Upgrading of X-ray CT technology for analyses of irradiated FBR MOX fuel

Abstract

The X-ray CT technology previously developed by JAEA was upgraded. The shape of the X-ray source beam was changed from a circular shape to an elliptical one and the collimator slit width was decreased from 0.3 to 0.1 mm. The X-ray detector was improved by changing a CdWO 4 scintillator to a highly sensitive silicon semiconductor detector. The analysis code of X-ray CT image was revised with respect to the number of points by using two kinds of experimental results and taking into account the effects of crack existence and deviation of the central void position from the radial center of a fuel pellet. As a result, high resolution X-ray CT images could be obtained on the transverse cross section of irradiated fuel assemblies. The error of the dimensional measurement was improved from +0.1 to +0.03 mm by upgrading the instrument and revising the analysis code of X-ray CT image. The discriminating accuracy of density difference could be increased, and the low density region (undisturbed region) and high density region (equi-axial and columnar regions) in the X-ray CT image on the cross section of irradiated fuel could be discriminated from each other. The reliability of fuel performance analysis improves because a large number of PIE data can be collected, compared with the conventional destructive PIE.Keywords: post-irradiation examination; X-ray CT; fast breeder reactor; irradiated nuclear fuel IntroductionThe Japan Atomic Energy Agency (JAEA) has been continuing the development of X-ray CT technology to investigate the performance of irradiated fast breeder reactor (FBR) MOX fuel [1]. Conventional post irradiation examinations (PIEs) give results on fuel rod performance after dismantling the fuel assembly [2,3], but X-ray CT technology has a great merit of being able to examine the fuel rods performance in the intact fuel assembly.Previously two-dimensional X-ray CT images on a number of transverse cross-sections have been obtained for irradiated fuel assemblies. From these images the deformations of fuel rods along the axial direction within the fuel assemblies were dimensionally investigated [4,5]. Next, the sizes of central voids formed in the fuel pellets by a steep temperature gradient were roughly measured and the relationship between the void sizes and linear heat rating was discussed [5,6]. In addition, two-dimensional X-ray CT images were synthesized to get three-dimensional X-ray CT images [7,8], and it was demonstrated as being