POSIVA OY

Olki luoto

FI-27160 EURAJOKI, F INLAND

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Tarja Jäppinen

March 2012

Working Report 2009-100

Process for Nondestructive Testingof the Disposal Canister Lid Welds

March 2012

Working Reports contain information on work in progress

or pending completion.

The conclusions and viewpoints presented in the report

are those of author(s) and do not necessarily

coincide with those of Posiva.

Tar ja Jäpp inen

V T T

Work ing Report 2009 -100

Process for Nondestructive Testingof the Disposal Canister Lid Welds

Process for Nondestructive Testing of the Disposal Canister Lid Welds ABSTRACT The lid of the copper disposal canister for high activity nuclear waste is sealed with a weld. Before accepting the canister to the final disposal the weld will be inspected by four non-destructive testing (NDT) methods. The methods are radiographic testing (RT), remote visual testing (VT) using cameras, eddy current testing (ET) and ultrasonic testing (UT). The methods used in this report are preliminary and they can be changed in the final disposal phase. In this process description the four NDT inspection methods (RT, VT, ET, UT) for in-spection of the electron beam (EB) weld are collected into the same process flowchart in the order they will be, according to current knowledge, used in the encapsulation plant. This description is a collection of the individual inspection procedures. All information in this report is collected and summarised from the procedures of each method and in-spection records. Also inspection personnel and experts have been interviewed for this description. All of the four NDT methods detect defects in slightly different directions and based on different physical principles. The four methods are therefore supplements to each other in the inspection procedure of the canister weld. The process chart of the four methods combined in the examination of one canister is visualised in this report. In the schematic presentation it is assumed that the radiography testing is performed first and after that the three other methods can be run at the same time, starting with eddy current and vis-ual testing and continuing with the ultrasonic testing. The described methods are pre-liminary inspection methods and they can be changed in the final solution for inspec-tion. Keywords: NDT, RT; VT, ET, UT, surface defect, EB-weld, disposal canister, spent nuclear fuel.

Loppusijoituskapselin EB-hitsin rikkomattoman tarkastuksen prosessikuvaus TIIVISTELMÄ Korkea-aktiivisen ydinjätteen loppusijoituskapseli suljetaan hitsaamalla kuparinen kansi kuparisen vaipan päälle. Ennen kuin kapseli hyväksytään loppusijoitettavaksi, sen hit-sisauma tarkastetaan neljällä eri rikkomattomalla menetelmällä (NDT). Nämä menetel-mät ovat radiografia (RT), visuaalinen tarkastus (VT) kameralla, pyörrevirta (ET) ja ult-raääni (UT). Tässä raportissa käytetyt menetelmät ovat alustavia ja ne voidaan vaihtua loppusijoitusvaiheessa. Tässä prosessikuvauksessa on kerätty kaikki neljä tarkastusmenetelmää samaan proses-sikaavioon siihen järjestykseen, jolla tällä hetkellä uskotaan EB-hitsisauman tarkastetta-van kapselointilaitoksella hitsauksen jälkeen. Tämä kuvaus kerää yhteen ja referoi eri menetelmien erilliset prosessikuvaukset. Raportin tiedot on kerätty erillisistä menetel-mien kuvauksista, tarkastuspöytäkirjoista ja haastattelemalla tarkastajia. Kaikki neljä tarkastusmenetelmää havaitsevat viat kuparikapselin hitsisaumassa toisis-taan hieman poiketen ja näin täydentävät toisiaan. Tässä raportissa on kuvattu yhden kapselin NDT tarkastusprosessi alkaen röntgentarkastuksella ja jatkuen muilla mene-telmillä. Esityksessä on oletettu, että röntgentarkastus tehdään ensin ja sen jälkeen muut kolme menetelmää ajetaan samanaikaisesti, aloittaen pyörrevirta- ja visuaalisesta tarkas-tuksesta ja jatkaen ultraäänitarkastuksella. Kuvatut menetelmät ovat alustavia tarkas-tusmenetelmiä ja ne voivat muuttua lopullisessa tarkastusratkaisussa.. Avainsanat: NDT, RT, VT, ET, UT, pintavika, EB-hitsi, loppusijoituskapseli, käytetty ydinpolttoaine.

