On certain aspects of alternative core disruption accidents in LMFBRs

Abstract

This paper examines the initial course of a postulated accident scenario in an LMFBR, involving the rupture of all piping connected to the reactor vessel, in the event of an earthquake (or an equivalent scenario involving both loss of heat removal and system rupture). The core is successfully shut down, but decay heat imposes a threat to core integrity. At the onset of pipe rupture, the fuel elements are cooled by natural circulation, followed by subcooling and nucleate boiling. Continuation of sodium evaporation leads to core dryout, clad melting, and subassembly wall failure. Clad melting is found to occur at a location close to the top of the core at a rate of 0.08 ft 3/s. The sodium vapor velocity is not high enough to carry the molten steel to the upper blanket region; therefore, flow-channel blockage in the lower axial blanket region is expected. At the time of clad melting, the fuel temperature rises by approximately 2°F/s, while the temperature-rise rate at the can wall, due to heat radiation from the fuel pin, is 10°F/s. Failure of the can wall initiates gross fuel motion. Continuation of the heat generation in the fuel pellet leads to the melting of the control rod support. Both the fuel motion and the control rod failure mark the start of reactivity insertion.