A practical cesium axial migration model for fast reactor fuel pins

Cesium is a volatile fission product which plays a major role in the formation of gap-filling compounds and internal cladding corrosion in Fast Breeder Reactor fuel pins operating at elevated temperatures and high burnup. In order to compute reliably the physical and chemical state of the gap, accurate fission product inventories are required. However, volatile species are known to significantly migrate down the radial and axial thermal gradients upon release from the fuel. In the present work, a pragmatic axial diffusion model for cesium is proposed, based on the proven concept of saturation concentration, and an empirical law for computing the vapor pressure of cesium at equilibrium with MOX fuel. This approach is shown to be predictive of the axial distribution of cesium measured through gamma spectrometry for a large database of fuels. Cesium accumulations, which are highly sensitive to local thermal gradients, are satisfactorily reproduced, contributing to the validation of the PLEIADES/GERMINAL V3 fuel performance code multiphysics coupling.