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Stefan Sand l i n

November 2010

Work ing Repor t 2009 -84

Defect Detectability in the Disposal CanisterLid-Weld Using the 9 MeV Linear Accelerator

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 -84

Defect Detectability in the Disposal CanisterLid-Weld Using the 9 MeV Linear Accelerator

Defect Detectability in the Disposal Canister Lid-Weld Using the 9 MeV Linear Accelerator ABSTRACT The spent nuclear fuel will be placed in specially designed canisters, which consists of a cast iron insert with cannels for the fuel bundles and an outer corrosion barrier of copper with a wall thickness of 50 mm. The copper wall should meet the acceptance criteria regarding soundness of the weld. The canister to lid weld is a critical part in this aspect and it has to be inspected by non-destructive methods. The objective of this work was to investigate the defect detectability in the lid-to-canister weld using the high energy X-ray system at SKB (Svenska Kärnbränslehantering AB) in Oskarshamn in Sweden. The system is based on a digital X-ray line detector, a 9 MV linear accelerator and a mecha-nism for rotating the canister. A test object consisting of a 70 degree sector of the elec-tron beam (EB) welded lid-to-canister joint was used in the measurements. The test ob-ject has been equipped with several kinds of artificial defects, mainly in the weld asea. The smallest artificial defects are a groove of 0.5 mm cross-section extending 50 mm from the top of the weld to the root, a 1 × 1 × 1 mm3 cubic defect in the root and a flat-bottom bore hole of diameter 1 mm and depth of 1 mm also in the root of the weld. These small defects are clarely observed in the X-ray pictures taken during the test pro-gram. Further, image quality indicators (IQI) of wire-, duplex wire-, hole- and step-hole types were used to compare the inspection results with European standards. The mini-mum requirements set by the standards was fulfilled, however, there are no minimum image quality values given for copper, so the corresponding values for steel were used (as recommended by the standard). The X-Rays project a picture of the weld on the line detector through a varying thickness of copper, therefore there is a risk that small de-fects may be unobserved if they happen to be situated in the shadow of a sharp thickness change of the copper wall. The presented results are, however, very encouraging and with some improvements in the detector-accelerator setup and in the image processing software are even better results than those presented in this work can be achieved. Keywords: Digital radiography, X-ray detector, high energy X-ray, final deposition, spent nuclear fuel, copper canister, image quality indicator, defect detectability, linear accelerator

Ydinjätekapselin sulkuhitsin vikojen havaittavuus käyttäen 9 MeV lineaarikiihdytintä TIIVISTELMÄ Ydinjätekapseli koostuu 50 mm paksusta kuparikuoresta ja valurautaisesta sisäosasta, jossa on kanavia polttoainenippuja varten. Kuparikuori toimii korroosiosuojana ja pitää täyttää hyväksymiskriteerit koskien hitsin virheettömyyttä. Kapselin sulkuhitsi on tässä mielessä kriittinen osa. Hitsin eheyttä on tästä syystä tutkittava ainetta rikkomattomilla menetelmillä. Tämän työn tarkoituksena on tutkia sulkuhitsin pienten vikojen havaitta-vuutta korkean energian radiografialla SKB:n Oskarshamnissa sijaitsevalla kiihdytin-pohjaisella laitteistolla. Laitteiston perustana on 9 MV:n lineaarikiihdytin (Varian), kol-limaattorilla varustettu digitaalinen viivadetektori sekä kapselia pyörittävä mekanismi. Mittauksissa käytettiin referenssikappaletta, joka on todellisesta putki-kansiliitoksesta leikattu 70 asteen sektori. Referenssikappaleeseen on tehty joukko erilaisia keinovikoja, pääosin sulkuhitsiin. Pienimmistä keinovioista voidaan mainita hitsin läpi pystysuun-nassa kulkeva 0,5 mm levyinen ura, 1 × 1 × 1 mm3 mittainen kuutio hitsin juuressa sekä porausreikä halkaisijalla 1 mm ja syvyydellä 1 mm, sekin hitsin juuressa. Kaikki pienet-kin keinoviat havaittiin selvästi koekuvauksissa. Kuvauksissa käytettiin myös kuvan laadun indikaattoreita (IQI), jotta kuvaustulosta voitiin verrata eurooppalaisiin radiogra-fiastandardeihin. Standardeissa mainitut minimivaatimukset täyttyivät sillä poikkeuksel-la, että standardeissa ei anneta minimiarvoja kuparille; tästä syystä käytettiin teräksen minimiarvoja, kuten standardit suosittelevat. Koska kappaleen paksuus vaihtelee paljon hitsin pystysuunnassa, on olemassa vaara, että pieni vika jää huomaamatta etenkin jos se sattuu olemaan kohdassa, missä kuparin paksuus muuttuu jyrkästi. Tässä työssä saadut tulokset ovat kuitenkin hyvin rohkaisevia pientenkin vikojen havaittavuuteen nähden. Etenkin jos detektori ja kiihdytinkokoonpanoa sekä kuvankäsittelyä optimoidaan pääs-tään vielä parempiin tuloksiin kuin tässä työssä. Avainsanat: Digitaalinen radiografia, röntgendetektori, korkeaenergiaradiografia, lop-pusijoitus, käytetty polttoaine, kuparikapseli, kuvan laadun indikaattori, IQI, vian ha-vaittavuus, lineaarikiihdytin.

