possible d'obtenir tíoíí informations objectivesqui lui assurent une audience équivalente à .celle ducontrôle radiograph 3 que .

De l'expérience acquise, il ressort qu'il est néces-saire- d'effectuer simultanément leñ deux types de contrôleC*ost la conclusion qui ressort de l'étude comparativefigurant sur l'annexe If étude que nous devons auService do ContrôJe de la Qualité de la Société CREUSOT-LOIRiî.

PAîm ClîLTnnS-CXTRQLR VVS SOV DTÏÏÎES ArSTE\I ,

Parmi les problèmes particuliers posés par lesenceintes sous pression des réacteurs prototypes, lecontrôle non destructif der; aciers austénitiques figureen bonne place. Un contrôle effectué sur de tels maté-riaux s'est posé» en particulier, à propos du réacteurPhénix., Une étude importante a dono tSté meneo sur cesu jot par la Section des Techniques Avancées du C15N-Snclay*

437

1/ce odor.** uunténi tiques se prêtent pJtu?difficilement u un controle ultrasonoro que .lo» aclfrsíerrjtiques. La variai ion <Jo la color i te'" do« ondos ultra»sonoros avec 1 * orientation du cristal est on effet plusmar que o dans 1'austé nito, 31 on résulte» do» réflexionsplus importantes aux jointe do ftrain qui ont pour consé-quence pratique un"bruit de-fond" notable qui 0ôno conui-durablement l'identification dos échos de défauts. Enoutre l1amortissement est plus important.

Coa difficultés sont mises on relief díuis l'étudeJointe dans 3 «annexe II. Cette étudo duo a M.M.MAHf.KT otROULE, indique quelles voies ont otó utiiiscos pourvaincre cos difficultés, {ondes longitudinal os, tranduc~tours focalisés, contrôles automatiques.)»

Cos O.tude» ont abouti à la réalisation d'un matérieldo contrôle automatique qui a Otó utilisé do façon indus-trieJle au contrôle des soudures austónitiquos du réacteurPhénix.

YKCHNTQIHO.S KOUVICLI,K$~_ J. * l'>Î 1.85;TON AOO*:STTQW

Les techniques claasiquos de contrôle non destructifpermettent un contrôle dos oitcointos sour, pression quel*on pout considérer comme assois satisfaisant, mais one-oreloin de fournir tous 3es renseignements quo l'on désire.Ceci explique l'intérêt présenté par d'autres méthodes aupremier plan desque!les figurent les techniques d'émissionacoustique.

L*émission acoustique est potentiellement une méthode• de choix pour 3 a détection des fissures, en particulier»&ùv les enceintes achevées ou déjà en service. Cependanten, l'état actuel, eon utilisation ne peut être considéréequo comme expérimentaio.

Lo CKA a entrepris une étude sur cotte technique quia,d'ores et déjà, abouti à la nij.so au point d'un matérieldont la commercial i sut ion ost iintainente. Un point- de cetteétude fait-J«objet de l'annexe III établie par Mme CURKT1KNde la Section des Techniques Avancées»

438

Los techniques injsoa au point font actuellementl*objot .d'une» expérimentation sur tíos cas indu»triolaconcrets. Cette expérimentation porto également sur duwnlériol déjà commercialise à l'étran/çor» Parmi ces oa-sais, il convient do citer 1*¿couto effectuée sur la cuvedu réacteur J'1/K do Choou lors do 1* éprouve hydrauliquedfétanchéité exécutée h l'issue du rechargement du com-bustible on Août 1971,

Cotte expérimentation industrielle nouo n permisde cerner certaines difficultés d*npplication inhérentesà 1*environnement, l'armicos difficulté»,se trouve, au prp>».mier plan l'effet du bruit ot des vibrations ambiantes .Des études sont en cours pour définir les différentes ca-ractéristiques des impulsions propres à 1'émission acous-tique- ot colles provoquées par les bruits étrangers(forme do ltirapulnion, durée, fréquence ,..) « Le -but estde mmur les iippareils, ei cola est possible, d'un sys-tème éliminant la plus grande part des bruits étrangers,A notre sens la mise au point de dispositifs perntettnntde neutraliser l'influence do l'ambiance industrielleest l*une dos principales actions à cntx'eprendre pourconcrétiser les résultats obtenus e>t laboratoire

439

UTILISATION !»KS ULTRASONS ET DR I,A RADTOUKAMITKCOMME HOYMNS DR CONTROLE 1JBS CUVES DB RKAGTKURS NUCLEAIRES

INTRODUCTION ..

CREUSOT-LOIKE fabrique des cuves de réacteurs nucléaires pour l'Europe enparticulier E.D.F,, et les U.S.A.

Les cuves pour les U.S.A. sont fabriquées suivant le code ASME qui prévoiten particulier la radiographie dés soudures mais n'envisage le recours aux'ultra-sons que dans le cas où Jla radiographie n'est pas applicable.

En Europe, les soudures sont généralement controlées suivant les 2 méthodes,bien qu'une tendance commence à se faire jour* de supprimer la radiographieau profit d'un contrôle par ultra-sons excitisii".

"* Etendue dos control <«s par ul traçons et rn«l 8 o^raphi c _p«»ur_ les .ctiyçs'

aï Vît ra-stms

Contrôle à 100 % de toutes les pi?«ce$; constitutives (tôles, piècesboulonnait»)» des soudures (Cnntnut ~1x>irc eCtec.tue ce contrôle rocine lorsqu'iln'est pas exigé comne c'est le cas pour les fabrications suivant code ASME),de l'adhérence des revêtements.

RadiographieContrôle et 100 % de toutes les soudures accessibles à ce type de contrôle.

ÏI - Efficacité deg méthodes par ultra-sons et radiographieDe façon générale, les ultra-sons permettent de déceler les défauts à2 dimensions dans la masse des pièces ou des soudures. La radiographie estplus apte à détecter les défauts voluniiquos, quel que soit leur emplacementdans la pièce.La radiographie ne détecte les défauts à deux dimensions que s'ils sontorientés parallèlement au rayonnement.De façon plus précise, les ultra-sons mettent bien on évidence les fissures- et même les microfissures aussi bien dans les pièces constitutives que dans

440

les soudures. Les inclusions - en particulier les Inclusions de laitier dansles soudures - sont également b.ien décelées, de même que les manques de liaisonau sondage.La radiographie met en évidence les manques de pénétration dans les soudures,les porosités/ les fissures orientées parallèlement à la direction durayonnement.

III - Avantages et inconyéni ents comparés dos niéthodos de contrôle par ultra-sons et r ad i o era ph i c

a) Ultra-sons— Avantages :

- mise en oeuvre simple» ne nécessitant pas une infrastructure lourde etcomplexe, donc facilement utilisable sur les pièces lourdes et encombrantes,-,difficiles à manoeuvrer, que constituent les cuves de réacteurs.

- permettent une détection sure des fissures,-- l'utilisation des ultra-son.. n'est pas limitée par i*épaisseur à traverser

pour les fortes épaisseurs des pièces ou des soudures constituant les cuves.Ils permettent le contrôle de tout le volume de brides de plus de 60 cmd'épaisseur.

""• ,T "forwent cnt s :- l'Interprétation des indications reste délicate et exige du contrôleur

une formation et une expérience poussées.- certaine défauts volumiques peuvent échapper à ce mode do contrôle.

Il est à noter que ce type de défaut, qui de toutes façons n'est pas le plusdangereux, n'existe pratiqucwcnl pas dans les cuves de réacteur, coiaptc-tcnude leur node de fabrication.

- les défauts proches de la surface ne sont pas détectés. C'est la raisonpour laquelle, un contrôle de surface est toujours effectué pour les cuves deréacteurs nucléaires (roagnéfcoscopie ou ressuage)»

b) IRadiographie«• Avantages t

-• les films sont conservés et peuvent erre consultés à tout montent, ycompris lorsqu'un réacteur est déjà en service.

- Les défauts à 3 dimensions sont détectés.- le'contrôle est particulièrement efficace pour les faibles épaisseurs.« Visualisation de défauts déjà- détectés aux ultra-sons.

441

- ïycpiwpnitvnts :

•••CoÔt élevé pour les contrôles en fortes épaisseurs, surtout lorsque lataise en oeuvre de la radiographie implique des déplacements de pieces lourdeset oncctnbrantes et lorsqu'il faut, faire appel à des générateurs de rayons X& haute énergie, ce qui est fait pour toutes les soudures des cuves de réacteurs

- Perte de sensibilité de la méthode lorsque l'épaisseur radiographiéedépasse 30 cm.

Les deux modes de contrôle sont complémentaires. Pour cette raison, la plupartdes constructeurs de cuves de réacteurs nucléaires les utilisent l'un etl'autre pour le contrôle des soudures.

La mise en oeuvre simultanée des 2 modes de contrôle assure l'absence de toutdéfaut préjudiciable à la tenue en service des cuves de réacteurs nucléaires»

442

CONTROLE AUTOMATIQUE PAR ULTRASONSDE LA SOUDURE D'ANGLE DU TOIT PHENIX

P. MQNNÏER *M. ROULE *R, SAGLIO *

I. INTRODUCTION.-

Dans le cadre de la réalisation du réacteur à neutrons rapides PHENIXle problème du contrôle par ultrasons de soudure d'angle entre destôles en acier inoxydable austénltlque, d'épaisseur 60 mm, a été poséà la Section des Techniques Avancées.

En effet, le cahier des charges du toit PHENIX ne prévoyait-qu'un con-trôle radiographique de ces soudures, mais il est. vite apparu qu'untel contrôle était incapable, en raison de la géométrie de la soudureet ries épaisseurs à traverser, do détecter certains types de défauts, lecontrôle ultrasons, s'il était possible, devait constituer un complément.

Les rapports (1) ot C2) décrivent les essais qui ont été faits en labo-ratoire et exposent le principe de la méthode retenue. Ce rapnort décritla machine et les résultats obtenus sur le site.

II. DESCRIPTION DE, LA MACHINE:.-

II.1. Principe de la mechinB.-

La figure 1 schématise l'ensemble de la machine aua l'on peut décomposeren quatre parties que nous décrivons maintenant.

* DMECN/irTEC/SECS/STA - Sac lay

443

11.1.1.Ensemble mécanique.-

Deux suspentes qui s'accrochent sur Î8 haut de la viroîe supportent unrail circulaire. 1s rayon de courbure du rail est tel qu'il est en touspoints equidistant de la virole. Ce roil supporte un chariot équipé dedeux vérins hydrauliques. Un premier vérin ayant une course d'amplituderéglable (maximum 105 mm) supporte le palpeur et sa cuve, il génère undéplacement vertical en va-et-vient, Ja vitesse de déplacement étantcomprise entre 5 et 10 cm par seconde. Un second vérin, porteur d'unecrémaillère, entraîne par l'intermédiaire d'un réducteur de vitesse etd'une roue libre, un pignon qui attaque une courroie crantée solidaire du*rail à ses extrémités. Chaque fois que le vérin rie déplacement verticalarrive en fin de course haute ou Lasso,le deuxième vérin est alimentégénérant ainsi le pas qui peut être rêplé en limitant plus ou moins lacourse de ce second vérin.

te balayage ainsi obtenu est conforme à celui représenté sur la figure 1.

La figure l: est une photographie de le machine pendant le contrôle sur, le toit.

11.1.2.Centrale_hvdraultque.-

Uns centrale hydraulique classique délivre l'huile sous pression néces-saire au fonctionnement des vérins.

11.1.3.Baie_électronique.-

Cette baie comprend t

- Un appareil à ultrasons de type USE 1 KrautKraroer équipé d'un sélecteurd'écho.

. - Un quantificateur Harwell qui quantifie an intensité le signal délivrépar le sélecteur d'éc'ho afin d'alimenter la plume de l'enregistreur àpaptar électrosensible.

444

- Un tiroir de commanda des vérins hydrauliques. Ce tiroir comprend : unréglage de la vitesse de déplacement verticale, le vérin correspondantétant alimenté par l'intermédiaire d'une servovaive. deux réglages quipermettent de limiter les positions haute et baose de la course» laposition de référence se situant au milieu de le course maximum de105 mm. Chaque réglage haut ou bas a une plage maximum d'action de50 mm. Cette possibilité de réglage est obtenue h partir d'un capteurde position fixé sur la machine. Le potentiel délivré par ce capteurétant continuellement comparé aux potentiels de référence affichés parles réglages haut et bas.

II.1.4.Enregistreur.-

L'enregistreur est constitué à là base d'un cimstic fi 100 aue nous avonsmodifié en l'équipant d'une table de défilement da papiar mufax, seule lavoie X est utilisée et son déplacement est synchrone de celui du palpeur.A chaque fin de course le papier avance par l'intermédiaire'd'un moteurpas-â-paa. Les possibilités de réglage de la sensibilité sur la voie Xe't du nombre de pas permettent d'obtenir la cartographie de la soudure àl'échelle 1/2, 1, 2-, et 4, nous avons toujours utilisé l'échelle 1.

Nous pouvons ainsi obtenir en demis teinte grâce à l'utilisation du oaoieréîectrosenslble la cartographie de la soudure sans limite de longueur.

11.2. Palpeur, mini-cuve.-

H.2.1.Palpeur. -

le palpeur fabriqué par nos soins est un 4 Wz de 60 rmi de diamètre foea-liaé à 300 mm par uns lentille. Son comportement & été particulièrementétudié dans la référence (3).

Sa tache focale en émetteur-récepteur a dans l'eau un diamètre de 2 nunet une j>r;.u urd'environ 80 mm pour une chute de 6 db. Dans l'acier, sila surface eau-acier est plane le diamètre est d'environ 2 mm et la lon-gueur 20 mm ce qui permet de couvrir la totalité de la soudure comme in-diqué sur la figure 3.

445

II.2.2.rtirii~cuve,-

t.es essais en laboratoire (1) et (2) avalont été faits en immersion to-tale. Pour l'application industrielIR du procédé nous avons conçu un» mloi-cuve tells que calls schématisée sur le figure ">. Le Joint à lèvre plis-sant an sllastftne est olaqué sur la virole par la oression de l'eau nulaï-rive et repart dans la haut de la mini-cuve. Un très loger débit d'eaues%%assuré. Les fuites sont nulles en parties courantes et très faiblesau- passage des soudures de raboutage des tôles de la virale. Le volumed'eau contenu dans la cuve est de "150 cro\

Par commodité sur le chantier nous avions alimenté notre cuve directement' 'sur le réseau d'eau mais un circuit fermé muni d'un bac de dégazage au-rait parfaitement pu être utilisé.

III. RFSULTATS «I CONTROLE.-

III»1. Etalonnage.-

Aucun critère n'ayant été fixé pour ce contrôle par ultrasons il a éténécessaire de choisir une référence, (a machi.no n'ayant «té disponible»que trop tardivement il n'était plus possible de faire des essais variéssur des échantillons significatifs. Nous avons donc choisi de rechercherpar ultrasons dans la moitié de l'échantillon d'agrément de procédé clnsoudure les défauts éventuels. Nous avons augmenté le gain de l'appareilà ultrasons jusau'à ce quo nous ayons vu apparaître les premiers échos.L'enregistrement obtenu est donné Figure 4.

Les réglages do l'USE 1 était :

Puissance 2Gain 74 dbSeuil 2.0Guérite 15 - 70Distance 250 mm acier longitudinal avec synchro s»ur 1er écho.

C'oat ce replace OUR nous avons utilisé nour lu contrôle sur chantier.

446

L' échantillon a ensuite été étudié par radiographie et micrographie parl'Institut do Soudure. Cet examan a fait l'objet du rapport n* 5210 dett. OU8RESSON.

Les défauts détectés présentaient tous une très petite section compriseen 1/4 et 1 tnm^ pour une langueur variant cfe 2 à 12 iwn. Tous ces défautsétaient très nettement plus petits que ce que les spécifications établiespour le contrôle radiograohique prévoyaient.

Le môme réglaga eysnt été utilisé sur le chantier cela a conduit à unetrop grande sensibilité. La comparaison antre la figure 4 et les résuî-tets des examens microgrsphiqueg montra que îa dimension des défautsdans le sens de défilement du contrôle est correcte tout en étant légère-ment surestimée de 2 mm environ, par contre îa dimension da l'image desdéfauts dans la ssns perpendiculaire au défilement du contrôle {donc //eu balayage X ds l'enregistreur) est très largement surestimée. Ceciest dû à l'enregistreur qui possède un certain traînage visible sur l'en-registrement. Ce traînage a deux origines :

a) élactroniquQ, le mouvement de la plume n'ast pas parfaitement syn-chone avssc le mouvement du traducteur mais est en retard,

bî physique, 1 'Slectrolysa du papier "encrasse* le oluma qui continue àécrire un certain temps variable après avoir cessé d'êtra alimentée.

