Effectiveness evaluation methodology of the measures for improving resilience of nuclear structures against excessive earthquake

過大地震時の原子炉構造レジリエンス向上策の有効性評価技術

栗坂 健一 ; 西野 裕之  ; 山野 秀将   

Kurisaka, Kenichi; Nishino, Hiroyuki; Yamano, Hidemasa

本研究の目的は破損拡大抑制技術によって過大地震時の原子炉構造レジリエンス向上策の有効性評価手法を開発することである。安全上重要な機器・構造物のレジリエンス向上策によって耐震裕度が増すとみなす。同向上策の有効性を評価するため、炉心損傷頻度CDFを指標に選び、CDFの低減を 地震PRAによって定量化する。崩壊熱除去機能喪失に至る事故シーケンスがナトリウム冷却高速炉SFRの地震時CDFに有意な寄与を示す。また、同事故シーケンスは超高温を経て炉心損傷に至る。本研究では過大地震時の振動への対策のみならず超高温での対策も評価するよう手法を考案した。手法の適用性を検討するため、ループ型SFRを想定して試計算を実施した。仮定した範囲内では、レジリエンス向上策は設計地震動の数倍の地震までCDFを有意に低減する効果を示した。適用性検討を通じて、有効性評価手法が開発された。

The objective of this study is to develop an effectiveness evaluation methodology of the measures for improving resilience of nuclear structures against excessive earthquake by applying the failure mitigation technology. This study regarded those measures for improving resilience of important structures, systems, and components for safety to enlarge their seismic safety margin. To evaluate effectiveness of those measures, seismic core damage frequency (CDF) is selected as an index. Reduction of CDF as an effectiveness index is quantified by applying seismic PRA technology. Accident sequences leading to loss of decay heat removal are significant contributor to seismic CDF of sodium-cooled fast reactors (SFRs), and those sequences result in core damage via ultra-high temperature condition. This study improved the methodology to evaluate not only the measures against shaking due to excessive earthquake but also the measures at the ultra-high temperature condition. To examine applicability of the improved methodology, a trial calculation was implemented with some assumptions for a loop-type SFR. Within the assumption, the measures for improving resilience were significantly effective for decreasing CDF in excessive earthquake up to several times of a design basis ground motion. Through the applicability examination, the methodology for the effectiveness evaluation was developed successfully.