Development of safety assessment methodology on fuel element failure propagation in SFR and its application to Monju

高速炉における燃料ピン破損伝播評価手法の開発と「もんじゅ」への適用

深野 義隆  

Fukano, Yoshitaka

ナトリウム冷却高速炉(SFR)の開発当初から炉心局所事故は過酷事故の一つの原因と考えられ、多くの国で研究が実施された。燃料ピンの自然破損は、その発生頻度の高さと破損伝播の可能性から、最も支配的な起因事象と考えられている。炉心局所事故の最新レビューによって燃料ピンの自然破損からの損傷拡大(FEFPA)に至る4つのメカニズムを抽出した。本研究では、熱的,機械的,化学的破損伝播を含むFEFPAの評価手法を開発し、全てのSFRに適用可能な安全評価コードとして整備するとともに、「もんじゅ」のFEFPAを評価した。その結果、「もんじゅ」ではFEFPAが起こる可能性は低く、仮に損傷拡大を仮定しても、損傷は最大でも1集合体内に留まることを明らかにした。この結果は「もんじゅ」の経済性を向上させる燃料ピン破損後の長期運転の可能性を示すものである。

Probabilistic and deterministic safety assessments and experimental studies on local fault (LF) propagation in sodium cooled fast reactors (SFRs) have been performed in many countries because LFs have been historically considered as one of the possible causes of severe accidents. Adventitious fuel pin failures were considered to be the most dominant initiators of LFs in these probabilistic assessments because of high frequency of occurrence during reactor operation and possibility of subsequent pin-to-pin failure propagation. Four possible mechanisms of fuel element failure propagation from adventitious fuel pin failure (FEFPA) are extracted from the state-of-the-art review of open papers. All the methods for FEFPA including thermal, mechanical, chemical propagation are modeled into a safety assessment code which is applicable to all the SFRs with the development of some methods. Furthermore the assessment on FEFPA in Monju was performed using this methodology. It was clarified that FEFPA was highly unlikely and limited within at most one sub-assembly in Monju by the multiple and diverse detection and shutdown systems for FEFPA even assuming the propagation. This result also suggests future possibility of run beyond cladding breach which enhances the economic efficiency in Monju.