SAS4A analysis on hypothetical total instantaneous flow blockage in SFRs based on in-pile experiments

炉内試験に基づく仮想的集合体入口瞬時完全閉塞事象のSAS4A解析

深野 義隆  

Fukano, Yoshitaka

炉心局所事故(LF)はナトリウム冷却高速炉(SFR)において、その開発当初からシビアアクシデントの一つの要因と考えられてきた。LFの3つの主要な起因事象、燃料ピンの自然破損、局所過出力、大規模閉塞の影響に対する「もんじゅ」の安全評価は既に実施され、1集合体内の損傷に留まる結果が得られている。仮想的集合体入口瞬時完全閉塞事象(HTIB)の発生頻度は無視できるが、HTIBはLFの中で最も厳しい結果を与えるため、炉内試験が行われてきた。このため、本研究では、過去の実験を基に最も早い事象進展シナリオを新たに提案するとともに、HTIBに関連する現象に対して追加的にその妥当性を確認したSAS4Aコードを使用して、HTIB事象の影響に対する「もんじゅ」の安全評価を実施した。本評価によって、HTIB事象の影響は、「もんじゅ」のシビアアクシデントの代表的事象であるULOF事象よりはるかに小さいことを明らかにした。したがって「もんじゅ」における全てのLFの影響はULOF事象に包絡されると言える。この評価手法は他のSFRにおける保守的な安全評価にも適用可能である。

Local fault (LF) has been historically considered as one of the possible causes of severe accidents in sodium-cooled fast reactors (SFRs). Safety assessments on the consequences of three major LF initiators, adventitious-fuel-pin-failures, local over-powers and large flow blockages, have already been performed for the Japanese prototype fast breeder reactor (Monju) and resulted in limited damage of within a fuel subassembly. Although the frequency of hypothetical total instantaneous flow blockage at the coolant inlet of a fuel subassembly (HTIB) is negligible, in-pile experiments on this event have been performed because it gives the most severe consequences among the above-mentioned LF initiators. Therefore a most rapid damage propagation scenario was newly proposed in this study based on past experiments, and a safety evaluation was performed on the consequences of HTIB in Monju using the SAS4A code, additionally validated by an in-pile experiment especially on the HTIB-related phenomena. It was clarified through the evaluation that the consequences of HTIB were much less severe than that of the unprotected loss of flow (ULOF) events which represent the most severe accidents in Monju. Therefore it was concluded that all the consequences of LFs in Monju can be comprised in that of ULOF. This evaluation method would be also applicable to enveloping safety assessments on LFs in arbitrary SFRs.