An experimental study on flow characteristics of homogeneous and stratified debris beds

This paper is concerned with reducing uncertainty in quantification of debris bed coolability in a severe accident of light water reactors (LWRs). The experimental results on the flow characteristics of particulate homogeneous and stratified beds are reported here and the objective is to get an idea of how the particles bed characteristics (such as the stratified information and its hierarchical arrangement) affects its flow resistance which is crucially important to the debris bed coolability analysis. Three type 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 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 gradients and its flow 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's model are more comparable with the measured pressure drops. For a bed packed with uniform mixture of particles, the measured pressure gradients 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. Comparing with the homogeneous bed with the same particles, the stratified bed will generate a lower flow resistance, and consequently resulting in a higher dry-out heat flux under boiling conditions.