3D simulation in the PLEIADES software environment for sodium fast reactor fuel pin behaviour under irradiation

In the framework of the basic design of ASTRID (Advanced Sodium Technological Reactor for Industrial Demonstration), the GERMINAL fuel performance code is developed in the PLEIADES software environment. In order to improve one dimensional modelling of GERMINAL, a 3D simulation for the SFR fuel pin behavior under irradiation has been proposed. The 3D model represents a single pellet fragment and its associated piece of cladding. The scale transfer between this single fragment model and the fuel pin scale is achieved through appropriate boundary conditions given by GERMINAL results. A preliminary 3D thermo-mechanical computation scheme has been implemented in the LICOS code of the PLEIADES platform. In this approach, chemo-physical state variables are pre-computed by the GERMINAL code and are introduced in the 3D computation scheme as some input data in a two-step procedure. First studies have been achieved in order to analyze pellet-to-cladding gap closure mechanisms at the beginning of irradiation. Two mechanisms of fuel relocation have been identified through the 3D simulation. The first one is linked to the hourglass shape of the fragmented fuel pellet under thermal gradient, and the second one is induced by the mass transfer due the central-hole formation and fuel restructuring. Thanks to our 3D results, the gap closure rate given by the fuel relocation displacement model of GERMINAL can be interpreted. The next step is now to propose a fully coupled formulation between fuel mass transfer model and the radial relocation displacement model. This new formulation will be used to improve the 1D computation scheme of GERMINAL and will prepare further developments to extend the GERMINAL fuel performance code to the 3D simulation. (author)