NUMERICAL SIMULATION OF TEMPERATURE FIELD IN STRATIFIED CORIUM AND EVALUATION OF NUCLEAR REACTOR WALL MELTING DURING SEVERE ACCIDENT
Numerical simulation of combined heat transfer for the in-vessel corium retention problem is presented. An asymptotic description of the natural convection in the corium pool proposed by L. I. Zaichik for high Rayleigh number limit is employed. By use of the effective conductivity model, the transient thermal state of the corium pool with a surface layer of molten steel and the heating and melting of the reactor vessel are considered simultaneously as a two-dimensional conjugated problem, including the formation of oxide crust layers. The calculations confirm a high local heat flux density toward the wall in the region of the molten steel layer. It has been also found that the steel surface temperature increases considerably to the pool center. The latter result is important for correct evaluations of the radiative flux to overlying in-vessel elements and the vessel wall.
A combined theoretical model is developed for the quasi-steady thermal state calculations. This model is based on heat balance equation for oxide pool, one-dimensional effective conductivity model for the molten steel layer and two-dimensional conduction problem for the melting vessel wall. The combined model gives more correct (and not so high) values of specific heat flux to the vessel wall than those obtained by use of the known isothermal models, which do not tak-into account temperature variation along the molten steel or on the corium pool surface.