HFTM RIG

Fusion Engineering and Design 84 (2009) 1847–1851

Contents lists available at ScienceDirect

Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes

Assessment of X-ray tomography for irradiated IFMIF/HFTM RIG Ion Tiseanu a,∗ , Teddy Craciunescu a , Anton Möslang b a b

National Institute for Lasers, Plasma and Radiation Physics, Bucharest-Magurele, Romania Forschungszentrum Karlsruhe, Germany

a r t i c l e

i n f o

Article history: Available online 20 January 2009 Keywords: Fusion materials X-ray microtomography Irradiated materials Monte Carlo simulations

a b s t r a c t An inspection procedure to assess the mechanical integrity of IFMIF capsules and rigs during the irradiation campaign is necessary. In a number of experiments, we previously proved that X-ray tomography is a reliable solution for official inspections of the structural integrity of IFMIF complete assemblies before and after various tests (tensile, creep-fatigue, fracture toughness, crack growth, microstructure swelling). The next main challenge is to qualify the transmission microtomography inspection as a tool for non-destructive inspection during maintenance (beam-off) periods and after irradiation campaigns. The influence of the sample gamma radioactivity (produced by neutron activation during irradiation campaigns) on the tomographic reconstruction quality is investigated in this paper. The evaluation is performed for the specific configuration of newly manufactured IFMIF irradiation capsules. Experimental X-ray radiographic data together with simulated gamma radiographic images were considered. Latest activation data were incorporated. For the numerical simulations, a working environment able to provide a realistic numerical simulation was constructed on the basis of two well established Monte Carlo computer codes: SIMIND and ITS. The optimal parameters of the collimation system, essential for the investigation of the irradiated samples, were determined. It is proved that transmission tomography is practicable, providing good quality reconstruction which allows accurate geometrical measurements. The procedure developed for tomography of irradiated HFTM can be applied for the inspection of other irradiated structures for which X-ray tomography represents a non-destructive inspection technique (NDT). © 2008 Elsevier B.V. All rights reserved.

1. Introduction The International-Fusion-Material-Irradiation-Facility (IFMIF) [1] is planned to be an accelerator driven neutron source able to create a displacement rate per full power year of 20–50 dpa/fpy in fusion DEMO reactor candidate materials within the High-FluxTest-Module (HFTM) [2]. More than 1000 qualified specimens will be irradiated simultaneously within its limited volume of 0.5 l. They will be housed in capsules, which will be inserted in rigs. A total of 12 rigs filled by identical sets of irradiation specimen will be installed within the HFTM container (segmented by stiffening plates into four compartments). Cooling and temperature control will be ensured by rigs and distance holders which will create narrow cooling channels. Helium is chosen as coolant as it will not be activated nor it is corrosive. The temperature level will be adjusted independently in each rig by additional segmented electrical heaters within tolerances up to ±15 ◦ C at most. They are wound and brazed into grooves rising spirally around the capsule wall. The heat transfer between the specimens as well as between specimens

∗ Corresponding author. Tel.: +40 214574490; fax: +40 214574243. E-mail address: tiseanu@infim.ro (I. Tiseanu). 0920-3796/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2008.11.063

and capsule wall is guaranteed by filling up with sodium or eutectic sodium–potassium liquid metal [3]. The limited space within the HFTM requires a sophisticated arrangement of specimens, equipments for temperature control and heat removal through narrow channels. Therefore an inspection procedure to assess the mechanical integrity of IFMIF capsules and rigs and individual specimens is necessary. Presently, the Xray microtomography is a reliable solution for official inspections of the structural integrity of IFMIF complete assemblies [4]. The 3D model of miniaturized samples is the main result provided by tomography. By further processing of 3-D reconstruction the accurate positioning of the specimens, heaters and thermocouples can be determined. In addition to the accurate 3-D model of the miniaturized specimens, t. A number of X-ray tomographic measurements of the HFTM rig were performed and representative results were reported previously [5]. We have demonstrated that for the HFTM rig prototype the geometry resolution is about 30 ␮m for maximum characteristic dimension of 50 mm. Thus, voids of 30 ␮m diameter and cracks of 20 ␮m width can be detected. Such an absolute error of geometrical measurements should be sufficient for the assessment of the structural integrity of the irradiation capsule and for the geometry description of the thermal-hydraulic modeling. NaK filling height

