Severe accident analysis to evolve insight for severe Accident Management Guidelines for Large Pressurised Heavy Water Reactor

The Pressurised Heavy Water Reactor (PHWR) contains both inherent and engineered safety features that help the reactor become resistant to severe accident and its consequences. However in case of a low frequency severe accident, despite the safety features, procedural action should be in place to mitigate the accident progression. Usually for all these designs the Emergency Operating Procedures (EOPs) are developed in support of detailed accident analysis, which gives an adequate coverage for design basis accidents. Currently the designers are making provisions in design to mitigate progression of accidents arising from multiple failure accidents like Large Break Loss of Coolant Accident with failure of Emergency Core Cooling System and failure of moderator as heat sink. Many designs of Large PHWRs have adopted the approach of symptom based EOPs to handle multiple failure events as currently practiced for Light Water Reactors (LWRs). Severe accident analysis is an important aspect which complements Severe Accident Management Guidelines (SAMG) development process. These analysis provide insight into the accident progression and basis to develop the SAMG. The order of uncertainty in modelling the phenomena is very high. Hence it is emphasized that different computational models be used so that an un-biased "insight" can be evolved which can be used for SAMG development. The paper discusses two categories of severe accident analyses for such large reactors for multiple failure transients involving a high pressure scenario (initiation event like SBO) and low pressure scenario (initiating event like LOCA). The insight evolved from these analysis is being discussed in the paper.