On doit &£alemant signaler que la rigidité de la machine n'était pas par-faits 0t suivant le sens du déplacement vertical du paloeur son orienta-tion variait légèrement, ce qui accroissait encore l'effet da traînage.Ce phénomène était très gênant cor son amplitude variait avec l'effortde frottement du joint à lèvres sur îa viroîe, effort constamment variable.

III. 2. Contrôle sur le site. -

ÏI 1 . 2 .

Le contrôle a dû être fait dans l'atelier sits en intervenant uniquementpendant les périodes ou le toit était en oosîtion "endroit" sans nourautant entraver lea différentes onérations en cours sur ce toit. Cela

447

noua a conduit à intervenir durant trois périodes de toit "endroit", tepas d'exploration étant de 0,5 mm, la vitesse de contrôle, manutention

• C " '" * ; ? ' • i .''...'comprise, a été de S m de soudure par jour, es qui a conduit è 8 tourspleins de contrôle plus une perte de temps de 3 lours environ liée àl'interpénétration de notre opération de contrôle avec les autres opéra-tions en cours et eux conditions climatloues (circuit d'eau gelé, nulletrop épaisse.' etc...) du moment.Mous noterons"sImp!ement Qu'une rood Mication a depuln été apportée àta machine et quo H vitesse de contrôle de cette môme machine estmaintenant de î,5 nii 5 l'heure, soît une douzaine de mètres par jour. .

IîI•2.2.Rêsuitats^obtonus.-

Comme nous l'avons dit dans lo paragraphe 111'.t. la sensibilité utiliséea été. trop forte car nous voyons" que-"trof? de choses ont' été détectées,ce qu! auraif pu avoir pour éffot de'ifïé pas mettre suffisamment enévidence les zones de défauts nocifs. Nous n'avons pas modifié la sensi-bilité car notre pièce étalon était pendant la période de contrôle enexamen à l'Institut de Soudure, cette pièce constituant notre seule réfé-rence, nous avons préféré foire 'la totalité du contrôle avec le gainqui avait été défJnf malgré Pexcès.de sensibilité.

La totalité de la soudure (moins deux mètres) a été contrôlée1.i . . - . « , • .

IV. CONCLUSION.-

. i . -,. *La machine de contrôle a rempli son rôle. La reproductIbI IIté des résultatsa été parfaite'. On peut regretter quo nous n'ayons pas eu le tercps suffi-

! . •sant pour refaire un deuxième contrôle avec un réglage plus conformeaux défauts qui étaient recherchés. Cette expérience qui est la premièreen France est très encourageante pour l'avenir. La procédure de contrôlequi devrait présenter le inaxlaum d'efficacité et de rapidité pour uncoût minimal nous semble pour ce type de soudure la suivante :

1. Contrôle de la totalité de la soudure par ultrasons-en automatique.

2. Contrôle par radiographie des zones trouvées défectueuses auxultrasons.

448

3. Comparaison des résultats et réparations éventuelles.

Cetto procédure appliquât* au toit PHFrNIX en rfi'tin3«cernent df» cello quia 6té utilisée aurait conduit à divi&er la dur&o du contrôle par nuotre,le coût par deux et e pn augmenter J *eff icacH6.

ta bonna marche de cette première expérience industrielle devrait cons-tituer un tremplin pour la muïtipHcotitm des machinas de contrôle au-tomatique.

B Ï D L i n n R A P H T C

C 1 Î EÏIH3R ET REALISATION P'l:finTBIRS UlTRASONORFS FOCALISANTS.

APPLICATION AU CUNTROlt DtC SOUOUKfS D 'ANGIE OAK'S L ' A C I E R

INOXYDABLE.

Communication présentée ?i 1*IKSTN

Y. BCHJRncniS - J. MARINÏ - fi. POULE.

C23 CONÏROLF AUTOMATIQUE PAR ULTRASONS n'ASSF^LAnr. 5 D'ANGLEAClfrR AUSTfcNI TIQUE.

Communication présentée à l'INTSN

J. mRINT Bt M. ROULE.

13) LES TRADUCTEURS FOCALISES.

Communication présentée à 1*ÏKSTN

t". ROULE et R. SAGLIO.

449

Venn avance pas à pas charte*

v»o

CommandeEnr«a»streur

, x Cd« Centrale

LHoteur avance pas a pas papier

FIG.1_ ENSEMBLE DE CONTRÔLE SUR CHANTIER DE SOUDURE D'ANGLE — PRINCIPE

rffcen

Lentille focalisante

Transducteur aUltrasons

Circulationeau

Cuve .pour imnersion partielle

Joint glisr.ant

Assemblage soudé

//•'.•'s/ X/ .•' /' -X /./'/. . ./,-VV ','

F(G.2« TRANSDUCTEUR A ULTRASONS FOCALISé AVEC DISPOSITIF

A IMMERSION PARTIELLE _ PRINCIPE -_

Traducteur

v

Plan contenant îe

E_paisseyr intéressée parle con t rô l e

" Dard

FiG.3 _ CONTROLE DE SOUDURE D'ANGLE

452

;l«iS>;: r}.,;:<!I«:','•' ::• *• -t -.! < •'V-;': .••:-x-è:"~K.'• " '!: ,--,*' -'..V>' ". «• - »,"nVH is rrSF':.' ',.". ,"'"" Mr-"A:"H i """*'-' à '> •'."'•• F•>.S\ .SriiiÇV."'1 '• •." <H B*«8i:.*'':'AL:ï,«r«i-iw,'''. /•;* " - . *- :

Stt ...i;ï«.„ «?ïI-S<'-»'"'"»" '.-- "Xï;

"M:î.T 4'

Fig. 4 ENREGISTREMENT DE LA MOITIE DE L'ECHANTILLON D'AGREMENT

Fig. 5 MACHINE DE CONTROLE AUTOMATIQUE DESSOUDURES D'ANGLE DU TOIT DE PHENIX

454

ANNEXE HI

APPLîCAÏïON DE L'EMISSION ACOUSTIQUE AU CENTRE D'ETUDES

NUCLEAIRES DE SACLAY

pur

N, CHRETIEN*

E. TOMACHEVSKY**

1 - INTRODUCTION

Le CEÂ s'est îni-éressé depuis 1968 aux possibilités d'investigation

offertes par les techniques d'écoute d'émission acoustique et dans ce'but îl a

utilisé aussî bien du matériel commercialisé que du matériel construit ou modifié

par ses propres spécialistes,

H a tour à tour expérimenté :

- tes matériaux sur éprouvettes

- {es jonctions par soudures

- les structures complètes.

Cette note, forcément succînte, ne donne qu'un aperçu des travaux déjà

effectués ou en cours.

2 - CARACTERISTIQUES ACOUSTIQUES DÈS MATERIAUX

Comme if est nécessaire de connaître parfaitement les caractéristiques de

l'émission d'un matériau particulier avant d'envisager une méthode de contrôle non

destructif de ce matériau, basée sur l'analyse des ondes de contrainte, les matériaux

à étudier sont soumis à un test mécanique simple, en l'occurence un essai de flexioneffectué sur des éprouvettes, soit simplement entaillées, soit entaillées, puis

fissurées par fatigue.

* DMECN/STA Sac lay

* *DEDR/SEMT Saclay

455

Pour les acîers à haute résistance de Iimite élastique supérieure à 60 h bars

(feuiliard de précontrainte, 35 CD4, etc), {'essai le mieux approprié est ï'essaï de

traction, Mais pour ies matériaux moins résistants (aciers inoxydables et aciers de

caisson) et compte tenu des recommandations de l'ASTM sur l'épaisseur, J'essai de

flexion en trois points est préférable car ii nécessite des efforts beaucoup plus faibies

que l'essai de traction pour une même section d'éprouvette. Cet essai de flexion a

finalement été retenu pour tous les matériaux.

Les résultats obtenus sur quelques matériaux sont analysés un peu plus en

détail.

" Acier austênttique Z3 CNJ8-1Q (AiSI 304L)

La charge et.i'émission acoustique cumulée sont portées en fonction du

temps pour deux barreaux, l'un simplement entaillé, ('autre entaillé eî fissuré

par fatigue (Rg. 1). Le comportement des deux éprouvettes est différent. Pour le

barreau simplement entaillé, l'émission acoustique accuse une montée raide dans

le zone de décharge plastique fusqu'à un point où semble se produire une diminution

de la pente moyenne. A partir de ce point, l'activité acoustique est donc moins

grande,

Dans le cas du barreau fissuré, l'émission acoustique débute à un niveau

de charge plus faible et croît suivant une courbe dont !a pente moyenne est

sensiblement constante.

Quelques photos de signaux d'émission acoustique obtenus sur une éprouvette

fissurée sont montrées en Fig. 2 . La figure 3 présente l'évolution des signaux au

cours de l'essai de flexion pour une autre éprouvette également fissurée. On peut

voir îa différence entre les signaux de déformation plastique (signaux de faible

amplitude) comme dans la zone 1 et les signaux qui correspondent a la progression de

la fissure, début des zones 6 et 7 par exemple, qui sont d'amplitude très importante

et qui donnent sur la courbe d'émission cumulée des marches caractéristiques du

développement d'un défaut.

456

au carbone A 56

La charge et l'émission acoustique cumu-ée sont portées en fonction du

temps pour deux barreaux, l'un simplement entaillé (Fig. A], l'autre entaillé et

fissuré par fatigue .

L'émïssîon acoustique de l'éprouvette fissurée commence à croître un peu

avant que ]Q charge ait atteint son maximum. Au cours de ia décharge progressive,

l'augmentation de l'émission est régulière et très importante. Par contre, i 'éprouvefte

simplement entaillée commence à émettre lorsque ia décharge est déjà fortement

amorcée .

~ Ail îage jéger AG 5

L'émission acoustique cumulée et ia charge sont portées en fonction

du temps pour deux essais de flexion, l'un d'un barreau simplement entaillé

(Fig. 5), l'autre d'un barreau fissuré. Le phénomène d'émission acoustique lié

à la déformation plastique seule se voit nettement dans le cas de l'éprouvette

simplement entaillée. Par contre, l'éprouvette fissurée émet de façon plus

régulière dès qu'une certaine décharge est atteinte (320 daN environ). Pour ce

matériau, fa différence des émissions acoustiques en fonction de la charge est !&

encore très neîte.

3 - CONTROLE DES SOUDURES

La création d'une fissure au cours du refroidissement d'une soudure se

traduit par l'émission de signaux de même nature que !a création d'une fissure sous

l'effet de contraintes d'origine mécanique. Une des applications pratiques de

l'étude de l'émission acoustique est, de ce fait, !e contrôle de ia qualité d'un

cordon de soudure au cours de son exécution,

TSG

L'éprouvette est un disque en Z3 CNT 25 de Î60 mm de diamètre, bridé

sur sa périphérie et fléchi au centre ; par soudage automatique TIG, on fond un

cordon circulaire de diamètre 60 mm. Une étude précédente a determine

l'influence de la fièche sur la fissilité du cordon.

45?

Une expérience a permis de démontrer que l'émission acoustique recueillie

par îe capteur était bien due à la libération des contraintes thermiques (Fïg. 6).

Sur une iôîe avec une flèche très importante (24 mm), une première fusion a été

effectuée sur Sa première moitié de la longueur totale de soudage (courbe en trait

fin), Après refroidissement, la torche a été amenée au début du cordon et une

deuxième fusion a eu lieu sur route sa longueur de soudage (courbe en trait fort),

il apparaît très nettement qu'une émission acoustique se manifeste au cours du

deuxième épisode, seulement au moment où fa torche aborde la partie de la tôle

quî n'a pas été précédemment soudée, les contraintes ayant été libérées dans la

première partie du cordon par ics première passe de soudage.

Trots fusions successives sur une même tôle avec des flèches croissantes

de 18 ô 22 mm ont montré une augmentation brutale de l'émission acoustique

aux mêmes endroits du cordon par suite de la création puis de ia propagation des

fissures.

- Soudage 6E (bombardement êiectî'ontque)

Par les méthodes classiques de soudage (soudage sous flux et même soudage

TIG), il peut se produire des formations de crasses ou d'oxydes qui, en craquant,

produisent une émission acoustique qu'il est diffîciîe actuellement de différencier

de l'émission propre des matériaux soudés. Sous vide, par contre, on n'observe pas

de signaux parasites de cette nature. On constate cependant, que le bruit de fond

augmente pendant ie soudage par bombardement électronique (Fig, 7). Néanmoins,

au cours d'un essai de soudabîiiré, un certain nombre de signaux de forte amplitude

se sont manifestés à îa fin du soudage, f is correspondent à !'apparition d'une

fissuration au début du cordon. La fissure s'est propagée dans ies minutes qui ont

suivi l'arrêt du faisceau. La pièce obtenue présentait dans l'axe du cordon, et

pratiquement sur toute sa longueur, une fissure très nette,

4 - RECHERCHE DES DEFAUTS DES STRUCTURES

La détection et la localisation des fissures de longueur critique, c'est-à-

dire de longueur suffisante pour se propager dans ies conditions de service est d'un

intérêt majeur pour le contrôle et la surveillance des structures ; c'est pourquoi ie

458

CEA a entrepris un programme d'essai d'application des techniques d'émission acoustique

sur des structures complètes tant sur des maquettes en laboratoire que sur des

enceintes réelles industriel Ses.

4,1 - Essais en laboratoire

Un programme-d'étude sur la propagation des fissures de type ductile est en

cours de réalisation, iî est effectué sur des groupes de récipients sphériques ayant les

caractéristiques suivantes :

- acier de cuve AMMO composition : C --0,12 V - 0,07

Mn =1,20 $=0,015

Mo ^0,45 P=0,07

Cr -0,03

propriétés mécaniques : R = 58 hb

E = 47 hb

A= 24%

- diamètres de 363, 918 et 1 800 mm

- 'épaisseurs respectives de 3, 7 et 15 mm

- les fissures sont représentées par des entailles en V pénétrant à 80%

de l'épaisseur et de longueur, variable.

Au cours des mises en pression hydraulique de ces enceintes on a effectué

des essais de détection d'émission acoustique ; le but étant d'essayer d'établir une

relation expérimentale entre les signaux acoustiques et îa longueur des défauts

d'un type donné. L'existence d'une telle relation permettra d'affirmer qu'il est

possible de déterminer, a partir d'une base expérimentale, des relations de ce type

pour des enceintes en vraie grandeur contenant divers types de défauts.

Un exemple d'essai sur une enceinte de 363 mm de diamètre est montré

en figure 8. Le défaut fraisé à une longueur de 70 mm. L'épaisseur en fond d'entaille

est d'environ 5/10 mrn. La longueur totale de la fissure, après rupture, est également

de 70 mm. Les augmentations de l'émission acoustique à chaque augmentation de

charge correspondent à la création et à la propagation de la zone fissurée. On peut

constater ici l'effet Kaiser ou effet d'irréversibilité de l'émission acoustique.

459

En effet, au cours des charges et décharges successives,, on n'observe pas d'émission

acoustique font que l'on n'a pas atteint la charge maximum du cycle précédent .

4.2 - Essais sur des str uc t res industrie! les

Le C.E.A. a pu effectuer un esse" d'écoute acoustique de la cuve

d'un réacteur à l'occasion d'un arrêt technique afin de constater dans quelle mesure

les maîérîeis utilisés éf-aient opérationnels au cours d'une intervention "In situ",

l'objectif visé étant de pouvoir enregistrer les signaux acoustiques érnls par !a

cuve d'un réacteur pendant son épreuve hydraulique et plus tard pendant ses ré-

épreuves afin de comparer ces signaux et en déduire s' s i y a ou non une fissuration

dangereuse ,

Les équipes de montage ayant poursuivi leur activité pendant

l'épreuve, ie niveau du bruit ambiant a été anormalement élevé, de plus le

bruît de la mise en pression hydraulique a été, trop fort également, i! en est

résulté que, les signaux acoustiques détectés se sont trouvés noyés dans un bruit

élevé et n'ont pas pu être triés par l'appareillage utilisé. Voir la figure 9, qui

donne un exemple du speclre des bruits.

Les matériels se sont bien comportés dans une ambiance d'une trentaine

de degrés pour ia majeur partie des appareils et d'une cinquantaine de degré

pour les organes de détection.

Cet essai a fourni de nombreux renseignements sur ie comportement du

matériel et des données techniques sur les améliorations à apporter aux dispositifs

existants.

