High burnup rim project. (IV) Threshold burnup of rim structure formation

High burnup extension of LWR fuel is progressing to reduce the amount of total process flow in the nuclear fuel cycle and eventually to reduce the fuel cycle costs. As a result, the local burnup is now exceeding the anticipated range of the UO2 fuel that was investigated in the great time of the 1960's. A 'new phenomenon', a crystallographic re-structuring, is commonly observed at the rim area of high burnup fuel pellets in LWRs, and also in FBRs to some extent. The objectives of the High Burnup Rim Project (HBRP) are to identify the conditions of the rim structure formation as functions of burnup and temperature, and to investigate physical and chemical properties of fuels following this re-structuring. After the irradiation, the rods were transported to the Institute for Transuranium Elements (ITU), Karlsruhe for Post Irradiation Examinations (PIE). This report shows recent progress of PIE, and discusses threshold burnup and temperature of the rim structure formation (author) (ml)