Preliminary analysis on downward discharge of the molten metallic fuel in lower strucures of the fuel Assembly

Abstract

Downward discharge behavior and porosity analysis were conducted in this paper. In the HCDA of the metal fueled SFR, fuel melts. The molten fuel is ejected into the coolant channel by accumulated pressure inside of the cladding owing to fission gas. Some amount of the ejected fuel goes out from the core by levitation and other part of the molten fuel discharges downward and influences further progression of the accident in the point of coolability. Molten fuel can form not coolable geometry and decay heat from the debris bed can induce further progression of the accident. Wood’s metal and water were selected as simulants of fuel and sodium coolant. Experimental conditions were based on the ULOF analysis in SAS4A code. Two kinds of experiments about discharge of the molten fuel through lower structures of the fuel assembly from the active core region to out of the core were conducted, LOF-DT01 and LOF-DT02. Experimental apparatus for LOF-DT01 had scale-downed, simplified geometry with similar hydraulic dimeter and apparatus for LOF-DT02 had 1/2 scale-downed exact same geometry with lower structures of the fuel assembly in PGSFR. Analysis on the distribution of the debris was conducted by high speed camera in LOF-DT01 and radiography in LOF-DT02. Porosity was measured in case of debris bed with significant amount. In both experiments, about 80 % of the melts was founded in lower reflector and 15 % of the melts was founded in nosepiece. Different to original concern, melt was not accumulated at the receptacle orifice because the most of the debris was stuck before receptacle. Except one case, porosity was measured as high as about 80 %, which can secure low pressure drop. Porosity of the debris bed formed at the three-forked branched flow path inside of the lower reflector showed 51 % and 60 % porosity, which is still large enough to ensure low pressure drop and coolability.