Performance analysis of the second stage accumulators in BNPP-1 in response to MB-LOCA with SBO: A support for practical modification in the injection line

Abstract

During the last decades, the substitution of passive safety systems instead of active safety systems has increased the reliability of new commercial nuclear power plants. This effort is much more significant for generation (Gen) III + reactors in comparison with Gen II and III reactors. Hydro-accumulators, as a part of the emergency core cooling system (ECCS), is a common passive system among different Gens to encounter the loss of cooling accidents (LOCAs). Recently, the application of advanced and multi-stage accumulators has become successful in GEN III + reactors. Bushehr nuclear power plant (BNPP-1) as a Russian VVER-1000 is somehow categorized between II and III Gens, but it is the first VVER-1000 equipped with two stages of accumulators. This article investigates the influence of Second Stage Accumulators (SSAs) of BNPP-1 on postponing the core damage time in an MB-LOCA with Station Blackout (SBO). Also, SSA's injection line is upgraded after the engineering judgments of start-up tests, and accordingly, each line was equipped with a Flow Restriction Component (FRC). Besides, the effect of FRCs on postponing of the core damage time is assessed in this work. Extracted results achieved from both start-up tests and RELAP5 simulations showed that, although the SSAs are useful in supplying water in MB-LOCA with SBO as a Design Extension Condition (DEC), the application of FRCs are much more effective in postponing the core damage time and without them, SSAs discharge instantly. The results of this study verify the utilization of SSAs with their associated FRCs. Also, the results confirm that some design modifications in BNPP-1 as a commercial reactor can provide resistance against a DECs. Moreover, this experience gained in BNPP-1 could be turned into a basis for safety systems designs in newer VVER-1000s and also VVER-1200 reactors.