Mechanistic modelling of urania fuel evolution and fission product migration during irradiation and heating

The models of the mechanistic code MFPR (Module for Fission Product Release) developed by IBRAE in collaboration with IRSN are described briefly in the first part of the paper. The influence of microscopic defects in the UO2 crystal structure on fission-gas transport out of grains and release from fuel pellets is described. These defects include point defects such as vacancies, interstitials and fission atoms, and extended defects such as bubbles, pores and dislocations. The mechanistic description of chemically active elements behaviour (fission-induced) is based on complex association of diffusion–vaporisation mechanism involving multi-phase and multi-component thermo-chemical equilibrium at the grain boundary with accurate calculation of fuel oxidation. In the second part, results of the code applications are given to different situations: normal LWR reactor operation, high temperature annealing, loss of coolant accident (LOCA) and severe accidents conditions.