Rim characteristics and their effects on the thermal conductivity in high burnup UO2 fuel

Characteristics of high burnup UO2 fuel such as threshold burnup for the fo rmation of high burnup microstructure (rim), rim average burnup and rim wid th were estimated and then the thermal conductivity degradation due to the porous rim region was investigated. The threshold burnup for rim formation was estimated as a function of tempe rature and fission rate using Rest's model. The calculated threshold burnup , which shows a particular dependence on temperature, ranges from 40 to 50 MWd/kgU at typical fuel periphery temperatures of 400 to 600 degrees C. In addition, the rim average burnup and the rim width were obtained by statist ical analysis of the data available in open literature. To consider the additional degradation of thermal conductivity in the rim r egion, a formula for rim porosity was presented with the assumption that ri m pores are overpressurized and that all the produced fission gases are ret ained in the rim pores. To estimate the thermal conductivity in the porous rim using the general co rrection method applicable to two-phase structure, it was assumed that the rim region consists of pores and fully dense materials composed of UO2 matr ix and solid fission products. Then by combining the general model for two- phase with the rim porosity developed in the present paper and HALDEN's the rmal conductivity model, a thermal conductivity model for the porous rim re gion was developed. The predicted thermal conductivity shows an additional reduction of similar to 20% due to the porous rim structure which would cause to increase the f uel temperature of high burnup fuel during steady-state operation and trans ient irradiation.