Akihiro Ishimi

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

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 CdWO4 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.

Fuel performance of hollow pellets for fast breeder reactors

Three fuel rods containing hollow mixed oxide (MOX) pellets of uranium and plutonium oxides were fabricated and irradiated at a high linear heat rate (LHR) to burn-up of nearly 30,000 MWd/t in the experimental fast rector, JOYO MK-II. After irradiation, one of the fuel rod pellets was examined by X-ray CT and conventional nondestructive and destructive methods. Swelling rate was evaluated by both dimensional change and radial density distribution. There were no differences between both types of results and it was concluded that swelling rate can be examined in detail by the X-ray CT technique without dismantling the assembly. In addition, the swelling rate of hollow pellets was nearly the same as values reported for the fuel rods containing solid pellets. LHR was higher in the examined fuel rod containing hollow pellets than in the conventional fuel rod containing solid pellets, but fission gas release rates for both fuel rods were nearly the same.

Distributions of density and fission products in the reaction product between irradiated MOX fuel and molten zircaloy-2

ABSTRACT Two- and three-dimensional images were obtained by X-ray CT in the reaction product between zircaloy-2 cladding tube and MOX fuel. The gamma-ray intensity distributions in the same specimen were also obtained by gamma-ray measurements of two fission products (Cs-137 and Eu-154) and one neutron-activated nuclide (Co-60). The average values of the fuel density (about 10.5 g/cm3) and the cladding density (about 6.55 g/cm3) were obtained in the metallic phase region by evaluation of the density distributions on two-dimensional X-ray CT images. The distributions of the crushed fuel pellet and the pores were also clearly observed in the three-dimensional X-ray CT images. The following results were found from the gamma-ray measurement. First, Cs-137 was observed in the unreacted fuel region and the pore region in the metallic phase region. Second, Eu-154 was widely distributed to all regions. Finally, Co-60 was confirmed only in the metallic phase region.

Development of a High Resolution X-Ray CT Technique for Irradiated Fuel Pellets

A nondestructive method making use of X-ray computer tomography (X-ray CT) has been applied to post irradiation examinations (PIEs) of fast breeder reactor (FBR) fuel assemblies. In the X-ray CT system, a 12 MeV X-ray pulse was used in synchronization with switching on the detector to minimize the effects of gamma ray emissions from the irradiated fuel assemblies. This technique enables a clear cross section CT images of the irradiated fuel assembly to be obtained.