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TABLE OF CONTENTS

ABSTRACT

TIIVISTELMÄ

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

2 GOAL ........................................................................................................................ 3

3 THE HIGH ENERGY X-RAY MEASUREMENTS AT SKB ........................................ 4

3.1 Technical specifications of the accelerator ....................................................... 4 3.2 The detector ...................................................................................................... 6 3.3 The inspected reference object ........................................................................ 7 3.4 The artificial defects .......................................................................................... 9 3.5 The used image quality indicators .................................................................. 10

4 ANALYSIS AND RESULTS .................................................................................... 12

4.1 Detectability of the artificial defects ................................................................ 13 4.1.1 The four 50 mm long notches through the dept of the weld ............... 13 4.1.2 The artificial J- and U-shaped defects 2 and 9. .................................. 14 4.1.3 The artificial defect 3, four notches at the top of the weld .................. 16 4.1.4 The artificial defect group 4, the quadratic defects ............................. 16 4.1.5 The artificial defect group 7, cylindrical holes ..................................... 16 4.1.6 The artificial defect group 8, notches in the middle of the weld .......... 17

4.2 Determination of image quality values ............................................................ 18 4.2.1 Wire type IQI, EN 462 W6 Cu 50 ........................................................ 18 4.2.2 Basic un-sharpness from duplex wire measurements ........................ 20 4.2.3 The step-hole IQI ................................................................................ 22 4.2.4 Hole IQI or ASME penetrameter ......................................................... 24

4.3 System performance ....................................................................................... 26 4.3.1 Detectability of small defects .............................................................. 26 4.3.2 Comparability of image quality to standards ....................................... 27

5 ALTERNATE X-RAY INSPECTION SETUPS ......................................................... 29

5.1 Digital X-ray detectors .................................................................................... 29 5.2 The Gallium-Arsenide based linear detactor................................................... 30

6 CONCLUSIONS ...................................................................................................... 31

REFERENCES ............................................................................................................. 32

APPENDICES ............................................................................................................... 33

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1 INTRODUCTION

The canister for spent nuclear fuel consists of an outer copper cylinder (wall thickness 50 mm) with an insert made of cast iron with cannels in which the spent fuel bundles are inserted. The copper cylinder acts as a corrosion barrier while the iron insert gives the construction its mechanical strength. In this work the defect detectability in the lid to canister electron beam (EB) weld is investigated using the SKB (Svenska Kärnbränsle-hantering AB) X-ray system. This system consists of a 9 MeV linear accelerator manu-factured by Varian and a linear collimated digital detector provided by BIR (Bio Imag-ing Research Inc). This work contains a brief presentation of the technical properties of the accelerator and the detector. The test object used for this investigation is a sector of an EB-welded lid to canister joint designed by Posiva Oy. The test object is equipped with ten groups of artificial de-fects at different locations in or close to the EB-weld. As special problem in radio-graphic investigation of the lid to canister weld (as well as the test object) is the varying thickness of the copper. This is compensated for by a special software module. A total of 16 digital X-ray pictures were taken of weld area at four different beam angles rela-tive to EB-weld. Even the smallest artificial defects were visible in most of the X-ray pictures. For example, a notch of diameter 0.5 mm and extending from the top to the root of the weld (50 mm) is visible in most of the radiographs. Further, wire-, duplex wire-, hole- and step-hole-type image quality indicators were used to compare the in-spection results with European standards. There are no minimum required image quality values for copper in the standard, but use of those given for steel is recommended. To this background the requirements of the standards are fulfilled. The results of the system evaluation are thus very encouraging. However, some alternative detector designs that possibly could improve the perform-ance of the high energy X-ray system are proposed. The spatial resolution could proba-bly be increased by using a direct conversion line detector instead of the present scintil-lator detector. Further, increase in spatial resolution could be achieved by reducing the pixel pitch from the present 400 �m to 200 �m. This new direct conversion detector could be based on Gallium Arsenide (GaAs) as this material has good radiation endur-ance in the MeV energy range. The GaAs detector will probably also be more sensitive than the present scintillation detector, thus allowing the reduction of the pixel size with-out prolonging the examination time.

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2 GOAL

The goal of this work is to evaluate the performance of the high energy accelerator and detector system at SKB in Oskarshamn in Sweden as applied to an EB-welded reference specimen designed by Posiva Oy. The evaluation is based on the detectability of refer-ence flaws machined into the reference specimen and on the use of different image qual-ity indicators (IQI). The use of the IQIs provides a method for comparing the results to standards. Further, the work provides a short overview of alternative detector concepts based on a direct conversion Gallium Arsenide detector.

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3 THE HIGH ENERGY X-RAY MEASUREMENTS AT SKB

The high energy X-ray equipment used by SKB in Oskarshamn for pilot examination of the lid to canister weld consists of a 9 MeV linear accelerator (Varian Linatron 3000), a collimated line detector and a manipulator system. The manipulator system is used to rotate the canister with a constant angular velocity of 360 degrees per hour during the X-ray inspection. Further, the angle of the central X-ray beam relative to the canister lid can be pre-set (in this case angles of 5, 10, 20 and 30 degrees were used). Fig. 1 shows a picture of the high energy X-ray equipment at SKB. The accelerator can be seen in up-per right part of the picture while the gray detector supported by a black steel construc-tion can be seen in the lower left. The large copper cylinder is a final deposition canister which in this case is used as a turn table for the specimen placed on top of the canister. Both the accelerator and the detector are attached to the blue steel frame which can be used to adjust the beam angle relative to the EB-weld. In this work an EB-weld segment (XK002 B665) designed by Posiva Oy was inspected. In this chapter the technical specifications of the accelerator and the detector are briefly described. Software for data acquisition, image processing, control of the accelerator and the manipulator movement is also of great importance, but a description of these is beyond the scope of this report.

Figure 1. The test specimen (XK002 665) is on the top of a rotating canister used as a turn table during the X-ray examination. Both the accelerator (upper right) and the de-tector (the gray part at the lower left side) are attached to the blue steel frame by which the beam angle relative to the weld in the specimen can be adjusted.