- Essai en ate Her

Une cuve en acier de 3 m de long, de '* -28 rr» de diamètre intérieur

et d'épaisseur de virole de 240 mm, pour une pression de service de 666 bars a subi

l'épreuve hydraulique réglementaire de 1 000 bars imposée par îe Service des Mines

en fin de l'année Ï971 .

Avec un objectif identique à celui de l'essai précédent, ie C.E.A. a été

autorisé à effectuer un essai d'écoute d'émission acoustique sur cette cuve.

460

Le bruit ambiant était celui o£yn îrnporranr atelier de mécanique en

activité, normale de [our et réduite de nuit. Le bruit de la pompe de charge n'a

été gênant qu'au grand débit. Le secteur n'était pas stable et de nombreux parasites

éiecfrîques ont quelque peu perturbé ie fonctionnement des appareils électroniques

malgré la présence d'un transformateur dMsoiemenf.

Les bruits captés par se? détecteurs e* comptés par Ses Totalisateurs n!ont

pas tous pour origine l'émission acoustique de l'acier de la cuve. Au cours de cet

essai un capteur de 500 KHz à été expérimenté e\ comparé aux copieurs de

120 KHz utilisés principalement et les résultats obtenus sont très encourageants.

Une fuite au joint du tampon d'obturation de la cuve a été détectée par

tous les capteurs insiaUés $ur la cuve, Sur la figure 10 on voit parfaitement

l'apparition de ict fuite/ puis l'établissement du régime d'écoulement.

5 - METHODE DE LOCALISATiON DES SOURCES EMISSIVES

Dans le cadre du contrôle ou de la surveillance par émission acoustique

de structures lors de leur essais ou en cours de fonctionnement, \\ devient vite

indispensable de pouvoir localiser les sources émertrtces de signaux. Le C.E.A. a

étudié une méthode d'acquisition de l'information et de premier traitement avant

!a phase de caicul de locafisalion,

l'appareillage se décompose en 4 parties :

- un ensemble de détection à plusieurs capteurs (3 minimum)

- un ensemble de reconnaissance de î'ordre dans lequel les capteurs

reçoivent l'émission acoustique significatrice.

- un ensemble de mesure des différences des temps de parcours de î'onde

arrivant successivement sur deux capteurs différents, ceci étant fait pour

!'ensemble des capteurs pris deux à deux.

- un ensemble de classement statistique de ces informatîons.-

La sortie de cet appareillage est couplé à un calculateur qui fait la

localisation géométrique de !a ou des sources émettnces à partir de ces données.

Cette phase calcul est basée sur la triangulation hyperbolique décrite par îcfaho

Nuclear (programme ACOUST). La théorie de ces calculs est relativement simple

461

en géométrie plane. On sait en effet que l'hyperbole est le Heu géométrique des

points dont fa différence des distances aux deux foyers est une constante. Chaque

couple de deux capteurs représente les foyers d'une branche d'hyperbole sur laquelle

se trouve ra source. La zone de rencontre des branches des différentes hyperboles

déterminées par je calcul définit la région ou se trouve !a source émettrïce.

6 - CONCLUSION

L'émission acoustique est un nouveau moyen d'investigation notamment

dans les domaines suivants :

- Analyse et comportement'des maténaux

Les principaux laboratoires d'étude des matériaux effectuent des écoutes,

d'émission acoustique lorsque ces matériaux sont sournts à diverses sollicitations.

Par exemple, des études de fragilisation par l'hydrogène sous contrainte de câbles

en acier à très haute résistance sont en cours. La détection acoustique permet de

mettre en évidence, d'une façon tout à fait originale, i"influence de différents

traitements thermiques sur la tendance à fa fragilisation,

- Contrôle en cours de fabrication

Des études entreprises pour déceier la fissuration due au soudage, peuvent

être étendues et développées chaque fois que l'on aura à contrôler des pièces

chères, foitement soiiicitées,

- Contrôle global en fin de fabrication (cuve de réacteur par exemple)

L'écoute de l'émission acoustique effectuée au cours d'une épreuve

hydraulique en atelier apporte des informations indéniables à l'Inspecteur qui ne

disposait que de l'examen visuel jusqu'à présent.

- Controle global en fin de montage sur je sîte

Alors que l'essai précédent intéresse le composant principal, l'essai global

intéresse une structure complète voire tout un ensemble tel qu'un circuit primaire

de réacteur.

462

- Controle périodiqueC'est encore un contrôle global mais effectué après un certain temps de

fonctionnement, î! nécessite donc une instrumentation capable de supporter les

conditions de fonctionnement.

- Surveillance en cours de fonctionnement

I! s'agît du moyen idéal de connaître !'état de ia structure à tout moment

mais suppose que !'on puisse discerner Ses informations intéressantes des bruits de

fonctionnement,

tes études effectuées un peu partout dans le monde, montrent l'intérêt

de cette méthode de contrôle et peu à peu elfes apporteront ies solutions techniques

aux problèmes rencontrés actuellement.

463

EMISSION ACOUSTIQUE DE L'ACIER Z3 CN 18 10

464FIG.1

ZONE 1 ZONES 2 .3

ZONE ZONE S

ZONE 6 ZONE 7 ZONE 8

dv/cm.Si/cm)

466

-, 6000

2000

1500

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VARIATION DES SIGNAUX EMiS PAR L'ACIER 23 CN 18.10

467

FIG. 3

EMISSION ACOUSTIQUE DE L'ACIER XC 18 f

468ESSii.

DEBUT SOUDAGE SOUDAGE ARRET SOUDAGE

PL 4?7-25 Annexe I I Il

R£FRO!DISS£MENT

2 v/cm0,5$/cm64 kHz

2 v ' c m

* S.'C-n

64

PARAMETRES SOUOA&E60 k v . 360 mAS, SA Foc

Distance tir 65 mm

EMiSSÎON ACOUSTiOUE DU SOUDAGE BE (Bombardement électronique)&&.?

KJQQC -

15000 -

10000

5000

EMfSSiON ACOUSTIQUENOMBRE DE COUPS

CAPTEUR OUNEGAN lï CAPTEUR H

EMISSION ACOUSTIQUE D'UNE ENCEINTE SPHERIQUE

F1G.8

472

EXAMENS IfOff PgS'mUCTIFS DÎSS ELEMENTS COMBUSTIBLES IRRADIES

M» Watteau

FRANCE

Résumé

Cette contribution concerne essentiellement les sxamens non destructifspratiqués, après irradiation, sur les éléments combustibles de réacteur à eau.

Gém-raierrent, ces examens sont pratiqués dans la piscine de stockage'attenante au réacteur et leur but principal est de permettre de décider si,oui ou non, les éléments examinés peuvent être rechargés pour un nouveau cycled'irradiation.

L'objectif à atteindre est donc de contrôler un grand nombred'éléments dans un temps minimum. Cela nécessite une organisation pousséedes programmes d'examens.

Les examens non destructifs, praticables en piscine, dont il estquestion ici sont les suivants ;

- Contrôles d'étanchéité- Scrutation gamma- Contrôle par ultrasons- Contrôle par courants de Fouc-ult- Inspection par télévision.

En complément, on décrit la méthode de neutrographie utilisée parle CEA à l'aide de l'installation associée su réacteur OSIRIS.

EXAMENS NON DESTRUCTIFS DES ELEMENTS COMBUSTIBLES IRRADIES

I - INTRODUCTION

Cette contribution concerne essentiellement les sxamens nan destructifspratiqués, après irradiation, sur les éléments combustibles de réacteurs à eau.

475

Généralement, ces examens sont pratiquas dans la piscine-1 de fai.uckagaattenante au réacteur et leur but principal est dn permettre de décider si,oui ou nons les éléments examinas peuvent être rechargés pour un nouveaucycle d'irradiation.

L'objectif à atteindre est donc tis crjntrSlnr un grand nombre d'élémentsdans un temps minimusn. Cela nécessite unu organisai ton poussée*, des programmesd'examens.

Les examens non destructifs, praticables en piscine, dont ji sera questionici sort les suivants :

- Contrôle d'étanchéité~ Scrutation gamma- Contrôle par ultrasons- Contrôle par courants de Foucarit- inspection par télévision,

En complément, on décrira la méthode de neutrngi^phie u ta..l i&ée par leCFA à l'aids de l'installation associée au réacteur OSIRIS.

II - CDNTROLF B'ETAiCHEITC

La méthode u-tilisée est connue sous le nom de "Sapping test".

La mesure de l'activité gamma d'échantillons d'eau, prélevés a l'intiîrieurdes éléments combustibles, est utilisée pour dépister ceux dont certainscrayons ne sont pas étanches.

Si les éléments combustibles sont dotés de boîtiers 'sans perforations, leprélèvement d'échantillons d'eau peut s'effectuer c-ans qu'il ooit nécessrîired'extraire 3es éléments de la cuve du réacteur (celle-ci étant ouverte bienentendu).

En raâson de la puissance résiduelle du combustible, il existe une circu-lation naturelle de l'eau, de bas en haut, à l'intérieur du boîtier qui enve-loppe latéralement chaque élément. Cette circulation naturelle est bloquéeen criffant d'une cloche étanchs le sommet d'un élément. Si certains descrayons qui le composent, ne sont plus étsnches, l'eau qu'il contient SB charyeen produits de fission (et en radionuclides, descendante de ces fjrtsduits de

476

fission). Après quelques dizaines de minutes d'accumulation, un échantillond'eaus d'un demi litre environ, est prélevé à 3'a.ide d'un dispositif traversantla cloche étanche. L'activité gamma de l'échantillon est ensuite mesurée,La figura 1 schématise une installation possible de "sipping test",

Cette méthode est une méthode relative, permettant de comparer entreelles les activités d'échantillons provenant de ehpcun des éléments combus-tibles et de comparer CBS activités à celle de J'eau du réacteur.

Si les éléments combustibles ne possèdent pas de boîtierst au bienpossèdent des boîtiers perforés, le contrôla d'étanchéité peut se pratiquer,dans la piscine de stockage, selon la même méthode, mais en utilisant cettefois une cloche étanche capabJe de loger la totalité de l'élément. Pouraccélérer l'accumulation des produits de fission, il est passible d'emplircette cloche avec de l'eau chaude.

On peut penser que les possibilités du "sipping test" sont limitées,d'une part :

parce que les produits-de fission ont tendance à s'échapper des crayons**

non étanches, pendant lé temps qui sépare l'arrêt du réacteur des opérationsde contrôle ;

d'autre part ;parce qu'à importance égale de défauts, une fuite proche de la zone de

prélèvement - c'est à dire du sommet de l'élément - donne un signal plusimportant qu'une fuite éloignée de cette zone -. c'est à dire située vers lebas ûss. l'élément.

La position la plus réaliste vis à vis du "sipping test" consiste à direqu'un signal important est l'indice de défauts d'étanchéitê - à préciser, sibesoin est, par une inspection plus fins — mais qu'un signjil "normal" n'est pasuns preuve de l'étanchéité des crayons.

Une autre méthode dite de "dry sipping" se pratique comme indiquéci-dessous t

L1element à contrôler est dans la piscine de stockage. On le recouvred'une cloche étanche. L'eau contenue dans cette cloche est refoulée et rempla-

477

cée par un gaz (air ou ga? neutre). L'élément reste ainsi un certain tun.psconfiné en atmosphère gazeuse, L'activité gamma du gaz contenu dans la clocheest ensuite mesurée.

Le "dry sipping" est certainement une methods beaucoup pJus sensible quele nwct sipping"î

- dans un gaz, la position du défdt-1 par rapport à la zone de prélèvementest moins importante {les produits de fission gazeux qui s'échappent del'élément, se mélangent au milieu ambiant de façon homogène, plus facilementdans un gaz que dans l'eau).

- le gaz de remplissage de la cloche refroidit moins bien l'élément quel'eau ds la piscine de stockage. Il en résulte, du fait de la puissancerésiduelle du combustible, un échauffement qui dilate les défauts d'étancnéitééventuels des gaines et fait dégager par le combustible uno partie des produitsde fission retenus.

Cette technique du "dry sipping" présente ct-pendant des inconvénients :

- On peut se demander dans quelle mesure ce n'est pas une méthode des-tructive, surtout pour des combustibles à forte puissance résiduelle,

- Elle est plus délicate à mettre en oeuvre et demande plus de temps quele "wat sipping" lorsque les éléments combustibles sont nombreux et de grandesdimensions.

III - SCRUTATIOM GAMMA

Les spectrographies gamma pratiquées sur les éléments combustibles ontpour hut de matérialiser les répartitions axiale et radiale de puissance dans leréacteur. Elles servent également à déterminer les taux de combustion atteints jceux-ci faisant généralement l'objet de garanties contractuelles.

Lorsque les spectrographies gamma sont effectuées quelques jours seulementaprès l'arrêt du réacteur -» ce qui est le cas pour les examens réalisés à

- l'occasion de l'arrêt annuel d'une centrale - les mesure* d'activité exploita-bles sont essentiellement celles concerna."ît les rayonnements émis par 3elanthane 140 et le couple zirconium niobium 95 (périodes respectives : 40 heureset 2 mois environ).

478

Pour des temps de refroidissement de quelques mois» il est possiblesd'utilisar 3 ES rayonnements émis par le prassodysss 144 - descendant ducérium 144 - qui a une période apparente d'environ 9 mois et demi,

Au delà, on peut prendre en compte le césium 137 (période de 30 ans)»

La figure n° 2 schématise une implanta'ion possible ds l'installation descrutation gamma en pasuine.

L'ensemble collimateur~détecteur (germanium - lithium) fixe est immergédans la piscine. Une "cheminée", débouchant à la surface, assure 3e passage descâbles électriques et d'une canalisation permettant de refroidir en permanencele détecteur à l'aide d'azote liquide.

L'élément combustible à tester est placé verticalement, en face de lafenêtre du collimateur, sur un bâti élévateur permettant des mouvements verti-caux dans les deux sens.

Les signaux sont recueillis par un analyseur multicanaux et les spectresenregistrés sur bande magnétique.

IV - EXAfCNS PAR ULTRASONSt COURANTS DE FOUCAULT et TELEVISION DES CRAYONS COMBUSTIBLEj

Certains éléments aombustibJes sont conçus de façon qu'ij soit possible,après irradiation, d'enlever individuel]ement, en piscine, des crayons de1'assemblage pour les examiner. Ensuite, ces crayons peuvent être remontésdans l'élément ou remplacés par des crayons différents. Cette conception permet,dans certains cas, de procéder au remplacement des crayons défectueux d'unélément avant de le recharger peur un nouveau cycle d'iriadiation.

Les crayons extraits d'un élément peuvent subir, en.piscine,tdes examenspar ultrasons, courants de Foucault et télévision.

Les examens par ultrasons visent à mettre en évidence la présence d'eaudens les crayonn. Ceux par courants de foucault permettent de détecter lesruptures de gaines.

Les créions, réputés wauvais à l'issue des contrôles par ultrasons etcourants je Foucault, peuvent être soumis à une inspection par télévision ?

479

cette inspection ayant pour but de préciser la nature des défauts. II estévidemment possible d'enregistrer sur magnétoscope les images les plus inté-ressantes.

Le matériel de télévision développé et commercialisé par 3.a sociétéfrançaise THOHSON--CSF est particulièrement bien adapte à ci* type de contrôle.

V - EXAMENS PAR TELEVISIDN DES fi. CME NTS COMBUSTIBLES

En 1968, les éléments combustibles du réacteur Équipent 3 a centrale deCHOQZ ont fait l'objet d'examens particulièrement détaillés par télévision.Le but était de localiser, à l'intérieur de res éléments, équipés de boîtiersperforés, des débris métalliques provenant des structures internes du réacteur.

La figure 3 illustre la technique d'inspectirm utilisée. Les examenss'effectuaient au travers des faces latérales des boîtiers perforés. La caméra,le projecteur et le réflecteur étaient solidaires et pouvaient se déplacerverticalement. L'élément en cours de contrôle pouvait tourner sur lui-même selon

—1son axe vertical et être déplacé à très faible vitesse (1 era.s } suivantdeux axes horizontaux perpendiculaires.

Le réflecteur était destiné à atténuer et mieux répartir la lumière duprojecteur.

Cette solution a permis d'inspecter correctement les éléments combustibles,notamment au niveau des grilles où l'on pouvait craindre un rassemblement desdébris métalliques par un effet de filtrage. Elle a permis également de localiseravec précision la position des débris, d'une part en mesurant la cote parrapport au pied de 1*assemblage, et d'autre pprt en effectuant deux visées à90° au travers des alignements de crayons. La mise au point de l'optique de Jacaméra étant télécommandée, il était possible d'examiner toute l'épaisseur de1'assemblage.

