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Stefan Sand l i n

November 2010

Work ing Repor t 2009 -82

High Energy Radiography for Inspectionof the Lid Weld in Disposal Canisters

November 2010

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.

Stefan Sand l in

V T T

Work ing Report 2009 -82

High Energy Radiography for Inspectionof the Lid Weld in Disposal Canisters

High Energy Radiography for Inspection of the Lid Weld in Disposal Canisters ABSTRACT The copper canister is mainly designed to act as a corrosion barrier for the cast iron in-sert, which in turn contains the spent fuel bundles. The canister needs to be absolutely tight and the canister-to-lid weld is a critical part. Therefore this weld has to be in-spected by non-destructive methods. The objective of this work is to investigate the ap-plication of high energy radiography to inspection of the sealing weld. The approach is based on available written material in the field. The canister is filled with radioactive waste when the inspection will be done. This fact sets the requirement that the inspec-tion must be completely automated. A further challenge is the thickness of the copper wall. Due to an estimate based on a canister drawing the X-rays need to penetrate vary-ing thicknesses of copper from about 40 mm to nearly 120 mm in order to cover the whole weld area. For these thicknesses X-ray energies of a few MeV are required. To get a high enough photon flux a linear accelerator is assumed to be needed. The scat-tered radiation from the working accelerator will itself cause an intense field of radia-tion in the inspection room. All these aspects make the high energy X-ray inspection of the canister sealing weld very special. SKB in Sweden has done initial tests with accel-erator based X-ray inspections of this kind. The weld geometry, the X-ray attenuation and scattering in copper is presented in this study. The radiation field around the nuclear waste-filled canister is investigated. Some X-ray sources and digital detector concepts are presented. X-ray film is not well suited for this application. Also X-ray modelling tools available and their potential for this special problem are described. Most tools for simulation of X-ray inspections are designed for ordinary X-ray inspection and are not directly suited for accelerator based inspections. Keywords: High energy radiography, final depository canister, X-ray detector, linear array detector, flat panel detector, accelerator, X-ray-inspection modelling, NDT

Ydinjätekapselien sulkuhitsin tarkastus suurenergiaradiografialla TIIVISTELMÄ Kuparikapseli on pääasiassa suunniteltu toimimaan korroosioesteenä valurautasisäosal-le, joka puolestaan sisältää käytetyn polttoaineen ydinjäteniput. Kapselin on oltava eh-dottoman tiivis, näin ollen kapselin sulkuhitsi kuparikannen ja putken välillä on kriitti-nen osa. Tämä huomioiden sulkuhitsi on tarkastettava ainetta rikkomattomin menetel-min. Tämän työn tavoite on tutkia suurenergiaradiografian soveltuvuutta sulkuhitsin tarkastukseen. Tutkimus perustuu käytettävissä olevaan aineistoon. On todettava, että kapseli on tarkastuksen aikana täytetty radioaktiivisella jätteellä. Tämä edellyttää, että tarkastukset on suoritettava automaattisella kauko-ohjauksella. Toinen korkea vaatimus tarkastuksen toteutukselle on kuparihitsin paksuus. Piirustukseen perustuva arvio edel-lyttää, että röntgensäteet läpäisevät vaihtelevan kuparipaksuuden, joka on välillä 40 mm ja 120 mm, jotta koko hitsin alue voidaan 100 % kattaa. Nämä paksuudet edellyttävät muutaman MeV:n energiaa radiografia-tarkastuksessa ja jotta saataisiin synnytettyä tar-kastukseen riittävä fotonivuo, tarvitaan lineaarikiihdytintä. Siroava säteily toimivasta kiihdyttimestä synnyttää vahvan säteilykentän tarkastushuoneessa. Huomioiden kaikki nämä erityispiirteet tekevät suurenergisestä röntgentarkastuksesta sulkuhitsin kunnon-valvonnassa vaativan. Ruotsissa on SKB suorittanut alustavia kokeita tämäntyyppisillä röntgentarkastuksilla käyttäen lineaarikiihdytintä. Tässä tutkimuksessa esitetään hitsin geometria ja röntgensäteiden vaimentuminen ja siroaminen kuparissa. Säteilykenttä polttoaineella täytetyn kanisterin ympärillä on tarkasteltu. Joitakin mahdollisia röntgen-lähteitä ja detektorityyppejä on esitelty. Röntgenfilmi ei sovellu tähän tarkastukseen. Myös röntgentarkastuksen mallinnus ja sen mahdollisuuksia tähän erikoistarkastukseen on kuvattu. Röntgentarkastukseen luodut mallinnustyökalut käsittelevät yleensä tavan-omaista röntgentarkastusta eivätkä sellaisinaan sovellu kiihdytinpohjaisen tarkastuksen mallinnukseen. Avainsanat: suurenerginen radiografia, loppusijoituskapseli, röntgendetektorit, rivi-detektori, litteätaulu detektori, kiihdytin, röntgentarkastuksen mallinnus; rikkomaton aineenkoetus.

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TABLE OF CONTENTS

ABSTRACT

TIIVISTELMÄ

1 INTRODUCTION........................................................................................................ 2

2 THE CANISTER AND THE INSPECTION GEOMETRY ........................................... 4

2.1 The weld inspection ............................................................................................. 5 2.2 X-ray attenuation in copper ................................................................................. 8 2.3 Needed defect sensitivity .................................................................................. 10

3 THE RADIATION FIELD AROUND THE FILLED CANISTER ................................. 12

3.1 Estimation of the radiation field around the canisters ........................................ 12 3.2 Gamma radiation dose rates ............................................................................. 14 3.3 Neutron radiation dose rates ............................................................................. 16

4 HIGH ENERGY X-RAY SOURCES ......................................................................... 17

4.1 X-ray tubes ........................................................................................................ 17 4.2 Accelerators ...................................................................................................... 18 4.3 Isotope sources ................................................................................................. 19

5 DIGITAL X-RAY DETECTORS ................................................................................ 21

5.1 Film versus digital detectors .............................................................................. 21 5.2 Examples of digital detectors ............................................................................ 24

6 MODELLING OF X-RAY INSPECTIONS ................................................................. 29

6.1 Modelling principles ........................................................................................... 29 6.2 Available modelling software packages ............................................................ 30

7 X-RAY INSPECTION GUIDELINES AND STANDARDS ......................................... 32

7.1 The development of standards for digital radiography ...................................... 32 7.2 Definitions .......................................................................................................... 33

8 DISCUSSION AND RECOMMENDATIONS ............................................................ 37

REFERENCES ............................................................................................................. 38

