The elimination of severe power excursion during core disruptive accidents is a key issue for the enhanced safety of sodium-cooled fast reactors. In order to prevent the formation of a large-scale molten fuel pool within a reactor core, which is one of the factors leading to the severe power excursion, the Japan Atomic Energy Agency (JAEA) is considering 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 is closed, and therefore it is expected that the molten fuel will be discharged from the reactor core towards the 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, and thereby to confirm the effectiveness of FAIDUS. The possibility of upward discharge of a high-density melt driven by coolant vapor has been confirmed by the JAEA's experiment, in which molten Wood's metal 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 the 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, the melt-enthalpy-injection rate. The experimental knowledge obtained from the JAEA's experiment suggests that the coolant pressure buildup could act as one of the driving forces 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 simulant experimental condition.

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