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TABLE OF CONTENTS ABSTRACT TIIVISTELMÄ FOREWORD

1 INTRODUCTION ................................................................................................... 3

2 PURPOSE OF THE PROCESS DESCRIPTION ................................................... 5

3 NDE METHODS .................................................................................................... 6 3.1 Radiographic testing (RT) ............................................................................ 7 3.2 Visual testing (VT) ........................................................................................ 9 3.3 Eddy Current testing (ET) .......................................................................... 11 3.4 Ultrasonic testing (UT) ............................................................................... 13

4 COMBINATION OF SEVERAL NDT-METHODS ................................................ 15

5 CONCLUSIONS .................................................................................................. 17

REFERENCES ............................................................................................................. 18

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FOREWORD This report has been prepared at VTT and it was ordered by Posiva Oy. This report is written by Tarja Jäppinen from the individual inspection procedures. Subscriber and project supervisor at Posiva Oy was Jorma Pitkänen.

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1 INTRODUCTION The final disposal canister for high activity nuclear waste consists of a copper overpack, a copper lid and a nodular cast iron insert. The nodular cast iron insert gives strength to the structure of the canister to withstand the mechanical stress originating from the bed-rock in the underground repository. Oxygen-free high conductivity copper overpack forms a corrosion resistant shell to the canister. The copper lid of the canister will be sealed with electron beam (EB) welding to the copper tube. During manufacturing defects may be generated in the components. The welding proc-ess may produces different types of welding defects, for instance internal root defects, porosity and cavities [1,2]. Lifting of the canister at the encapsulation plant is one factor that could cause defects or make small defects to grow. Thus, there is a limited number of lifts that are allowed. Possible handling incidents could cause defects on the outer surface of the copper overpack. The quality of the weld has to be verified according acceptance criteria before the final disposal of the canister. Those criteria for accepting the canister are created to control and ensure the wanted strength and corrosion resistance levels for the canister. Non-destructive testing (NDT) methods give possibility to reveal defects in the weld. The four NDT methods used are: radiographic testing (RT), remote visual testing (VT) using cameras, eddy current testing (ET) and ultrasonic testing (UT). Non-destructive testing of the canister weld is carried out in the encapsulation plant be-fore the decision to move the canister to final disposal. The encapsulation plant is lo-cated at the ground level above the actual repository, Figure 1. All NDT methods will be remotely controlled. Nondestructive testing is operated automatically in an isolated area because of the radiation originating from the waste filled in disposal canister. The objectives of the NDT tests are to acquire and analyse the data to detect possible manu-facturing defects in the closure weld of the disposal canister. In NDT there are different phases such as data acquisition and data analysis. In the NDT -process it is important to divide the whole system in subsequent parts. The duration for each activity will be esti-mated and the process chain will be visualised as an activity chain in a function of time. Each NDT-process will be processed separately and combined into a whole procedure.

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Figure 1. Encapsulation plant [3].

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2 PURPOSE OF THE PROCESS DESCRIPTION This description is carried out as a collection of the individual inspection procedures. All information is summarised from the procedures, inspection records and by inter-viewing the inspection personnel and experts. The purpose of the process description is to collect the four non-destructive testing methods (RT, VT, ET, UT) into the same process flowchart in the same order they will be applied in the encapsulation plant. This process description will change if changes are made in the individual NDT-processes. The reasons for changing an individual NDE-method could be its development or changed practice in testing. The objective for the nondestructive testing is to detect defects after welding and han-dling. Defects will be sized separately according each NDT-technique. A size picture of a defect as a combination of all used method will be constructed. Indications will be then classified and checked if they meet the joint acceptance criteria. Indications are also detected and evaluated by one separate NDT-technique. If an indication exceeds the acceptance criteria for one method it is enough for rejection. The criterion for each testing method is defined separately.

