SCDAP/RELAP5 Modeling of Heat Transfer and Flow Losses in Lower Head Porous Debris

iv Designs are described for models to calculate the heat transfer and flow losses in porous debris in the lower head of a reactor vessel. The COUPLE model in SCDAP/RELAP5 represents both the porous and nonporous debris that results from core material slumping into the lower head. Previously, the COUPLE model had the capability to model convective and radiative heat transfer from the surfaces of nonporous debris in a detailed manner and to model only in a simplistic manner the heat transfer from porous debris. In order to advance beyond the simplistic modeling for porous debris, designs were developed for detailed calculations of heat transfer and flow losses in porous debris. Correlations were identified for calculating convective heat transfer in porous debris for the following modes of heat transfer; (1) forced convection to liquid, (2) forced convection to gas, (3) nucleate boiling, (4) transition boiling, (5) film boiling, and (6) transition from film boiling to convection to vapor. Interphase heat transfer is modeled in an approximate manner. Correlations were identified for calculating the flow losses and interphase drag of fluid flowing through the interstices of the porous debris. These calculations for flow loss and interphase drag were integrated into the momentum equations in the RELAP5 part of the code. Since the models for hea