High Energy Radiography for Inspection of the Lid Weld in

Working Report 2009-82
High Energy Radiography for Inspection
of the Lid Weld in Disposal Canisters
Stefan Sandlin
November 2010
POSIVA
OY
Olkiluoto
FI-27160 EURAJOKI, FINLAND
Tel
+358-2-8372 31
Fax +358-2-8372 3709
Working Report 2009-82
High Energy Radiography for Inspection
of the Lid Weld in Disposal Canisters
Stefan Sandlin
VTT
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.
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 insert, 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 inspected by non-destructive methods. The objective of this work is to investigate the application 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 inspection 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 varying 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 scattered radiation from the working accelerator will itself cause an intense field of radiation 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 accelerator 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 valurautasisäosalle, joka puolestaan sisältää käytetyn polttoaineen ydinjäteniput. Kapselin on oltava ehdottoman tiivis, näin ollen kapselin sulkuhitsi kuparikannen ja putken välillä on kriittinen osa. Tämä huomioiden sulkuhitsi on tarkastettava ainetta rikkomattomin menetelmin. 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 edellyttää, 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 tarkastukseen riittävä fotonivuo, tarvitaan lineaarikiihdytintä. Siroava säteily toimivasta
kiihdyttimestä synnyttää vahvan säteilykentän tarkastushuoneessa. Huomioiden kaikki
nämä erityispiirteet tekevät suurenergisestä röntgentarkastuksesta sulkuhitsin kunnonvalvonnassa 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 röntgenlähteitä ja detektorityyppejä on esitelty. Röntgenfilmi ei sovellu tähän tarkastukseen.
Myös röntgentarkastuksen mallinnus ja sen mahdollisuuksia tähän erikoistarkastukseen
on kuvattu. Röntgentarkastukseen luodut mallinnustyökalut käsittelevät yleensä tavanomaista röntgentarkastusta eivätkä sellaisinaan sovellu kiihdytinpohjaisen tarkastuksen
mallinnukseen.
Avainsanat: suurenerginen radiografia, loppusijoituskapseli, röntgendetektorit, rividetektori, litteätaulu detektori, kiihdytin, röntgentarkastuksen mallinnus; rikkomaton
aineenkoetus.
1
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
2
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 mechanical strength. The fuel elements are inserted into the canisters in a specially designed 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 inspection 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 assumed to be needed. This is because an accelerator can deliver several orders of magnitude 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 inspections 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 inspection 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 usable 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-
3
ergy in the MeV range are presented. Finally the work is concluded with a brief review
of available modelling software for X-ray inspection.
4
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 improve 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 canister. 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).
5
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 optimized 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).
6
Figure 3. The Swedish design of the weld between the canister and the lid (Stepinski
2006).
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 detector remain stationary (Ronneteg & Moberg 2003).
7
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 experimental Swedish X-ray inspection system. The Fig. illustrates how three defects are projected 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 distribution suitable for the linear array detector. The detector output picture is built up of vertically 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 Moberg (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 (Ronneteg & Moberg 2003).
8
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.
I
I 0 e Px
where I0 is the initial intensity, I is the intensity when the X-rays have travelled a distance x in the material and μ is the linear attenuation coefficient. Linear attenuation coefficients for some materials are given in Tab. 1. Copper has a higher coefficient of attenuation 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 present, 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 radiation reaches the film (or detector), the image formation is therefore more complicated
than the above equation suggests. Radiographic equivalence factors are used to approximately 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 scattering cross-section, is often measured in a unit called barn (1 barn = 10-28 m2). The scattering 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 number, Z) of the scattering atom and on the energy of the incoming photon. The total crosssection is the sum of cross-sections for different scattering mechanisms, such as photo
9
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 dominating scattering mechanism (Radiation Environment, 2005).
10
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).
Material
Steel
Copper
150 keV
1.0
1.6
X-ray energy
220 keV
400 keV
1.0
1.0
1.4
1.4
4 to 25 MeV
1.0
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 dependence 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 thickness”. 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 detectable 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.
11
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).
