Experimental and numerical investigations on debris bed coolability in a multidimensional and homogeneous configuration with volumetric heat source

Within the framework of severe reactor accident studies, we present experimental and numerical parametric studies on debris bed coolability. Data are provided by the SILFIDE multidimensional experimental facility at Electricite de France. The bed is composed of inductively heated steel sphere beads (diameters ranging from 2 to 7.18 mm) contained in a 50- X 60- X 10-cm vessel. Numerical computations are obtained with MC3D REPO developed by Commissariat a l'Energie Atomique. Because of heterogeneous power distribution within the bed, two definitions (mean and local) for the critical heat flux (CHF) are proposed. Even in the first case, the CHF was higher than the Lipinsky one-dimensional flux. As the power is being increased, temperature plateaus above saturation temperature are observed. An analysis is proposed, based on possible different hydrodynamic flow configurations occurring in postdryout regimes. In some experiments, some spheres were superficially molten and stacked together, but globally, the bed was still coolable. The influence of operational parameters such as bottom coolant injection, height of the water, fluidization of upper particles, and subcooled liquid injection on dry-out phenomena and CHF values are also described. The MC3D-REPO calculations assuming a thermal equilibrium between the three phases gives results in accordance with experimental data.