3.1 Technical specifications of the accelerator

The acceleration voltage of the Varian Linatron 3000 is 9 MV. This means that the elec-trons hitting the target to produce X-rays have a kinetic energy of 9 MeV, however, only very few of the electrons loose “all” their kinetic energy in a single collision with a target atom, producing a 9 MeV X-ray photon. Instead, most electrons undergo multiple collisions with the target atoms and thus produce several X-ray photons of considerable

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energy lower than 9 MeV. According to an estimation presented in Müller et al. (2006) the intensity spectrum of the X-rays leaving the accelerator will have the form shown in Fig. 2. As can be seen the maximum intensity of the X-ray photons is found around 3 MeV. The X-ray radiation leaving the accelerator is pulsed and the dose rate can be adjusted by selecting an appropriate pulse repetition frequency (PRF). The maximum dose rate achievable by this accelerator is 30 Gy/min at about one meter from the accelerator. In the measurements presented in this report a pulse PRF of 250 s-1 was used for a time of 400 ms i.e. 100 pulses was delivered by the accelerator during 400 ms. After that the accelerator rests for 10 ms and after that the process is repeated. This means that 100 pulses, or equivalently, 400 ms is used for the exposure of one picture element. The pic-ture element is read out from the detector during the 10 ms before the accelerator deliv-ers the following 100 pulses for the next picture element (Ronneteg 2006).

Figure 2. The estimated form of the X-ray spectrum leaving the Varian Linatron 3000 accelerator target (Müller et al. 2006).

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Figure 3. The result of an experimental determination of the target size for the Linatron 3000 accelerator using the pin-hole method. The results are 2.6 × 2.4 mm (Müller et al. 2006). The X-ray beam leaving the accelerator is collimated by a 30 mm high and 3 mm wide slit. As the target is about 520 mm from the collimator the opening angle of the fan-shaped beam in the vertical plane is about 3 degrees. Another important parameter of the accelerator is the target size. In Müller et al. the target size has been measured using the pin-hole (EN 12543-2) method. The results for the target size are 2.4 mm in the ver-tical direction and 2.6 mm in the horizontal direction (se Fig. 3).

3.2 The detector

As mentioned above the detector is a collimated line detector. The detector was deliv-ered by Bio-Imaging Research Inc (BIR). The slit of the collimator has a width of 0.4 mm in the horizontal direction. The scintillator elements of the detector have a total length of about 100 mm in the direction parallel to the slit and the detector array is composed of 2048 lines perpendicular to the slit. Each line has width of 0.05 mm. As 8 lines are used to form one pixel we get a pixel size of 0.4 mm in the direction of the slit. In the perpendicular direction the pixel size is determined by the width of the slit which is also 0.4 mm as already stated. When the fan-shaped X-ray beams from the accelerator has penetrated the test specimen and enters the detector in front of the scintillator ele-ments scattered radiation from the object and the surrounding will effectively be hin-dered from hitting the detector elements. Less than 3 % of the scattered radiation will reach the detector elements (Zscherpel 2006). The scintillator material in the detector elements is CdWO4 and the elements have a thickness of 5 mm to effectively convert the incoming X-ray rays to light. This light is further converted to electrical signals us-ing an array of photodiodes. A picture of the detector, or strictly speaking, the detector housing and the massive collimator plates can be seen in Fig. 4.

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Figure 4. A picture of the detector housing and the collimator blocks in front of it. The collimator slit goes in the vertical direction. The X-rays enters through the tests object to the left.

3.3 The inspected reference object

A photo of the reference specimen can be seen in Fig. 5. Figs. 6 and 7 show drawings of the specimen, a top view and a cross-section respectively. To be able to machine artifi-cial defects into the weld, the ends of the specimen were cut of and thereafter the re-maining part was cut in two along the EB-weld as indicated in Fig. 7. The specimen was re-assembled after the artificial defects had been machined into it.

Figure 5. The test specimen used in this work. See also the drawings in figs. 6 and 7.

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Figure 6. A top-view drawing of the test specimen with the dimensions given in millime-tres and degrees. The middle of the EB-weld is situated at the diametrical position 954.8 mm and the depth of the weld is about 50 mm in the direction perpendicular to the paper. To manufacture artificial defects into the weld the ends were cut off and after that the specimen was cut into two pieces along the weld (again perpendicular to the paper). The specimen was re-assembled after the defects had been machined into it.

Figure 7. A drawing of the lid to canister joint. The EB-weld area is marked by an el-lipse. Dimensions are in millimeters.

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3.4 The artificial defects

Figure 8 shows an X-ray picture taken of the test object using the above described X-ray system. The artificial defects machined test object can be seen in this picture. As men-tioned above the test object was cut along the weld to make it possible to machine de-fects into the weld. The picture is a projection of the weld region in such a way that the top of the weld is projected to the left while the root is projected a bit to the right of the middle of the picture (defects 4 and 7 are machined into the root). Defects 1, 2, 9 and 10 are situated in the end pieces (see Fig. 6). The artificial defects are numbered from 1 to 10 and described below:

1. Notches machined into the middle of the weld extending from the top to the root. The depths of the notches are 50 mm. The “diameters” of the notches are from above: 0.5 mm, 1.0 mm, 1.5 mm, and 2.0 mm.

2. Two J-shaped defects in the cross-sectional plan of the weld shown in Fig. 7. The J:s are directed in the depth direction of the weld with the arcs pointing in the root direction. One is 6 mm high with the arc situated 30 mm from the top of the weld. The other is situated with the arc in the root and has a height of 12 mm. The height is measured in the weld depth direction. The “depth” of the J-shaped defects, i.e. their extension in the circumferential direction of the weld is also 6 respectively 12 mm.