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1 INTRODUCTION

The canister for spent nuclear fuel consists of an outer copper cylinder with an insert made of cast iron with cannels in which the spent fuel elements are inserted. The copper cylinder acts as a corrosion barrier while the iron insert gives the construction its me-chanical strength. The fuel elements are inserted into the canisters in a specially de-signed encapsulation plant where the canister is sealed by welding a copper lid to it. The welding is done either by electron beam welding (EBW) or by friction stir welding (FSW). At the present Posiva Oy in Finland has mainly concentrated on the EBW method, while SKB (Svenska Kärnbränsle) has directed its main interest towards the FSW-method. Irrespective of the welding method used, the integrity of the welds needs to be ensured by NDT-methods. Ultrasonics, high-energy radiography and Eddy-current testing are the methods to be employed. Eddy current testing can only assess defects close to the surface of the weld but both high-energy radiography and ultrasonics can be used the test the whole internal weld volume. The applicability of high-energy radiography is the topic of this work. Available high energy X-ray sources and detection systems will be reviewed. Main attention is drawn on digital radiography because the inspection of the filled canister needs to be fully automated. This work starts with a presentation of the weld geometry. Further, the radiation field around the filled canister is studied as the radiation may have influence on the performance of both the detection system and the radiation source. The background radiation is not expected to disable the X-ray inspec-tion but background subtraction may be desirable. X-ray attenuation and scattering in copper is briefly addresses and compared to other materials. Copper has a higher linear coefficient of attenuation than iron, the percentual difference being 51 % at 50 keV, 11 % at 1 MeV and 17 % at 10 MeV. Due to an estimate based on a canister drawing the X-rays need to penetrate varying thicknesses of copper from 40 mm to 120 mm in order to cover the whole weld area. These thicknesses are assumed to require X-ray energies well above one MeV. To get a high enough photon flux an accelerator (type Linac) instead of isotope sources is as-sumed to be needed. This is because an accelerator can deliver several orders of magni-tude higher photon fluxes than isotope sources. Further, the scattered radiation from the working accelerator will itself cause a huge field of radiation in the inspection room. All these aspects make the high energy X-ray inspection of the canister-to-lid very special. Only SKB in Sweden has done initial tests with (9 MeV) accelerator based X-ray in-spections of this kind. Further BAM has used a 12 MeV Linac (40 Gy/h, focus 1.5 mm) together with a linear diode array detector (CdWO4) and a specially designed collimator to make CT (Computer tomography) of HLW canisters (High Level Waste). Similar accelerators are also used at border-crossing stations around Europe for security inspec-tion of trucks; one accelerator and detector system depict the truck from above while another accelerator-detector system produces a side view of the truck as it slowly moves through the X-ray beams. However, these security systems are of course not as such us-able for NDT-inspection. After a review of different X-ray sources we present some digital detector schemes available together with a short comparison of radiographic film and digital detectors. Some examples of digital detectors used together with X-ray en-

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ergy in the MeV range are presented. Finally the work is concluded with a brief review of available modelling software for X-ray inspection.

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2 THE CANISTER AND THE INSPECTION GEOMETRY

The canisters designed for spent fuel from VVER 440, EWR and the new EPR differ in length and with respect to the design of the cast iron insert as shown in Fig. 1. The wall thickness of the canister and the lid is 50 mm in all cases. Only the geometry of the lid and the weld is of interest in this report. In the present Finnish design the lid will be sealed to the canister by an electron beam weld of depth 50 mm parallel to the axis of the canister (see Fig. 2). In the Swedish design the weld goes in the radial direction as shown in Fig. 3. The welding will be done ether by electron beam welding (EBW) or by friction stir welding (FSW). The fact that the canisters are filled with highly radioactive spent nuclear fuel when the X-ray examination takes place also calls for further demands on examination geometry. Especially the detector is expected to need shielding against the background radiation from the canister and scattered radiation from the primary X-ray beam in order to im-prove image quality and detector life. The radiation from the canister mainly consists of gamma radiation, but there are also neutrons and electrons present as will be seen later. Further, the intensity of the radiation varies with the azimuthal angle around the canis-ter. This is mainly due to the geometry of the insert.

Figure 1. Artist’s view of the canisters designed for spent fuel from VVER 440, BWR and EPR reactors. The canisters differ in length and in the design of the insert (Raiko 2005).

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2.1 The weld inspection

The weld geometry shown in Figs 2 and 3 is the starting point for defining the needed X-ray inspection setup. The geometry and the material (copper) differ from that usually encountered in X-ray inspection. Assuming, for example, that incident X-rays form an angle of 45 degrees with the axis of the canister and that one ray goes through the root of the weld and another ray though the surface bead in Fig. 2 we can estimate that the rays have to penetrate a thickness of copper varying from about 63 mm to 128 mm in order to get a projection of the whole 50 mm deep weld. The penetrated thickness will vary with the X-ray angle and therefore the angle of the incident X-rays need to be op-timized for the final set-up. An angle giving a penetrated thickness between 40 mm and 120 mm may be close to the optimum.

Figure 2. Present Finnish canister-to-lid weld area design. The vertical electron beam weld is 50 mm deep and marked “EB-weld”. The wall thickness of both the canister and the lid (both copper) is 50 mm. The depth of the weld is also 50 mm. The steel lid covering the cast iron insert is seen under the copper lid (Mikkola 2006).

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Figure 4. X-ray inspection design by SKB in Sweden. The radiation source is a 9 MeV accelerator (Linatron 3000) by Varian. The transmitted radiation is detected by a linear array detector of height 100 mm. The canister is rotating while the source and the de-tector remain stationary (Ronneteg & Moberg 2003).

Figure 3. The Swedish design of the weld between the canister and the lid (Stepinski 2006).

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Fig. 4 shows an X-ray inspection setup used by SKB in Sweden (Ronneteg & Moberg 2003). The setup consists of a linear array detector and a 9 MeV linear accelerator source (Varian Linatron 3000). During the inspection the canister is rotating while the source and the detector are stationary. This system was designed to be able to detect 1 mm pores in a 100 mm thick piece of copper. In this case the inspected weld geometry is that of Fig. 3. In this case incident beam forms an angle of 35 degrees with the plan of the weld. Fig. 5 shows a more detailed view of the image formation using the experi-mental Swedish X-ray inspection system. The Fig. illustrates how three defects are pro-jected onto the detector while the canister is rotating around its axis. The linear detector array produces horizontal line elements of the picture seen to the right in Fig. 5 for each 0.4 mm of circumferential movement of the outer canister surface. A collimator system is used to form the X-rays from the accelerator to a thin fan-shaped radiation distribu-tion suitable for the linear array detector. The detector output picture is built up of verti-cally piled line elements in the mentioned Fig. The array detector has a length of 100 mm and a resolution of 0.4 mm in the length direction. According to Ronneteg and Mo-berg (2003), reference measurements are used to compensate for the large variations in material thickness. If the same inspection geometry is used for the Finnish weld design (Fig. 2) then an X-ray interacting with a defect in the outer part of the weld will have to travel a comparable longer part in copper before reaching the detector. Fig. 6 shows an X-ray picture taken with the system shown in Fig. 4. The EB-weld tested in Fig. 6 was fabricated by Posiva Oy.

