Multi-physics modelling of the pellet-to-cladding gap closure phenomenon for SFR fuel performance codes

The reference fuel design for french Sodium nuclear Fast Reactor (SFR) consists of fuel pins made of (U,Pu)O2 pellets inserted in a steel alloy cladding tube. Fuel pin behaviour under irradiation is complex and simulated with SFR fuel performance codes through the world. Concerning the thermal behaviour, the pellet-to-cladding gap evolution has a strong impact on the fuel maximal temperature and hence has to be precisely modelled. Based on experimental observations, the gap size evolution seems to be related to two phenomena: one related to the effect of pellet fragmentation and the second one related to a porosity migration phenomenon. The second phenomenon is due to the presence of high thermal radial gradients and leads to a fuel restructuration. The aims of this work are first to investigate with 3D simulation the impact of the fuel restructuration and of the fuel fragmentation on the pellet fragments radial displacement, and then to propose a new 1D physically based relocation model and its coupling formulation with the SFR codes multi-physics computational scheme. Thanks to these new developments the pellet-to-cladding gap closure simulation and hence the pellet temperature assessment, can be improved in a 1D fuel performance code.This work has been done in the framework of a cooperative program between CEA, FRAMATOME and EDF, devoted to the development of the fuel elements for GENIV Reactors.