1848

I. Tiseanu et al. / Fusion Engineering and Design 84 (2009) 1847–1851

and distribution in the specimen volume can be identified. Even more challenging, the quality of the brazing layer can be assessed. This is of crucial interest in order to assure the best heat coupling of the heater wires and thermocouples. The main challenge to the well established transmission microtomography inspection is the influence of the sample radioactivity, due to neutron activation, on the quality of the tomographic reconstruction. The goal of this study is the evaluation of this influence. It has been carried out by 3-D Monte Carlo radiation transport simulations corroborated with previous tomographic measurements on HFTM irradiation capsules. 2. Materials and methods In computed X-ray transmission tomography the 3-D model of the investigated object is reconstructed from a number of X-ray radiographies obtained at different rotation angles of the investigated object. Let IR be the radiographic image at fixed rotation angle. In the case of a radioactive sample, gamma radiation is emitted from the sample and detected. The image induced by the gamma radiation is called the auto-radiography IAR of the object. The auto-radiography produces a smearing effect superimposed on the transmission radiographies. The effective image formed on the detector is Ieff = IR + IAR . This of course alters the X-ray tomographic reconstruction. In order to try to remove the effect induced by the gamma radioactivity of the sample, a supplementary measurement must be performed: the sample is measured in the absence of the X-rays, in the same conditions as in the case when the X-ray tube was powered on. This enables the acquisition of auto-radiographic  images IAR similar to the auto-radiographies incorporated in Ieff   IR and IAR differs only due to statistical reasons. IAR can be sub , which is another stracted from Ieff in order to obtain IR = Ieff − IAR instance of IR . Of course, IR and IR differs due to statistical fluctuations. This difference induces residual effects that produce artifacts in the tomographic reconstruction. For the evaluation of the influence of the gamma radioactivity of irradiated HFTM capsule on the reconstruction in X-ray transmission tomography we used both experimental and numerically simulated data: (i) X-ray radiographies IR were measured [5] (ii) auto-radiographies were calculated using Monte Carlo simulations of gamma ray emission, transport and detection. The main parameters describing the tomographic experiment and the Monte Carlo simulation setup are listed in Table 1. The working environment able to provide a realistic numerical simulation was established on the basis of SIMIND Monte Carlo simulation code [6] and the Integrated TIGER Series (ITS) [7]. SIMIND Table 1 The main parameters of the tomographic experiment. Source parameters

HV = 215 kV I = 650 ␮A

Filter

2 mm Cu

Source

VISCOM micro-focus open X-ray tube 225 kV/300 W

Detector

GOS (Gd2 O2 S) – KODAK Scintillation LANEX screen filter, 409.6 mm × 409.6 mm × 0.53 mm, 1024 × 1024 pixels, 16 bits digital output

Sample

HFTM irradiation capsule

Geometry

Source–object distance: 228 mm Source–detector distance: 1503 mm Magnification: 6.6 720 equidistant angles

