Study on the law of melting and melt migration of single fuel plate

BackgroundOnce the flow channel of plate fuel assembly is blocked, it is easy to cause a fuel melting accident. It is of great significance for the safe operation of the plate fuel reactor to study and master the melting of plate fuel and the migration and expansion process of molten material, so as to avoid the continuous expansion of molten material.PurposeThis study aims to simulate the melting and melt migration behavior of monolithic fuel plate and to master the melting and melt migration behavior characteristics of the fuel plate after the coolant channel is completely blocked.MethodsFirstly, based on the standard fuel assembly of JRR-3M research reactor, the geometric model of a single fuel plate was extracted, and a half fuel plate model with symmetrical boundary was established based on the mirror symmetry. Secondly, the geometric structure of the fuel plate was greatly simplified by replacing the fuel meat and its cladding on both sides with equivalent fuel. Furthermore, based on the assumption that the axial power distribution of the fuel did not change with time and the heat release rate of the fuel was only related to the volume fraction of the fuel, the calculation model of the heat release rate of the fuel was obtained. Finally, based on the commercial computational fluid dynamics (CFD) software Fluent, the Volume of Fluid (VOF) method and the enthalpy-porosity method were used to simulate the fuel melting and melt migration behavior of JRR-3M fuel plate.ResultsThe simulation results show that the time period from the start to the end of molten material migration is no more than 6 s after the fuel plate melts; the migration process of the molten material expands obviously in the transverse direction, and quickly migrates to the adjacent fuel plate along the vertical direction of the fuel plate, and then stops at 25~30 mm from the central symmetry plane along the radial direction of the fuel plate. That means the molten material does not penetrate the aluminum plate at the edge of the assembly.ConclusionsThe coupling of VOF and enthalpy-porosity method proposed in this study offers a new solution for the analysis and research of plate fuel melting accident, providing technical support for the analysis of assembly-level melting accident and the development of fuel melting and melt migration mechanism model.