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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ト壁の早期・高熱流束破損の発生機構に関する研究

A Study on mechanism of early failure of inner duct wall within fuel subassembly with high heat flux from molten core materials based on analysis of an EAGLE experiment simulating core disruptive accidents in an LMFBR

豊岡 淳一; 遠藤 寛*; 飛田 吉春; 二ノ方 壽*

Toyooka, Junichi; Endo, Hiroshi*; Tobita, Yoshiharu; Ninokata, Hisashi*

本研究では、FBRにおいて炉心損傷事故が発生した場合でも、燃料集合体内に設置した内部ダクトを介して溶融燃料を炉心外に排出し厳しい再臨界を防止する設計対策(FAIDUS)の原理的有効性を確認する観点から、カザフスタンIGR炉で実施されたID1試験結果に伝熱挙動解析コードTAC2D及び炉心損傷事故解析コードSIMMER-IIIを適用し、高温融体と接触したダクト壁の伝熱・破損機構を明らかにする目的で体系的な伝熱挙動評価を行った。本研究で得られた知見から、ID1試験で見いだされたダクト壁の早期の高熱流束破損機構はFAIDUS集合体への要求条件「炉心損傷事故時に燃料集合体のラッパ壁が破損する以前の早期段階においてFAIDUS集合体の内部ダクト壁が破損し、燃料集合体内に生成された溶融燃料が集合体外に流出すること。」を大略満足し、FAIDUS概念の原理的成立性を支持していると判断される。

In the design of JSFR (Japan Sodium-cooled Fast Reactor), a design measure (FAIDUS: Fuel sub-Assembly with an Inner DUct Structure) is considered to prevent severe re-criticality events even in case of core disruptive accidents by molten fuel ejection out of the core region through the duct equipped within the fuel subassembly. Confirming principle effectiveness of such design measure is important. In this study, systematic heat transfer behavior of the ID1 test, which was conducted in IGR (Impulse Graphite Reactor) in Republic of Kazakhstan, was evaluated applying a heat conduction code TAC2D and a reactor safety analysis code SIMMER-III focusing on the clarification of heat transfer from high-temperature mixture of molten fuel and steel to the duct. As a result, the duct failure by high heat flux from the mixture was identified as one of an important mechanism 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 without the presence 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 to FAIDUS, i.e., the inner duct of FAIDUS should fail at an early phase of core disruptive accident in advance to wrapper tube failure so that produced molten fuel can escape from the core region, and it supports feasibility of the FAIDUS concept.

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