Experimental and analytical investigation of quenching of superheated debris beds under top-reflood conditions. Final report

A final report is presented of a combined experimental and analytical investigation of the quench characteristics of packed beds of superheated particles which are cooled by water supplied by an overlying pool. The objective of the work was to develop a model, for use in containment loading calculations with codes such as MARCH or CONTAIN, to predict the steam generation rate resulting from quench of high-temperature particulate debris beds in light-water reactor severe accidents and, in addition, to assess coolability of such debris beds under quench conditions. The results suggest that bed quench proceeds via a single- or two-stage frontal propagation process. In the two-stage process liquid penetrates the bed leaving the pockets or channels of unquenched particulate which remain hot until the final cooling front reaches the particles. Only 25-50% of the bed stored energy is removed during the initial penetration period. The bed heat flux, observed to be nearly constant during both frontal stages, is compared with the available bed dryout heat flux data. The quench and dryout heat flux data overlap and agree reasonably well with the steady-state Lipinski dryout heat flux model. From this result it is concluded that the mechanism which limits the bed cooling rate in both the transient quench and steady-state dry-out scenarios is the two-phase countercurrent bed hydrodynamics