12
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 materials in the detector and the source which reduce the lifetime of these components or
change their performance. Electronic components are known to be especially susceptible 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 irradiation (Smith et al. 2001); therefore also damages of this kind of materials may have indirect 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 radiography) 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 materials these electrons are not expected to have influence on the X-ray inspection equipments. However, secondary electrons created in the detector itself or in the related electronics 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 using the Monte Carlo code MCNP4C. The calculations was done for three different canister 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 difference 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
13
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 radiation 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).
14
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 yellow 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 azimuthal angle (the angle between a radius and the horizontal diameter in the crosssections 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 informative. However, the angular variation is well illustrated.
15
1,0E+15
Photons/(s MeV tU)
1,0E+13
1,0E+11
1,0E+09
VVER
BWR
EPR fuel 1
EPR fuel 2
1,0E+07
1,0E+05
0,01
0,10
1,00
10,00
Photon energy, MeV
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).
350
Gamma dose rate, mSv/h
300
VVER
BWR
250
EPR
200
150
100
50
0
0
45
90
135
180
Azimuthal angle, degrees
Figure 13. Calculated gamma dose rates of the outer surface of the Finnish final disposal canisters as function of the azimuthal angle. Here 180 degrees is shown, corresponding to the upper or lower part of the canisters in Figs. 9 to 11. The data are from
Anttila (2005).
16
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.
18
Neutron dose rate, mSv/h
16
VVER
BWR
EPR
14
12
10
8
6
4
0
45
90
135
180
Azimutal angle, degrees
Figure 14. Calculated neutron dose rates of the outer surface of the Finnish final disposal canisters as function of the azimuthal angle. Here 180 degrees is shown, corresponding to the upper or lower part of the canisters in Figs. 9 to 11. The data are from
Anttila (2005).
17
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 energy. 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 accelerate electrons in an evacuated tube to high kinetic energy. As these energetic electrons strike a metallic target they lose their kinetic energy in one or usually many collisions with the target atoms. These multiple collisions are responsible for the energy distribution of the X-rays as the electron produce photons of different energy in each collision. 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 continuous 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 voltages 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.
18
Figure 15. Cross-section of an X-ray tube. Electrons are emitted from the heated filament and accelerated towards the target where they produce X-rays as they are retarded, 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 accelerate 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 tomography (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 acquisitions while a new one, EuroMeV in France, has appeared. This new linac has an
especially small focal spot.
19
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 wavelengths (for example se Fig. 25), gamma-ray sources emit one or a few discrete wavelengths. A gamma ray with energy of 0.5 MeV is equivalent in wavelength and penetrating 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 energy 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 practical thickness limits for the different sources (note that this applies to film radiography). Digital detectors needing considerably shorter exposure times than film may expand the useful application range.
20
Table 4. Isotope sources used in industrial radiography (Quinn & Sigl 1980).
Table 5. Gamma ray applications according to Ouinn & Sigl (1980).
21
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 absorb 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 intensifiers 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 detectors is limited to lower (< 250 keV) X-ray energies (Ewert et al. 2005). Detectors operating in the MeV energy range exist, as shown for example in Fig. 4, but detailed experience 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 radiography 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 alternative in the cited report. Further, the amorphous silicon detector is said to be inherently insensitive to radiation damage. The supporting electronics surrounding the detection 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 highenergy and high resolution applications. Current amorphous silicon detectors can resolve 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):
22
x
x
x
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 influence 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 performance of digital detectors by comparing the SNR for the detector to the SNR for film
as given in Tab. 6 for different standards.
23
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:
x
x
x
x
x
Exposure and energy response
MTF (Modulation Transfer Function)
Penetrameter sensitivity
Speed
Throughput
Fig. 18 shows a summary of the penetrameter sensitivity tests. The penetrameter sensitivity 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 aSe indirect arrays. These data show that the direct a-Se achieves penetrameter sensitivities 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 NDTfilm. 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 indirectdetection 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.
24
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 detectors. 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 detectors 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 without 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 separated 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
25
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 accumulate on the capacitor (Kasap & Rowlands 2002).
26
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 accelerator 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.
27
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 accelerator. The large field screen is a scintillator and the shielded camera captures the
X-ray picture through the mirror (Moulin et al. 2005).
28
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 Moscow. 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 accelerated 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 optical 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 detector. 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 Computed 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 mentioned article of the detector performance in MeV energy X-ray beams or neutron
beams.