Figure 5. The radiation from the accelerator (not shown) is formed to a thin fan-shape to fit the linear array detector. Three defects in the weld (1, 2 and 3) are projected onto the detector. The picture to the right is built up of vertically piled line elements each of which are captured for every 0.4 mm circumferential movement of the canister (Ron-neteg & Moberg 2003).

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Figure 6. A digital radiograph of an electron beam test weld made by Posiva Oy and captured by the X-ray inspection setup shown in Fig. 4. The weld geometry is shown in Fig. 2.

2.2 X-ray attenuation in copper

If radiography were done with mono-energetic radiation, that is, with an X-ray beam containing but a single wavelength, the laws of attenuation of X-rays by matter could be stated mathematically with great exactness as.

xeII ��� 0 where I0 is the initial intensity, I is the intensity when the X-rays have travelled a dis-tance x in the material and μ is the linear attenuation coefficient. Linear attenuation co-efficients for some materials are given in Tab. 1. Copper has a higher coefficient of at-tenuation than iron, the percentual difference being 51 % at 50 keV, 11 % at 1 MeV and 17 % at 10 MeV. The above equation describes removal of photons from the initial mono-energetic beam. It is important to notice that it does not represent all photons pre-sent, since some have scattered to lower energy via different scattering mechanisms. In reality a broad band source is usually used and a considerable amount of scattered radia-tion reaches the film (or detector), the image formation is therefore more complicated than the above equation suggests. Radiographic equivalence factors are used to ap-proximately compare absorption in different materials. Table 2 shows the radiographic equivalence factors of copper, when steel is taken as reference i.e. the value for steel is put to 1. Note that the relative absorption of the two materials depends of X-ray energy and as the energy increase the difference between the materials become less (Quinn & Sigl 1980). As can be seen copper absorbs more X-ray energy than steel in all indicated energy ranges. A deeper insight into the absorption behaviour can be gained by studying the scattering mechanisms at different X-ray energies. The efficiency of photon scattering, the scatter-ing cross-section, is often measured in a unit called barn (1 barn = 10-28 m2). The scat-tering cross-section can be looked upon as a “dartboard” around the scattering electron, atom or nucleus. The larger the area of the “dartboard” is, the greater the probability for scattering. Further, the scattering cross-section is dependent on the mass (atomic num-ber, Z) of the scattering atom and on the energy of the incoming photon. The total cross-section is the sum of cross-sections for different scattering mechanisms, such as photo

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effect, Rayleigh scattering, Compton scattering and pair production. Fig. 7 shows the total scattering cross-section and the scattering cross-section for different mechanisms for copper. The total scattering cross-section decreases steeply with photon energy for energies used in ordinary radiography, but planes out for energies above 1 MeV when pair production becomes the dominating scattering mechanism. This is the energy range where X-ray generating accelerators work.

Figure 7. The contribution of different scattering mechanisms to the total scattering cross-section for copper. The cross-section decreases steeply with photon energy for low energies but planes out at higher energies when pair production becomes the domi-nating scattering mechanism (Radiation Environment, 2005).

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Table 1. Linear attenuation coefficients (cm-1) for copper and some other elements for X-ray energies from 50 keV to 30 MeV (from Radiography, 2006).

Table 2. Approximate radiographic equivalence factors for copper as compared with steel at different X-ray energies from Quinn & Sigl (1980).

X-ray energy

Material 150 keV 220 keV 400 keV 4 to 25 MeV Steel 1.0 1.0 1.0 1.0

Copper 1.6 1.4 1.4 1.3

2.3 Needed defect sensitivity

Information about the maximum allowable flaw size can be found in the references by Stepinski and Müller et al. (2006). Stepinski (2006) give the recommended schematic curves shown in Fig. 8 without exact numbers. The minimum detectable flaw size (blue) grows in two steps from the outer surface of the weld. The red curve gives the depend-ence of the maximum acceptable flaw size as function of dept in the weld as measured from the outside canister surface. The highest sensitivity is required just below the outer surface (up to a depth of 2 mm). In another report by Stepinski (2004) the following statement about radiographic detection probability of lack of penetration (LOP) defects can be found: “Radiographic inspection performed at Lockheed Martin using film and digital methods on FSW test panels demonstrated 90 % probability/95 % confidence ability to detect LOP discontinuities greater than or equal to 30 % of the material thick-ness”. In the above mentioned article by Müller et al. the following statement can be found: “The flaw radial size has to be limited so that a remaining wall thickness of 15 mm is guaranteed against ground water corrosion. The maximum allowed flaw size in the radial direction is 35 mm”. To guarantee that no canister has a flaw larger than this the minimum detectable flaw size is must be considerable smaller. This minimum de-tectable size is dependent of the exact X-ray inspection set-up and therefore not a topic for this type of state-of-the-art report. However, it is worth mentioning that Müller et al.

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present a couple of curves showing radiographic maximum contrast as a function of the defect size penetrated by the X-rays. These curves illustrate how the problem can be approached.

Figure 8. Required sensitivity of the NDT method as function of defect depth (blue) and (red) the corresponding acceptable flaw size (Stepinski 2006).

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3 THE RADIATION FIELD AROUND THE FILLED CANISTER

The X-ray inspection of the canister-to-lid weld has to be done after the canisters have been filled with spent nuclear fuel. The radiation from the spent fuel inside the canister may have various effects of the inspection equipment. First, this background radiation may interfere with the image forming X-rays at the detector making it more difficult to identify defects. Secondly, the radiation may have time dependent effects on the materi-als in the detector and the source which reduce the lifetime of these components or change their performance. Electronic components are known to be especially suscepti-ble to radiation damage; both malfunction and permanent damage may occur (Rybka, 2005). It is also well-known that polymers (plastics) suffer from damage due to irradia-tion (Smith et al. 2001); therefore also damages of this kind of materials may have indi-rect effects of the inspection result. The background radiation is not expected to disable the X-ray inspection, but background subtraction (this can be done in digital radiogra-phy) may be desirable or necessary. Further, the background radiation probably reduces the service life time of certain components. All these reasons make it important to have a detailed knowledge about the radiation field around the filled canisters.

