Abstract
The introduction of Fuel Assembly with Inner Duct Structure (FAIDUS) is being studied 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 Core Disruptive Accidents (CDAs) of Sodium-cooled Fast Reactors (SFRs). In the current reference design for FAIDUS, it is expected that the molten fuel will be discharged from the reactor core toward the upper sodium plenum through the inner duct. An out-of-pile experiment with visual observation was conducted to clarify the fundamental mechanism for upward discharge of a high-density melt through the inner duct. In the experiment, molten Wood's metal was injected into a simulated inner duct, which was filled with water. Through this experiment, the following mechanisms have been observed. 1) At the early stage of melt discharge, a coolant void was formed by vaporization of the liquid coolant, and subsequent upward void development excluded the liquid coolant from the melt discharge path. 2) Coolant vapor pressure was built up at the lower void boundary, and the melt was discharged upward by the coolant vapor flow. These experimental results support the possibility of upward discharge of a high-density melt by coolant vapor through the inner duct structure.