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3 NDE METHODS Four non-destructive methods are selected to be used in the examination of the closure weld. The inspection is fully automated, controlling and monitoring occurs in the iso-lated control room. The inspection of the weld will follow the procedures written in separate working instructions of every method. All of the methods need to be calibrated with a special calibration block before testing. In the calibration block some defects such as side drill holes (SDH), flat bottom holes (FBH), notches, V-grooves and special kind of defects are simulating manufacturing defects like planar and voluminous defect types. It is under evaluation if this calibration block can be the same for all of the methods. During the canister inspection, the calibra-tion block needs to be located nearby the inspected canister for example right on the top of the canister. In that case the calibration for radiography inspection can be done to-gether with the inspection. If the calibration block is located on the top of the lid during the inspection there will not be any artificial defects on the bottom edge of the calibra-tion block. The canister is transported from the welding area to the inspection with a special carrier. That carrier works also as a manipulator for canister during the testing. It turns the can-ister around clockwise in a determinated speed during the inspection and the NDT-equipments are stationary. It is important for the inspection methods that the canister is carefully centred to the canister transfer trolley. Before carrying the canister to the NDT-area the surface of the weld is machined of. The requirement for the maximum surface roughness for all of the four inspection methods is Ra = 12 µm. The surface of the canister is also required to be dry and clean. This requires for instance exact timing between visual testing of the outer surface of the lid and pouring on the coupling medium in UT. It is possible to run VT and ET inspection techniques at the same time. Also UT Tech-nique can be run simultaneously with VT and ET. Because of the water coupling UT needs to be located after VT and ET instruments in circumferential direction. The simul-taneous inspection would save time radically but increases the requirements for the in-spector. RT inspection needs to be done separately because the radiation from the accel-erator. Also the reference specimen positioning for X-ray inspection on the surface of the lid is disturbing the performance of other methods. Details for each method are listed below.

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3.1 Radiographic testing (RT) Radiographic examination for closure welds is carried out with a 9 MeV linear accelera-tor. The centreline of accelerator beam is directed at a 10° angle to the lid surface. For radiographic examination a digital x-ray detector is used. The detector is placed behind the outer surface of the canister, Figure 2.

Figure 2. The accelerator and the detector. The canister will be rotated in a carrier. Rotation speed is controlled by the RT-software that controls the radiation source and the detector. Before placing the canister on the carrier it is important to center it, the distance tolerance is max 1.5 mm. Area to be ex-amined is 100 % of the weld and the heat affected zone, 10 mm at both sides of the weld. There will be a marked zero-point on the canister surface. In the circumferential direction scanning exceeds the total circumference with suitable overlap. The 0 -point and the rotation direction will be marked also on the examination plan. Before the actual measurement of the canister, calibration test for intensity setting will be carried out to check out proper intensity in the detector. A separate calibration block made of the same material as the cylinder with known wall thickness is used for calibra-tion and calibration of the measurement is carried out simultaneously with the weld in-spection. An image quality indicator (IQI) is placed on the detector side surface of the canister so that the calibration hole in IQI is visible on the x-ray picture. Because the quality of the image can not be controlled during scanning, the quality of the qpicture will be moni-tored afterwards with IQI sensitivity and intensity value.

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For examination the exposure voltage is nominal voltage of the x-ray accelerator. The minimum distance from source to object is defined as a factor of maximum allowed geometric unsharpness (Ug). The images are saved by its identification number. In order to evaluate data properly a minimum contrast ratio is defined for the display. The picture evaluation is carried out using suitable software. The images will be analysed partly already during the scan in the encapsulation plant. The operator is required to have a level 1 certification according SFS-EN 473 and the industrial radiation safety examination passed. Also the operator for digital radiography should have 40 hours extension course for digital radiography. Inspector who analyses the radiographic images needs to have a level 2 certification according to SFS-EN 473. Image analyser needs also 40 hours training for digital radiography and 40 hours train-ing for operation of the equipment used for inspection. Checking and calibration of the x-ray detector is done annually. Table 1. Process chart for the RT examination of the closure weld. Time [min]