HFTM rig dimensions

19 mm × 50 mm × 140 mm

is a computer code dedicated to emission tomography providing fast simulation of the image formed on the detector. It describes a standard camera and can easily be customized for almost any type of calculation or measurement encountered in SPECT imaging. It was used for obtaining detailed and realistic simulations of the auto-radiographies. SIMIND has the advantage of using numerically defined 3-D sources and numerically simulated samples (“phantoms” in tomographic terms). For an auto-radiography experiment the source coincides with the phantom. For its numerical description we used the 3-D model obtained by tomographic reconstruction of the non-irradiated HFTM capsule. A tool for calibrating the intensity of the simulated auto-radiographies in respect with the measured radiographies is also necessary. In principle, SIMIND provides the possibility to simulate also the radiographic images IR . It allows simulations of transmission experiments, but for monoenergetic radiation. As the X-ray emitted in tomographic experiments is polychromatic, it means that the spectrum must be, in principle, discretized and SIMIND must be run for each discrete component of the spectrum. For our tomographic geometry, SIMIND needs around 50 h, on a 2-GHz ATHLON CPU, to generate images corresponding to a specific value of the energy. In consequence the use of SIMIND for calculating the radiographic images is a very time-consuming procedure. In order to avoid such extremely inefficient calculations, ITS was used instead of SIMIND. It has the advantage to accommodate the energy input in the form of a spectrum. However ITS does not accept numerically defined 3-D objects. Therefore, it can be used to provide accurate global parameters describing the HFTM measurement, but it is not suitable for obtaining fine resolution radiography and auto-radiography images. A strategy for an efficient simulation of the formation of the X-ray radiography is to divide the problem in two steps: 1-D simulation of electron transport and formation of X-ray on the tungsten target and 3-D simulation of photon transport through pre-filters, the object, post-filters to the detector. Taking into account the X-ray source operating parameters and the gamma activity, the ratio R between the total pulse height generated by auto-radiography and X-ray radiography, respectively, can be calculated. This value is used to calibrate the intensity ratio of the simulated auto-radiographies to the measured radiographies. For a long irradiation time (up to several years—typical for IFMIF experiments) experimental data concerning the activation of HFTM due to neutron activation was recently evaluated in Forschungzentrum Karlsruhe [8]. Neutron activation analysis has been carried out for the whole IFMIF test cell employing a recently developed 3D geometrical model which describes in detail the deuteron beam delivering pipe, the lithium loop, the test modules for the sample irradiation and auxiliary systems. In our analysis we used the neutron induced gamma activity calculated for 1 year irradiation (see Fig. 1). This evaluation was performed, as described in Ref. [8], using the McDeLicious Monte Carlo transport code [9], an extension of MCNP-4C, which simulates the IFMIF neutron source extending up to 55 MeV. Neutron cross-sections from general purpose high energy libraries [10] and [11] were used for the transport and nuclear response calculations. The induced radioactivity was calculated by the ALARA inventory code on the basis of the IEAF2001 activation library. To represent IFMIF test cell for the Monte Carlo calculations a comprehensive 3-D model (version md33) that was recently developed [12]. It reflects the latest IFMIF design modifications and describes in detail the geometry and material specifications of the IFMIF test cell. For the IFMIF characteristic neutron spectrum and intensity, the gamma activity, specific for HFTM case, increases significantly especially due to 51 Cr up to specific saturation activity of about 1014 Bq/kg. 55 Fe disintegrates by electron capture and a gamma transition arises at 126 keV but it can be neglected due to its small probability (1.3 × 10−7 %). Of course 3 H is not gamma radioactive.

I. Tiseanu et al. / Fusion Engineering and Design 84 (2009) 1847–1851

Fig. 1. Total and specific isotope activities induced in the Eurofer (solid lines) and SS-316 (dashed lines) steels after 1 year irradiation in the HFTM of the IFMIF.

In comparison with other gamma emitters, 51 Cr has a contribution several orders of magnitude higher. Consequently, in our evaluation we determined the energy deposition in the detector for the gamma line of 320 keV of 51 Cr after one life-time (approximately one month) of cooling period. This cooling period is adequate in order to avoid the detection of gamma rays from isotopes with short half-life (up to ∼1 day). It also makes sense for the duration of scheduled interventions at the facility. 3. Results The combined experimental and numerically simulated evaluation shows that, without a collimation system, the ␥-rays completely perturb the X-ray radiography. This can be explained by comparing the high value of the ratio between the energy depositions by auto-radiography and X-ray radiography, R(ray/X-ray) ∼ = 2.1 × 103 , with the nominal dynamic range of the detector array (SNR—signal to noise ratio) which is around 2000. This means that the intensity of the radiographic image is

