Development of the mechanistic code MFPR for modelling fission-product release from irradiated UO2 fuel

The models of the mechanistic code MFPR developed in collaboration between IBRAE (Moscow, Russia) and IRSN (Cadarache, France) are described. Exhaustive description of fission-gas behaviour in grain and out of grain is given in relation with individual validation results on analytical experiments under various conditions (steady irradiation, transient, post-irradiation annealing). It is shown that microscopic defects in the UO2 crystal structure can strongly influence fission-gas transport out of grains and release from fuel pellets. These defects include point defects such as vacancies, interstitials and fission atoms, and extended defects such as bubbles, pores and dislocations. A model for the dislocation generation and evolution in irradiated UO2 fuel has been developed and implemented in the mechanistic code MFPR along with a fuel densification model. Being combined with the set of equations describing evolution of point defects (vacancies and interstitials) and their interactions with fission-gas bubbles, a completely self-consistent consideration of the whole system of point and extended defects in irradiated UO2 fuel has been obtained. The mechanistic description of chemically active-elements behaviour is also presented. It 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. Examples of application to the VERCORS 4 and 5 experiments show the possibilities of the code in the frame of severe-accident interpretation