Investigation on Flow Characteristics in Radial Stratified Debris Bed

During the severe accident of light water reactors (LWRs), the particulate debris bed with porous structure may be formed at different places in the reactor after molten corium fuel coolant interaction (FCI). The coolability of the debris bed therefore plays an important role in corium risk quantification, which is crucial to the stabilization and termination of a severe accident in LWRs. Many experimental and analytical studies have been conducted towards quantitative understanding of debris bed coolability. However, most of previous studies were conducted based on the homogeneous debris beds packed with single size particles, and only a few investigations were performed with the heterogeneous debris beds like stratified debris bed. In fact, scoping studies on debris bed formation and configuration based on FCI experiments indicate that the stratified debris bed would be most expected. In order to study the flow characteristics in heterogeneous debris beds, the packed porous beds with radial stratification were constructed in the present study using two different sizes glass spheres with the diameter of 2 mm and 8 mm respectively. Besides, the homogeneous packed beds packed with single size particles and uniform mixture by the above two size particles were also constructed for comparison. The particles were packed in a cylindrical test section with the inner diameter of 120 mm and the height of 600 mm. Single-phase flow tests were performed on the homogeneous beds and heterogeneous bed firstly to investigate the flow resistance characteristics in the packed beds with different configurations. Then numerical simulation was also conducted to reveal the flow redistribution of stratified bed, especially on the flow field at the stratified interface. The experimental results show that the pressure drops of single-phase flow in the homogeneous beds can be well predicted by Ergun equation. For the radial stratified packed bed with different permeability layers, the pressure drops in each layer of the stratified bed are almost equal and increase with the liquid inlet flowrate. Comparing with those in the homogenous beds packed with the same size particles as those in different layer of stratified bed, the pressure drops in the stratified bed are much lower than those of homogenous bed packed with smaller size particles, while slightly higher than those with larger size particles. The numerical simulation results state that there is a two-dimensional flow phenomenon in the radial stratified bed. In addition to dominate upward flow in the stratified bed, a lateral flow flows from low permeability layer to high permeability layer. The two-dimensional flow in stratified bed decreases the flowrate and pressure drops in low permeability layer and increases the pressure drops in high permeability layer. With the increase of liquid flowrate, the average lateral flowrate at the stratified interface increases, but the ratio of lateral volume flowrate to total fluid volume flowrate decreases.