Abstract
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.
Highlights
During a severe accident of the light water reactor, the reactor core would gradually heat up, degrade and relocate into the lower plenum of the vessel
The severe accident strategy employed by Nordic BWR is to flood the cavity into a deep water pool
The coupling of MELCOR and COCOMO is developed to extend the MELCOR capability for analysis of ex-vessel debris bed coolability, in order to satisfy the contemporary need on the analysis of quenching process for ex-vessel debris beds formed in postulated severe accidents of Nordic BWR
Summary
During a severe accident of the light water reactor, the reactor core would gradually heat up, degrade and relocate into the lower plenum of the vessel. The ‘‘dryout” heat flux or power density, considered as the debris bed maximum capability for the decay removal, has received extensive studies both on experiment and model development (Hu and Theofanous, 1991; Lipinski, 1982; Thakre et al, 2014). These studies normally assume the initial condition as a thermal equilibrium state between the debris bed and the saturated water pool. The COCOMO code is used to perform the numerical simulation for the quench process of the prototypical ex-vessel debris bed, for the safety analysis of the severe accident strategy of Nordic BWR.
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