3. Four notches machined into the top of the weld (parallel to the circumferential direction of the weld).

4. Three groups of quadratic defects each containing four defects of dimensions 10 × 10 mm, 5 × 5 mm, 3 × 3 mm and 1 × 1 mm. The depths of the defects in each group are from above: 4 mm, 2 mm and 1 mm. In this case the depth means the radial direction of the canister.

5. A cylindrical bore hole next to the weld with a diameter of 3 mm and extending to a depth of 30 mm. This hole is on the canister side of the weld and parallel to the axis of the canister.

6. A cylindrical bore hole next to the weld with a diameter of 3 mm and extending to a depth of 30 mm. This hole is on the lid side of the weld and parallel to the axis of the canister.

7. Three groups if cylindrical defects each containing four defects with diameters 8 mm, 4 mm, 2 mm and 1 mm. The depths of the defects in each group are from above: 4 mm, 2 mm and 1 mm. In this case the depth means the radial direction of the canister.

8. Three groups of notches, all with a width of 1 mm in the middle (depth direc-tion) of the weld. Each group contains three notches with depths from above: 4 mm, 2 mm and 1 mm. The lengths (in the depth direction of the weld) of the notches in each group are from above: 12 mm, 6 mm and 3 mm.

9. Two U-shaped defects in the cross-sectional plan of the weld shown in Fig. 7. The U:s are directed in the depth direction of the weld with the arcs pointing in the root direction. One is 6 mm high and situated about 30 mm from the top of the weld. The other is situated with the arc in the root and has a height of 12 mm. The height is measure in the weld dept direction. The “depth” of the U:s, i.e. the extension in the circumferential direction is 12 mm for both defects.

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10. Four notches machined into the top of the weld (perpendicular to the circumfer-ential direction of the weld).

3.5 The used image quality indicators

The image quality indicators (IQI) used in this work was wire type (EN 462 W1 Cu 25, EN 462 W6 Cu 25, EN 462 W6 Cu 50), duplex wire type (EN 462-5), hole type and step-hole type indicators (ASME). The duplex wire IQI is used to measure image un-sharpness while the others are used to measure image contrast.

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Figure 8. An X-ray picture of the test object showing the groups 1 to 10 of artificial de-fects. See the text for a description of the defects. In this picture the top of the weld is to the left (defects 3 and 10 are surface defects). The rightmost parts of defects 4 and 7 are in the root of the weld.

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4 ANALYSIS AND RESULTS

The measurements were carried out using four different angles for the central beam relative to the plan of the canister lid. These were 5, 10, 20 and 30 degrees. The follow-ing analyses concentrate mainly on the results for the angle of 10. Appendices 1 to 5 show all the 20 X-ray pictures taken during the measurements. There are four pictures for each angle corresponding to different locations of the image quality indicators (IQI). Some conclusions can be drawn directly from the pictures, however, in order to avoid subjective interpretations gray value profiles were plotted over all artificial defects and over the wires or holes of the IQIs. Due to the large amount of profile plots only a part of them can be reproduced here. A computer software called ImageJ was used for image processing and for plotting the gray value profiles. Fig. 9 shows an example of how the data were selected for plotting the gray level profiles. The profiles are plotted in the length direction to the rectangles and each profile is an average of all individual line profiles included in the vertical (the word “vertical” refers only to the way the X-ray picture is presented in Fig. 9) size of the rectangles; i.e. the number of averaged profiles is equal to the number of pixels in the vertical direction in each rectangle in Fig. 9. In the following we analyze the detectability of the artificial defects described in section 3.4 above using the numbering given in Fig. 8. If a peak or dip in the gray level profile resulting from an artificial defect or an IQI wire or hole is small, a signal to noise ratio (SNR) evaluation is needed according to EN 13068-1 in order to see if the indication can be regarded as detected. According to EN 13068-1 the SNR shall be larger or equal to 2 for a detected indication. The SNR is defined as:

S

SSNR��

� (1)

where �S is the amplitude of the peak or dip from the mean of the background, while �S is the standard deviation of the background signal (next to the peak or dip). Here the signal S refere to the gray level values. The software ImageJ was used to determine �S and �S and hence the SNR..

Figure 9. Some examples of data selection for plotting the gray level profiles. For ex-ample for the square defects the width (here height) of the rectangle was selected ac-cording to the smallest (1 × 1 mm) rectangle. For the J- and U-shaped defects is the end parts of the test object (not shown here) the rectangles were drawn in the perpen-dicular direction compared to those shown here.

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4.1 Detectability of the artificial defects

In this section we investigate the detectability of the artificial defects machined into the test object. We concentrate on the smallest defects as the larger ones are clearly visible in all radiographs (see Appendices 1 to 4). The most interesting small defects (see Fig. 8) are the 0.5 mm notch (length 50 mm) in area 1, the 1 × 1 × 1 mm square hole in area 4 and the 1 mm diameter cylindrical hole with depth of 1 mm in area 7. If a defect is close to the noise level in the gray level plot a SNR analysis according to Eq. 1 is made.

4.1.1 The four 50 mm long notches through the dept of the weld

Here the results from the using the X-ray angle 10 degrees is presented. As can be seen from Appendices 1 to 4 the four notches can be observed for all X-ray angles. In the fol-lowing a more detailed analysis is presented for the X-ray angle 10 degrees. In area number 1, three rectangles covering all four notches, were selected for plotting gray level profiles. The first rectangle was placed to the left in area number 1, the second in the middle and the third to the right in the mentioned area. These correspond to the top, middle and root of the weld respectively. The width of the rectangles is approximately equal to the side of the third square in the groups of shown in area 4. Figs. 10 to 12 show the profile plots from the top, middle and the root areas of the weld. The notches can be seen as dips in the gray level profiles. As can be seen all notches, even the small-est with diameter 0.5 mm, can be observed. Only for the smallest notch a signal to noise ratio (SNR) analysis is need in order to determine if the notch can be regarded as de-tected according to EN 13068-1. An ocular estimate of the profiles in Figs. 10 to 12 shows that the smallest notch in Fig. 11 has (middle of the weld) the worst SNR. A cal-culation according to Eq. 1 gives the value SNR = 2.14, i.e. this notch can be regarded as detected also in the middle of the weld.

