In-vessel retention analysis for a typical PHWR

Abstract

A very unlikely sequence of events with multiple failures of safety systems and human actions may lead to severe accidents like simultaneous occurrence of loss of coolant accident $$\left( \text {LOCA} \right)$$ , failure of emergency core cooling system $$\left( \text {ECCS} \right)$$ and moderator cooling system. This may result in the core disassembly and debris formation in the calandria vessel bottom. It is imperative to study the coolability/retention of the debris/corium in the calandria vessel in the presence of the vault water as the only available heat sink. Analysis is performed with the objective to study the in-vessel retention and coolability of the corium in the calandria vessel by external cooling through calandria vault inventory. In-vessel retention capability with respect to the failure criterion based on failure due to reduced external cooling which is one of the six failure modes cited in the literature is investigated. Duration of the availability of the calandria vault water for heat removal from the debris is also estimated. This study is performed using system thermal hydraulic code RELAP5 and various cases are analysed with different initial debris temperature and initial time of the transient $$\left( \text {decay power} \right)$$ . Additional cases are analysed to study the effect of oxidation of $$\text {Zr}$$ , porosity of the debris and the gap between the debris and calandria vessel. It is observed that the presence of gap and porosity decreased the heat transfer and consequently increased the debris temperature. Generally, the heat flux is found to be lower than the critical heat flux values which are reported in the literature for downward facing thick vessels. The coolability of the corium is achieved in all the cases investigated as long as the external calandria vault heat sink is available.