Simulation of Transient Melting of CANDU Debris Bed under Severe Core Damage Accident

In a Loss-of-Coolant Accident (LOCA), the fuel channels inside of a CANada Deuterium Uranium (CANDU) reactor would collapse to the bottom of the calandria vessel and continue to heat and melt due to decay heat generation. The transient melting process of the terminal debris would impact the melt pool thermohydraulic behaviours and consequently affect the exiting heat flux, which is a key parameter for reactor safety assessments under severe accidents. Here, the transient heating and melting of a collapsed CANDU solid debris bed after moderator dry-out is simulated using computational fluid dynamics incorporating an enthalpy-porosity melting model. A time-varying decay heat source is considered, and the transient evolution of the debris bed temperature profile, flow field, and exiting heat are presented. Results from the current study can assist future modelling efforts on melt migration and debris bed deformation.