3.1 Estimation of the radiation field around the canisters

The fuel has been cooled for about 20 years after removal from the reactor before it is inserted into the final depository canisters. The radiation around the canister mainly consists of gamma- and neutron radiation; some secondary electrons are also emitted from the canister surface (Lundgren, 2004), but due to their short range in solid materi-als these electrons are not expected to have influence on the X-ray inspection equip-ments. However, secondary electrons created in the detector itself or in the related elec-tronics may have serious influence of the inspection result. Despite the direct influence of neutron radiation, these particles may also cause indirect damage through activation of other materials. In the following the data about the radiation around the canister are taken from the report by Markku Anttila (2005). Anttila has calculated the radioactive properties of the spent nuclear fuel using a software package called ORIGEN 2.1 and assuming that the fuel bundles in the canister had a discharge burnup of 60 MWd/kgU and that the fuel has been stored for 20 years after removal from the reactor. Once the radioactive properties are determined as function of discharge burnup and storage time, Anttila calculated the gamma and neutron dose rates and fluxes outside the canister us-ing the Monte Carlo code MCNP4C. The calculations was done for three different can-ister types, VVER, BWR and EPR, where VVER-440 stands the Loviisa-type pressure water reactors, BWR stands for the Olkiluoto type boiling water reactors and EPR stands for the new European pressure water reactor to be built in Olkiluoto. The differ-ence in radiation on the surface of the three canister types is mainly due to different sizes of the fuel bundles and the different geometry of the inserts. Due to the form of the inserts, the intensity of the radiation will vary as function of the azimuthal angle around the canister. Figs 9 to 11 show the canister cross-sections used by Anttila in modelling the radiation field. In these Figs. the fuel bundles are shown in dark colour while the cast iron is green and the copper is shown in yellow. The azimuthal angle is the angle between a radius and the horizontal diameter of the canisters. As seen from the Figs. the radiation has to penetrate different thicknesses of iron at different azimuthal angles and

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therefore the intensity of the radiation will vary with the azimuthal angle. This fact need to be accounted for in the X-ray inspection as it cases a variation in the background ra-diation due to canister rotation. More details about the dose rates around the canisters will be shown below. Tab. 3 gives the maximum and average dose rates for gamma- and neutron radiation.

Figure 9. Cross-section of the VVER-canister used in the MCNP-model. This canister contains 12 hexagonal fuel bundles. The green surface represents the cast iron while the yellow ring represents the copper lining (Anttila 2005).

Figure 10. Cross-section of the BWR-canister used in the MCNP-model. This canister contains 12 square fuel bundles. The green surface represents the cast iron while the yellow ring represents the copper lining (Anttila 2005).

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Figure 11. Cross-section of the EPR-canister used in the MCNP-model. This canister contains 4 square fuel bundles. The green surface represents the cast iron while the yel-low ring represents the copper lining (Anttila 2005).

Table 3. Maximum and average dose rates on the radial outer surface of three canister types (Raiko 2005).

3.2 Gamma radiation dose rates

Fig. 12 shows the gamma spectrum emitted from fuel in three different canister types (VVER, BWR and EPR). The Fig. shows the number of gamma photons emitted per second from one ton of uranium as function of energy. In this Fig. no account is taken to the attenuation caused by the fuel itself and the canister material (i.e. the insert and the copper canister). Low energy gamma photons will be more strongly attenuated than more energetic photons. However, in the calculation of the dose rates, Fig. 13, on the surface of the canisters the attenuation has, of course, been accounted for. This Fig. shows the dose rate on the outer surface of the copper canister as function of the azi-muthal angle (the angle between a radius and the horizontal diameter in the cross-sections shown in Figs 9 to 11). The data are given as dose rates (mSv/h), which mean that biological quality factors are included. From point of view of X-ray inspection, photon flux (Gy/h) as function of energy (and azimuthal angle) would be more informa-tive. However, the angular variation is well illustrated.

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1,0E+05

1,0E+07

1,0E+09

1,0E+11

1,0E+13

1,0E+15

0,01 0,10 1,00 10,00

Photon energy, MeV

Phot

ons/

(s M

eV tU

)

VVERBWREPR fuel 1EPR fuel 2

Figure 12. Photon spectra (1/s/tU/MeV) for VVER, BWR and EPR fuel for a discharge burnup of 60 MWd/kgU. It is assumed that the fuels have been stored for 20 years after removal from the reactor, data from Anttila (2005).

0

50

100

150

200

250

300

350

0 45 90 135 180

Azimuthal angle, degrees

Gam

ma

dose

rate

, mSv

/h

VVERBWREPR

Figure 13. Calculated gamma dose rates of the outer surface of the Finnish final dis-posal canisters as function of the azimuthal angle. Here 180 degrees is shown, corre-sponding to the upper or lower part of the canisters in Figs. 9 to 11. The data are from Anttila (2005).

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3.3 Neutron radiation dose rates

The neutron radiation is mainly due to spontaneous fission of Curium (Cm244). Fig. 14 shows the neutron dose rate on the outer surface of the copper canister as function of the azimuthal angle for the three different canister types. As in section 3.2 the values are dose rates (mSv/h) including biological quality factors (large for neutrons). From point of view of X-ray inspection, neutron flux as function of energy (and azimuthal angle) would be more informative. However, the angular variation is well illustrated.

4

6

8

10

12

14

16

18

0 45 90 135 180

Azimutal angle, degrees

Neu

tron

dose

rate

, mSv

/h

VVERBWREPR

Figure 14. Calculated neutron dose rates of the outer surface of the Finnish final dis-posal canisters as function of the azimuthal angle. Here 180 degrees is shown, corre-sponding to the upper or lower part of the canisters in Figs. 9 to 11. The data are from Anttila (2005).

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4 HIGH ENERGY X-RAY SOURCES

X-rays are characterized by the photon energy E, which is given by E = h�, where � is the frequency and h is Planks constant. According to this the radiation consists of a stream of individual energy packages or photons, each having the energy h�. Further, the intensity of the radiation is defined as the total number of photons striking a unit area per unit time. A radiation source may deliver photons of different energies; the source is therefore characterized by the intensity distribution of photons of different en-ergy. i.e. the number of photons in each energy interval. X-rays are produced by X-ray tubes, accelerators or isotope sources. X-ray tubes are in fact also accelerators, but real accelerators are much more sophisticated in order to produce higher dose rates and more energetic X-rays. The operating principle of an accelerator (or an X-ray tube) is to ac-celerate electrons in an evacuated tube to high kinetic energy. As these energetic elec-trons strike a metallic target they lose their kinetic energy in one or usually many colli-sions with the target atoms. These multiple collisions are responsible for the energy dis-tribution of the X-rays as the electron produce photons of different energy in each colli-sion. The X-rays produced by isotope sources are called gamma rays to illustrate that they originate from decaying atomic nuclei. As isotopes emit photons of one or a few energies the isotope sources have a discrete energy distribution contrary to the continu-ous photon energy distribution given by accelerators.

