Evaluation of Effectiveness and Adverse Effects of Independent Severe Accident Measures of OPR1000 Using MELCOR

Abstract

Since the Fukushima accident occurred in 2011, many researches related to severe accident have been conducted experimentally and numerically. Considering the catastrophic consequences from the severe accident, more vigorous measures to mitigate hypothesized severe accident need to be secured. In Korea, currently 11 Optimized Power Reactor 1000 (OPR1000) are under operation and electrical power from these fleets take up almost half the total power generation. For the OPR1000, Severe Accident Management Guidelines (SAMG) were prepared based on the Westinghouse Owner’s SAMG but it is necessary to perform more vigorous quantitative analysis to investigate effectiveness and adverse effects of independent severe accident mitigation measures suggested in the SAMG. Thus, in this study, seven severe accident mitigation measures (three in-vessel mitigations and four ex-vessel mitigations) incorporated in the OPR1000 SAMG were carefully analyzed and employed to mitigate the severe accident under the hypothesized severe accidents of Small Break Loss of Coolant Accident (SBLOCA) without Safety Injection (SI), Station Black Out (SBO), and Total Loss of Feed Water (TLOFW). MELCOR 1.8.6 code was used to analyze various accident progressions of thermal-hydraulic response, fuel cladding oxidation, fuel melting, core relocation, reactor pressure vessel failure, containment leak, to mention a few. Effectiveness and adverse effects of individual severe accident mitigation measure were analyzed in terms of delay of reactor pressure vessel failure, oxidation heat generation, hydrogen concentration, and delay of containment leak. The MELCOR simulation shows that the first mitigation strategy, Mitigation 1, feeding water into the steam generators is the most effective among the other mitigations. In addition, Mitigation 2, Reactor Coolant System (RCS) depressurization alone could not mitigate the severe accident sufficiently. Among four ex-vessel mitigation strategies, Mitigation 7, a reliable operation of Passive Autocatalytic Recombiners (PARs) was essential in reducing the hydrogen risk. Nonetheless, local heating and steam generation due to catalytic reaction could be an issue threating the containment integrity. It was also shown that use of spray to control the fission products and containment state may result in undesirable adverse effect of condensing steam and subsequent increase of partial pressure of hydrogen, which may contribute on raising the possibility of hydrogen combustion. In conclusion, this study suggests that mitigation measures should be carefully selected and encountering measures should be prepared for the possible adverse effects.