Transient thermal response of a hemispherical lower head upon contact with a heat-generating melt pool

A numerical study is performed to determine the thermal response of a reactor vessel lower head following a sudden contact of the core melt with the vessel wall during a severe accident. The major objective is to determine the transient wall temperature distribution, including possible hot spots within the lower head. The mathematical formulation involves a nonlinear coordinate transformation which serves the dual purpose of fixing the moving interface between the crust and the melt pool, and simplifying the computational domain by converting the crust and wall regimes into two adjacent square regions of equal length. The system is nodalized using an orthogonal grid with equal spacing in both the crust and steel regions. Results from the actual crust thickness distribution indicate a sharp decline in thickness from 4.37 × 10^-2 m at the bottom center to 3.04 × 10^-3 m at the upper edge. These results indicate that a hot spot is likely to form at the upper edge of the vessel. However, it would take an increase in molten pool heat flux of 80% to initiate wall melting at the upper edge. This means that the pool heat flux in an actual core-meltdown accident would have to be 80% larger than the anticipated heat flux to initiate wall melting. Being that this scenario is highly improbable, flooding the reactor cavity with water proves to be an effective method for removing the decay heat generated during a core-meltdown accident.