L'objectif poursuivi a été atteint puisque cette inspection a permis deprocéder, dans de bonnes conditions, au nettoyage des éléments qui ont étérechargés et ont ensuite fonctionné de façon satisfaisante.

480

- METHODE 0'EXAMEN NCUTROGRAPHIQUc. .UTILISFE,__AJMRI5. (pile piscine)

La neutrographie des crayons combustibles permet d'examiner I'-eispact desconstituants internes après irradiation. Cette méthode a Également été uti3i5.eepour expertiser des ruptures de gaines en zircaloy.

L'examen est effectué à l'aide d'un faisceau de neutrons sur le trajetduquel on interpose l'objet à inspecter.

te faisceau, modulé par l'objet, est reçu sur un convertisseur constituéd'une feuille de dysprosium. Cette activité est utilisés pour impressionnerun film qui, après développement, donne l'image neutragraphique de l'objet.

La figure n" A schématise le dispositif de neutrographie utilisé àQSÏRIS.

Ce dispositif est immergé dans la piscine du réacteur.

L'ensemble de collimation est constitué d'un diaphragme et d'une chambre,de forme pyramidale, dont les parois sont tapissées de carbure de bore.

Le diaphragme est situé à une centaine de millimètres du coeur du réacteur.

Derrière le collimateur, une chambre ou "chaussette" reçoit l'objet àneutrographier -par exemple un crayon combustible ~. Les parois de cette cham-bre sont en aluminium. L'échantillon est isolé dans un conteneur, égalementen aluminium.

L'eau étant un milieu très diffusant peur les neutrons, il est nécessairede 1'évacuer de la chambre contenant l'échantillon. Pour cela, la chambre estisolée du -reste de, la piscine par un joint de glace - un groupe réfrigérantassure le maintien de ce joint pendant toute la durée de l'exposition - etl'eau est expulsée de la chambre par de l'air comprimé,

Une cassatte en aluminium est appuyée fortement, par des vérins, contrela chambre. Elle se trouve à environ 30 millimètres de l'échantillon à nsutro-graphier. Cette cassette contient une feuille de dysprosium tree pur.

Après exposition et désactivation de la cassette, le convertisseur endysprosium est utilisé pour impressionner un film. On utilise des films àgrain très fin dont le support reste stable au cours du traitement.

481

L'ensemble des opérations, pour neutrpgraphier des crayons combustibles~ mise en place- exposition du convertisseur,

demande, actusl3en'S!tts environ trois heurss.

Cette Fiéthoda permet, après irradiation, d'examiner l'aspect des consti-tuants internet; aes crayons ta plus particulièrement celui des pastillescombustibles {fissures, trou central, comblement des évidemeniSj etc .,.}.

Dans le cas de gaines en zircsioy on a pu, par neutrographie, mettreen évidence de fortss'concentrations d'hydrures eu voisinage immédiat deruptures.

Vîï - CONCLUSION

Les examens non destructifs sur éléments combustibles irradiés, dont i1vient d'être question, ont évidemment leur place dans les programmes d'étudesdestinées au développement.

Mais, de plus, ils jouent un rôle important dans l'exploitation des cen-trales en service ;

- Détermination par scrutation gamma des taux de combustion atteints.- Détection pour élimination, ou réparation éventuelle, des élémentsdéfectueux.

Les éléments dotés de crayons amovibles permettent de pousser très loinles examens non destructifs en piscins. Cl est possible de procéder, parrestrictions successives à l'aide d'inspections de plus en plus fines, à lalocalisation des crayons défectueux - ceux-ci étant remplacés par des crayonssains si l'on souhaite recharger les éléments ï

- Dans un premier temps, tous les assemblages sont soumis au "sippingtest".

- Puis, tous les crayons des assemblages suspects au test d'étanchéitésont contrôlés par ultrasons et courants de Foucauit.

- Enfin, les crayons suspects aux contrôles par ultrasons et courants deFoucauit sont examinés par télévision.

482

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483

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niveau de travail

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486

CONTROLES NON DESTRUCTIFS MIS EN OEUVRE LORS DE LA FABRICATIONDES ELEMENTS COMBUSTIBLES

M. WATTCAU

.Résumé.

Cette contribution concerna les contrôles non destructifs mis enoeuvre lors de la fabrication des éléments à combustible céramique fritte etgainage métallique.

Ceux-ci sont généralement constitués d'un faisceau de crayonscombustibles, maintenu selon une disposition géométrique régulière par despièces de structure : grilles, entretoises ou espacsurs. Dans certains cas,le faisceau de crayons est logé è l'intérieur d'un boîtier.

Les contrôles non destructifs pratiqués sur les crayons combustiblessont les plus nombreux et les plus importants pour la tenue en service del'élément j ce sont ceux sur lesquels on insiste particulièrement ici.

Les pièces de structure et les assemblages en tant qu'ensemblessont aussi l'objet de contrôles non destructifs qui sont également évoqués.

ï - INTRODUCTION

Cette contribution concerne les contrôles non destructifs mis en oeuvrelors de la fabrication des éléments à combustible céramique fritte et gainagemétallique.

Ceux-ci sont généralement constitués d'un faisceau de crayons combustibles,maintenu selon une disposition géométrique régulière par des pièces de struc-ture : grilles, entretcises ou espaceurs. Dans certains cas, le faisceau decrayons est logé' à l'intérieur d'un boîtier.

Les contrôles non destructifs pratiqués sur les crayons-combustiblessont les plus nombreux et les plus importants pour la tenue en service del'élément ; c<u sont ceux sur lesquels on insistera particulièrement ici.

Les pièces de structure et les assemblages en tant qu'ensembles, sontaussi l'objet de contrôles non destructifs qui seront également évoqués.

487

II - CONTROLES NOM DESTRUCTIFS PRATIQUES LORS DE LA FABRICATION BC5 CRAYONSCOMBUSTIBLES.

2.1 - ContrSles des constituants des cramons

Les contrôles non destructifs pratiqués sur les gaines et lespastilles de combustible fritte sont traités par ailleurs.

îl est iiïipjrtant d'insister sur le contrôle de santé des barresdestinées à la réalisation des bouchons de gainage. Ce contrôle a pourbut d'élimirîer les barres présentant des porosités ; car les bouchonsusinés dans de telles barres mettent en danger l'étanchéité des crayonsqu'ils équipent.

Le test peut consister en un simple exarcen mëtalloçraphiqus.C'est une méthode longue qui ne permet de traiter qu'un échantillonnage.

En Franco, le contrôle à 100 % des barrea à bouchons par ultrasons,courants de Foucsult, rayons X et gamma, fait l'objet d'études au CEAet dans l'industrie. Une installation industrielle da contrôle parultrasons des barres en alliages de zirconium est en service. D'autrepart, des gammes de fabrication, éliminant les opérations les plussusceptibles ds provoquer l'apparition de porosités su sein das barres,ont été mis au point,

2.2 - Contrôles des soudures

Les contrôles non destructifs sont indispensables mais non suffi-sants pour s'assurer de la qualité des soudures des crayons ctw»bustib3es.

Ils doivent être accompagnés d'essais destructifs : examens mêtal-lographiques et tests mécaniques qui ont en particulier pour but decontrôler la profondeur de pénétration des soudures, l'étendus de lazone affectée par le chauffage accompagnant le soudage, les caractéris-tiques mécaniques de cette zone. Ces essais sont des essais de mise aupoint de fabrication mais il est nécessaire ds les prrtiquer égalementen production sur des échantillons.

Certains contrôles non destructifs n'intéressent pas les seulessoudures, mais la totalité du gainage : c'est 1s cas du test de tenue à

488

la corrosion pour les gairiages en alliages de zirconium et du testd'ëtanchëité qui sont abordés plus loin.

Le contrôle rsdiographique permet de mettre en évidence lesdéfauts des tordons de soudure i soufflures, porosités, décollementsde gains» C'est un moyen qui donne de bons résultats avec les souduresréalisées par bombardement électronique ou à l'arc. Par contre, il estpratiquement inutilisable dans le cas des soudures réalisées par résis-tance, en raison de la forme irrégulière du bourrelet de refoulement.

Pour des soudures par résistance, le meilleur contrôle en fabrication est celui des paramètres de soudage : intensité et durée du chauffage, effort de refoulement. Un contrôla rigoureux ties paramètres rested'ailleurs la règle imperative à reapec^er queJ que soit le mode desoudage utilisé.

2.3- Contrôle de 3 a .tenue à Ici

Une bonne résistance du gainagn à la corror.ion est essentiellepour assurer un comportement en service satisfaisant des crayons coi?>bus~tiblcs .

Dans le cas des crayons gfiinés en alliage de zirconium et destinasà fonctionner dans l'eau, vers 300°C et au dessus, le problème de latenue à la corrosion est partirulièreroent important.

ïl est possible de contrôler cette tenus, de mani&re non destruc-tive, par un essai «sous eau en autoclave.

Lors de cet essai, une bonne résistance à 3 a corrosion du gainageSB traduit par 3 a formation d'une couche superficielle, adhérent? etuniforme de zircois noirs. Cette couche n'évolue pratiquement plus dèsque son épaisseur a atteint quelques microns.

Si la tenue à la corrosion de tout ou partie du gainagt estdéfectueuse - du fait d'une mauvaise répartition des précipités dansl'alliagte ou d'uns teneur trop élevée en azote - le passage en autoclaveentraîne la formation, dans les zones suspectes, de zircons blanche. Si leséjour dans l'eau, en température, se prolonge, cette zircons continueà se développer jusqu'à ronger tout Je métal résistant mal à la corrosion,

489

Généralement, le test en autoclave consiste en un séjour d'unedcuzaine d'heures» dans la vapeur d'eau surchauffée à 4QOSC, sous unecentaine de bars. Cet essai met bien en évidence une mauvaise tenus a lacorrosion quand elle trouve son origine dans une répartition défectueusedes précipités. Far contre» on a constaté que les résultats étaientmoins nets en cas d'amoindrissement du la résistance à la corrosiondû à3a présence d'azote dans le raétal» C'est un inconvénient certain,surtout pour le contrôle des soudures dont la cause la plus fréquentede mauvaise tenue à la corrosion est uns pollution par l'azote de L'ai*.

Sur ce point, la sensibilité du test est renforcée sa. l'on faitprécéder le séjour en vapeur surchauffée d'un maintien de plusieursheures cri phase liquide vers 300°C,

Les fabricants de crayons combustibles ne conçoivent pas tousde la même façon l'essai dn tenue à la corrosion. Certains procèdentseulement à des contrôles sur échantillons, d'autres testent systémati-quement toutes les gaines avant fabrication des crayons,

La solution qui consiste, les crayons étant terminés, à lespasser tous en autoclave ; d'abord en phase liquide, puis en vapeur, estle moyen le plus efficace da détecter - quelle que soit son origine -une mauvaise tenus à la corrosir.n des gaines, des bouchons ou dessoudures.

2.4 - ContrSle d^êtsnchéité

On contrôle l'étanchéité des crayons combustibles par ressusgehélium.

Les crayons sont maintenus pendant quelques heures dans uneenceinte emplie d'hélium sous pression. AU cours de cette opération,l'hélium s'accumule à l'intérieur des crayons présentant des fuites,

A l'issue de cette periods d'accumulation, on laisse IOP crayonsséjourner quelque temps è l'eir et on les nettoie à l'alcool j ceciafin de réduire les rétentions superficielles d'hélium.

490

Enfin, les crayons sont mis sous vide et l'on compare, parspeetroroétrie de masse , les quantités d'héliuro qu'ils dégagent aveccelles fournies par une fuite étal.tn définissant le seuil de rebut pourdéfaut d'étanchéité.

Si l'on souhaite mettre en évidence des fuites de 10 à 10 lusse,l'élimination aussi complète que possible des rétentions superficiellesest indispensable, ïl faut pour cela que la surface des crayons présenteune rugosité microscopique particulièrement réduite. Pour des gainesen zircaloy,' un décapage, suivi ou non d'un passage en autoclave, donneun état de surface satisfaisant. Par contre, l!état de surface engendrfipar un sablage à l'aide de micr.o-billes da verre entrainn des rétentionsd'hélium beaucoup trop importantes,

2.5 ~

Une fois les crayons combustibles soudés, il est importantd'uriD part, de vérifier quo leu constituants internes {pastilles combus-tibles, pastilles isolantes, ressorts, etc ...} sont bien disposés commeprévu ; d'autre part, de contrôler que les manipulations, précédant lemontage en élément des crayons, n'ont pss eu pour conséquence desdétériorations affectant en particulier Irn pastilles combust5.blcs.

Pour ce faire, on procède généralement au contrôle des empilementspar rayons X ou

2.6 - CqntrBlp de santé des gaines sur rayons terminés

II est intéressant de disposer d'un moyen permettent de contrôlerla santé des gaines une fois les crayons fabriqués.

Soit pour pratiquer un super-rcontrôle destiné à vérifierqu'aucune gaine défectueuse n'a été utilisée lors de 3 a fabricationdes crayons,

Soit pour contrôler que les gaines n'ont pas été endomnagêesau cours de cette fabrication.

Les courants de Faucault permettent de réaliser un tel contrôledans de bonnes conditions.

491

HI - CONTROLES NON DESTRUCTIFS PRATIQUES LORS DE LA FABRTÇATIDN DES STRUCTURES :Grillas, entretoisesA _: esDoceurSi^ _b_o£tiera

On pratique sur IBS grilles, entretoisas et espaceurs, des contrôlesraétrologiques d'encombrement, de régularité de pas (conditionnant l'êcartementdes crayons après assemblage ) , de dimensions des logements destinés au passagedes crayons. Pour les boîtiers on vérifie l'encombrement, la flèche, levrillage»

Pans le cas des grilles, on procède également au contrôle des effortsexercés par les ressorts as maintien des crayons,

Pour les structures de réacteurs à eau, on vérifie la tenue à la corro-sion des pièces en alliage de zirconium (voir 2.3).

Généralement, les brasures et soudures réalisées sur cts pièces sontcontrôlées visuellement, avec ou sans ressuage.

IV - Cyjn^sjQj^

Ce sont essentiellement des contrôles dâmensionneln. Ils concernent larégularité d'espacement des crayons combustibles et Is encombrement des éléments.

L'espacement des crayons est vérifié à l'aide de jauges d'épaisseur oubien par des moyens optiques.

L'encombrement des éléments, les défauts de for&e éventuels* sontgénéralement contrôlés à l'aide de gabarits.

CONCLUSION

Parmi les contrôles non destructifs pratiqués lors de la fabrication deséléments combustibles, ceux concernant directement les crayons sont les plusimportants car ils ont pour objectif d'éliminer les défauts dont l'é/olutionen service conduirait à des ruptures de gaines.

Mis à part le contrôle de la bonne disposition des constituante internesftous les autres intéressent le gainage. Ils permettent de vérifier son étan-chéité initiale et l'absence de défauts pouvant entrainer la perte de celle cimauvaise tenue è la corrosion, défauts de santé.

492

THJ3 INSPECTION OF SCHÜR PRESSURE SÜBBS

"byR B Cockaday

U.K.

(Abstract of Paper presented to IAEa Conference

Vienna 1971)

The paper describes the pre- and post-servi ce inspectionof the zirconium alloy pressure tubes installed in theSteam Generating Heavy Water Moderated Heactor ( SGBWR)situated at Winfrith, Dorset UK, and in particular givesdetails of the pressure tube bore measuring gauge andinternal viewing equipment used for periodic surveillanceinspection.

The pressure tubes in SGHWR are of Zircaloy~2 in the cold worked conditionand are about 15 ft (46 30 ram) long and 5»14 in. (130 nan) internal diameter. The\call thickness is 0,2 in. (5 sui) and the tubes are required to be straight to 1part in 3000. The tubes aro connected to the reaainaor of the primary circuit(aastenitic stainless steel) "by means of roll-expanded mechanical joints. Thedesign conditions are 1050 psig at 298°G, and opeiating conditions are somewhatless so that the tubes are stressed in service in the hoop direction at 14000 Ibs/in^ a-t 293 C. Before installation in the reactor the tubes were subjected torigorous inspection including ultrasonic exeaii nation for defects to ensure thatthey were fit foi' long service. Before the reactor first went to power some of thetubes were given a further dimensional check so -%s to provide accurate bore meas-urements which could be used as reference diameters during post-service surveil-lance measurement for in-service creep déformation. At intervals? conveniexitlyduring annual reactor shutdown period, certain of the tubes are measured using aprecise bore measurement gauge to próvido creep assessment date,. Other inspectionsv/hich can be carried out during shutdown periods consist of remote visual inspec-tion bjr TT camera or introscopo and» if necessary, probe measurements of the depthsof fretting marks or scoring due to fuel cluster movement.

