Flow resistances characteristics in a particulate bed with the configurations of uniform mixture and stratification

During severe core melting accidents of light water reactors with failures of all cooling systems, the molten core fuel (corium) would meet and interact with residual coolant water (FCI), then break up and fragment into a porous debris bed. Therefore, the debris bed coolability depending mainly on the flow friction laws, will be in great significance to nuclear reactor safety. This paper is concerned with reducing uncertainty in quantification of debris coolability in a severe accident of light water reactors (LWRs), and the experimental results on the flow resistance characteristics of homogeneous and stratified particulate beds are reported here. The objective is to get an idea of how the particles bed characteristics (such as the stratified information and its hierarchical arrangement) affect its flow resistance which is crucially important to debris bed coolability analysis. Three types of beds are packed in a cylindrical test section with the inner diameter of 120 mm and the height of 600 mm. Type-1 bed is a homogeneous bed packed with single size spheres. Type-2 bed is also a homogeneous bed with uniformly mixing by two sizes of spheres. Type-3 bed is the axially stratified bed which is composed of two sizes of glass spheres same as that in the Type-2 bed. Both single and two phase flow tests are carried out, the pressure drops and its flow resistance characteristics are measured and recorded during the tests. The results show that for gas-water co-current flow through a homogeneous bed, the predictions of Reed model are more comparable with the measured pressure drops. For a bed packed with uniform mixture of particles, the measured pressure drops are close to the predictions of Ergun equation with area mean diameter at low flowrate (e.g. Rep < 7), but the length mean diameter should be considered as increasing of the Reynolds number of fluid. Compared with the homogeneous bed with the same particles, the stratified bed will generate a lower flow resistance, and consequently result in a higher dry-out heat flux under boiling conditions.