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

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

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.