4.1 X-ray tubes

Industrial X-Ray tubes typically work with currents in the range 20 – 200 mA and volt-ages in the range 120 - 450 kV. The dose rate in air from an industrial X-ray tube at a distance of one metre can be about 2 Gy per minute (Radiation protection and safety for industrial X-ray equipment, 2003). Fig. 15 illustrates the working principle of an X-ray tube. Electrons are emitted from a heated filament and accelerated through the applied high voltage against the target where the electrons produce X-rays when they collide with the target atoms.

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Figure 15. Cross-section of an X-ray tube. Electrons are emitted from the heated fila-ment and accelerated towards the target where they produce X-rays as they are re-tarded, a phenomenon called Bremsstrahlung (Podgorsa 2005).

4.2 Accelerators

Accelerators used for industrial radiography produce photons in the MeV range at high dose rates. For example, a 3 MV Linatron accelerator can produce a dose rate of 3 Gy per minute at a distance of one metre and a similar device operating at 9 MV can yield a dose rate of over 30 Gy per minute at the same distance (Radiation protection and safety for industrial equipment, 2003). The best known accelerators in connection with NDT are linacs, betatrons and microtrons. They all differ in construction, but they all acceler-ate electrons to high energies and produce X-rays by letting the accelerated electrons collide with a target. We already presented the 9 MeV accelerator used by SKB in Fig. 4. A 12 MeV linac can be seen in Fig. 16. This accelerator is used to computer tomo-graphy (CT) of high level waste drums. One such drum (without radioactive material) can be seen in the middle of the picture. The detector, a linear array (photodiodes with a CdWO4 scintillator), with a specially designed collimator can be seen to the left (Haase et al. 1999). Accelerator manufacturers: AECL, CGR, Elektra, General Electric, Siemens, Toshiba and Varian. Some of these names may have disappeared lately because of corporate ac-quisitions while a new one, EuroMeV in France, has appeared. This new linac has an especially small focal spot.

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Figure 16. A 12 MeV Linac (right) used for high energy tomography of a radioactive waste drum, the dark vertical cylinder. The linear array detector to the left (Haas et al. 1999).

4.3 Isotope sources

Radiography with gamma rays has the advantage of simplicity of the apparatus used, compactness of the radiation source and independence from outside electrical power. In contrast to X-ray machines (tubes & accelerators), which emit a broad band of wave-lengths (for example se Fig. 25), gamma-ray sources emit one or a few discrete wave-lengths. A gamma ray with energy of 0.5 MeV is equivalent in wavelength and penetrat-ing power to the most penetrating radiation emitted by an X-ray tube operating at 0.5 MV. Most of the photons emitted by the tube will have energy less than 0.5 MeV. Therefore the X-rays emitted by the 0.5 MV tube are less penetrating than the 0.5 MeV gamma radiation. Tab. 4 lists the isotope sources used in industrial radiography (Cobalt 60, Cesium 137, Iridium 192 and Thulium 170) together with their half-life, gamma en-ergy and dose rates. Cobalt 60 is the only one having gamma energies over one MeV (Quinn & Sigl 1980). The same reference gives the application range (Tab. 5) and prac-tical thickness limits for the different sources (note that this applies to film radiogra-phy). Digital detectors needing considerably shorter exposure times than film may ex-pand the useful application range.

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Table 4. Isotope sources used in industrial radiography (Quinn & Sigl 1980).

Table 5. Gamma ray applications according to Ouinn & Sigl (1980).

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5 DIGITAL X-RAY DETECTORS

Both conventional film radiography and digital radiography are based on absorption of X-rays inside the examined object. Discontinuities such as flaws, pores and cracks ab-sorb less radiation than the parent material and they can therefore be observed on the projection images produced by these techniques. During many decades of development film radiography has reached a high level sophistication. In radiography the film method is still usually the reference method that various other methods are compared to (Sood 2005). However, many technologies are available for the acquisition and display of digital radiographic images. Film replacement in radiography started by the development the bulky image intensifi-ers in fluoroscopy and of the so called computed radiography (CR). In CR the X-ray image is stored by trapped electrons in a phosphor layer on a film like, but reusable plate. The latent image on this image plate (IP) is read by a special device and converted to a digital image. By emergence of linear array detectors and flat panel detectors the reading the IP was eliminated and the image could be transferred to the computer more or less in real time. These new techniques also reduced the amount of radiation needed per image considerably. However, the application range of most of these digital detec-tors is limited to lower (< 250 keV) X-ray energies (Ewert et al. 2005). Detectors oper-ating in the MeV energy range exist, as shown for example in Fig. 4, but detailed ex-perience of the long term performance in applications like inspection of the canister weld in the real surrounding is lacking. Due to the fact that the X-ray inspection of the canister weld will be fully automated, it is self-evident that a digital detector is needed.

5.1 Film versus digital detectors

The Non-destructive Testing Team at Los Alamos National Laboratory has determined that the latest generation of digital imaging can be viewed as equivalent to film imaging for most applications. To achieve this equivalence, however, the application must be held against the capabilities of both detection systems. Also, the parameters of radiogra-phy must be adjusted to optimize the performance of either detection systems (Davis et al. 2000). Amorphous silicon detector plates are said to be the best film replacement al-ternative in the cited report. Further, the amorphous silicon detector is said to be inher-ently insensitive to radiation damage. The supporting electronics surrounding the detec-tion area will, however, need shielding from the primary X-rays. In Fig. 17 Davis et al. illustrate the practical range of digital imaging for different source energies and required resolution (line pairs per millimetre). According to this, film is still preferred for high-energy and high resolution applications. Current amorphous silicon detectors can re-solve 4 line pairs per millimetre, where standard M film can easily resolve 20 line pairs per millimetre. Film is limited by grain size, and the randomly arranged grains are nominally 20 microns in diameter. However, for the majority of applications, including some high-energy applications, digital imaging performs as well or better than film. Digital radiography has several advantages over conventional film based radiography (Blakeley & Spartiotis 2006):

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� The digital detectors require less radiation to create an image, typically only 1 to 4 % of that normally required for a D7 film.

� The image can be stored, emailed or processed on PC. � Automated defect recognition systems can be used to reduce the influ-

ence of the subjective assessment of an inspector.

Figure 17. Digital imaging and film can be used in the green area while only film will perform well in the red area. Some high-energy applications (yellow) are still possible only with digital detectors with special scintillators (Davis et al. 2000).

