MECHANISM OF UPWARD FUEL DISCHARGE DURING CORE DISRUPTIVE ACCIDENT IN SODIUM-COOLED FAST REACTORS

摘要

The elimination of severe power excursion with significant mechanical-energy release during Core Disruptive Accidents (CDAs) is a key issue for the enhanced safety of Sodium-cooled Fast Reactors (SFRs). In order to prevent the formation of a large-scale molten fuel pool within a reactor core, which is one of factors leading to the severe power excursion during CDAs, Japan Atomic Energy Agency (JAEA) is studying the introduction of Fuel Assembly with Inner Duct Structure (FAIDUS). In the current reference design for FAIDUS, the top end of the inner duct is open whereas the bottom end of the inner duct is closed, and therefore it is expected that the molten fuel will be discharged from a reactor core towards an upper sodium plenum through the inner duct. The objective of the present study is to clarify the fundamental mechanism for upward fuel discharge through the inner duct structure in FAIDUS, and thereby to confirm the effectiveness of FAIDUS. In the previous paper, the possibility of upward discharge of a high-density melt driven by coolant vapor was confirmed by visual observation in the JAEA's out-of-pile experiment, in which molten Wood's metal (density at the room temperature: 8700 kg/m3, melting point: 352 K) simulating the molten fuel was injected into a coolant channel (equivalent inner diameter: 30 mm, total height: 2 m, fluid content: water) simulating the inner duct structure. In this paper, the mechanism of upward discharge of a high-density melt driven by coolant vapor pressure and/or flow in this experiment is discussed in terms of the application to reactor conditions. Through this discussion, the following mechanisms were clarified. 1) Coolant vapor pressure is built up within the coolant channel after the melt injection. The magnitude of the pressure buildup becomes larger with increase of melt-enthalpy-injection rate which is defined by the product of melt-mass-injection rate into the coolant channel and melt specific enthalpy. 2) Following the pressure buildup, the melt is discharged upward being driven by the coolant vapor flow directing towards the top opening end of the coolant channel. The upward discharge mass rate becomes higher with the increase of the magnitude of the pressure buildup and therefore melt-enthalpy-injection rate. From these experimental knowledge, it was suggested that the coolant pressure buildup could act as one of the driving force for the upward discharge of a high-density melt through the inner duct structure in FAIDUS under reactor conditions with higher melt-enthalpy-injection rate than the current experimental condition.