Direct numerical simulation of turbulent natural convection in a heat generating fluid within a shallow horizontal enclosure

Core catchers are a significant safety feature of Generation-IV nuclear reactors for the safe dissipation of decay heat and maintaining the integrity of the reactor containment in the event of a severe accident. Direct numerical simulation is used to simulate turbulent natural convection in a shallow layer of dense oxidic corium, with significant internal heat generation ( R a i = 4.5 × 1 0 8 ), at the base of a core catcher. The simulation revealed a large persistent convection cell adjacent to the pitched lateral boundary, smaller eddies comparable in dimension to the fluid depth closer to the centre of the core catcher and enhanced dissipation of turbulent kinetic energy in the upper half of the fluid. Around 73 % of the heat generated within the fluid is dissipated from the upper boundary. The surface and time-averaged heat flux at the upper and lower boundaries are in good agreement with Emara and Kulacki predictions in the shallow limit. Significant spatiotemporal fluctuations in local Nusselt number are observed, with local flares in heat flux manifesting in local Nusselt numbers of several times the surface average. These local flares are less frequent at the lower and lateral boundaries but greater in amplitude relative to the surface-averaged heat flux. Further research is required into the duration and frequency of these flares, given the potential of persistent local flares to induce crust remelting or localised thermal attack of the core catcher.