A 3D numerical study on the breakup and fragmentation of corium jet in liquid sodium pool following a severe accident in SFR

Understanding the fragmentation of the corium jet which is molten nuclear fuel in liquid sodium during a severe accident is crucial for the safety analysis of Sodium cooled Fast Reactors (SFR) safety analysis. This study numerically investigates the breakup and fragmentation of mixed oxide (MOX) fuel jet in the liquid sodium using three dimensional simulations. Volume of Fluid method coupled with adaptive mesh refinement is adopted in the present study for hydrodynamic fragmentation. A surface solidification model is also harnessed to estimate the final size distribution of the fragments after thermal fragmentation. The computational model is first validated with an MFCI experiment of molten alumina poured in liquid sodium from published literature. Further, breakup of MOX jet in liquid sodium at different jet breakup regimes was analysed and the size distribution of fragments generated was studied. The results show that the mass median diameter of the fragments due to thermal effects is about one order of magnitude smaller than that from hydrodynamic effects, but the number of fragments is about two orders of magnitude higher. This study also estimates the increase in surface area after corium fragmentation in liquid sodium. The particle size distribution is an important factor which influences the coolability of degraded core in liquid sodium pool. • Numerical study on the breakup and fragmentation of MOX jet in liquid sodium is done. • Hydrodynamic fragmentation is modelled using VoF-AMR in BASILISK code. • Surface solidification model is used for secondary thermal fragmentation of drops. • The model is validated with MFCI experiment of molten alumina in liquid sodium. • Size distribution of fragments generated during MFCI are evaluated.