Defect 1, top, 10 degrees, file 5007

12800129001300013100132001330013400135001360013700

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Figure 10. A gray level profile plot over the artificial defects in area 1 in Fig. 8 taken close to the top of the weld. The artificial defects diameters from the left are 0.5 mm, 1.0 mm, 1.5 mm and 2.0 mm.

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Defect 1, middle, 10 degrees, file 5007

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Figure 11. A gray level profile plot over the artificial defects in area 1 in Fig. 8 taken near the middle of the weld. The artificial defects diameters from the left are 0.5 mm, 1.0 mm, 1.5 mm and 2.0 mm. Here we get an SNR = 2.14 for the smallest notch (the leftmost dip).

Defect 1, root, 10 degrees, file 5007

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Figure 12. A gray level profile plot over the artificial defects in area 1 in Fig. 8 taken near the root of the weld. The artificial defects diameters from the left are 0.5 mm, 1.0 mm, 1.5 mm and 2.0 mm.

4.1.2 The artificial J- and U-shaped defects 2 and 9.

The profiles plotted over the two J-shaped defects are shown in Fig. 13. The first dip from the left originate from the smaller J while the next one originate from the larger J having its arc in the root of the weld. At the rightmost parts of each dip an even deeper

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part can be seen. This is due to the arc of the J:s where a maximum of material is miss-ing in the direction of the X-rays. Fig. 14 shows a similar plot for the two U-shaped de-fects. The two dips from the U-defects are deeper then the dips from the J-defects be-cause the U-defect provide an extra “wall” in the X-ray direction compared to the J-defects.

J-defects, 10 degrees, file 50021

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Figure 13. The gray level profiles over two J-shaped defects in the weld. The lowest part of the first J is at a depth of 30 mm (left dip), while the second J-defect has its arc in the root. Both dips have a deeper part to the right because a maximum of material is missing in the direction of the X-rays in the arc part of the J-defects.

U-defects, 10 degrees, file 50021

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Figure 14. The gray level profile over two U-shaped defects in the weld. To the left the smaller U-defect (the arc at about 30 mm from the top of the weld) and to the right the larger U-defect in the root of the weld. Both dips have a deeper part to the right be-cause a maximum of material is missing in the direction of the X-rays in the arc part of the U-defects.

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4.1.3 The artificial defect 3, four notches at the top of the weld

As can be seen from Appendices 1 to 4 these defects are clearly detected for all beam angles and no further analysis is needed.

4.1.4 The artificial defect group 4, the quadratic defects

As can be seen from Appendices 1 to 4 the smallest square defect can be observed for all X-ray angles. The detectability of the square formed defects using an X-ray angle of 10 degrees is presented in Fig. 15 using a gray level profile plot. Note that the duplex wire IQI is also seen in Fig. 15 from about pixel 358 to pixel 552. Only the SNR of the smallest defect (1 × 1 × 1 mm) need to be calculated according to Eq. 1 to see if the de-fect can be regarded as detected according to EN 13068-1. The SNR of the rightmost dip in Fig. 15 was found to be 2.80, i.e. the defect can be regarded as detected.

Square holes, 10 degrees, file 5003

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Figure 15. A gray level profile plot over the square defects of area 4 in Fig. 8. Note that the largest defect in the last group to the left if covered by the end of the duplex wire IQI. The SNR for the leftmost dip (1 × 1 × 1 mm) was found to be 2.80, so the smallest square defect can be regarded as detected.

4.1.5 The artificial defect group 7, cylindrical holes

As can be seen from Appendices 1 to 4 the smallest cylindrical defect can also be ob-served for all X-ray angles. The detectability of the cylindrical defects using an X-ray angle of 10 degrees is presented in Fig. 16 using a gray level profile plot. Only the SNR of the smallest (1 × 1 × 1 mm) need to be calculated according to Eq. 1 to see if the de-fect can be regarded as detected according to EN 13068-1. The SNR of the rightmost dip in Fig. 16 was found to be 1.73, i.e. the smallest defect is under the detection limit even if it can be distinguished in the X-ray pictures.

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Circular bore holes, 10 degree, file 5002

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Figure 16. A gray level profile plot over the cylindrical defects of area 7 in Fig. 8. The SNR for the leftmost dip (1 mm in diameter and 1 mm deep) was found to be 1.73, so the smallest cylindrical defect is under the standard detection limit even if it can be seen in the X-ray picture.

4.1.6 The artificial defect group 8, notches in the middle of the weld

As can be seen from Appendices 1 to 4 the smallest notch defect can also be clearly ob-served for all X-ray angles. The detectability of the notch defects using an X-ray angle of 10 degrees is presented in Fig. 17 using a gray level profile plot. Also the smallest dips in Fig. 17 are so deep that no SNR calculations are needed to ensure detectability.

Notches, 10 degrees, file 5002

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Figure 17. A gray level profile plot over notches of area 8 in Fig. 8. All notches are clearly detected with enough SNR.

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4.2 Determination of image quality values

In this section the detectability of wires and holes by using IQIs is investigated. A du-plex wire IQI is used to determine basic unsharpness while the other wire and hole type IQIs are used to measure image contrast (Halmahaw & Kowol). The detection criteria is the same as above, i.e. based on Eq. 1 and EN 13068-1. Minimum image quality values for wire and step-hole IQIs are given in Annex B of EN 1435 for different penetrated thicknesses. Further, the maximum allowed duplex wire unsharpness values and maxi-mum pixel size when using X-rays with energy over 1 MeV are given in EN 14784-2. The following analysis corresponds to a beam angle 10 degrees with the plan of the lid.

