EAGLE試験解析に基づく高速炉炉心損傷事故時の燃料集合体内部ダク卜壁の早期•高熱流束破損の発生機構に関する研究

摘要

日本原子力研究開発機構(JAEA:Japan Atomic Energy Agency)では,高速増殖炉サイクル実用化研究開発 (FaCTプロジェクト:Fast Reactor Cycle Technology Development Project)を推進している。ここでは,高速増 殖炉（FBR),再処理,燃料製造などに関するこれまでの研究開発により得られた知見や事故の経験等を踏まえ,幅広いFBRサイクル技術の中から技術選択肢の評価を行い,革新的な技術を取り入れつつ,競争力のある実用化候補概念の構築とその研究開発計画などの検討•策定を行つている。FaCTプロジェクトの中核的研究の1つは,将来の軽水炉に比肩し得る経済性を有するループ型ナトリウム冷却高速炉（JSFR~2):Japan Sodium-cooled Fast Reactor)の設計と安全性確保を実現するための技術開発である。%In the design of a Japanese sodium-cooled fast reactor (JSFR) , a design measure (fuel subassembly with an inner duct structure; FAIDUS) is considered to prevent severe recriticality events even in the case of core disruptive accidents by molten fuel ejection out of the core region through the duct equipped within the fuel subassembly. Confirming the principle effectiveness of such a design measure is important. In this study, the systematic heat transfer behavior in the ID1 test, which was conducted in the impulse graphite reactor (IGR) in Republic of Kazakhstan, was evaluated by applying a heat conduction code TAC2D and a reactor safety analysis code SIMMER-Ⅲ focusing on the clarification of heat transfer from a high-temperature mixture of molten fuel and steel to the duct. As a result, the duct failure caused by high heat flux from the mixture was identified as one of an important mechanisms of early duct failure in FAIDUS. It was also suggested from this study that the high heat flux from the mixture is caused by the direct contact of molten steel in the absence of fuel crust on the duct wall. Based on these findings, it is judged that the mechanism of early duct failure with high heat flux obtained in the ID1 test satisfies the required condition for FAIDUS, i.e., the inner duct of FAIDUS should fail at an early phase of core disruptive accident in advance of wrapper tube failure so that the produced molten fuel can escape from the core region, which supports the feasibility of the FAIDUS concept.