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3.2 Visual testing (VT) Visual inspection in the encapsulation plant will be done with cameras. The method for the remote controlled optical vision will be developed. In this report the visual testing process is carried out using VT inspection procedure for naked eye inspections. The area to be inspected is the surface of the upper part of the lid. Especially 100 % of the weld area and the entire upper end of the lid are inspected. Also the vertical upper outer surface of the lid shell and the vertical inner surface of the lid are inspected, Figure 3. The illuminance degree is required to be high enough (500 lx). Additional lightning is needed from multiple lightning directions due to the variable reflection of the copper surface because of characteristics of copper and its oxides. The surface to be examinated is required to be clean. The purpose of the visual testing is to verify, document and evaluate the surface area. During this inspection both manufacturing originated surface defects and handling de-fects can be detected. This information is critical for the integrity of the canister but also for verifying the indications primarily detected in eddy current testing and secondly by ultrasonic and radiography testing especially outer surface defects which can be seen in radiographic images. Indications that exceed reporting values will be reported by loca-tion, dimensions and defect type. Indication sizes correspondingly locations are tabled and compared to the relevant acceptance criteria. There are stamped identification marks in the weld, lid and shell that need to be documented and checked. This visual testing technique will be adjusted for remote visual testing and an examination inspec-tion procedure will be produced.

Figure 3. Remote visual testing of EB weld. Inspector is required to be qualified to level 2 in visual inspection according to EN 473. The vision of the inspector will be checked in every 12 months.

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Table 2. Process chart for the VT inspection. Time [min]

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3.3 Eddy Current testing (ET) Eddy current testing is a surface inspection technique. There are 2 approaches for eddy current testing with low and high frequency. With high frequency technique surface breaking defects and surface extensions can be accurately measured. With low fre-quency eddy current deeper surface breaking defects and defects having small ligament can be evaluated. The eddy current testing will be carried out at the present with 32 channels equipment. In the current inspection procedure inspection is done using 4 channel eddy current equipment. In 4 channel electronic board it is possible to use 4 probes or 4 frequencies. At the present maximum scanning speed is 160 mm/s. 5 samples/mm are recorded. The scan is visualised with in a form of colour coded C-scan. A probe array consisting of pancake coils is held on the surface of the copper lid with a probe holder. The array and holder is moved by a manipulator, Figure 4.

Figure 4. Eddy Current probe array and the holder. The upper planar surface of the lid is inspected using a coil probe array in order to de-tect defects. In inspection surface breaking defects will be detected, sized and classified. The inspection speed depends on the effective width of the probe array and on the fre-quency of the probe. At the beginning of the inspection the probe array is positioned at the zero point of circumferential coordinate. There are necessary steps to set up eddy current equipment and choose suitable parameters for the data acquisition like sampling rate, encoder pulse setting for different coordinate movements, sensitivity and frequency. The stability of the eddy current system shall be verified with the sensitivity and equip-ment check with a reference block. The check is needed before and after the examina-tion, after power loss or cable damage and after a long break. Every 12 hours calibration must be carried out.

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Data acquisition operators shall be certified to level 2 according to the EN 473 standard. They also are required to have documented training of the equipment. Data analysis per-sonnel are required to have level 2 or 3 certification to perform data analysis. They also need to pass a specific qualification to perform qualified inspections. Operators, who carry out data acquisition have to check the calibration of the equipment but are not al-lowed to make changes in the set up of the equipment. The qualified data analysis per-sonnel are allowed to make the calibration and adjust the equipment set-up. Table 3. Process chart for the ET inspection. Time [min]

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3.4 Ultrasonic testing (UT) The inspection is performed using the Multi-2000 phased array ultrasonic system. Dur-ing the examination an ultrasonic transducer is scanning the outer surface of the lid in circumferential direction. Simultaneously the phased array probe also carries out elec-tronic scanning in axial direction. Each measurement stores A-scan data in RF-form. There are about 1500 measurement positions in circumference. Two different focus depths will be used which demands own focal laws for both. Also three different tech-niques using +20°, 0° and -20° refraction angles will be used. In total about 1.5 GB of acquired data from one EB-weld will be stored. The ultrasonic phase array probe is positioned on the outer vertical surface of the canis-ter at the distance of 0.1 mm from the scan surface. The probe is positioned vertically and the element number 1 is at upper end of the probe, Figure 5. An electronic scan is performed in the direction of the weld penetration (axial direc-tion). The velocity of the longitudinal wave in copper is v = 4760 m/s or 4.76 mm/µs. The attenuation of the sound is dependent on the grain size in copper, bigger grains at-tenuate sound more [4]. Large grains extending over 2 mm occur in the weld in circum-ferential direction. In radial direction the grain size is smaller but large enough to pro-duce higher attenuation compared to base material of the tube and lid. In some cases also in the tube and lid can be large variations in grain size. In the manufacturing proc-ess of the base material the grain size is controlled in order to minimise the grain size and its distribution. At the encapsulation plant the ultrasonic testing will be carried out in local immersion. Before the examination the performance and the stability of the system is verified with a reference block. Also the setting values and equipment correction need to be check. The calibration block has the same geometry and material properties as the canister. There are different reflectors in the block. The calibration check is performed at the beginning and at the end of the inspection, after changing components, after alternations in the sys-tem performance and at the beginning and at the end of the inspection shift. Also the check on contact and signal is done before and after every canister inspection.