4.2.1 Wire type IQI, EN 462 W6 Cu 50

The profile plots are shown for three different positions (see X-ray pictures in Appendix 2). A SNR analysis is performed for the weakest peaks. The results are shown in Figs. 18 to 20. As can be seen wires W6, W7, and W8 are detected at all positions (left, mid-dle and right). At the middle position (Fig. 19) also wire W9 is detected just above the detection limit. In all these cases the wire IQI was placed on the detector side. In Fig. 21 the IQI was placed on the source side for comparison. As can be seen from the Figs. there is no radical difference compared to the three cases where the IQI was on the de-tector side. Note, however, the larger distance between the peaks in Fig. 21 due to the larger projection magnification.

EN 462 W6, 10 degrees, left, file 5007

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Figure 18. The IQI was at the left position on detector side (Appendix 2). Wires W6, W7 and W8 from the left are detected above the noise level. For W8 the SNR is 2.19 and for the following hardly distinguished wire W9 the SNR is 1.5 or below the detection limit.

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EN 462 W6, 10 degrees, middle, file 5007

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Figure 19. The IQI was at the middle position on detector side (Appendix 2). Wires W6, W, W8 and W9 from the left are detected above the noise level. For W8 the SNR is 3.20 and for the following hardly distinguished wire W9 the SNR is 2.16 or just above the detection limit.

EN 462 W6, 10 degrees, right, file 5007

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Figure 20. The IQI was at the right position on detector side (Appendix 2). Wires W6, W7, W8 from the left are detected above the noise level. For W8 the SNR is 2.68 and the SNR for the following hardly distinguished wire W9 is 1.75 or below the detection limit.

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EN 462 W6, 10 degrees, middle, source side, file 5008

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Figure 21. Here the IQI was placed upside down on source side (Appendix 2). Wires W6, W7 and W8 from the right are detected above the noise level. There is no radical difference compared to the three cases above where the IQI was on the detector side. Note, however, the larger distance between the peaks due to a larger projection magni-fication.

4.2.2 Basic un-sharpness from duplex wire measurements

The different placements of the duplex wire IQI can be seen in Appendix 2 for a beam angle of 10 degrees. In the first three cases the IQI is on the detector side (Figs. 22 to 24). In the fourth case (Fig. 25) the IQI is placed on the source side. In the gray level profile plots the wire pairs are seen as twin peaks. The wire pairs are considered to be resolved if the dip between the twin peaks is at least 20 % of the maximum (standard EN 14784-1). In practice the twin peaks may have different heights (not mentioned in the standard), in such cases the height of the lower peak is taken as the maximum. The maximum is measured from the mean noise level to the top of the peak. Further, we need to determine which of the twin peaks distinguished from the noise level. The weaker of the twin peaks is used also for this purpose. In Fig. 22 wire pairs 1D and 2D are clearly resolved. The line pair 3D should also be regarded as resolved as the dip is 76.5 % of the maximum, however, according to the SNR analysis the line pair 3D cannot be regarded as detected as the SNR is 1.86. Hence, the first unresolved wire pair is 3D. From Fig. 23 it can be concluded that wire pairs 1D, 2D and 3D (from the left) are clearly resolved. The wire pair 4D is also resolved as the dip is 55.2 % of the maximum; this twin peak can also be regarded as detected as the SNR is 2.23. Therefore the first unresolved wire pair is 5D. From Fig. 24 it can be concluded that the peaks from wire pairs 1D, 2D, 3D and 4D are all clearly detected. The wire pair 4D is also resolved as the dip is 34.7 % of the maxi-mum. Therefore the first unresolved wire pair is 5D.

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The profile in Fig. 25 is plotted with the duplex wire IQI on the source side. No further analysis is needed, it is clearly seen that wire pairs 1D, 2D, 3D and 4D are resolved and detected and that the first unresolved wire pair is 5D. Note that the separation between the peaks as larger in this case as compared to the three profiles where the IQI was on the detector side. This is, as before due to the larger projection magnification.

Duplex wire, 20 mm, 10 degrees, file 5007

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Figure 22. The IQI was at position 20 mm on detector side (Appendix 2). Wire pairs 1D and 2D (from the left) are clearly resolved. The wire pair 3D is also resolved as the dip is 76.5% of the maximum; however, this twin peak cannot be regarded as detected as the SNR is 1.86. Therefore the first unresolved wire pair is 3D.

Duplex wire 40 mm, 10 degrees, file 5002

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Figure 23. The IQI was at position 40 mm on detector side (Appendix 2). Wire pairs 1D, 2D 3D (from the left) are clearly resolved. The wire pair 4D is also resolved as the dip is 55.2 % of the maximum; the twin peak can also be regarded as detected as the SNR is 2.23. Therefore the first unresolved wire pair is 5D.

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Duplex wire, 60 mm, 10 degrees, file 5003

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Figure 24. The IQI was at position 60 mm on detector side (Appendix 2). Wire pairs 1D, 2D 3D (from the left) are clearly resolved. The wire pair 4D is also resolved as the dip is 34.7 % of the maximum; the twin peak is also clearly detected. Therefore the first unresolved wire pair is 5D.

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Figure 25. Here the IQI was on source side. No analysis is needed (Appendix 2). Wire pairs 1D, 2D, 3D and 4D are clearly resolved and detected. The first unresolved wire pair is 5D. Note that the distance between the peaks is larger than in the previous three cases. This is due to the larger projection magnification.

