Numerical investigation of turbulent natural convection in the lower plenum of sodium cooled fast reactor under core relocation scenario

Turbulent natural convection inside the lower pool of pool-type sodium cooled fast reactor (SFR) is numerically studied for the modified Boussinesq number (Bo∗) range of 5 × 109 to 2 × 1011. The enclosure considered is a geometrical model of the lower plenum of a typical pool type SFR main vessel with the in-vessel type core catcher assembly. Aim of this study is to analyze the decay heat removal rate from horizontally spread core debris on the core catcher during post accident heat removal (PAHR) condition under core meltdown scenario in SFR and correlate the same with the associated non-dimensional parameters. The mass, momentum and energy conservation equations have been numerically solved in cylindrical co-ordinates using finite volume method and using SIMPLE algorithm for pressure-velocity coupling. Turbulence has been modelled using k-ε model and the computational model is validated against benchmark numerical and experimental studies on natural convection found in literature. PAHR has been simulated under two scenarios of decay heat removal (DHR), viz., under pump driven DHR condition above the lower plenum (i.e., under Total Instantaneous coolant Blockage (TIB) scenario) and under completely passive DHR condition (i.e., under Unprotected Loss Of Flow Accident (ULOFA) scenario). Nusselt number (Nu) is correlated as a function of Bo∗ for natural convection above the debris bed on the top surface of the core catcher and also below the bottom surface of the core catcher. These conservative correlations can be used in the design of in-vessel core catcher in pool-type SFR in order to ensure safe retention of radioactivity within the primary containment system even under whole core accident scenarios (WCA).