1849

less than the dark (offset) image plus one standard deviation: IR < Dark + std dev(Dark), where std dev(dark) ≈ 30 digital units. In order to reduce the factor R, a collimation system can be used. By collimating the X-ray cone to a fan, the scattering artifacts will be also reduced. We evaluated rectangular collimators with different slit heights and full width (the width of the collimator covers the entire width of the detector). Using the collimator, only a slice of the object is measured. The measurements must be repeated successively, each time shifting vertically the position of the collimator in order to obtain the data needed to reconstruct the whole object. This is the price paid for diminishing the influence of the auto-radiography in the effective image formed on the detector: a significant increase of number of measurements and of the total time needed to perform the experiment. However, taking into account the shape of the collimator, it is reasonable to consider the replacement of the 2-D matrix detector (typical for cone-beam X-ray tomography) with a linear detector array which provide fast image acquisition and fast data transfer, since they are made of individual scintillators with detection efficiency much larger than typical flat panel arrays. These characteristics contribute to significantly diminish total acquisition time. We considered slit heights that lead to a value of R close to unity. In this case the influence of the ␥-radioactivity of the irradiated HFTM capsule is presented in Fig. 2. For a slit width of 0.2 mm the influence of ␥-radioactivity is relatively reduced and it consists in an additive noise. The level of this noise increases significantly for a slit width of 0.4 mm; even so, almost all the fine details are still present in the image. For a slit width of 1.0 mm the blurring induced by noise is considerable and only coarse details can be visualized. Tomographic reconstructions proved to be much more sensitive in respect with the perturbative effect of the gamma rays, as it is illustrated in Fig. 3. For a collimator with a slit height of 0.2 mm geometrical shapes are strongly distorted and for a slit height of 0.4 mm the reconstruction is almost meaningless. This result enforces the use of lower value for the slit height in order to get improved reconstructions, as illustrated in Fig. 4(a) (0.1 and 0.05 mm slit heights). Accurate geometrical measurements can still be performed. This can be proved quantitatively. Two line segments (AB and CD in

Fig. 2. Influence of the ␥-radioactivity of HFTM when using a collimation system with different slit heights: 0.2 mm (top), 0.4 mm (middle) and 1.0 mm (bottom). Effective  (right image Ieff = IR + IAR ) of the irradiated HFTM (left column) and simulated radiography of the irradiated HFTM after subtraction of the auto-radiography, IR = Ieff − IAR column).

1850

I. Tiseanu et al. / Fusion Engineering and Design 84 (2009) 1847–1851

Fig. 3. Axial cross-section of the tomographic reconstruction for a collimator with a slit height of 0.2 mm (left) and 0.4 mm (right).

Fig. 4. (a) Axial cross-section of the tomographic reconstruction of HFTM rig: (i) non-irradiated (top), irradiated and collimator with slit height of 0.05 mm (middle) and 0.1 mm (bottom). Two line segments (AB and CD) were defined for line profile plots. Line profiles depict the variation of the intensity along the white lines superimposed on the axial cross-sections. (b) Intensity plot along line segment AB (a) for the non-irradiated (top) and irradiated (bottom) HFTM rig. (c) Intensity plot along line segment CD in (a) for the non-irradiated (top) and irradiated (bottom) HFTM rig.

Fig. 4(a) were defined. Profile plots of the intensity along the two line segments can be obtained and they are presented in Fig. 4(b and c). The peak-shaped structure obtained for the non-irradiated rig is correctly reproduced in the case of the irradiated one. For geometrical measurements full width at half maximum (FWHM) of the peak is used to delimit the borders of objects inside the cross-section. As, Fig. 4(b and c) reveal a very good agreement of FWHM values for non-irradiated and irradiated HFTM rig, respectively. It results that accurate geometrical measurements can be performed using the reconstruction of the irradiated rig. In conclusion we appreciate that the simulations prove that good quality reconstructions can be obtained for a practicable geometry of the collimation system. Shapes are correctly reproduced and accurate geometrical measurements can be performed. The procedure developed for tomography of irradiated HFTM can be applied for the inspection of other irradiated structures. For example, within a fusion ceramic tritium breeder pebble bed, the location and fragmentation of pebbles due to burn-up of lithium could be studied. Other examples of nuclear and active applications of