In this paper are described the principal areas of pressure tube inspectionconcentrating primarily en the p^o-service defect inspection technique and the postservice bore measuring technique.

493

_SPBCIPI CATION FOR PRSSgURE T1TBSS

Apart from having to meet the normally accepted requirements for chemicalcomposition and mechará cal properties? the pressure tubes for SGííVÍR wore requiredto be closely controlled for dimcnaional tolerances and freedom from defects. Thedimensional tolerc?.nces required weres—

Length s 15 ft 2¿- in. + .125 in-Bironeter s 5» 14 -f «01 in. (including ovality)Wall Thickness î .105/215 i*1- (including eccentricity)Straight-ness s 1 in 3000 overall j 1 in 900 over any 3 ft

length.

These tolerances vjere all assessed by conventional techniques.

All tube.s were inspected ultrasonic ally for defect?? end vmll thicknessvariation» All tubes which on ultrasonic examination exhibit od the prononce ofdefects giving a greater signal than the "standard" defect (0.2fjn long x .005"deep x .005" vade (6. 35 cim x .13 ram x .13 ni-n) ) were set asido for furtherexamination, and if net corrected, were rejected.

A!P

At the manufacturer's works the tubes in the semi-finished condition, i.e.straightened, stress relieved, aud honed in the bore, and flasheâ pickled on theoutside surfaces but not finally pickled and autoclaved, were measured for lengthand outside dicmeter (vernier tape), inside diameter (air gauge and plug gaxage)and straightness (run-out measured by dial gauge with the tube mounted on rollersabove a. flat gauging tr.ble). Wall thickness variations and incidence of defectswere assessed by an ultrasonic method essentially as developed by Heactor MaterialsLaboratory, Culcheth and described previ ousl y\. 1 ). Incidence of defects was asses-sed on a 'Continuous trsvorae basis using separate transmitting and receivingprobes for both longitudinal and truieverse orientation of defects. Wall thicknessvariations wo.ce measured by making discrete manual circumferential scans at 12 in.intervals along the tube.

I.ABOBATOHISS

Is an additional check on the thoroughness of the flaw detección techniqueused at works and on the validity of the discrete wall thickness measurements»-certc-in of the early tubes produced, and subsequently at intervals during themanufactura of the batch of tubas for the z*eactor core, were --c-assessed usingthe installation available at Oulcheth Laboratories. The samo basic techniquewas used but for flav,1 detection single transmitter/receiver probes for both cir-cumferential and longitudinal defects were us^d and wall thickness variationswere assessed ?.âth a probe mounted on the traversing carriage carrying the flawdetectors so that a complete helical scan of about 3/16" pitch was made for thefull length of each tube. A general view of the equipment is ehown in Pig 1.These check insasursaaents confirmed that the procedure used at the works wassatisfactory.

It has been variously reported^- }^}4) ^^ ^Q creep rate of zirconium alloysis increased by exposure to fast neutron flux, and in order to check the creeprate of the pressure tubes in SGHW under operational conditions against the

494

behaviour predicted by results obtained in experiments in toet reactors it wasaeoided to make periodic measurements of the borss of certain pressure tubes atconvenient intervals during the lifetime of the reactor. To provide datummeasurement s of boro diameters at start of reactor life it was necessary to use ameasuring device of greater accuracy bhsn tho air gauges and plug gauge usedduring but, manufacturing process control (note that the specification permits avariation of up to .020 in. (0.5 rara) i a boro dimeter). Ideally a device capableof measurement to 1 x 10~4in. (2.5 x 10- 3mm) was desirable since the expectedcreep strain was only of the order of 5 x 10~ -"^in/year? (0.13 rata). Such a precisedevice wa-s not available at the time SGHÏÏR was commissioned and the initial "datum11

measurement and a few of the earlier "postservice" measurements were obtainedusing gauging instruments accurate and reproducible to only ¿ 1 x 10"" 3 (2.5 x 10mm). Two such gauges were used both making use of a sensor of the linear variabledifferential transformer (LVDT) type. Later measurements (since 1968) have beenmade utilising a bore gauge» again employing an LTDT transducer, accuracte to ¿2 x 10~4in. (5 x 10~3mra).

BORE, I'EAgJRIITG GAUGE

The bore measuring gauge consists of tho following basic component ss~a) Measuring head - a cylindrical unit 24.5 in* (620 mm) long by 4*75 ia. (120nm) diameter which contains the actuatiag motor, gearbox, and cans for driving andretracting the positioning probes and measuring probes. Also incorporated is acalibrating can device and a thermocouple. The head is suitably sealed to permitunderwater operation at temperatures up to 95 C.

b) Telescopic suspension, which permits the head to be lowered to the bottom oftho pressure tube? and which permits location of the head to any desired axialposition in one inch steps to within ¿ .050 in. (1.3 mm) and which transmits radialmovement in steps of 30° to ¿ 1°.

c) Hoisii frame and heist drives to provide axial movement to the measuring if- advja the lifting chain, with which is associated the cable assembly . A storage drumis provided for compact storage of cable v;hen the head is in the raised position»

a) The bjuso plate and axial rotation drive v?ith associated trip switches to pe.nr.ibsutowatic radial traverse and location of the measuring head.

e) 'J?he control console unit v/hioh houses the measurement hend transducer meterand a digital thormcnetor, indicating Icnps and control switches. The latter pro-vide facilities for auto scanning, radial scanning and manual axial scan. Provisionis made for a visual display of vertical position and radial position of the acao~uring head, temperature, and measured oi.am.cter (in units of 1 x 10~4in. ?»5 x 10~"3ma), additionally these dati are permanently recorded by an automatic printer.The drive unie and measuring head of the bore monitoring device are shown in Fig 2.

^i^

The measuring gauge has been used on a number of occasions to measure creepdeformation of selected pressure tubes in SGHM« It has not been possible tomeasure particular tubes at successive reactor &hutdovm periods because of therequirements of the fuel management progrcnrie, but over the 3 years during whichmeasurements have been made some 17 channels have been measured. The resultsobtained have demonstrated that the Wood-Y/atkins equation relating the creep of

495

2ircaloy as a function of neutron flux, temperature, and stress (4)estimates of creep strain under operating conditions witnin _+ 20$ of the measuredvalues v;heri an appropriate allowance for the primary creep contribution is made.In particular the Eoasureiaant of t?;o channels, firstly "before the statutory bi-annual pressure test, and secondly after the test, showed a maximum divergence"between individual pairs of readings of 3 x 10~%n. (7-5 x 10~3isn), the averagedifference between the two sets of reading "being zero. This provides confirmationthat both accuracy of measurement, and precision of location of the probes withinthe tubes is well vdthin the specified units.

fc complete s. full autosean of a tube ( 160 elevations each of 6 di?.ncters)occupies a period cf 3 hours after the device has "bwen installed on the reactortop above a defuelled channel. In practice, to reduce on-line tine, only a fewchannels were initially coi.ipiei.ely scanned to indicate tho zone of aaxiaua creepdeformation. More recent inspections covered only a few readings at the top andbottom of the tubes and a zone cf about 2 feet in thy region of sjaxiavsn neutronflux. Such an inspection can be conpleted in about 30 minw. The overall timeincluding setting up tho assembly on the reactor top and locating above a partic-ular channel depends of course on the particular design of the reactor.

¡En addition to the bore measuring ¿-:auge a further device is available forsurveillance of the SGEÏÏR pressure tubes. This item of equipment, employingggain an inctrunented head or rabbit capable of being lowered under water to thelower end cf the pressure tube, embodies a bore measuring probe unit capable ofmeasurement to 1 x 10~3in. (2.5 x 1U~2mm) and additionally is fitted with & closedcircuit television camera for inspection of the condition of the tube interiorplus a micrometer probe device for measurement of surface flawsj score marks»cracks and fretted areas.

The upper part of the vievdng head consists of a \vater~tigbt comparimenthousing a remotely controlled TV camera, remote focusaing controls, light source»,self-centring clamping facility and motor drives to the rotatable lower part»

The rotatable end section enables tho W caaera to view both directly andvia a tilted mirror a large section of the channel wall, and also contains amicrometor dial gauge and remotely controlled stylus operating mechanism whichpermits the probing of surface detects of the order of. 2 x 10~3in. (.05 «ffl) deepand 20 x 10~3in. (0.5 mm) wide.

This vievring and measuring instrument has been used on a nmber of occasionsto locate and identify possibly damaged arc¡as caused by fuel vibration a/;d charge/discharge. operations. To date, no defects of significance have been observed.

1. MS STOCK R P5 LI3M3 R P, and WALKER D G B. «Ultrasonic Inspection of Tubes'1,Ultrasonics, July- Sept 1964, Toi 2, pp 109-119*

2. KCDLEBISY. "Uniaxial in-reactor creap of zirconium alloys". J Nuol Mats.*>., 1, (1968), 51.

3. ROSS-ROSS P A and HUNT C iî L. "In reactor creep of cold worked Zr-2 andZirconiun 2-gf¿ Nb pressure tubes". Ibid, 2.

4« ÏÏATKIHS B, WOOD D S. "The significance of irradiation induced creep onreactor performance of a Zircalcy-2 pressure tube". Paper presented, at anÁSTH Synxp at Philadelphia, Oct (1?69).

496

FIQüRgS

FIGURE 1 is Pig 7 of TRG Report 693(0). (Unclassified).

FIGURE 2 ie Pig 5 of IAEA/SÎ/127/35- (Prague Paper). (Unclassified)

499

"Cperatioq Surjye^^^ Components, byr^and Noise Analysis"

by J.W. Ehr entre ichCommission of the European Communities

Directorate-General for Industrial, Technolofical and Scientific AffairsIII. C.I.

ATTRACT

Damage occurred to coolant recireulating pumps and to reactor internals inpressuriaed-water reactor plants which led to long shutdown periods and enormouslycomplicated and costly repairs to activated components.

Nineteen instances of damage to turbines in Coranunity nuclear power plants havebeen reported since 19 5 which likewise involved long outages.

Continuous surveillance of these components could probably nave facilitated theearly detection of chances likely to cause damage, provided that suitable techniqueshad been available* thus cutting down outages and repairs.

It is in the plant operators' interest to develop methods and instruments for thecontinuous operational surveillance of the vibration characteristics of importantnuclear power plant components and to demonstrate their effectiveness.

By initiative of the Commission of the European Communities, the experts workingin this field in the Member States have been brought together for coopei*ation undera study contract with the title :"Critical comparison of operational surveillance techniques for componentsof nuclear power plants by vibration and noise analysis for the early detectionof damage and theoretical studies for their further development".

RESUME

Des poapes de recirculation de réfrigérant et des structures internes de réac-teur ont subi, dans certaines centrales à réacteur à eau pressurisée, desavaries qui ont entraîné de longs arrêts et dss réparations extrêmement compli-quées et coûteuses sur des composants activés.

501

Depuis 1963» on a enregistré dans les centrales nucléaires de la Communautédix-neuf cas d'avariés qai ont ainsi entraîné de longues mises hors service.

Une surveillance continue de ces composants aurait probablement facilité la dé-tection précoce des modifications susceptibles de provoquer des avaries si l'osa^ait disposé de techniques appropriées, ce qui aurait permis de réduire lesarrêts et les réparations,

Les exploitants de centrales ont intérêt à ce qu@ l'on mette au point des mé-thodes et des instrumente de surveillance opérationnelle continue des caracté-ristiques vibratoires d'importants composants des centrales nucléaires «t à cequ'on en déaoatre l'efficacité.

A l'initiative de la Coffifflission des Coasamnautés Européennes, les experts travail-lant dans ce domaine dans las Etats membres coopèrent dans le cadre d'un contratd'étude intitulé:

"Comparaison critique des techniques de surveillance opérationnelle descomposants des centrales nucléaires par analyse des vibrations et du bruiten vue de la détection précoce d'avaries et étude théorique en vue de leurdéveloppeaent".

It will ?;HS recalled that damage occurred to coolant recirculating pumpsand, more seriously, to the reactor internals in the Trino Vercellese andChooz pressurized-water reactor plants which led to long shutdown periodsand enormously complicated and costly repairs to activated components»Operation of the Trino Vercellese plant was interrupted from March 1967to June 19?0 and of the Chooz plant from January 1968 to May 1970. A largenumber of connecting, bolts were broken on trie core barrel of the two reactorsowing to vibrational stress due to insufficiently understood hydrodynamicforces.

Nineteen instances of damage to turbines in Cofisnimity nuclear power plantshave been reported since 1963 which likewise involved long outages. Thedamage mainly consisted of broken turbine blades and the effects of same.

502

Continuous surveillance of these components by, for example, acoustic orneutron flux noise analysis could probably have facilitated the early detectionof changes likely to cause damage, provided that suitable techniques hadbeen available, thus cutting down outages and repairs. In some cases it wasthe consequential malfunctions (jamming of control rods, presence of fragmentsin the boxes of the steam generators, damage to the turbine condenser and torupture discs) which first indicated the breakage of the connecting boltsin both belts of the core barrel in one cese and failure of some of the tur-bine blades in another.

In view of the difficulties and dasaage encountered in the Trino Vercelleseand Chooz plants, special attention was paid to the behaviour of the reactorinternals during the startup of the Obrigheitn plant, a comprehensive seriesof measurements being carried out» This enabled suitable measures to be taken(i.e., rigid mounting of the thermal shield) as a result of which a repetitionof similar damage in this reactor was avoided.

Detecting and rectifying the causes of damage is without question the job ofthe nsanufaeturers. There can, however, be no doubt that unforeseen stresses,due in particular to vibration, are still to be expected. It is in the plantoperators' interest, therefore to develop methods and instruments for thecontinuous operational surveillance of the vibration characteristics of im-portant nuclear power plant components and to demonstrate their effectiveness.

As much as three years ago, on the occasion of a Meeting; held in Brusselsin October I$?c8 under the programme for the exchange of experience betweenthe nuclear power plant operators and the Commission, we drew attention tothe need tçr joint action in this field,

Comprehensive monitoring laeasureraents, especially on the reactor internals,were performed during the renewed startup of these installations after repairand on the Obrigheim plant; this enabled those concerned to sain experienceand to develop special methods for carrying out and evaluating suoh vibrationand noise measurements. Mr. Cioli (of ENEL) will explain details of thesemeasurements and the conclusions drawn from them in Ms report.

503

On 17 December 1970 we held a conference on the subject of "VibrationPhenomena Inside Li nt Water Reactor Vessels", which was attended by expertson noise analysis from Member States arid delegates from some of the depart-ments of the Cojrsnission, This included reports and discussions on theoreticalresearch and experiments concerning mechanical vibrations in pressure vesselinternals, particular attention being giver, to measurements raade on operationalpressurized water plants (Trino VerGellese^ CHOOK, Obrigheim).

At the meeting it was agreed to set up a working party of all specialistsactive in this field and to expand the scope of future meetings to includenoise analysis and vibration monitoring in other reactor types (notably fastbreeders).

We have contacted various groups of experts within the Community in an attemptto induce them to cooperate in this field» We wanted to involve at leastone organization from eacn Community country which represents the circle ofnational experts and already possesses practical experience in such measurementson ligftt water reactors or turbines.

The Allianz-Zentrum fu*r Technik GmbH (Germany), the Laboratoriuro fur Reaktor-reglung und Anlagensicherung Garching. (Germany), the HDP (France), the MEL(Italy) and the Laboreiec (Belgium), with the assistance of the TNQare at present beginning work on a study - limited for tne tirae being toone year - on the subject of :

"Critical comparison of operational surveillance techniques forcomponents of nuclear power plants by vibration and noise analysisfor the early detection of dataag-e and theoretical studies fortheir further development".

First of all a report is being drafted on tne present state of the art.This is intended to provide a rundown of the measuring devices, techniquesand evaluation procedures currently in use in all fields involving theuse of noise analysis for the early detection of damage, namely :1. Crack formation and propagation (stress wave emission)2* Vibrational phenomena in reactor pressure vessel Internals3. Vibrational phenomena in turbines,

504

A summary will also be given of measurements made to date.

An attempt will then be made to carry out a comparative analysis of theresults of measurements performed hitherto on light-water reactors andturbines in order to arrive at a uniform interpretation of the peaksobserved in the spectra.

This task forms the main section of the study in view of its major importancefor the practical application later on of noise measurements in nuclearpower stations for the early detection of damage.

