Tsuyoshi Nagamine

Measurement of central void diameter in FBR MOX fuel by X-ray computer tomography

Metallurgical restructuring takes place in fast breeder reactor mixed-oxide (FBR MOX) fuel during irradiation at high linear heat rating. Under the influence of steep temperature gradient in radial direction, the pores homogeneously distributed in the as-fabricated fuel migrate toward fuel center, and result in the formation of central void, leaving columnar grains behind. 1) The diameter of the central void provides important information for estimating the temperature distribution in the fuel pins. Until now, the central void diameter has been measured by the metallurgical observation on a polished cross section of the fuel, but this is a procedure that calls for onerous work, and which also generates much radioactive waste. Then, the number of measurements of central void diameter has been limited. In the present study, nondestructive technique using X-ray computer tomography (X-ray CT) was applied to the measurements of central void diameter of MOX fuel pins irradiated in the experimental fast reactor “JOYO”. This technique was recently developed for use in post irradiation examination (PIE) work on fuel assemblies. 2,3) This technique made it possible to show the distribution of the central void diameter of all the fuel pins in a fuel assembly at same time without requiring the pins to be sectioned.

Application of X-Ray Computer Tomography for Observing the Deflection and Displacement of Fuel Pins in an Assembly Irradiated in FBR

A nondestructive method making use of X-ray computer tomography (X-ray CT) has been applied to post irradiation examination of fast breeder reactor (FBR) fuel assemblies. In the study, an examination is made of the deflection and displacement of fuel pin in a fuel assembly irradiated to 74.2GWd/t peak burnup in the fast reactor “JOYO.” In the examination, X-ray CT images of transverse cross sections of fuel pin were obtained at different heights of fuel pin along its axis. Analysis of the resulting images indicated that: 1. The hexagonal wrapper tube had its lateral wall faces slightly bulged outward; 2. The fuel pins loaded in the outermost array were markedly displaced in the direction of wrapper tube, particularly in portions of fuel pin intermediate between positions constrained by wrapping wire. The latter behavior of fuel pins was substantiated by the contours of fuel pin along its axis, which were derived from cross section images obtained at different levels along axis. Such fuel pin displacement is surmised to have been caused by thermal stressing of the affected fuel assembly cladding.

THREE-DIMENSIONAL X-RAY CT IMAGE OF AN IRRADIATED FBR FUEL ASSEMBLY

Abstract Three-dimensional X-ray computer tomography (CT) images were successfully taken of a fast breeder reactor fuel assembly that had been irradiated to high burnup. The interior and outside of the fuel assembly can be clearly observed on any cross section from any angle. These images make it possible to analyze deformations and microstructural changes in the fuel pins and abnormalities in the fuel assembly. An analysis was made for 127 central voids, i.e., one in each fuel pin of the traverse cross section, and the void sizes were tentatively related to the linear heat rating. Compared with conventional nondestructive and destructive postirradiation examinations (PIEs), this X-ray CT technique has great advantages including acquiring large numbers of PIE data in a short time, reducing PIE costs, reducing the amounts of radioactive waste generation, and physically protecting nuclear materials.

Establishment of Post Irradiation Examination Technique Using High-Energy X-Ray Computer Tomography

The non-destructive examination technique using X-ray computer tomography was developed for the purpose of post irradiation examination. This X-ray CT technique is characteristic of using a pulsed high energy X-ray source in order to reduce the effects of gamma ray emissions from an irradiated fuel assembly, and then made it possible to obtain the clear cross section image.

FMF-Extension Project and Post-Irradiation Examination for MONJU Fuel.

高速増殖原型炉「もんじゅ」の運転に伴い,照射された炉心燃料等の炉心構成要素が炉心より取り出されるが,これらは建設中のFMF増設施設において照射後試験を実施する。そのデータは貴重であり,照射挙動を十分に把握することは,今後の高速炉燃料開発にとって不可欠である。 本稿では,「もんじゅ」炉心構成要素等の照射後試験計画および試験実施施設,試験機の概要・特徴について解説する。