4.2.3 The step-hole IQI

The different placements of the step-hole IQI can be seen in Appendix 5. In the first three cases the IQI is on the detector side (Figs. 26 to 28). In the fourth case (Fig. 29) the IQI is placed on the source side. From Figs. 26 to 28 it is easy to see that the three

23

first holes from the right are detected. In Fig. 29 a forth hole may be detected. However, due to the complicated background (cannot be handled by the ImageJ software) in this case, no attempt was made to confirm this by a SNR analysis.

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Figure 26. The IQI was at position 20 mm on detector side (Appendix 5). The three first holes from the right are detected.

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Figure 27. The IQI was at position 40 mm on detector side (Appendix 5). The three first holes from the right are detected.

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Step-hole, 60 mm, 10 degrees, file 50023

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Figure 28. The IQI was at position 60 mm on detector side (Appendix 5). The three first holes from the right are detected, possibly also the fourth.

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Figure 29. Here the IQI was on source side (Appendix 5). The three first holes from the right are detected, possibly also the fourth.

4.2.4 Hole IQI or ASME penetrameter

The three different positions of the hole IQIs can be seen in Appendix 2. This IQI was used only on the detector side. In this case the profiles are such that the detectability of the holes can be determined directly from the profiles without SNR analysis. In Fig. 30 the two largest holes can de detected while the smallest hole between them cannot be distinguished from the background. In Figs. 31 and 32 all three holes can be detected.

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Hole IQI at 20 mm, 10 degrees, file 5008

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Figure 30. The IQI was at position 20 mm (Appendix 2). The two largest holes can be detected.

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Figure 31. The IQI was at position 40 mm (Appendix 2). All three holes can be de-tected.

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Figure 32. The IQI was at position 60 mm (Appendix 2). All three holes can be de-tected.

4.3 System performance

This is a short summary of the results from the previous sections. First the detectability of small artificial defects in the test object is reviewed. Next we examine to what extent the image quality fulfil the demand of standards.

4.3.1 Detectability of small defects

As shown above all of the artificial defects in the test object were detected in the X-ray inspection. The detectability was confirmed visually directly from the radiographs and more objectively by plotting gray level profiles over the defect and applying a signal to noise (SNR) analysis according to EN 13068-1. The smallest defects are a 50 mm long notch with diameter 0.5 mm, a 1 × 1 × 1 mm square hole and a cylindrical hole of di-ameter 1 mm and depth 1 mm. The small cylindrical defect, however, caused some problems. Even if this defect can be seen on the X-ray pictures the calculated SNR is 1.73 which means that the defect strictly speaking cannot be regarded as detected. This is probably due to some accidental high noise near the defect. The “diameter” of the 50 mm long notch (0.5 mm) is close to size of the pixels (0.4 mm). This hole is however detected by 2 to 3 pixels. A simple trigonometric calculation based on target to object distance, object to detector distance and target size (2.6 mm) shows that an object of diameter 0.5 mm will have a shadow of diameter of about 0.8 mm at the detector. A 0.5 mm hole should therefore be detected by at least 2 pixels (3 pixels if the two edge pixels are partially illuminated). The origin of this projection magnification is illustrated in Fig. 33.

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Figure 33. The origin of projection magnification from geometry and finite target size. Picture from Blakely & Spartiotis (2006).

4.3.2 Comparability of image quality to standards

Table 1 summarizes the detectability of the wires in wire type IQI EN 462 W6 Cu 50 with the IQI placed at four positions (three positions on the detector side and one posi-tion on the source side). Here wires W6 to W8 are detected at all positions. Further, W9 is detected also at the middle position on detector side. Table 1. Detected wires in the EN 462 W6 Cu 50 image quality indicator for four IQI positions.

IQI position Wire Left Middle Right Source side W6 yes yes yes yes W7 yes yes yes yes W8 yes yes yes yes W9 no yes no no

W10 no no no no W11 no no no no W12 no no no no

Table 2 gives the results for the hole IQI or ASME penetrameter. This IQI has three holes with diameters 1T, 2T and 4T, where T is the thickness of the plague, in this case 1.02 mm. Only the hole 1T at position 20 mm remained undetected. The hole diameters are 1.02 mm, 2.04 mm and 4.04 mm.

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Table 2. Detected holes using the hole or ASME penetrameter image quality indicator for three IQI positions. The thickness T of the penetrameter is 1.02 mm (0.04 inch).

IQI position Hole 20 mm 40 mm 60 mm 1T no yes yes 2T yes yes yes 4T yes yes yes

Table 3 summarizes the results for the step-hole IQIs. The three largest holes are de-tected at all positions. In some cases a forth hole may well have been detected (this is indicated by a star (*) in Table 3), however, no attempt was made to verify this by SNR analysis, because of the difficult background (because of the varying background, Im-ageJ would give wrong values for the noise). Table 3. Detected holes using the step-hole image quality indicator for four IQI posi-tions. The star (*) means that the detection was not verified by SNR analysis.

IQI position Hole 20 mm 40 mm 60 mm Source side

1 yes yes yes yes 2 yes yes yes yes 3 yes yes yes yes 4 no no * * 5 no no no no 6 no no no no

Table 4 presents the results from unsharpness measurement using the duplex wire type IQI for three IQI positions on the detector side and one position on the source side. The unsharpness in millimetres is given as one half of the spacing between the first un-resolved wire pairs. The worst result is from position 20 mm on the detector side where the first unresolved wire pair is D3. This corresponds to an unsharpness of 0.32 mm. Note that the pixel size is 0.40 mm. A unsharpness better than the pixel size is possible due to the projection magnification (see Fig. 33). Table 4. Measured unsharpness using the duplex wire image quality indicator for four IQI positions.