The study should conclude by giving draft recommendations and proposalsfor further research and development work and by stimulating the continuationof this cooperation in future years (e.g., work on improving methods andmeasuring devices, developing new equipment or performing further experiments},

505

AiiitNOTS ^ON ACOUSTIC .EKESSIOM MEASÜRZMEHTS AT

by

N Kirby and P G Bentley

Eisley Engineering and Materials LaboratoryUnited Kingdom Atomic Energy Authority

Wigshaw LaneCulchethWarringtonLanes.

AbstractThe development of high sensitivity instrumentation for detecting and

processing acoustic emission signals is outlined. Details are given of testson fracture toughness test pieces of low strength steels and the benefits andlimitations of such data are discussed. A summary of development work toestablish location techniques is given.

The ability of metals to enit sounds when deformed has long been recognised butit it only in i%ooparativ€1.y recent years that the full technological implicationsof acoustic emission phenomena have been appreciated. Of the varied applicationsthat have becïi proposed, one of the most important is the assessment of structuralintegrity*-1»2'.Work on acoustic emission within the UKASA Keactor Engineering and MaterialsLaboratory began in 1968» At that tiste some exploratory studies were roade onartificially flawed pressure vessels '. A long term programme was then initiatedto develop a technique suitable for use primarily on pressure vessels, during proofpressure tests. The separate parts of the programme may be described as follows*(1) Development of a high sensitivity instrumentation for detecting and processing

emission signals.(2) Development of instrumentation and analysis for defect location.(3) Characterisation of emissions and patterns of eiuission for low and nsedium

strength pressure vessel steels.This paper describes sotas of the work done in parts (2)-(5) of the programme*

507

BASIS OF THF.. ACOUSTIC . SCSST OH. APPROACH

When a metal containing defects is stressed, &. stress concentration occurs at thetips of the defects* under increasing stress, relaxation first occurs an theseregions and may take the forra of plastic flow, micro cracking or in the extremelar^e scale cracking. Acoustic emission is part of tho energy released by theseprocesses ajad so provides an indication of the presenceof defects.

Emissions can be detected remote from their señoree and so it is possible to inspecta large area of a structure with a single detector and provide complete inspectioncoverage with a number oi detectors.

The emission is converted to an electrical signal by ne ans of a piezo electrictransducer. The transducer used is generally a 1 in» diam» x ^ in. disc (70-100 KHzresonant frequency) which is contained in a metallic housing, Fig 1, and ray bebonded to the test speciirsn with adhesive or a thin film of grease. The signal isthen amplified often using a prs-amplificr near the transducer, filtered to removelow frequency electrical fend mechanical noise and then recorded or analysed directly.

gStressing of/saetal can produce continuous erdssion or discrete bursts of higheramplitude, Fig 2. Analysis of these signals can be made in a number of ways* Eachring of the transducer above a set threshold level may be counted (ring âcwxx count) oralternatively each burst -type emission may be counted es a discrete pulse. Thenumber of counts per second or the integrated count is then compared with some otherparameter such as applied load, pressure or strain. The trends may be displayedgraphically on a chart recorder or X-Y plotter by converting the digital outputfrom the countar to a simple voltage. A single transducer system used formonitoring small scale tests is shown diagraramatically in Fig 3»A source of emission can be located by using a series of transducers at knownlocations and by measuring the differences in the times taken for a given emissionto reach the transducers. A small computer is used to record and store the timedata which can then be printed or plotted automatically using a conventional plotteror, for greater speed, displayed on an oscilloscope, î'o calculate the position ofa defect, the time delays are converted tc distances either rranually or with theaid of the computer. Fig k shows a system of 10 transducer channels v?hich providesfor on line location with ;uay A- channels as well as tape recording, and subsequentanalysis. The system has recently been raounted in a trailer unit to facilitate i touse in on site testa on plant and structures.CHABACTgRISATION STULIES

The possible use of acoustic emission to estimate defect severity and proximityto failure is being examined using notched test pieces end artificially flawedpressure vessels. This vork is still in progress and the observations are nsainlyqualitative»TESTS, ON S&VLL SPECIMENS

Hotched bend specimens and compact K specimens (CKS) generally i in. (2.5 cm)thick have been used in the investigations. These were fatigue cracked and thentested according to the recoiaaiended procedure for plane strain fracture toughnessmeasurement^ '. The tests were done in an hydraulic tensile machine which wasmanually controlled to give a steady rate of loading, 'A CKT specimen mounted ready

508

for testing with a clip gauge and an acoustic emission transducer is shown inHg 5» During test the rate of emission (or cumulative emission) and the openingof the notch (indicated by the clip gauge) were recorded as a function of theapplied load using a 2 pen X-Y plotter.Steels re-aging from JO ton/in (4,?x10kg"Vcm) to 100 ton/in2 (15 x 10-* kg/cm2)ultimate tensile strength have been ¿tudiod» At the highest strength levels amaraging steel and .a medium alloy quenched and tempered steel were tested. Thesegave large burst type emission signals and in some tests there was an audible'release of energy when a 'pop in1 occurred at the end of the crack» Clear warningof impending failure was given by a dramatic rise in the emission count whichbegan at about 8CC¿ of the ultimate failure load. In the low and raediuiB strengthsteels of 15 tons (2.3 x 10? kg/ca ) to 30 tons yield strength (*f.7 x 103 kg/cm2)the omission signals were basically similar to those of the high strength steel buthad generally lower amplitudes; also the proximity to failure was cot alwayscharacterised by a rapidly increasing rate of emission.Correlation of the emission occurring during loading with the load displacementcurves showed that the increasing rate of emission given by the ultra highstrength steels had taken place under linear elastic plane strain conditions whereasthe low and isedium strength steels had failed in non plane strain and had given asharply rising emission rate only when there was gross plastic flow at the root ofthe notch*Observations made on the latter steels can be summarised using the result from aC-Ma steel specimen Fig 6a. Specimens having the least ductility and failingbetween the equivalent of points A and B on the load displacement curve gave aconstant rate or a steadily rising rate of emission through to failure» specimenshaving greater ductility and failing between B and C on the curve showed a sharprise before failure; at still higher levels typified by the C-Hn steel specimens,the emission rate rose and then decreased again before failure. In some teststhere was a further increase in emission rate at failure, also, when specimensfailed entirely by slow tearing there was a continuously high level of noise(possibly due to rotation of the specdoens on the loading grips) .as the failurewas occt raring *Failure of the specimen used to obtain the result shown in ?ig6a was predominantlybrittle but was preceded by a small amount of fibroxis tearing. (This subcriticalcrack extension is represented by a dark fracture region adjacent to the slightlybowed fatigue crack (ilg 6a & b)>. The influence of the tearing was investigatedby jaeans of a series of tests in which sitnilar specimens were loaded part way tofailure, unloaded and then broken at a brittle temperature to reveal whether ornot a crack had initiated during the loading operation. By these tests, the tearwas shown to have developed mainly during the relatively quiet period before finalfracture and to have made no significant contribution to the peak in acousticactivity occurring in the earlier part of the test. The results therefore lendsupport to the view that the emission observed in the low and usecüum strength steelstested was caused chiefly by the occurrence of plastic flow.Studies have also been made of relationships which may permit quantitative assess-ment of defect/stress combinations using acoustic emission. 3*he approaches examinedhave been based on the cumulative emission count !ZD and the fracture mechanicsparameters, stress intensity factor, K/5) Or the crack opening displacement CODor 6V°'. For conditions of limited plasticity before failure the emission can beexpressed in terms of the relationship

(7)where A and a are constants ,

E5 * A Kn

509

However for conditions of greater plasticity appropriate to low and medium strengthsteels in thin sections and exposed to normal design and operational conditions thecrack openirg displacement approach has appeared more relevant.Plots of 2E against 6 for three steels of differing yield strengths are shown inFig ?. Since the axes are logarithmic the approximately linear variation shown byeach of the steels implies a relationship OD = A* on .where A' and n' are constants,This is sinalar to the linear elastic expression, and is to oe expected as K and 6can be shown to be directly related when the plastic soné at the tip of the crackis small(8). The values of n (derived by regression analysis) given in the figuresuggest no appreciable material dependency, however, further work is needed toinvestigate whether or not this is true for differert steels and test conditions.SHALL VSSSSL TESTSTests on small mild steel vessels were made early in the test programme with theobjectives of developing techniques and caking a preliminary investigation ofemission phenomena in a vessel containing a flaw.The vessels were essentially tubes, 12 era thick x ¡50 nan diara» x 100 cm long»Each vessel contained an axial through well thickness defect sealed to preventvrat*»r leaJkage by applying a patch of neoprene rubber reinforced with stainlesssteel s'u.et to the inner circumference of the tube, The ends of the tube weresealeci \atfa. metal end plugs and rubber sealing rings. These end seals proved tobe a potent source of emission, particularly during the early part of the test.However during repeated pressurisation the noise from the end fittings becamenegligible so that in pressure cycling tests it was possible to study emission•with greater confidence.It is v?ell known that a metal once stressed gives little or no emission during asubsequent stressing unless there has been some increase in stress concentrationwith-n tha Be£al, (for example due to the growth of a defect). This so calledKaiser eifect^S/ can give considerable operational confidence if observed, forexample, in a repeat proof test on vessel after a period of .service. Converselythe reOLscyse of acou&tic emission could imply that now defects had been introducedor that existing defects had grown since the previous pressurisation» However insome ste-sls recovery can occur so that emission in p. repeat test does notnecessarily imply the growth or appearance of a new defect.Two of the ndld steel vessels were cyclicly pressiirised 9 and 18 timesrespectively, with delays of up to 5 weeks between cycles. During most of thecycles the vessels gave negligible emission so that there was no evidence of anyprogressive recovery of emission. In one cycle of each series, however, appreciableemission x/as detected, and in this cycle the defect had grown significantly bywidening at the tip of the notch, Fig 8. la the other cycles where there wasvirtually no emission, there was no significant widening» Although the wideningVías thought to indicate the initiation of a fatigue crack» the evidence on thispoint was inconclusive, nevertheless the result served to demonstrate thesensitivity of emission to relatively small changes occurring at a defect.The.-.o tests also demonstrated the effect that stress cycling can have on theemission that occurs subsequently at higher stresses, When pressurised to failurethe vessels were exceptional in giving a much greater emission than any of the

510

many other vessels tested. Specifically* the maximum rate was 10,000 counts per1000 psi compared with rates generally of 500 counts per 1000 psi. ïhe steep risein emission is rhovra in Fig 9«

STUBIB5Initial development of the source location system was done in the laboratory- withthe aid of a steel plate test bed and artificial emission sources generated eitherelectrically or by some mechanical means. Since that time experience has beengained from tests on large plate specimens and on experimental pressure vesselscontaining machined slits to represent natural defects. Mere recently a number offield trials have been conducted..An example is a test made on a low alloy steel plate, 2.5 cm thick x 100 cm x150 cm, which contained a 10 cm long central transverse slit» The plate was beingstressed to failure in a machine of 000 ton (40 x 1C- Kg) load capacity forfracture evaluation purposes. Fig 10 shows the plate mounted in the machinebetween two end lugs. Cooling trays were attached to the plate for cooling to thetest temperature of -80 C« Four trar-sducers were used all being glued to theaccessible area on the upper end lug. Analysis of the eiaissions released as thespecimen was loaded revealed three sources. Each source was diffuse indicating thatthe emissions came from an area leather than a specific point on the plate. On®source was associated with the slit and its computed position was close to theactual position of the slit. The other sources were situated at opposite edges ofthe plate and since no defects were confirmed ultrasonically in these regions itseemed probable that these emissions had been caused by sotae local yielding of theplate.The opportunity to examine the problems of field operation was taken when anexperimental» 5 in. thick pressure vessel had been repaired and was about to bepressure tested at tlw fabricators» Although the vessel was situated centx'allyin a boiler making shop, where the normal activities of grinding, welding etc wereiri progress» no special problems due to interference from transmitted mechanicalnoise or spurious electrical signals were encountered and it was possible to checkarid calibrate the acoustic emission system without the need to halt productionwork» During the pressure test several leaks developed which were immediatelyS€¡nsed by the instrumentation and so demonstrated the known value of acousticemission in leak detection* However, the noise from the leaks was sufficient toobliterate any genuine emission in the vesnel» A later test was made to a higherpressure at RE-iL and emissions were detected satisfactorily.

'She vessel currently contains a slit to represent a flaw and is being cycliclypressurised to promote growth of a fatigue crack« Emission from the growingcrack is beiug used for further développât and proving of tlie source locationtechniques and to study the factors which caa complicate the quantitative ne assuré-ment of acoustic emissi.on (eg count rate)*Other work has involved measurements during a proof test on a thick walled nuclearreactor pressure vessel and the investigation of problems associated with themonitoring during the return to operating conditions of a 50 ft. diam» gas cooledreactor pressure vessel» Details of this wo.dc are to be published shortly* *-3'.DISCUSSION AND CONCLUSIONSV «WBl»t««»H>ii».»lEli|iiM»m»l.lll» -I lllMlui>»il»illl«l»««ll.«Ha»W».i>«P.M.HUK-il I III II "I I I

Acoustic emission provides a means of inspecting all parts of a vessel simultaneouslyanaj applied in a final proof test it permits a full assessment of the vessel in itscciiuition immediately before operation» Development of instrumentation and analysis

511

í'oi detecting ana locating emission sources in the relatively quiet conditions ofa proof test is well advanced although scope remains for improvident of detail.Having located regions of above average activity, these way be related to physicalfeatures on the vessel; from this it nay be possible to decide whether the emissionhas originated in the vessel itself and is, therefore»'likely to be due to, thepresence of a defect or whether it has come from some extraneous source such as abolted cojjiiection, support structure stc» Further deductions on the nature of thesource may be made from the shape and size of the emii'.ing region.Interpretation of the strength of an emission source in terms of the severity of apossibly defect isat present qualitative and based on experience. For instance,a sliarp increase in the rate of emission under steadily rising load conditionsmight be taken as an indication of a severe defect and the likelyhood of iesiineîïtfailure» Quantitative analysis of large vessels is not yet possible and regionsof possible weakness indicated by acoustic emission would normally be examinedfurther by conventional NET methods.Progress towards a more quantitative approach may be possible by relating acousticemission to the fracture mechanïBo/Kôaîm ô - both a function of applied stress anddefect sise ano geometry, The derivation of such relationships and proof of theirgeneral validity may be obtained in the course of monitoring commercial pressurevessels; however, information from such tests will be slow to accumulate and testson experimental vessels \;ith known sizes of defect subjected to known stress fieldsappear a necessary supplement.Acountic measurements on tost pieces have the advantage that many parameters canbe investigated using available equipment. Although there are inherent difficultiesin applying qujmtit&tively the results from a test piece to a structure, a usefulqualitative indication of etructural behaviour may be obtained. Specitcen geometiesare required which simulate reasonably closely the state of stress or strain in astructure. For conditions of plane strain test piece thickness may differ fromthat of the vessel, whereas for conditions of non plane strain, specimen size maybe a critical factor and an adequate representation of the emission in structurenay require the use of specimens of JTull plate thickness.These geometry requirements are generally met in specimens used to roeasure thefracture toughness of a material (KT or 5C). Heneo, the monitoring of fracturetoughness tests is a convenient way~£o obtain relevant acoust? c emission data.In addition, the emission detected cuxy be useful as an aid to define the onset offracture in the epecitaen and so permit a more accurate measurement of the fracturetoughness. This could be particularly important in tests to measure criticalvalues of COD (oc) where the onset of fracture by a slow growth process isdifficult to determine by other methods.

REFERENCES

1. BUTTON P H and PA2RY D L. Materials Research and Standards.>fERSA Vol 11. No. 3, p25.

2. NICHOLS 3 U and COV/AN A* Preprints of 1st Int. Conf. on StructuralKechaiúcs in Reactor Technology, Berlin 1971» Vol 5» pl»6l.

512

3* KIBLSKN À» LAÏKAM F G, KIR3Ï N. Acoustic Emission from Steel PressureVessels» UKAEA 2EG 1983(0).

b. Proposed Recommended Practice for Plane Strain Fracture Toughness Testingof Metallic Materials, ASTH Standards Pt J1, AS2M, Philadelphia USA (1969)P1099.

