INTEGRAL ANALYSIS OF DEBRIS MATERIAL AND HEAT-TRANSPORT IN-REACTOR VESSEL LOWER PLENUM

An integral, fast-running, two-region model has been developed to char acterize the debris material and heat transport in the reactor lower p lenum under severe accident conditions. The debris bed is segregated i nto the oxidic pool and an overlying metallic layer. Debris crusts can develop on three surfaces: the top of the molten pool, the RPV wall, and the internal structures. To account for the decay heat generation, the crust temperature profile is assumed to be parabolic. The oxidic debris pool is homogeneously mixed and has the same material compositi on, and hence the same thermophysical properties, as the crusts, while the metallic constituents are assumed to rise to the top of the debri s pool. Steady-state relationships are used to describe the heat trans fer rates, with the assessment of solid or liquid state, and the liqui d superheat in the pool being based on the average debris temperature. Natural convection heat transfer from the molten debris pool to the u pper, lower and embedded crusts is calculated based on the pool Raylei gh number with the conduction heat transfer from the crusts being dete rmined by the crust temperature profile. The downward heat flux is tra nsferred to the lowest part of the RPV lower head through a crust-to-R PV contact resistance. The sideward heat flux is transferred to the up per regions of the RPV lower head as well as to the internal structure s. The upward heat flux goes to the metal layer, water, or available h eat sink structures above. Quenching due to water ingression is modele d separately from the energy transfer through the crust. The RPV wall temperature distribution and the primary system pressure are utilized to estimate challenges to the RPV integrity. Should the RPV be submerg ed, the heat removal is enhanced by the ex-vessel cooling due to nucle ate boiling. The convection heat transfer correlations for the molten debris pool were validated against available experimental data and the oretical predictions. Testing of the model for a range of conditions i n a PWR lower plenum produced consistent results. In addition, a compa rison of the integral approach to a more detailed, special purpose mod el showed good agreement.