Figure 5. Phased array ultrasonic testing, the probe and the probe holder.

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The scanning is started at the circumferential zero-point. The scan step in the circumfer-ential direction is 0.22°. The work sequence during data acquisition is as follows: move the transducer in the correct probe holder system to the start position, activate the set-ting file, verify the setting files, define the acquisition data file name, activate the acqui-sition module, check contact and signals and start scanning. Recorded indications from the scan have coordinates as A (axial), R (radial) and C (circumferential). The data acquisition operator is required to be certified to level 2 according to EN-473 and have documented training. Data analysis personnel shall be certified to level 2 or 3 according to EN-473. They also have the final responsibility for the interpretation. The phased array ultrasonic system and the probe are calibrated annually. Table 4. Process chart for the UT inspection. Time [min]

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4 COMBINATION OF SEVERAL NDT-METHODS All of the four NDT methods used in the encapsulation plant are described above. The four NDE methods detect slightly different defects and are supplementary to each other in the inspection procedure of the canister weld. The process chart of the four methods combined in the examination of one canister are visualised in Table 5. In the schematic presentation it is assumed that the radiography is performed first and after that the three other methods can be run simultaneously, starting with eddy current and visual inspec-tions and continuing with ultrasonic method. There are some limitations that need to be taken under consideration. For example the width of the eddy current probe array at the present is only 10 mm and with that width it takes several rounds to cover the whole upper planar area of the lid. Also at the present a full turn in radiographic test takes 52 minutes. Improvements are under development in the RT inspection technique in order to double the speed of the inspection. All four NDT inspection methods between the welding of the canister lid and the accep-tation of the final disposal are done to one canister at a time. It is planned that the in-spection of one canister weld takes no more that 60 minutes. Also the time consuming defect analysis is supposed to be done simultaneous with the measurement, on line.

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Table 5. Schematic chart of the processes of the NDE methods for closure weld of one canister. Time [min]

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5 CONCLUSIONS This report is written to give an overall idea of the four non-destructive testing methods (RT, VT, ET, UT) used for the examination of the copper canister sealing EB-weld at the encapsulation plant. The inspection of the weld will be fully automated and the con-trolling and monitoring will occur in the isolated controlling room. The NDT inspection of canister welds is highly important. The canisters need to resist corrosion even in unfavourable environments for a hundred thousand years. By NDT methods it is ensured that there are no detrimental defects in the weld and that the weld meets the acceptance criteria before transferring the canister to the final disposal. The information in the report is gathered from the procedures written for every separate NDT method, from inspection records and by interviewing the inspection personnel and experts. All of the four inspection methods are changing due to the improvements or changes in the proceeding. This report gives an overview of the methods at the present and visions for the future inspection in the encapsulation plant.

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REFERENCES [1] Pitkänen, J., Salonen, T., Sandlin, S. and Ronneteg, U., Defect detectability in

EB-welded copper disposal canister with 9 MeV accelerator radiography, 6th Int. Conf. on NDE in Relation to Struct. Intergrity for Nuclear and Pressurized Com-ponents, Budapest, Oct. 2007.

[2] Ollonqvist, Pyry, Microstructural characterization and mechanical properties of electron beam welded thick phosphorous microalloyed oxygen free copper (Cu-OFP), Master’s Thesis, department of mechanical engineering, TKK, 2007.

[3] Työraportti 2006-95, Kukkola, T., Encapsulation Plant Preliminary Design, Phase 2. Repository connected facility

[4] Pitkänen, J. and Hirsekorn, A.S., The effect of grain size on the detectability in copper components in ultrasonic testing, 6th Int. Conf. on NDE in Relation to Struct. Intergrity for Nuclear and Pressurised Components, Budapest, Oct. 2007.