Thermal analysis of a reactor lower head with core relocation and external boiling heat transfer

A thermal analysis has been performed to assess the viability of external water flooding as a cooling strategy to prevent reactor vessel thermal failure in the case of a severe accident with partial core melting and core relocation to the reactor vessel lower head. Vessel wall temperatures and heat fluxes were predicted over a range of decay heating values using a one-dimensional heat conduction model in conjunction with literature correlations for the local heat flux distributions associated with natural convection inside the molten debris pool. A finite-element prediction of flow and heat transfer inside the internally heated molten pool was also obtained. Vessel wall temperature results were highly dependent on the distribution chosen to represent heat transfer along the lower surface of the melt region. The literature correlations, which were obtained for low Rayleigh number laminar natural convection in two-dimensional rather than axisymmetric geometries predicted much higher heat fluxes for the upper melt surface compared to the lower surface. The finite-element computational fluid dynamic predictions, obtained for turbulent natural convection indicated similar heat flux levels for the upper and lower surfaces. Vessel wall temperature predictions were satisfactory when the literature heat flux correlations were used, but inside vessel wall temperatures in excess of the melting point were indicated when the finite-element heat flux distribution was applied.