Development of fission gas release model for MOX fuel pellets with treatment of heterogeneous microstructure

Abstract

ABSTRACT This study develops a new fission gas release (FGR) model for mixed oxide (MOX) fuel pellets with fundamentally heterogeneous microstructures depending on the mixing methods of UO2 powder and PuO2 powder in fabrication process. The Japan Atomic Energy Agency developed a fuel performance modeling software FEMAXI-8 with an original FGR model supposing the microstructure to be homogeneous. The new FGR model explicitly treats the heterogeneity by computing the fission gas migration in two substructures independently. The model was applied to analyses of irradiation tests at the Halden reactor, in which two types of MOX fuels had different heterogeneity in their microstructure, while other fuel specifications were similar. The homogeneous model significantly underestimated FGR from the fuel with a remarkably heterogeneous microstructure. The new FGR model gave better calculations of the FGR behaviors of the remarkably heterogeneous fuel, namely the earlier onset of FGR and larger FGR at the end of irradiation than the relatively homogeneous fuel. The improved agreement with the experimental observation confirmed a certain validity of the developed model for estimating FGR from MOX fuels and studying heterogeneity effects.