Duplex wire

value IQI position

20 mm 40 mm 60 mm Source side First unresolved

wire pair D3 D5 D5 D5

Unsharpness, mm

0.5 0.32 0.32 0.32

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5 ALTERNATE X-RAY INSPECTION SETUPS

In the following some alternatives to the high energy scintillator detector used in this work are concidered. The main emphasis is on new detector concepts based on direct conversion.

5.1 Digital X-ray 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 linear de-tector provided by BIR (Bio Imaging Research, see Internet) was already described in chapter 3. A flat panel detector, on the contrary, is a full two dimensional array of detec-tors (pixels). The detectors are based on the technology for manufacturing solid state semiconductor circuits. Two main detection schemes exist, indirect and direct conver-sion as described in Fig. 34. 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. Informa-tion from a large amount of pixels builds up the digital X-ray image. In the direct con-version detection scheme the incoming X-ray photon produces electric charge directly in the detection pixel without the intermediate conversion of the X-ray energy to light energy. This is believed to increase the spatial resolution.

Figure 34. 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 in cap-tured by a TFT-array. To the right the X-rays directly produce charge in the photocon-ductor and this is captured by a TFT-array (Kotter & Langer 2002).

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The main advantage of using a flat panel would be the speed up of the inspection time that arises from the fact that a 2D detector utilizes most of the useful X-ray beam from the accelerator. A shorter inspection time on the other hand could facilitate for example multi angle radiography for better defect sizing and detection. However, the greatest benefit of a collimated line detector is the effective reduction of the influence of scat-tered radiation (below 3 %). When using a flat panel detector the scattered radiation may destroy the above mentioned advantages (Zscherpel 2006).

5.2 The Gallium-Arsenide based linear detactor

The spatial resolution could probably be increased by using a direct conversion line de-tector instead of the present scintillator detector. Further, increase in spatial resolution could be achieved by reducing the pixel pitch from the present 400 �m to 200 �m. This new direct conversion detector could be based on Gallium Arsenide (GaAs) as this ma-terial has good radiation endurance in the MeV energy range. The GaAs detector will probably also be more sensitive than the present scintillation detector, thus allowing the reduction of the pixel size without prolonging the examination time. An important as-pect of using a linear detector is that it can be effectively shielded from scattered radia-tion. Scattered radiation may prohibit the use of flat panels in this application.

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6 CONCLUSIONS

All artificial defects in the investigated reference object were observed. Further, the use of Image Quality Indicators (IQI) showed that the requirements in the standards are mainly fulfilled. Some improvements are, however, proposed. One is the use of a new Gallium Arsenide linear detector as this is expected to provide higher sensitivity than the present scintillator based linear detector and therefore make it possible to use smaller pixel size without prolonging the inspection time. Flat panel detectors can hardly be used because of their sensitivity to scattered radiation. A further improvement may be the use of stereoscopic radiography for defect discrimination and location in the X-ray beam direction. This will be the topic of a later report.

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REFERENCES

Blakeley, B., & Spartiotis, K. 2006. Digital radiography for the inspection of small de-fects. Insight. Vol 48. No 2. pp 109-112. Halmshaw & Kowol. Image quality indicators in industrial radiography. IE-NDT Ltd. Internet Address: http://www.ie-ndt.co.uk/ Göbbels, J. 2006. Private communication. Keller, H., Hinderer, R., Glass, M., Jeraj, R., Schmidt, R., Fang, G., Kapatoes, J., Mackie, T., R. and Corradini, M.,L. 2003. Design considerations for efficient binary megavolt photon detector structures. IEEE Transactions on Nuclear Science. Vol. 50. No. 1. p. 117 – 121. Kotter, E. & Langer, M. 2002. Digital radiography with large-area flat-panel detectors. Eur Radiol. 12: 2562-2570. Müller, C., Elegin, M. Scharmack, M., Bellon, C., Jaenisch, G-R, Bär, S,. Redmer, B., Goebbels, J., Ewert, U., Zscherpel, U., Boehm, R., Brekow, G., Erhard, A., Heckel, T., Tessaro, U., Tscharntke, D. & Ronneteg, U. 2006. Reliability of nondestructive testing (NDT) of the copper canister seal weld. SKB report R-06-08. 158 pp. Ronneteg, U. 2006. Private communication. Zscherpel, U. 2006. Private communication. Bio Imaging Research. Internet Address: http://www.bio-imaging.com/Choosing_x-ray_detectors.asp

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APPENDICES

1. X-ray pictures a, b, c and d, angle 5 degrees 2. X-ray pictures a, b, c and d, angle 10 degrees 3. X-ray pictures a, b, c and d, angle 20 degrees 4. X-ray pictures a, b, c and d, angle 30 degrees 5. X-ray pictures a, b, c and d, angle 10 degrees with step-hole IQI

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Appendix 1. Angle 5 degrees, in a ,b, c IQIs on D-side. in d W1, W6 and Duplex-wire on S-side, a: file 5006, b: file 5005, c: file 5004 and d: file 50017.

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Appendix 2. Angle 10 degrees, in a ,b, c IQIs on D-side. in d W1, W6 and Duplex-wire on S-side, a: file 5007, b: file 5002, c: file 5003 and d: file 5008.

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Appendix 3. Angle 20 degrees, in a ,b, c IQIs on D-side. In d W1, W6 and Duplex-wire on S-side, a: file 50010, b: file 50011, c: file 50012 and d: file 5009.

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Appendix 4. Angle 30 degrees, in a ,b, c IQIs on D-side. In d W1, W6 and Duplex-wire on S-side, a: file 50013, b: file 50014, c: file 50015 and d: file 50016.

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Appendix 5. Angle 10 degrees, in a ,b, c step-hole IQI on D-side.. In d on S-side, a: file 50021, b: file 50022, c: file 50023 and d: file 50024.