computerized transmission X-ray tomography are the porosity and crack distributions in graphite, delamination in composites and kernel distribution in HTR fuels. Acknowledgements This work, supported by the European Communities under the Contract of Association between EURATOM and MEdC-Romania Associations, was carried out within the framework of the European Fusion Development Agreement. The views and opinions expressed herein do not necessarily reflect those of the European Commission. References [1] Möslang (ED.), IFMIF Conceptual Design Evaluation Report, Forschungzentrum Karlsruhe, FZKA 6199, 1998. [2] V. Heinzel, F. Arbeiter, B. Dolensky, U. Fischer, S. Gordeev, A. Möslang, K.-H. Lang, D. Leichtle, S. Simakov, V. Slobodchuk, P. Vladimirov, IFMIF high flux test module and test cell–design and design validation, Fusion Eng. Des. 82 (2007) 2444–2450.

I. Tiseanu et al. / Fusion Engineering and Design 84 (2009) 1847–1851 [3] V. Heinzel, F. Arbeiter, B. Dolensky, U. Fischer, S. Gordeev, K.-H. Lang, D. Leichtle, A. Moeslang, S.P. Simakov, V. Slobodchuk, E. Stratmanns, P. Vladimirov, IFMIF high flux test module and test cell—design and design validation, Fusion Eng. Des. 82 (2007) 2444–2450. [4] A. Ibarra, R. Möslang, R. Lässer, R. Ferdinand, E. Andreani, B. Surrey, V. Riccardi, H. Heinzel, U. Klein, R. Fischer, M. Forrest, Gasparotto, Recent EU activities for IFMIF EVEDA in the framework of the broader approach, Fusion Eng. Des. 82 (2007) 2444–2450. [5] Tiseanu, M. Simon, T. Craciunescu, B.N. Mandache, C. Volker Heinzel, E. Stratmanns, S.P. Simakov, D. Leichtle, Assessment of the structural integrity of a prototypical instrumented IFMIF high flux test module rig by fully 3D X-ray micro-tomography, Fusion Eng. Des. 82 (2007) 2608–2614. [6] M. Ljungberg, The SIMIND Monte Carlo program, in: M. Ljungberg, S.E. Strand, M.A. King (Eds.), Monte Carlo Calculation in Nuclear Medicine: Applications in Diagnostic Imaging, IOP Publishing, Bristol and Philadelphia, 1998, pp. 145–163.

1851

[7] J.A. Halbleib, R.P. Kensek, T.A. Mehlhorn, G.D. Valdez, S.M. Seitzer, M.J. Berger, ITS version 3—the integrated TIGER series of coupled electron/photon Monte Carlo Transport Codes, Sandia National Laboratories, IEEE Trans. Nucl. Sci. 39–4 (1992) 1025–1030. [8] S.P. Simakov, U. Fischer, A. Möslang, P. Vladimirov, F. Wasastjerna, P.P.H. Wilson, Neutronics and activation characteristics of the international fusion material irradiation facility, Fusion Eng. Des. 75–79 (2005) 813–817. [9] S.P. Simakov, U. Fischer, U. von Möllendorff, I. Schmuck, A.Yu. Konobeev, Yu.A. Korovin, J. Nucl. Mater. 307–311 (2002) 1710–1714. [10] M.B. Chadwick, P.G. Young, S. Chiba, S.C. Frankle, G.M. Hale, H.G. Hughes, et al., Nucl. Sci. Eng. 131 (1999) 293–328. [11] Yu.A. Korovin, A.Yu. Konobeev, P.E. Pereslavtsev, A.Yu. Stankovsky, C. Broeders, I. Broeders, et al., Nucl. Instr. Meth. A463 (2001) 544–556. [12] F. Wasastjerna, Forschungszentrum Karlsruhe Internal Report, April 2004.