Melting and multi-phase flow modelling of nuclear fuel in fast reactor fuel rod

Melting of nuclear fuel in a fast breeder reactor is accompanied by simultaneous release of gaseous fission products. These gases are trapped inside porosities present in the nuclear fuel and escape upon melting. The simultaneous release of fission gas and molten fuel results in a transient multi-phase flow. Initially, the fission gases are highly pressurized and exert hydrodynamic forces on the molten fuel. This in turn causes the molten fuel to undergo displacement. This displacement of molten fuel influences the reactor kinetics and the overall accident outcome. The present work is aimed at developing a mathematical model which simulates the melting and multi-phase flow phenomenon in a fast reactor fuel rod under a slow transient over-power accident scenario. The developed model is validated with the experimental data of the CABRI-E9 test. Thereafter, the behaviour of molten fuel is analysed with a nominal case study and parametric changes. Finally, the influence of molten fuel motion over reactor kinetics is analysed. The results show that under slow transients, molten fuel primarily relocates towards the lower portion of the fuel inner cavity. This behaviour of molten fuel can positively assist in the mitigation of a slow transient over-power accident.