Effect of porosity of frozen metal fuel on pressure drop inside SFR core structure under initiating phase of postulated severe accident

Abstract

After the Fukushima Daiichi accident in 2011, reactor safety studies have focused on accident mitigation strategies under postulated accident scenarios that exceed the design basis. With respect to safety issues of a metallic fuel in sodium-cooled fast reactor (SFR), a recent study has been conducted to categorize the severe accident scenario and establish reliable mitigation strategy for each accident phase. In an initial phase of metal-fueled SFR severe accident, cladding breach occurs with instantaneous eutectic reaction between the metallic fuel and the cladding material. The molten core material would be fragmented and frozen in core structures. It is verified that any molten core material formed during the accident progression is rendered in coolable state. Recently, Argonne National Laboratory (ANL) has conducted fragmentation tests where uranium was poured into a sodium-filled channel including the SFR core structure. They would reveal the effects of the core structure on the extent of melt fragmentation and coolability. To evaluate this coolability and structure of the fragmented molten material inside the core structure should be investigated. In the present study, the coolability was discussed by measuring a pressure drop of the fragmented masses using the simulants. A test facility for pressure drop measurement is constructed as an oil based loop which can measure the pressure drop of the test section resulted from the fragmentation tests. A series of pressure drop tests were performed to characterize the coolability of the fragmented masses inside the core structure. The pressure drops over a wide range of test conditions are described in this paper, having physical relationship between a coolability of the fragmented molten material and the pressure drop values.