OPR1000의 중대사고 시 다양한 SAMG 진입조건을 이용한 1차 계통 감압 전략의 평가

Abstract

Severe accident analysis for Korean OPR1000 with MELCOR 1.8.6 was performed by adapting a mitigation strategy under different entry conditions of Severe Accident Management Guidance (SAMG). The analysis is focused on the effectiveness of the mitigation strategy and its adverse effects. Four core exit temperatures (CETs) were selected as SAMG entry conditions; and Small Break Loss of Coolant Accident (SBLOCA), Station Blackout (SBO), and Total Loss of Feed Water (TLOFW) were selected as postulated scenarios that may propagate into severe accidents. In order to delay reactor pressure vessel (RPV) failure, entering the SAMG when the CET reached 923 K, 923 K, and 753 K resulted in the best results for SBLOCA, SBO, and TLOFW scenarios, respectively. This implies that using event-based diagnosis for severe accidents may be more beneficial than using symptom-based diagnosis. Preventing high pressure melt ejection (HPME), one of the objectives of mitigation strategy, was not achieved for three cases (SBLOCA-973K, SBO-923K and TLOFW-973K). Therefore, more active depressurization strategy is needed for those cases to prevent HPME. There is no significant difference among selected SAMG entry conditions in light of the operator’s available action time before the RPV failure. Potential vulnerability of the RPV due to hydrogen generation was analyzed to investigate the foreseeable adverse effects that act against the accident mitigation strategies. For the SBLOCA cases, mitigation cases generated more hydrogen than the base case. However, the amount of hydrogen generated was similar between the base and mitigation cases for SBO and TLOFW. Hydrogen concentrations of containment were less than 5 % before RPV failure for most cases.|한국표준형원전에 대한 중대사고 해석이 MELCOR 1.8.6 버전을 이용하여 수행되었다. 해석은 중대사고 관리지침서에 기술되어 있는 중대사고 관리전략을 적용하여 수행되었으며, 관리전략의 효과 및 부작용이 집중적으로 분석되었다. 네 개의 노심출구온도가 중대사고 관리지침서 진입조건으로 선정되었고, 안전주입이 없는 소형파단냉각재상실사고, 전원상실사고, 완전급수상실사고 등 중대사고로 진행될 수 있는 확률이 높은 세개의 초기 사건이 해석 대상으로 선정되었다. 소형파단냉각재상실사고, 전원상실사고, 완전급수상실사고는 노심출구온도가 각각 923 K, 923 K, 753 K일 때 관리전략을 적용하면 원자로용기 손상시간이 가장 지연 되었다. 이것은 중대사고 시에 사용되는 증상기반 진단보다 사건기반 진단이 더 효과적일 수도 있음을 시사한다. 완화전략의 목적 중 하나인 노심용융물 고압분출은 세 가지 경우 (SBLOCA-973K, SBO-923K, TLOFW-973K)에서 달성되지 못했다. 따라서 이 세가지 경우에 대해서는 노심용융물 고압분출을 막기 위해 더 적극적인 감압전략이 필요할 것으로 판단 된다. 운전원에게 가용한 조치 시간의 관점에서 보면, 중대사고 관리지침서 진입조건들 간에 큰 차이는 없었다. 감압전략의 부작용으로써 수소 생성량이 분석되었다. 소형파단냉각재상실사고의 경우, 완화전략을 취했을 때 취하지 않은 경우에 비해 더 많은 수소가 생성되었다. 하지만 전원상실사고와 완전급수상실사고의 경우에는 완화전략을 취하나 취하지 않으나 비슷한 양의 수소가 발생했다. 격납건물 내 수소 농도는 대부분의 경우에서 원자로용기 손상시까지는 5%를 넘지 않았다.; and Small Break Loss of Coolant Accident (SBLOCA), Station Blackout (SBO), and Total Loss of Feed Water (TLOFW) were selected as postulated scenarios that may propagate into severe accidents. In order to delay reactor pressure vessel (RPV) failure, entering the SAMG when the CET reached 923 K, 923 K, and 753 K resulted in the best results for SBLOCA, SBO, and TLOFW scenarios, respectively. This implies that using event-based diagnosis for severe accidents may be more beneficial than using symptom-based diagnosis. Preventing high pressure melt ejection (HPME), one of the objectives of mitigation strategy, was not achieved for three cases (SBLOCA-973K, SBO-923K and TLOFW-973K). Therefore, more active depressurization strategy is needed for those cases to prevent HPME. There is no significant difference among selected SAMG entry conditions in light of the operator’s available action time before the RPV failure. Potential vulnerability of the RPV due to hydrogen generation was analyzed to investigate the foreseeable adverse effects that act against the accident mitigation strategies. For the SBLOCA cases, mitigation cases generated more hydrogen than the base case. However, the amount of hydrogen generated was similar between the base and mitigation cases for SBO and TLOFW. Hydrogen concentrations of containment were less than 5 % before RPV failure for most cases.