In-pile study of thermal interactions between high-energy molten UO2 fuel and liquid sodium

Two in-pile transient experiments were performed in the TREAT Reactor Facility to investigate fuel-coolant interaction phenomena that might occur in a hypothetical prompt-burst disassembly accident involving extensive fuel vaporization in a nearly unvoided liquid-metal fast breeder reactor core. In these tests, a single fuel pin containing 28 g of UO2 was subjected to a self-limited 23-msec-period TREAT power excursion which deposited fission energy of 1700 cal/g of UO2 in the fuel. The pin was contained in an instrumented autoclave filled with stagnant sodium. Failure of the fuel pins occurred at a mean energy input of about 540 cal/g, corresponding to a UO2 vapor pressure of about 100 atm. Results of these tests indicated that no energetic fuel-coolant interaction was produced and that the measured transient pressures can be reasonably described by the time history of the fuel vapor pressure.