Prediction of Dryout Heat Flux of Volumetrically Heated Particulate Bed Packed with Multi-Size Particles

Abstract

This paper presents MEWA code calculations for the experiments performed on the POMECOHT facility to investigate the dryout heat flux of various particulate beds, with the objective to interpret the experimental data and validate the code. The code is then applied to coolability assessment for ex-vessel debris beds related to severe accident scenarios of a boiling water reactor (BWR). The characteristics of a prototypical debris bed, such as multidimensionality and multiple particle sizes are emphasized in this study. The volumetrically heated particulate beds of the POMECO-HT experiments are packed with multi-size particles and equipped with a downcomer to investigate the bottom-fed coolability by natural circulation which demands 2D simulation. The results show that the MEWA code is capable of predicting the coolability of the bed with a downcomer (2D) as well as the top-flooding bed whose dryout heat flux can also be predicted by the Reed model (1D). Given the effective particle diameter (1 mm) and porosity (0.45) defined from a few FCI tests, the ex-vessel debris beds for a BWR chosen here are coolable with varied margins: i) compared with a top-flooding bed (spreading over the entire floor of the cavity), the cylindrical configuration with an annular-gap water supply enhances the coolability comparison , but the gain is marginal since the large diameter of the bed prevents the side coolant from flowing into the center of the bed; ii) a heap-like debris bed reduces rather than improves coolability due to its considerable height and base diameter; iii) a stratified debris bed with a fine-particle layer on the top may challenge the coolability.