Analytical studies on fuel element failure propagation due to adventitious fuel pin failure in small to large size sodium-cooled fast reactors

小型から大型のナトリウム冷却炉における燃料ピンの自然破損からの燃料要素破損伝播に関する解析研究

深野 義隆  

Fukano, Yoshitaka

炉心局所事故はナトリウム冷却高速炉(SFR)においてシビアアクシデントの一つの原因となり得ると歴史的に考えられてきたため、確率論的、決定論的安全評価や実験研究が多くの国で実施されてきた。燃料ピンの自然破損は、その頻度の高さと破損伝播の可能性から、これらの確率論的安全評価において、炉心局所事故の最も支配的な起因事象と考えられてきた。既往研究では、燃料ピンの自然破損からの損傷拡大(FEFPA)に至る可能性のある4つのメカニズムが同定された。これらのメカニズムは任意のSFRに適用可能な安全評価コードにモデル化された。この評価手法を用いて常陽、「もんじゅ」(現行炉心及び高度化炉心)、JSFRの4つの炉心におけるFEFPAの安全解析を実施した。その結果、4つの異なる炉心設計によって解析結果は異なるものの、これらのSFRでFEFPAが生じる可能性は非常に低いことを本研究で明らかにした。これらの結果はSFRの経済性を向上させる破損後の継続運転の将来的可能性を示唆するものである。

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 have been considered to be the most dominant initiators of LFs in these probabilistic assessments because of its high frequency of occurrence during reactor operation and possibility of subsequent pin-to-pin failure propagation. The four possible mechanisms of fuel element failure propagation from adventitious fuel pin failure (FEFPA) were identified in the past study. All the mechanisms for FEFPA analysis including thermal, mechanical and chemical propagation were modeled into a safety assessment code which was applicable to arbitrary SFRs. Safety analyses on FEFPA of Japanese experimental fast reactor (JOYO), Japanese prototype fast breeder reactor (Monju), Japanese prototype fast breeder reactor with upgraded reactor core (Upgraded Monju) and Japan sodium-cooled fast reactor (JSFR) were performed using this methodology. Although analytical results were different owing to the different core designs in four SFRs, it was clarified in this study that FEFPA was highly unlikely in these SFRs. These results also suggest future possibility of long-term run-beyond-cladding-breach operation which would enhance the economic efficiency in SFRs.