A safety analysis for a core catcher concept in case of a hypothetical severe accident of an 875 MWth helium cooled fast breeder reactor

A hypothetical case has been considered in which the core of a 875 MWth Helium cooled fast breeder reactor is totally molten-down. A "core catcher concept" with a sacrificial steel-bed is suggested which should encapsulate the core melt in a safe way. Its thermal performance is surveyed through a CFD-analysis. The simulation was performed using the volume of fluid (VOF) and phase change models within the FLUENT CFD-code. The heat source within the two phase melt (steel and UO2) was considered as a function of the reactor thermal power, the time that the reactor has been operating under normal conditions, and the time after shut-down. Three options have been considered, with bottom-cooling using the RNG k-epsilon and the standard-k-epsilon turbulence models and without bottom-cooling using standard k-epsilon turbulence model. In all considered cases it was found that the suggested core catcher concept matches the safety requirements. Using SCALE-code oil the basis of CFD-simulation results criticality analyses were performed in addition to ensure if the situation in the core catcher can be considered as sub-critical from neutron-physical point of view. The results showed a sub-critical condition all through in core catcher.