5. Fracture Toughness Testing ana its Applications ASTM Special Tech.. Publ.No. 381 ASÏÏM Philadelphia USA 1965.

6. BÎ3RDEKIH F M in Proc. Symp, on Fracture Toughness Concepts for V/eldable•Structural Steel (Practical Fracture Mechanics for Structural Steels)UKAEA Bisley and Chapman and Hall, London 19 9, pC1,

?«, DÜNEGAN H L, HARRIS D 0 and TAHO C L» Engineering Fracture Mechanics 1968pío?.8. BÜEDEKIM F M, STON5 D E U* J0 Strain Analysis Vol 1, No. 2 1966, p1 5.9« KAISER «T. "üntersuchungen über das Auftreten von Gerauschen biera

Zugversuch!I* A doctoral dissertation presented to Fahultet fur V/aschinenwesen and Electrotechnik • Der ïehnischen Hochschule Munchen, Munchen»Germany 1950.

10. BURTON E J{ BEOTLSY P G, BU3NUP T S, C0.7AN A, KIRBY N.Paper to be presented at Conf. on Periodic Inspection of Pressure Vessels tobe held 9-11th Hay. I.Mech. S. London 1972.

513

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FIÛ 1t ACOUSTIC ÎMISSŒOKÏRAUSûuCER.

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517

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part loaded toand then broken

at a brittle temperature»

* «i 'a | |í S 'S«U It '°

FIG. 6a: ACOUSTIC EMISSION FROM 0.25$C-0.75£Mn STEEL CKS SPECIMENS.

Fatiguecrack

machinedslit\

slow crack_,,growth

brittlefracture

FIG. 6b! DIAGRAM OF FRACTURED SPECIMENS SHOWN IN FIG. 6a

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» Mn-Cr-Mo-V STEEL(~ 30 ton/in2, if.?x1o5kr/cm YS)

CRACK OPENING DISPLACEMENT 6 (INS x I O3}FIG. 7j INTEGRATED EMISSION AS A FUNCTION OF CRACK OPENING DISPLACDVÍIÍÍT FOR CKS

SPECIMENS OF LOW AND MEDIUM STRENGTH STRUCTURAL STEELS.

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ÎOî AC«f?7rïC MONITORING OF A FLAT PLATF F UfJTAJKTfe- A C OLÎT.

THE PREVENTION OP FRACTURE INITIATION INREáCTOR STRUCTURAL MATERIALS

R.W. Hichols and A. CosíanUKAEâ

Paper L 6/1 presented at Conference on Structural Mechanics of ReactorMaterials, Berlin, September 1971. To be published in "Nuclear Engineeringand Design", March 19?2.

525

CN NONDESTRUCTIVE TESTBTG IN THE U.S.A. OF PRESTRESSEDCONCRETE PRESSURE VESSEIS FOR FUCtEAR REACTORS*



ABSTRACT

Many nondestructive testing techniques are used to assure the integrityof prestressed concrete reactor vessels in the U.S. Metallic members areexamined by radiographie, ultrasonic, magnetic particle, and pénétrant tech-niques to meet the requirements of ASME Boiler and Pressure Vessel Code,Section III. The concrete is fabricated to meet the requirements of theAmerican Concrete Institute. After fabrication and during proof testing andoperation, the test methods include measurement of stress on the prestressingtendons, strain on both concrete and metallic members, temperature, heliumpermeation, and moisture' distribution in the concrete, leakage in the linerand penetrations, and ultrasonic examination of the liners for corrosionthinning. Acoustic emission is being investigated for possible application.

RESUME

De noiabrexises techniques de testo non destructifs sont utOpour s'assurer de l'éianchéité deo caisson» tie réacteur en "béton pré-coatralnt aux Ktats unis. LOB parties né tall i que s sont ©xniaineee pardos techniques radio^raphiques, ultrasouiquee, magnétique» ainsi qpar l'utilisation de procaixto pénétrants afin de vérifier qu'ellessatisfont aux exigences du Code ASKju des che.udiorecs ot ôoôde prcfíüion. l^e béton eat fabriqué en conformité {¿vec loo

^Research sponsored by the U.S. Atomic Energy Commission under contract•with the Union Carbide Corporation.

527

de l'Institut Áisáricaiii du Botón» Après la fabrication et peBtíani jesesf.&iG et. le foï;ctioy:neHïent en service continu, lee &é ùhoilo& de contrô-le comprennent dos nesuros des effortu uur les tendons de précont-i'i-.ir, te,de la déi'orioation des; parties en aétal et en béton,.de la température,de l'infiltration d'hélium, de la répartition do 1»humidité dans lobéton, des fuites et pénétrations ¿ans le rev6toraei.it interne, ainsiqu'un exaracm aux ultrasons des revêtements pour détecter l»íitóiuclsí>e-laent par eorro&ioa. L'émission acoustique fait actuellement l'objet,d'études «su vue d'une éventuelle a'm>licatiovx.

INTRODUCTION

The application of prestressed concrete pressure vessels for nuclearreactors is gaining increased attention throughout the world, with active con-struction in the United Kingdom, France, and the U.S.A. Major activities inthe U.S.A. have been a program of research and development sponsored by theU.S. Atomic Energy Commission at the Oak Ridge national Laboratory1 and. thedesign and construction of a vessel for a high-temperature gas-cooled reactor(HTGR). The latter was built by Gulf General Atomic for the Fort St. VrainNuclear Generating Station near Denver, Colorado. Most of the discussionabout nondestructive testing (ïIDT) in this paper is based upon activities andplans for the Fort St. Vrain Reactor (FSVR).2

The philosophy for the ÏÏDT of a prestressed concrete reactor vessel (PCRV)differs from that normally associated with metallic components (e.g., crackingof the concrete within certain specified limits is not only anticipated butacceptable). For most metallic components the existence of cracking isobjectionable. Among other considerations that allow a different 13DT philosophyis the extensive use of scale models that are tested to failure before con-struction of the actual full-scale PCRV. This includes not only both mortar

528

and "biréfringent eposy models during the research and development programs,3

but also concrete models as preliminary work before design and constructionof an actual full-scale vessel.*""6 In addition,, the continuous monitoringof strain, moisture, and temperature in the concrete and forces being devel-oped by the prestressing system, provides an instant awareness of the currentstate of the operating vessel.

Some of the examination or monitoring techniques do not fit the so-called cJ3.ssi.cal field of HBT that is frequently considered to involve onlyradiography., ultrasonics, eddy currents, and liquid pénétrants. However, allof the methods described here for PGR7 application fit the broad definitionof BDT in that they perform an examination without impairing the usefulnessof the component.

GfflEML CONSIDERATIONS

The basic functions of a PCRV* are to hold the pressure required by thereactor cycle, to shield against radiation, and to maintain leafctightnessagainst loss of coolant. It generally bas a cylindrical configuration withflat slab ends and is a composite assembly with an inner gas-tight steelliner, a concrete -wall, and prestressing cables. Other important features ofthe structure include a system of thermal insulation and cooling tubes toprevent overheating and excessive thermal stresses, reinforcement bars ofsteel, closures for the many wall penetrations, and anchors for the pre-stressing (or post-tensioning) tendons (cables).

The TOT requirements for each item are discussed briefly below, firston an individual basis, next collectively (as subunits or the entire PCRV)where appropriate at various stages including fabrication, and finally interms of proof testing and surveillance during and after service.

A new ASMS Boiler and Pressure Vessel Code on Prestressed ConcretePressure Vessels is in the late stages of approval by the appropriate ASME

529

coasadttee. It reportedly will contain many of the requirements noted inthis paper, such as use of Section III (ASME Code) requirements for examina-tion of metallic consponents, «sapping of visual cracks} and monitoring ofstresses and strains,

HDT OF PCE¥ COMPONENTS

Discussion of MDT on the individual coscponents of the PCRV in this sec-tion is devoted particularly to the metallic rnembers since little of thecurrent HDT technology is applicable to the thick sections of concrete. Mastof the metallic isaterials and their respective inspections comply with therequirements of Section III, Class A of the ASME Boiler and Pressure VesselCode,7

Cavity Liner

Because penetrations also contain liners, the primary liner for the PCRVis designated as the cavity liner. The cavity liner for the FSVR is a con-tinuous right circular cylinder 31 ft in diameter X 75 ft high with 3/4-in.-thick carbon steel walls. Penetrations also have liners that are welded tothe cavity liner to form a continuous boundary. Other welds include attach-ment pads in the internal surface for reactor internal structures, coolingtubes -welded to the outer surface of the liner, and studs welded to the outersurface to anchor the liner to other surrounding concrete.

The plate from which the cavity liner for the ESVR was fabricated wasexamined by longitudinal wave ultrasonic techniques in accordance with theASME Code7 to detect discontinuities that cause loss of back reflection.During construction of the liner, all welds were examined by ÏTOT methods.Magnetic particle examination was required on all surfaces prepared for weld-ing, on the root layer, and on the final layer. The purpose of this examina-tion was to detect any linear indications, any single rounded indications

530

larger than 3/16 in., or any multiple rounded indications in a linear arrayor -within a localized area. In addition, all liner welds were radiographedand inspected with liquid pénétrant in accordance with the requirements ofthe A5ME Code. The leaktightness of the liner was verified by systematicleak testing of all welds, including welds of the cavity liner to the penetra-tion liners. The test was conducted in accordance with MIL-STD-271D,8 Thewelds were divided into discrete lengths for examination with a mass spectrom-eter at a pressure differential of 3 to 3 psi. The total leakage from allwelds was not to exceed 1.5 X 10~2 cm3/'sec, the equivalent of an average leakof 6 X 10~6 cm3/sec per foot of weld.

Penetrations

There are numerous penetrations through the wall and heads of the PGWfor the FSVR. Each has a carbon steel liner 1/2 to 2 in. thick, dependingupon the inside diameter of the penetration. All penetrations have twoindependent closures in series — the primary and secondary closures. Theclosures were sealed by welds or gasketed joints. She penetrations werefabricated from plate, forgings, "bolting, and pipe that were inspected inaccordance with the ASM! ^ode for Class A vessels.

Plate

As indicated earlier on the cavity liner, the plate in the FSVR wasexamined by longitudinal ultrasonic techniques based upon observations of thedecrease of amplitude of the reflection from, the rear surface.

Forgings

-, The ASMS Code requires longitudinal wave examination of forgiugs todetect discontinuities that provide indications accompanied by a loss of backreflection. Ring or other hollow forgings were also examined by angle beam

531

ultrasonic techniques calibrated on a reference notch 1 in. long and havinga depth the lesser of 3/8 in. or 3% of the section thickness. Either liquidpénétrant or magnetic particle examination was also required.

Pipe

The pipe -was inspected in. accordance with the ASMS Code that allows -useof radiography, ultrasonics, liquid pénétrants, magnetic particles, or eddy-current techniques according to their applicability. If the inner surfacevas inaccessible for examination with liquid pénétrants or raagnetie particles,one of the other methods T»as used.

Bolting

All bolting -was examined visually to detect harmful discontinuities.Solting larger than 1 in. nominal was examined by magnetic particle orliquid pénétrant techniques; linear nonaxial indications and linear axialindications longer than 1 in. were unacceptable. Bolts larger than 2 in.nominal size were examined ultrasonieally before threading to detect dis-continuities that produced indications as large as 20$ of the back reflectionor that reduced the back reflection by 50$.

Tilners and Closures

In general, the assembled penetration liners and closures were subjectedto the same inspection requirements as the cavity liner with two exceptions.The maximum allowable leakage was 10"5 cm3/sec of heliw at 1 atm with theclosure gaskets excluded from the test. The secondary closure was inspectedin accordance with the requirements for ASMS Class B vessels. The latterrequires radiography unless a nonradiographable joint design is used; in thatcase ultrasonics, magnetic particle examination, or liquid pénétrant examina-tion is required.

532

Cooling Tubes

Cooling tubes are embedded in the concrete arid welded- to "both the cavityand penetration, liners to control the liner temperature. All of the tubingwas examined by the ultrasonic method before being formed to a square crosssection. The requirements of the ASMS Code specify reference standardscontaining notches less than 1 in. long with a depth not greater than thelarger of 0.004 in. or 5$> of the wall thickness. After being formed intothe square shape, all tubing was hydrostatically proof tested to 400 psig.Both butt o°iftts a&d socket joints were used to make the tube-to-tube weldsnecessary to fabricate the continuous path for coolant flew along the out-side of the liner*. All tube-to-tube welds were inspected with magneticparticles and checked for blockage. In addition,, the full penetration weldswere radiographed, pneumatically pressure tested at 189 pgig, and checkedwith a soap bubble solution. All tubing bends with an inside radius lessthan 9 in. were inspected with dye pénétrant on the outside surfaces. Con-tinuous fillet welds on both sides were used to attach the tube to the cavityliner. These welds were ejcamined by dye pénétrant or magnetic particles forat least 10$ of the length except for those tubes that transfer shear loadsfrcai the concrete to the liner; for these 100$ inspection was used. Eachtube was tested for blockage before and after welding as well as before andafter concreting to assure that at least 50$ of the cross-sectional area -vías

open.

Concrete

ÏŒO techniques of concreting were employed in the FSVR: (l) preplacedaggregate concrete for the bottom head, and (2) conventional (job-mixed)concrete for the side wall and top head regions. Construction methods andtolerances followed the recomaended practices of the American Concrete

533

Institute (Ad) Code.9 During the pouring of the grout into the preplaceclaggregate for the bottom bead, Time Domain ReflectroBietry (TDS) •«as used to

monitor the grout level.10 A cable (wire sensor) was inserted verticallyinto the region to be poured. A fast rise pulse was sent down the cable,and the amplitude characteristic of the reflected pulse was monitored. Dis-continuities in the cable •when the surrounding medium changed from air to•water were detected and displayed on an oscilloscope. An array of 94 pairsof sensor -wires provided a complete profile of the grout at any time duringthe pouring. This ms displayed by a three-dimensional plastic model in•»hieh readings from each TBR sensor were represented by means of adjustableplastic tubes located to correspond to the actual sensor location.

For the thick walls of the PCKV, differential shrinkage and curing-temperature-induced strain between the inside and outside concrete may causesurface cracking during construction. Reinforcement bars assure that stressesare distributed and that cracks remain small. Cracks -with a lateral openingof 0.015 in. are generally considered acceptable.

THE ASSEMBLED PCRY AID ITS PROOF TESTI3U

After a concrete vessel with its reinforcing bars, penetrations, andliners has been assembled, prestressing tendons, located in conduits embeddedin the concrete, are used to impart a ecmpressive load to the entire structurebefore prèssurization.

The PCRV is designed so that -when it is pressurized from atmosphericto the "reference pressure" (defined as 1.2 times the peak working pressure)the concrete will be unloaded from a full coiapressive state to a less com-pressive state, with the minimum condition being that the concrete remains incompression. A number of tests and examinations are performed on the completed

534

assembled PCRV before, during, and after the proof testing. The structuralresponse during both proof testing and operation is expected to be linearand elastic. Continuous monitoring of the response is by load, deflection,and strain sensors.

Leakage

The interspaces of the penetrations (between the inner and outerclosures) for the FSVR are maintained at a positive pressure of helium sothat any leakage will be from the volume of the penetration and not from thevessel. Leakage will be detected by the flow of replacement helium intothe penetration and monitored at the secondary closure. The helium permea-tion characteristics of the concrete are also checked by measuring the flow-rate through special pipes embedded in the concrete.

Stress and Strain

Load cells -were installed on selected prestressing tendons to allowcontinuous monitoring during proof test of changes in structural componentssuch as progressive tendon corrosion, concrete strength reduction, and steelrelaxation. As a check on the results from the monitoring with load cells,strains mil be measured by sensors embedded in the PCR? concrete andattached to the reinforcement bars. Most of the information about concretestrain -will be obtained from vibrâting-wire gages. The gage consistsprimarily of a taut steel wire mounted between end flanges. The resonantfrequency of the wire depends upon the distance between the flanges. AnelectroHjagaetic system excites the wire and determines its frequency ofvibration. A thermocouple -was incorporated for teisperature corrections.These thermocouples also monitor the concrete temperature at juany locationsthroughout the PCKV. Weldable strain gages were attached to the reinforce-ment bars, cavity liner, and penetrations to be monitored through the proof

535

testing stages. Because moisture causes drift in the gage reference reading,the usefulness of these strain gages as long-term monitors for time-dependent(creep) measurements is doubted.

Temperature

In addition to the load and strain monitors, thermocouples were embeddedin the concrete and attached to the liner and penetrations to provide assur-ance that the cooling system is functioning properly. Lifetime monitoringwith these thermocouples is not expected.