The main disadvantage of using digital detectors is, as indicated above, the resolution is lower than fine-grained film; typically 100 μm. Projection magnification can be used to increase image size, thereby increasing the effective resolution of the final image, but several factors must be taken into account for this. For example the focal spot size of the X-ray generator must be sufficiently small to avoid excessive unsharpness. Ewert et al. (2005) have analyzed the requirements of European and USA standards for film radiography to derive correct requirements for digital image quality and procedures for prediction and measurement of image quality. They conclude that USA standards seem to be more tolerant for these new innovative technologies. New standard proposal use signal/noise ratio (SNR) and unsharpness as dominant parameters for image quality. Ewert at al. also approach the problem of how the requirements given in standards for film radiography can be translated to the parameters used for characterizing the per-formance of digital detectors by comparing the SNR for the detector to the SNR for film as given in Tab. 6 for different standards.

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Table 6. Overview of film system classes in different standards and the corresponding SNR values (Ewert et al. 2005).

Soltani & Wysnewski (1999) have evaluated the performance of amorphous selenium (a-Se) direct radiography for industrial imaging using the following characteristics:

� Exposure and energy response � MTF (Modulation Transfer Function) � Penetrameter sensitivity � Speed � Throughput

Fig. 18 shows a summary of the penetrameter sensitivity tests. The penetrameter sensi-tivity of the a-Se array was evaluated by exposing a 25 mm thick stainless steel block to X-rays at 200 kVp with 1 %, 2 % and 4 % plaque type penetrameters. The exposure was varied and the resulting image optimized to evaluate visibility of the penetrameter holes. The exposure at which various holes could be seen was noted. These results are shown together with data for NDT-film and a medical fluorescent phosphor employed with a-Se indirect arrays. These data show that the direct a-Se achieves penetrameter sensitivi-ties comparable to fine grain NDT-film, but at about 1/50 of the exposure. It has been found that though the pixel pith of a-Se direct conversion flat panel array is limited to 139 microns, the contrast sensitivity is very high and comparable to fine grain NDT-film. Furthermore, it is found that because of the systems high signal-to-noise ratio, the system can detect the presence of flaws smaller than the pixel pitch as long as sufficient object contrast is produced. Partridge, Hesse & Müller (2003) have made a comparison of direct- and indirect-detection flat panel amorphous silicon imagers at a beam energy of 6 MeV, and in this study the conclusion is that the indirect–detection imager is a more appropriate choice for megavolt energies.

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Figure 18. Penetrameter sensitivity for different imaging methods at 200 kV (Soltani & Wysnewski 1999).

5.2 Examples of digital detectors

Terms like linear arrays and flat panel detectors are used to describe digital X-ray detec-tors. A linear detector is, as the name tells a linear array of detectors (pixels) with many detectors in the length direction and only a few in the transverse direction. A flat panel detector, on the contrary, is a full two dimensional array of detectors (pixels). The de-tectors are based on the technology for manufacturing solid state semiconductor circuits. Two main detection schemes exist, indirect and direct conversion as described in Fig. 19. In the indirect conversion the incoming X-ray photon is first transformed to lower energy light photons. One X-ray photon may produce a large amount of light photons. These light photons are then captured by the sensing pixels and converted to an electric charge which can be digitized and read by a computer. Information from a large amount of pixels builds up the digital X-ray image. In the direct conversion detection scheme the incoming X-ray photon produces electric charge directly in the detection pixel with-out the intermediate conversion of the X-ray energy to light energy. Fig. 20 illustrates the function of a direct conversion detector pixel. An incoming X-ray photon creates electron-hole pairs in the photoconductor layer; these charges are sepa-rated by the bias voltage and a charge proportional to the absorbed X-ray photons will accumulate on the capacitor. These charged capacitors form a latent image. To facilitate the readout of the latent image, all TFTs in a row have their gates connected, whereas all TFTs in a column have their sources connected. When gate line is activated, all TFTs in that row are turned on and N data lines (from j = 1 to N) read the charge on the pixel electrodes in the row i. The parallel data are multiplexed into serial data, digitized and

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fed into a computer. Next the scanning control activates the next row i + 1 and so on until the whole pixel matrix has been fed into the computer for imaging (Kasap 2002). Fig. 21 shows a picture of a flat panel detector used for X-ray pipe inspection together with a standard high power X-ray tube. This detection system is made by Varian. This detector was employed to replace a bulky image intensifier. The technical data are given in Tab. 7 (Liessem et al. 2005).

Figure 19. Different X-ray detection schemes. To the left a scintillator produces light which in turn is captured by a CCD-array. In the middle the scintillator light is captured by a TFT-array. To the right the X-rays directly produce charge in the photoconductor and this charge is captured by a TFT-array (Kotter & Langer 2002).

Figure 20. The principle of a direct-conversion X-ray detection pixel. The X-ray photons create electron-hole pairs and a charge proportional to the absorbed X-ray will accumu-late on the capacitor (Kasap & Rowlands 2002).

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Figure 21. A flat panel detector with control unit and power supply to the right. The number of pixels is 1920 ×1536 (Liessem et al. 2005)

Table 7. Some technical data for the detector shown in picture 21 (Liessem et al. 2005)

Some authors seem to think that flat panel detectors cannot survive when operating in the megavolt X-ray flux from an accelerator. However, the amorphous silicon detector (THALES) shown in Fig. 22 is said to operate together with a 9 MeV Linac (Perry et al. 2004). The report by Perry et al. also presents pictures taken by the detector and accel-erator system. In Fig. 23 we see a more traditional approach to the problem of detecting X-ray photons in the megavolt energy range. The X-ray image is formed of a large field scintillator screen and the picture is captured through a mirror using a CCD-detector. No potentially radiation vulnerable parts are in primary X-ray beam (Moulin et al. 2005). This detection concept is used together with an 8 MeV accelerator to inspect nuclear waste drums. The dose rate one metre from the accelerator is said to be 0.3 Gy/s.

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Figure 22. A THALES 14-bit amorphous silicon flat panel detector used together with a 9 MeV Linac. The dimensions of the detector are 12 × 16 inch (Perry et al. 2004).

Figure 23. A more traditional detection system designed to be used with an 8 MeV ac-celerator. The large field screen is a scintillator and the shielded camera captures the X-ray picture through the mirror (Moulin et al. 2005).

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Linear array detectors have already been mentioned, for example as used with the 12 MeV Linac shown in Fig. 16. In the following a linear array detector developed at the University of Bologna is presented (Casali et al. 2003). The main components are an intensified digital Electron Bombarded CCD (EBCCD) and seven optical fibre bundles (Fig. 24). The EBCCD unit was produced by the Geosphaera Research Center of Mos-cow. An EBCCD is a vacuum tube with a high potential applied (5 to 15 keV) between the photocathode and the anode, where the back-thinned CCD is positioned. The accel-erated electrons therefore hit the CCD directly without any intermediate conversion and a high gain is obtained due to the applied voltage (a gain of up to 2000 at 15 kV).

