The kwu fission gas release model for lwr fuel rods

Abstract

The KWU model for fission gas release of LWR fuel is based on the physical processes of fission gas release as reported in the published literature and on KWU post-irradiation examination results. The model is composed to two different submodels, which predict the steady-state and the transient fission gas release, respectively. The steady-state submodel can be divided into two main parts: (1) the fission gas produced is retained in the UO 2 matrix up to a certain saturation concentration, and all fission gas exceeding this matrix concentration is collected at grain boundaries. (2) the rate of fission gas release df/ dt is assumed to be proportional to the gas inventory g at the grain boundaries: df// dt = Kg. The factor K depends on temperature, burnup and open porosity. The submodel for transient fission gas release is presently based on the assumption that transient release is caused by grain boundary separations. The transient gas release calculated in the model depends on the gas inventory and on the power increase during the transient. The model is implemented in the KWU fuel rod computer code CARO and calibrated against measured fission gas release values of approximately 100 KWU fuel rods.