Experimental and computational studies of the coolability of heap-like and cylindrical debris beds

The COOLOCE (Coolability of Cone) test facility has been used at VTT for experimental investigations of the coolability of porous debris beds with different geometries. The main objective of the experiments was to compare the coolability of a heap-like (conical) debris bed configuration to that of a cylindrical, top-flooded debris bed. Few previous debris coolability studies have investigated the effect of the possible ex-vessel debris bed geometries, and the experiments aimed to provide new data on this topic. In a heap-like configuration, lateral flooding through the surface of the heap (or cone) is expected to increase dryout power while the height of the configuration can reduce it, and thus decrease coolability. The experimental results suggest that the coolability of the conical debris bed is poorer than that of the cylindrical bed assuming that the formation of the first dry zone is taken as the coolability limit. Computational analysis of the experiments and prediction of dryout power has been performed using the MEWA 2D code (developed at IKE, University of Stuttgart) to verify its applicability in 2D situations. This is of high importance concerning reactor scale assessment. In addition, 3D scoping simulations of the particle bed dryout process have been done by using the two-phase flow solver PORFLO developed at VTT. The COOLOCE experiments are performed considering a fully quenched water filled bed. However, when a debris bed is formed, particles will initially be hot and dry. Therefore, it is also very important to consider quenching of an initially hot and dry particle bed because quenching versus heat-up by decay heat determines the coolability in the initial stages of reactor scenarios. In this respect, an application of the MEWA code to reactor conditions by considering an initially hot and dry conical bed formed by settling of particles from breakup of melt jets flowing into a water-filled cavity is presented. It has been observed that quenching during bed formation indicates substantial coolability margins compared to quenching of an already established dry debris bed which was considered in a previous study