The models of the mechanistic code MFPR developed by IBRAE (Moscow, Russia) in collaboration with IRSN (Cadarache, France) 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 behavior (fission-induced) is based on complex association of diffusion-vaporization 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 in different situations : normal LWR reactor conditions, high temperature annealing conditions, Loss Of Coolant Accident (LOCA) and severe accidents situations.
(1) : Nuclear Safety Institute of the Russian Academy of Sciences, Moscow, Russia
(2) : IRSN