Figure 24. A linear array detector. From left the receiving CCD, the superimposed op-tical fibre bundles, the aligned fibre bundles forming the linear array, the EBCCD, the object, the pre-collimator and rightmost the radiation source (Casali et al. 2003). Seven optical fibre bundles (18.4 mm × 1.45 mm) form an important part of the detec-tor. At the EBCCD end the bundles are aligned to form a linear array (128.8 mm long and 1.45 mm high), which collect light produced due to electron bombardment from a thin strip on gadolinium-oxysulfide (GOS). The fibre bundles are superimposed on the output side in order to make it possible to project the linear image on a 1025 × 512 pixel CCD. This linear detector is said to be able to do Digital Radiography (DR) or Com-puted Tomography (CT) using a dose equal to about 1/100 of those usually needed. Casali et al. (2003) further state that because all electronics are outside the beam no damage is caused by radiation. They therefore claim that the detector can be used with high energy X-rays or with neutron beams. There is, however, no evidence in the men-tioned article of the detector performance in MeV energy X-ray beams or neutron beams.

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6 MODELLING OF X-RAY INSPECTIONS

During the last decades various computer codes for simulating radiography have emerged. Most of these simulators come with some common features. There seems to be an agreement on using CAD (Computer Aided Design) to model the test object and the flaws in it. One popular feature is the utilization of user-friendly interfaces to enable ordinary users to perform radiography simulations. The generation of a simulated X-ray image is a common feature of all simulation codes. Most of the simulators use a poly-chromatic primary beam and they use various approaches in generating the photon beam energy spectrums. One thing that is not very uniform among the simulators is the treat-ment of photon-object interaction physics. Both stochastic and deterministic methods are employed. A majority of the simulation efforts is based on first-order approxima-tions and ignores scattering. However, with materials examined in X-ray inspections the flux of scattered photons is usually larger than the uncollided photon flux component. Therefore, anytime scattering is neglected, total real photon flux incident upon the de-tector will be significantly different than the modelled flux (Inanc 2002).

6.1 Modelling principles

Modelling may be useful in creating an optimal X-ray inspection setup for the final de-pository canister weld. However, a useful modelling code should account for shattering effects; also pair production, as this will be the dominating shattering mechanisms for photons with energies above a few MeV. A useful modelling package includes four main steps:

� A module for generating the radiation spectrum from a linear accelerator. For an example spectrum see Fig. 25.

� Software for modelling the geometry of the object and typical defects in the ob-ject.

� Software for modelling the interaction of the incident high energy X-ray radia-tion with the object (including scattering effects). In the canister case it may be desirable to be able to include the effect of the background radiation from the fuel.

� A module for generating simulated X-ray images of different model defects typical to EBW and FSW produced welds. A key issue in this is the modelling of a non-ideal X-ray detector working high energy range.

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Figure 25. Calculated X-ray photon spectra produced by electrons with 5, 7.5 and 10 MeV energies (Kerluke 2002). The exact form of the spectrum depends on target mate-rial and target thickness. A thick test object will attenuate the lower energy more and move the top of the spectrum to the right.

6.2 Available modelling software packages

The following list of X-ray modelling software is based on the article by Inanc (2002) if not otherwise stated. XRSIM This is one of the oldest radiography simulation codes and it can be traced back to a project funded by NIST in USA. It has a sophisticated graphical user interface that can display simulated images. It lets the user to configure many parameters as done in actual implementations. Film and tube parameters are among these. It can generate primary photon beam energy spectrum. CAD files can be utilized for creating a geometric model of the object to be tested. The object may consist of materials with different attenuation coefficients. A new version which includes scattering is available. For more information see the XRSIM user’s manual on the Internet. The BAM simulation code BAM in Germany has also a group involved in the development of radiography simula-tion codes. Their code also permits the user to enter various parameters and to use CAD files for object representation. The scattering effect is included in terms of build-up fac-tors. For more information see Tillack et al. (2000).

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The CNDRI simulation code As with the two first models the CNDRI group also uses first-order deterministic ap-proximation to compute the photon flux incident on the detector. Scattering is neglected. They use a linear detector that can be oriented arbitrarily. The detector is assumed to be an ideal detector that absorbs all incoming photons. The source spectrum is obtained from experimentally measured databases. The interaction with the object is computed as function of energy. Some popular CAD object representations can be used. This code can also provide simulated data for tomographic reconstruction. RADICAD The RADICAD code has been developed out of a European collective project that in-volved British Aerospace and LETI-CEA-Technologies in France. CIVA CIVA is a French NDT-modelling code containing modules for modelling of Eddy cur-rent-, ultrasonic- and radiographic testing developed by CEA. The radiography module is quite new and better suitable for modelling of “ordinary” radiographic testing than for modelling of high energy accelerator based radiography. As a summary it can be concluded that most existing modelling packages for radio-graphic testing are intended for modelling of ordinary X-ray tube radiography or for modelling of inspection using isotopes and therefore not yet directly suitable for model-ling of accelerator based radiography.

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7 X-RAY INSPECTION GUIDELINES AND STANDARDS

X-ray film has been used in industry for more than 100 years. Industrial X-ray films are double coated and they are mainly used with metal (usually lead) intensifying screens. These films give a considerably better image quality than medical films, but a 10 to 100 times higher dose rate is needed. NDT films are usually exposed to an optical density (D) between 2 and 4, which is about two times the value used in medical applications. The application of industrial film radiography is well regulated in different standards (EN 584-1, E 1815, K 7627, ISO 11699-1, se for example Tab. 6). However, new digital X-ray systems are mainly developed for medical applications where the X-ray energy is less than 250 keV. Digital detectors are attractive in industry because of their higher dy-namic and shorter exposure time as compared with NDT film. The exposure time may be reduced to 5 to 25 % of that for film. Further, image processing software can readily be applied because the image is in digital form from the very beginning. As these digital detection systems are employed in industrial radiography the inspection result has to be comparable to the demands given in standards for industrial film radiography (Ewert et al. 2005). A procedure for comparing the image quality of industrial film radiography and digital radiography is therefore needed. Without such a procedure there is a risk of a serious drop in the probability of flaw detection. In the following we briefly discuss the standard development for digital radiography and finally we concentrate on some fun-damental aspects in comparing the quality of film and digital X-ray images.