29
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 polychromatic 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 treatment of photon-object interaction physics. Both stochastic and deterministic methods
are employed. A majority of the simulation efforts is based on first-order approximations 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 detector 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 depository 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:
x
x
x
x
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 object.
Software for modelling the interaction of the incident high energy X-ray radiation 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.
30
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 material 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 simulation 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 factors. For more information see Tillack et al. (2000).
31
The CNDRI simulation code
As with the two first models the CNDRI group also uses first-order deterministic approximation 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 involved British Aerospace and LETI-CEA-Technologies in France.
CIVA
CIVA is a French NDT-modelling code containing modules for modelling of Eddy current-, 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 radiographic 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 modelling of accelerator based radiography.
32
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 dynamic 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 fundamental 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 140962). 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 resolution. 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 systems 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 linear 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.
33
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:
D
lg IID0
.
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:
ID
I 0 10 D .
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:
x
x
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:
G
K
dD
u
log10 e dK
where K is the dose in Gray required for the density change (D – D0). Here D0 is the optical density of an unexposed and processed film including base (fog and base density).
34
Granularity D
Granularity D is defined (EN 584-1) as a stochastic density fluctuation in the radiograph 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 microdensitometer. 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):
S
1
K
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 radiation 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:
b
a f
p f
35
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 projection magnification should occur when the value of geometric unsharpness is equal to the
pixel- pitch of the detector (Blakeley & Spartiotis 2006).
Signal-to-noise-ratio for film
Film systems are characterised by the gradient G and the granularity D. G is determined 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-tonoise ratio (SNR).
Relative contrast.
This is the ratio of the intensity difference of two adjacent image areas representing different 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.
36
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 determines 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 interpretation. The human eye can distinguish between 60 and 100 gray levels. Radiographic images 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).
37
8
DISCUSSION AND CONCLUSIONS
For accelerator based X-ray examination of the sealing weld in the final deposition canister a collimated digital detector is assumed to be needed. Flat panel detectors are expected 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 potential of high energy stereoscopic radiography should therefore be investigated.
38
REFERENCES
Anttila, M. 2005. Gamma and neutron dose rates on outer surface of thee types of final
disposal canisters. Posiva Oy, Posiva working report 2005-14.
ASTM E 1025 standard. Standard practise for design, manufacture, and material grouping classification of hole-type image quality indicators (IQI) used for radiology.
Blakeley, B., & Spartiotis, K. 2006. Digital radiography for the inspection of small defects. Insight. Vol 48. No 2. pp 109-112.
Casali, F., Pasini, A., Bettuzzi, M., Brancaccio, R., Cornacchia, S., Giordano, M.,
Morigi, M., P. & Romani, D. 2003. A new system for digital radiography and computer
tomography using an intensified linear array detector. DGZfP-Proceedings BB 84-CD.
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Davis, A., W., Berry, P., C., Claytor, T., N., Fry, D., A., Jones, M., H. & White, S., M.
2000. An analysis of industrial nondestroctive testing employing digital radiography as
an alternative to film radiography. ESA-MT. Nondestructive testing and evaluation
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Deprins, E., Phelan, R., Marstboom, K., Kochakian & Bueno, C. 2005. Spanning the
range: Film to CR to DR. 3rd MENDT - Middle East Nondestructive Testing Conference & Exhibition – 27-30 Nov 2005, Bahrain, Manama.
EN 14096-2 standard. Non-destructive testing – Qualification of radiographic film digitisation systems – Part 2: Minimum requirements.
EN 444 standard. Non-destructive testing – General principles for radiographic examination of metallic materials by X- and gamma rays.
EN 462-1 standard. Non-destructive testing – Image quality of radiographs - Part 1: Image quality indicators (wire type) – Determination of image quality value.
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EN 462-3 standard. Non-destructive testing - Image quality of radiographs - Part 3: Image quality classes for ferrous metals
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EN 462-5 standard. Non-destructive testing – Image quality of radiographs - Part 5: Image quality indicators (duplex wire type), determination of image unsharpness value.
39
EN 584-1 standard. Non-destructive testing – Industrial radiographic film – Part 1:
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Ewert, U. 2002. Upheaval in industrial radiology. Internet address (2006-03-31):
http://www.ndt.net/article/ecndt02/414/414.htm
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Films. DGZfP Berichtsband 94-CD.
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