Coolability of corium spread onto concrete under water, the PERCOLA model

The paper presents a theoretical simplified model called PERCOLA whose objective is to estimate the time required to percolate the melt (interacting with concrete) into a debris bed (under water) such that the resulting upper and lower crusts can be cooled by conduction. It is supposed that the concrete decomposition gases sparging through the melt entrain part of the liquid through openings (cracks) in the upper crust. The entrainment coefficient is the main parameter of the model. The entrained melt solidifies in the water layer and is supposed to deposit to form a debris bed, which is cooled by the water. The interest of this model is to perform parametric calculations in order to state if the hypothesized scenario seems credible or not and if further development along this line makes sense or not. The results indicate that the transformation of the liquid melt into a debris bed is obtained for not unrealistic sets of parameters and low entrainment coefficients. The MACE scoping test also reveals the existence of a debris bed, despite the fact that the upper crust is separated from the liquid melt. It is concluded that further realistic experiments are needed. (orig.)