Coupled MELCOR/COCOMO analysis on quench of ex-vessel debris beds

The cornerstone of severe accident strategy of Nordic BWRs is to flood the reactor cavity for the long-term coolability of an ex-vessel debris bed. As a prerequisite of the long-term coolability, the hot debris bed formed from fuel coolant interactions (FCI) should be quenched. In the present study, coupling of the MELCOR and COCOMO codes is realized with the aim to analyze the quench process of an ex-vessel debris bed under prototypical condition of a Nordic BWR. In this coupled simulation, MELCOR performs an integral analysis of accident progression, and COCOMO performs the thermal-hydraulic analysis of the debris bed in the flooded cavity. The effective diameter of the particles is investigated. The discussion on the bed's shape shows a significant effect on the propagation of the quench front, due to different flow patterns. Compared with MELCOR standalone simulation, the coupled simulation predicts earlier cavity pool saturation and containment venting. (c) 2021 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http:// creativecommons.org/licenses/by/4.0/).

Automated summary

The cornerstone of severe accident strategy of Nordic BWRs is to flood the reactor cavity for the long-term coolability of an ex-vessel debris bed. In this study, the coupling of MELCOR and COCOMO codes was used to analyze the quench process of an ex-vessel debris bed, showing that particle diameter and bed shape have significant effects on the quench front propagation. The coupled simulation predicts earlier cavity pool saturation and containment venting compared to standalone simulation.