7.1 The development of standards for digital radiography

New standards for digital radiography are developed in several countries. Pioneer work was performed in USA (ASTM E2007-00, ASTM E2033-99) and Europe (EN 14096-2). The European standard EN 14096-2 defines a set of minimum requirements for the application of computed radiography (CR) to make the digital technique comparable with the film radiography as carried out according to the standard EN 444. The basic idea of the new standard EN 14096-2 is the application of two image quality indicators (IQI). These are the wire (or step hole) indicator for the wall thickness contrast (EN 462-1 to 4) and the Duplex wire IQI (EN 462-5) for the measurement of the spatial reso-lution. According to EN 14096-1 the system shall be characterised by signal-to-noise ratio (SNR) determinations as a measurement of contrast resolution and measurements of the best spatial resolution via modulation transfer function (MTF) or special IQIs. These standards have the goal of making CR comparable with film radiography (Ewert, 2002). The USA standard ASTM E 1025 define hole-type IQIs for X-ray and gamma ray radiology (dimensions and materials), but without reference to other detection sys-tems than film. As seen above, standards for digital radiography exist; however, most of them seams explicitly to refer to the use of CR (Computed Radiography, i.e. imaging plates utilizing storage phosphors) or film digitalizing; no references to flat panel or lin-ear detectors was found. A reason for this is that CR works in many aspects like film, while new digital detectors bring in many new aspects. According to Deprins et al. (2005) there is a need for a full slate of standards that deal with the same issues that film standards have dealt with, from acquiring and processing images, to assuring adequate image quality to dealing with new issues such as bad pixels and other artefacts.

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7.2 Definitions

In the following physical definitions of some basic concepts from both film and digital radiography are given. Optical density D The optical density D is a measure of the energy absorbed (i.e. dose, K) by the radiation sensitive emulsion of the film. The optical density is given by:

DIID 0lg� .

The above equation states that the optical density D is the “logarithmic value to the base 10 of the diffuse light intensity in front (I0) and behind (ID) the radiographic film” (EN 14096). Based on the above, the optical density D can be said to be a number, such that if the light intensity behind the film in I0, then the transmitted light intensity is:

DD II �� 100 .

According to for example EN 444 the exposure conditions should be such that the total density of the radiograph (including base and fog density) in the inspected area is greater than or equal to the following values:

� Film system class A: D � 2.0 � Film system class B: D � 2.3

A measuring tolerance of ±0.1 is permitted for optical density D. Ewert et al. (2005) that NDT film is usually exposed to optical densities between 2 and 4. Characteristic curve A Characteristic curve is a curve, which expresses film density (D) as function of the logarithm (base 10) of the relative exposure (Glossary of NDT Terms, 2006). Gradient G The change in density (D) of a radiographic film at a particular film density per unit change in the logarithm of the exposure received (dose K) by the film (Glossary of NDT Terms, 2006). The standard EN 584-1 define G as:

dKdD

eKG ��

10log

where K is the dose in Gray required for the density change (D – D0). Here D0 is the op-tical density of an unexposed and processed film including base (fog and base density).

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Granularity �D

Granularity �D is defined (EN 584-1) as a stochastic density fluctuation in the radio-graph which is superimposed on the image of the object. The granularity is determined by linear scanning of a film of constant diffuse optical density with a microdensitome-ter. Both emulsion layers shall be recorded. The optical density of the film shall be 2.00 ± 0.05 above fog and base. The diameter of the aperture of the microdensitometer shall be (100 ± 5) μm. Speed S The speed S (CEN) is the reciprocal value of the dose K measured in Gray (EN 584-1):

KS 1�

The CEN speed S is evaluated for diffuse optical density D = 2 above fog and base D0. The CEN speed shall be determined in accordance with tabulated values in the standard EN 584-1. The pixel size The pixel size (pixel-pitch) is the centre-to-centre distance between adjacent pixels in a row or a column (EN 14096-2). Projection magnification and geometrical unsharpness These two concepts are closely related as can be seen from Fig. 26. The projection magnification is simply caused by the diverging rays from the source. The geometric unsharpness on the other hand is caused by the size if the focal spot (target) in the radia-tion source. In theory the optimal projection magnification should occur when the value of geometrical unsharpness is equal to the pixel-pitch of the detector as shown in Fig. 26. It should be noted that a similar condition occur when using traditional film. In the case of film the geometric unsharpness is set equal to the film unsharpness (Blakeley & Spartiotis 2006). According to Fig. 26 we have the following relation between focal spot size f, the pixel-pitch p, the source to detector distance b and the source to object distance a:

fpbfa�

�

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Signal-to-noise-ratio for film Film systems are characterised by the gradient G and the granularity �D. G is deter-mined at D = 2 (giving G2) and at D = 4 (giving G4) and �D is determined at D = 2 (both above fog and base). The most important parameter for the perception of fine flaws is the gradient to granularity ratio G/�D because it can be used to calculate the signal-to-noise ratio (SNR). Relative contrast.

This is the ratio of the intensity difference of two adjacent image areas representing dif-ferent material thickness and the mean of these intensities.

Relative noise.

The noise is measured as the ratio of the standard deviation in an area of homogeneous exposure in the image and the mean of these intensities.

Figure 26. A schematic illustration of the origin of geometric unsharpness (due to the focal spot size f) and projection magnification. In theory, the optimum value of projec-tion magnification should occur when the value of geometric unsharpness is equal to the pixel- pitch of the detector (Blakeley & Spartiotis 2006).

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Contrast sensitivity (CS).

CS describes the smallest difference in material thickness that can be seen in the image. Contrast sensitivity is determined by the signal-to-noise-ratio and the influence of the scattered radiation. CS can be given in percent of the wall thickness.

Signal-to-noise-ratio (SNR).

SNR is the inverse of relative noise. Since it is independent of the object under test, it is suitable for the general characteristic of a DDA. In film radiography the granularity de-termines the noise level (see above).

Contrast to noise ratio (CNR).

CNR is the quotient of contrast and noise. It is also the quotient of relative contrast and relative noise and describes the detection of material thickness differences, see Fig. 27.

Gray level resolution.

This is the number of available gray levels for image viewing, printing and interpreta-tion. The human eye can distinguish between 60 and 100 gray levels. Radiographic im-ages have typically 1000 to 60000 gray levels, which require image processing to adapt to a human operator.

Figure 27. An example of the maximum radiographic contrast as a function of the defect size penetrated by the X-rays (Müller et al. 2006).

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8 DISCUSSION AND CONCLUSIONS

For accelerator based X-ray examination of the sealing weld in the final deposition can-ister a collimated digital detector is assumed to be needed. Flat panel detectors are ex-pected to the too sensitive to scattered radiation and to the background radiation from the spent fuel inside the canister. The pros and cons for scintillation detectors and direct conversion detectors (especially GaAs-based) need to be evaluated. It is also assumed that stereoscopic radiography will improve the defect resolution in the X-ray direction and help to determine the location of the defect in the mentioned direction. The poten-tial of high energy stereoscopic radiography should therefore be investigated.

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XRSIM Users manual. Internet address (2006-03-06): http://www.ndt-ed.org/TeachingResources/Downloadable/XRSIM/XRSIM_Users_Manual_499.PDF