小破断LOCA時高圧注入系不作動事象への統計的手法の適用: 破断口感度解析

摘要

統計的安全評価手法の適用により，最適評価結果に対する不確かさが定量的に評価され，過度の保守性を合理的に排除することで信頼性の高い評価結果が期待される.現在,小破断LOCA (冷却材喪失事故）時高圧注入系不作動事象におけるアクシデントマネジメント「2次系強制冷却による低圧注入」解析への統計的安全評価手法の適用を進めている.小破断LOCA時において，破断口のサイズ等の条件はPCT (被覆管最高温度）に影響するため，重要なパラメータである.そこで本研究では，2次系強制冷却を開始するタイミングが実際の事故時運転操作条件より厳しい条件のもとで，実機プラントを対象に破断口のサイズ，向き，位置を変更した感度解析を行った.感度解析の結果から，破断口の条件に依存して変わる事象進展の違いを評価するとともに，低温側配管3インチ下向き破断を統計的安全評価の基本ケースに決定した.%By applying a statistical safety evaluation method, the uncertainties of best estimate results can be estimated quantitatively, and as a consequence, excessive conservatism statistical evaluation method is being made to analyses of the "low pressure injection by intentional depressurization of the steam generator secondary side" which is an accident management approach in a SBLOCA (small break loss-of-coolant accident) with HPI (high pressure injection) failure. At the time of a SBLOCA, the break conditions such as the break size are important parameters since they influence PCT (peak cladding temperature). In this research, sensitivity analyses about the break size, direction and position were carried out for a system plant under a condition which the start timing of the steam generator secondary side intentional depressurization is severer than an actual abnormal operating condition. From the result of the sensitivity analyses, differences in the phenomena progression which change depending on the break conditions were evaluated, and a 3 inch facing-down break of the cold-leg was determined as the base case of a statistical safety evaluation.