Natural convection in a shallow pool heated from below and implications for the thermal focusing effect at the lateral wall

Convection within shallow pools of liquid metals heated from below is of significant interest for the In-Vessel Retention (IVR) strategy for Pressurised Water Reactors (PWR) as focusing of the lateral heat flux at the reactor wall presents a risk to the thermomechanical integrity of the reactor vessel. Under an IAEA Coordinated Research Project on corium melt retention, various international research institutions have performed CFD simulations to predict the thermal–hydraulic behaviour of a prototypic light metal layer of low Prandtl number ($\mathrm{Pr}=0.02$) and high external Rayleigh number ($R{a}_{\Phi }\sim {10}^{12}$) dissipating heat from the free surface and at the lateral reactor wall. Various computational approaches including LES-WALE, LES-Smagorinsky and spectral-DNS were validated under the conditions of two BALI-Metal experiments in water ($\mathrm{Pr}=6.9$), revealing promising agreement in the predicted repartition of the heat flux at the vertical and lateral boundaries. Simulations in a prototypic light metal layer indicated 30–34 % of heat dissipation due to thermal radiation at the free surface. Average thermal losses at the lateral wall corresponded to a focusing effect of 3.3–3.7 times the imposed heat flux. A spike in lateral heat flux close to the free surface equated to a local focusing effect 6-times the imposed heat flux from below. The fluid dynamics, driven largely by thermal losses at the reactor wall, were characterised by downwards acceleration adjacent to the lateral wall and ejection of a cold jet parallel to the lower boundary, forming a large convection cell comparable in size to the radius of the reactor.