Basic experimental study with visual observation on elimination of the re-criticality issue using the MELT-II facility (2). Effects of upper pressure drop in a coolant channel and melt enthalpy injection rate upon void development behavior

It is effective to demonstrate a rapid and massive relocation of molten fuel from the reactor core at an early stage of the core disruptive accident (CDA) in fast breeder reactors for elimination of the re-criticality issue in which meaningful mechanical energy release is concerned. The purpose of the present study is to clarify individual phenomena related to a fuel discharge behavior from the reactor core. In the present study, a series of experiments with simulant materials has been carried out. In the experiments, simulant of molten fuel (Wood's Metal: density∼8500kg/m3, melting point 78.8degC) is injected into a coolant channel (water-filled) simulating the discharge path (e.g. control rod guide tube) from its side wall. In the previous report, it has been clarified that void development leading to enhanced fuel relocation is initiated by a sufficient reduction of coolant subcooling in the vicinity of the melt injection site, and that an early initiation of void development is highly probable under the reactor condition (MOX fuel/sodium coolant). In this report, dependency of void development behavior on related parameters to the reactor condition is analyzed from the experimental data. The following results have been obtained through the present experiments. 1. It was confirmed under a condition with high pressure loss at the upper part of a coolant channel that coolant void is forced to develop downward up to the lower plenum located under the coolant channel. The experimental fact indicates advantage of a reactor design with large pressure loss in the upper part of a fuel discharge path for enhanced fuel relocation. 2. The injection amount of melt enthalpy necessary to initiate void development tends to be constant, irrespective of its injection rate or melt / coolant mixing time. On the other hand, the injection amount of melt enthalpy strongly depends on coolant subcooling. This tendency suggests that initiation of void development is dominated by heat balance between melt and coolant. 3. Void development is supported by continuous formation of vapor pressure which increases with melt enthalpy injection rate into the coolant channel. 4. Larger melt enthalpy injection rate leads to a higher possibility of FCL (Fuel-Coolant Interaction) events at void boundaries which have potential to disturb continuous melt relocation into the lower plenum. From the above results, it has been confirmed that rapid establishment of void development over the fuel discharges path is highly probable under the reactor condition. However, it has been recognized that another important subject is clarification of FCIs at void boundaries and their influence on fuel relocation for demonstration of a rapid fuel discharge during a CDA. (author)