Moisture

Moisture monitors are used to establish the moisture distribution inthe concrete across the •sails and heads and to detect moisture movementsduring curing,, heating, and cooling. One type of moisture detector measuresthe ac resistance "between two platinum electrodes embedded in the concretegrout. The change in resistance may be calibrated and related in a repeat-able manner to the amount of free -water in the grout. Knowledge of theamount of free water is important because of its effect on such properties ascreep strength and thermal conductivity. Other techniques that have been usedto monitor the moisture include measurement of capacitance, dielectric constant(with microwaves), and neutron bacfcscatter. For the latter a well is placedin the wall for probe insertion. For the neutron technique, the totalhydrogen in both free and combined water is measured. It is necessary toasstaae a constant amount of hydration and attribute the monitored changesto variations in the amount of free water. Another approach is to monitorthe humidity in a well in the concrete, assuming equilibration with thefree water.

536

Deformation

Overall deformation and deflections at the œidheight of the vessel andat both the center and perimeter of each head for the FSVR were measuredperiodically during initial heating and proof testing. Surveying equipmentcapable of measuring displacements with an accuracy of ±0.01 in. was usedto measure deflection relative to predetermined reference points.

Cracking

Visible crack patterns on PCEV surfaces are recorded before and afterproof testing. Cracks in the concrete wider than 0.015 in. are raarked onthe appropriate PCHV drawings for comparison at later inspection.

After proof testing, the hard-ware associated with the anchors for thetendons is examined by magnetic particle and liquid pénétrant methods.

PCRV SURVEILLANCE

Some of the testing systems described for use through the proof testingstages will also be used for monitoring the FSVR during its lifetime. Theseinclude visual examination and mapping of crack patterns f measurement ofdeformation and deflection with surveying instrumentsf and measurement oftendon loads, concrete strains, leakage, helium permeation, vessel tem-peratures, and concrete moisture distribution. In addition, several otherexaminations will be used.

Liner Corrosion Monitoring

Access wells at critical locations allow the insertion of ultrasonictransducers with remote handling equipment to detect localised thinning dueto corrosion and to detect defects such as cracks that may have occurred in.the area under surveillance, Such monitoring will be conducted from theconcrete side of the liner yearly only during reactor shutdown. The calibra-

537

tioa methods and acceptance standards -will be the same as required by theASME Code. Corrosion on the concrete side of the liner will be detected bysimilar methods from the inside surface of penetrations.

ADVANCED METHODS OF IXMtffiATION

Several other isodes of examination have been studied for the examinationof concrete, but may have limited application to PCKV's because of the largesection thicknesses. These include sonic techniques11 and gaiana attenuation.12

A more recent method that has gained significant attention is acousticemission, a technique involving the detection of acoustic "noise" or impulsesarising from within an object as a result of crack formation or propagationor other deformation processes. Experiments13 conducted on both cylindricalspecimens and mortar ssodels of PCKV's demonstrated that emissions due tostructural degradation were large in both number and amplitude. Grosscracking, onset of vessel failure and leakage, and prestressing rod failureveré all detectable from the acoustic emission data. In addition, trialstudies sponsored by equipment manufacturers are being conducted on the 5SVR.

SÜ6ÍÍARY

Many nondestructive testing techniques are used to assure the integrityof prestressed concrete reactor vessels in the U.S. Metallic members areexamined by radiographie,, ultrasonic, magnetic particle, and pénétrant tech-niques to meet the requirements of ASMS Boiler and Pressure Vessel Code,Section III. The concrete is fabricated to meet the requirements of theAmerican Concrete Institute. After fabrication and during proof testing andoperation, the test methods include measurement of stress on the prestressingtendons, strain on both concrete and metallic members, temperature, heliumpermeation, and moisture distribution in the concrete, leakage in the linerand penetrations, and ultrasonic examination of the liners for corrosionthinning. Acoustic emission is being investigated for possible application.

538

ACKNOWIEDGMEMT8

The significant assistance provided in techniea-u discussions withG. D. Whitman. Reactor Division, Oak Ridge national Laboratory, Oak Ridge.Tennessee, and T. E. Worth-up, Gulf General Atomic, Inc., San Diego,California, is gratefully acknowledged.

REFERENCES

G. D. Whitman, "Summary of a Program, of Research and Development for Pre-stressed Concrete Nuclear Pressure Vessels," presented at the SecondConference on Prestressed Concrete Reactor Vessels and Their ThermalInsulation, Brussels, Belgium, November IB-20, 1969.Fort St. Vrain Nuclear Generating Station, Final Safety Analysis Report,Public Service Company of Colorado, Denver, Colorado.J. M. Corum and J. E. Smith, Use : of Smll Mgdels,, in^Design^and Analysis

USAEC Report" ÔBNÏÎ 4346 (May 1970) ,

Preatressed Concrete Beact-or Vessel Model 1, USAEC Report GA-7097,General Atomic (Oct. 25, 1966), as cited in reference 3.Prestressed Goncrete Reactor Vessel odeJ- J jaBBlemental He-port.UBASC Report GA-7097 Suppir7"Geñeral Atomic (June 30, 1967), ascited in reference 3.Prestressea Concrete Reactor Vessel ; Model 2, USAEC Report GA-7150,General Atomic (Nov. ï7~1966j", as cited "in Preference 2.

Vessel Code,^ ^The American Society of Mechanical Engineers, New Yo, i960-

8- Nondestructive Testing B^cruirements for Metals, MIL-STB-271D ( SHIPS),11 iferch 1965, Department of the Navy, USA, Paragraphs 6.1-6.5.

9» ACI tfonual o Concrete Practice, Parts 1, 2, and 3, American ConcreteInstitute Publicatïon7~Ï9657~ s cited in reference 2,

10. Roñan Associates, Elec tronicar out_ i Depth Measurement s for the Full-Scale Mock-Up Pour, Dec. 16, 1963, as cited in reference 2.11. E. A. Whitehurst, Eyaluation of Concrete Properties from Sonic Tests,

American Concrete Institute Monograph No. 2, 1966, as cited byChen Pang Tan, j gJ £gj gd Cj Vessels — ACritical Review of Current Literature, USAEC Report~CML-i227""('May 1968) .

539

12. D. G. Harland, "A Radio- Active Method for Measuring Variations in Densityin Concrete Cores, Cubes and Beams," Mag . Concrete Res : . .jy|(55), 66-101(June 1966), as cited by Chen Pang Tan, Prestressed Concrete in IfuclearPressure Vessels — A Critical Review of Current Literature, IBAEC ReportOKHL-4227 (May 1968).

13. A. T. Green, Stress Wave Emission aud Fracture of Prestressed ConcreteReactor Vessel Sfeterials, Aerojet-General Corporation MC~4 190 (June 1969) .

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&

Vienna, ?9 November - 3 December 1971

Li st of Items

¥1)7 facilities awlied to: -,-**1. (00) Coated fuel particles elaboration (KKEA) » Germany, Italy jJftfy

?. (CO) Fuel nellets* production (JSA) fPranoe]*» w

}. (GO) Stainless steel can fabrication (UK) !,Swedon, France!\i tf

4. (GO) ?rr. base allov can fabrication (Sweden) | France]5. (RE) Assemblies1 manufacture (including í^idfí, s loe veo, placers)

(France)6. (RE) Post irradiation inspection (UK)

Pressure fígnt ai ninen fc?. (RE) WP procedures during fabrication of Rteel pressure

/ \*vessels (Germany)Coramnnic?tions iealinsr with:

a) The feasibilit.v of inrneotinç austenifeic weldrsjb) the advantages and ais;idxranta,^es of TïS and racHo^nrtphic inaoection;c) new ¥DT techniques such as acoustic emission;d) automatic methods of testing;e) the relationship between TfDV indications and the

significance of defectf?;f) the eToerience of usinée different WT)T teohnolo-nr to

various national

8. (RR) Base-line and in-servine inspection of steel Dressnre9. (CO) FDT techniques during fabrication and in-service inspection

%--%

of r>rer,Bure tubes (Canada^ ÍCJK, lISAl10, (CO) WP techniques durinf fabrication arid in~8ervice inspection

of concrete vessel, steel liner, incoroorated heat exchangersy.M-

(Prance) fUK, USA]Reactor LInt ernal s -(othfir than assemblies, moderators & rrenfiure vensels)

11 . (PA) Expérience of poecial 7ÎDT1 techniques developed to overcomeproblème during fabrication and in-service of In-core componentssuch as; neutron A thermal ffhields, bolts 4 pipinp1, internalpumps & moderator tanks.

* Rxnected communications from US, Sweden, UK, Germanv, Japan and others.** assisting countries

541

ANHEX B

IAEA PAffEL ON "îfDT FOR REACTOR GORE GOMPCÍNENTS *. PRESTIRÉ VESSELS'*

Vienna, ?9 November - } December 1971

I. IJEETTCHG ARRAWflEMEHTS

Titrée types of pipers should be presented at the Panel:(CO): Coordinated paners {.less than 4^ minutes) edited by nronosed

countries and oresertted "by one .country. Each coordinated paper shall befollowed by short comments and discussions.Type (RE) : Report papers (less than 41? minutes) summing up all the nationalcontributions on the same topic and presented by the proposed countries.Each report shall be followed by short comments from the various authorsand by short discussions»Type (PA): National comrauni cations (a 20 minute oral presentation byeach author followed by 1O minute discussions).

2. According to the number and contents of the par>ers sent in advance, ,.the Agenda, as stated in Para. IT, was proposed "by the Board and ad lustedin vivo.

3. Tn reference to Annex A, items 4?9 and 10 could be combined in theforro of coordinated m.pers and items 5, ,7 and B as report napers. On theseitems each author was allowed some additional comments and on all other itemf?individual papers were resented bv their authors.

4* In order to allow more time for round-table discussion, all thepresentations were ancomnliphed in ^ It1? rtavs dnrin,0" three rdenarv sessions.The conclusions and recommendations to the âfencv were then nrep''red during1 I/? davp separate conferences bv two different workinr rour.r. (each led bya chairman) «ncoranisninir the follnwinf areas?

for WG T: PHiel Elementsfor WO TIî PrenBîire Containment and Reactor TnternalsAll but four attendees of the plenary sessions narticipated in the above

working sjroun meetings»to the very tight time schedule, no provision could be arranged

during this Meeting for individual reflexion period.

542

I&Eá PAMIL ON "NOT FOR REACTOR CORS COMPOffiaiTS & PRESSURE VESSELS" AMEX B

IT. TOME TABLR

29 Efoyember

inpt Session : Adoption of Agenda 9.3O - 11,00

Session T Item T : -1? (Mr. Price) OBCD ^ 11 IS 1' 10Itero IT : -?4 & -?p (Mr. McGlung) USA . •? - - o

(Afternoon)Session I oontd. Item ITT : -?1, -13 * -?0 (Mr. W rsn) UK # , . IE- ,_ ,„

Item T? : -10, ™?0 & -14 (Mr. WiVlurd) Sweden 4 ° " '°.y, 3P November(Morning)

Session I contd. Item ? : ~??f -11 & -20 (Mr, Watteau) Prance ^ Q IA ~ 1? oItem FT : -?3, - 1, - 4, -•>, -8t -?^ &-20 (Mr. Mann)WC '3 °

(Afternoon)Session I Item VTI : -l?,-lf-,-?5,-20f-K>,-31 (Mr. ÏVumpfheller)FRG* 14.15 ~ ift.OOItem VITT : -22,- ?,- 6,- ?,- Q,-?0 (Mr. COwan)UK*WedneisdajS| 1^ December

(Morning)Session IT! Item TX

Item XItem XI

-Î, -15, ~?B (Mr. McClung) USA-IB, -3? (Mr. Roche) France*

* 9 ^0 --19, -?o, -29

(Afternoon)Session I? WG I A WO TJ 14.3O

Sersjon V WG T & WG TT 9.30 - 12.WG I fc HG II 14.15 - lo.

Sersion VT General Asnembly: Disci?sRion on WG reports 1 .15 - 17-FrTidrayt \ DecemberClosing Session: General Assembly: Adoption of WG Reports» Conclusions

and Recommendations 9-}0 - 12.- CLOSE OP PANE!, -

* Rapporteur or Coordinator/

543

List of Participant;

Annex G9 December 1971

PartieiffantsProf. Y. Ando

In-ç. Baez

Mr. Tullío Boazoni

Dr. A» Cowan

Mr, R. Filio

Mr» ake Junghera

Sir. R. de Knock

Dr. R.W, MeClung

Mr. Roland Roche

OrganisationFaculty of EngineeringUniversity of Tokyo7-3-1» Hongo» Bunkyoku, TokyoJapanComisión Wacional de EnergíaAtómica de la ArgentinaAvda Libertador 82 0Programma CÎRENSS, Maria di GaleríaRomeItalyUKAEAOulchethU.ï.Skoda WorksSuelear Power Plants DivisionPi 1 senCSSRDirectorTekniska Ro'ntg'en-CentralenFACKStockholm SOSwedenC E ÏTMolBelgitirn

Oak Rid^e National LaboratoryP.O. Box X"Oak Ridge, Tennessee 37830U.SJU

Ingénieur au Service des EtudesMécaniques et Thermiques du Centred*Etudes ÎTuclêaireB de Saclay

B.P. ïo. 291, Gif-sur-YvetteFrance

Working GroupII

II

TI

II

II

II

544

Mr. A, de Sterke

Mr. R. Truiïipfheller

Rent gen-Technische Dienst N.V.Delftweg 144RotterdamNetherlands

Rheinisch-Westfalisoher TechnischerUberwachungg Herein e.V.

43 Sssen 1Postfach 7041Fed. Rep. of Germany

II

II

ObserversMr. Jan BergstrSm

...ObserversOrganization

Reactor InspectorDelegationen for AtomenergifragorBox 4305BS-1007? Stockholm

Working GroupIt

Ing. Blondo

Prof. Jarme Garlsson

Mr. P. Cioli

Mr. Stig Dahn

Mr. J»W» Shrentreich

Mr. HShnel

Mr. J. Holliday

Wacional de EnergíaAtómica de la ArgentinaAvda LibertadorBuenos airesargentina

ft Royal Institute of TechnologyS-10044 Stockholm 70SwedenB H E L - CPITIfiale Regina MargjheritaRomeItalyEngineerTekmska RcSntgencentraien A.BPACK, 10405 StockholmSweden

European Communities200, rue de la Loi1040 Brussels

Institut fur Reafctorsioherheit derfeohnischan îîbsrwaohungsvereine e.V.

D 5 Kb'ln 1Glockengasse 2Fed, Rep. of Germany08 CD High Tenroerature Reactor ProjectDragon ProjectA.S.E. WinfrithDorchester, DorsetU.K.

II

II

II

545

Mr. P. Jehenson

Mr. C,A. Mann

Mr. H. Maurer

Dr. H.J. Meyer

Dr. Kurt $agel

Mr. Rudolf Pahnke

Mr. M.S.T. Price

Mr. A. Prot

Mr. G. Rottenberg

Mr. Luciano Sala

Mr, O. Sandberg

EuratomC C HI spraItalyUKAIà - SpringfieldsPrestonU.K.European Communities200, rue de la Loi1040 BrusselsBelgiumM A N - ACD 85 Mrnberg- 2PostfachFed. Rep. of GermanyBadische Anilin- &. Soda Fabrik AG

(BASF)D 6?0 LudwigshafenFed. Rep. of GermanySiemens Afí-Reaktortechnik HT 43D 852 SrlangenGunther-Scharowsky-Str « 2Fed. Rep. of GermanyOSCB Hi i Temperature Reactor ProjectDragon ProjectA.8.E. WinfrithDorchester, DorsetU.K.Département de Sûreté Nucléaire - SETSCEI SaclayB.P. îîo» 291, Gif-sur-YvettePranceCSNMolBeigi-umLaboratorio Technologie raaterialidel Centro Sueleare OH1ÏÏ délia Gasaccia

S. Maria di GaleríaRomeItalyCivil EngineerOskarharansverkets Kraftverksgrupp ABBirger Jarlsgatan 41AStockholmSweden

II

II

II

II

II

II

II

546

Dr. K. Schlosser

Mr. Setzwein

Mr* R.S. Sharpe

Osterreichische Studiengesellachaftfur Atosnenergie GmbH

Lenaugasee 10Vienna VIÏIAustriaInstitut fur Reaktorsioherheit der

Tecnnischen tfberwachungsvereine e.V.D 5 Koln 1Gioakengasse 2Fed. Rep. of GermanyÎÎDT CentreAERK.Harwell, Didcot, BerkshireU.K.

II

I & II

Mr. P, Spongier

Mr. M, Watteau

Mr. J<5rgen Wifclund

a 4020 LinsAustria

Dî\"ision de Metall.xi.rgie et dfEtudedes Gombustiljles Nucléaires

GEÎi SaclayB.P. No. 291, Qif-sur-YvetteFrance

EngineerSandvikens JernverkStockholmSweden

ÏI

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