Fuel Vapor Generation in LMFBR Core Disruptive Accidents

Methods to calculate bounding values for the generation of fuel vapor during disassembly and during expansion of the fuel after disassembly in a fast-reactor core-disruptive accident were developed. Isentropic expansion of the fuel following disassembly with no fuel mixing before expansion was assumed. It was necessary to develop consistent thermodynamic fuel properties for the analysis. The method was applied to a liquid-metal fast breeder reactor disassembly, first with sodium in the core and then with sodium removed. Bounding values were also compared to lower values obtained by assuming mixing and thermal equilibrium of the fuel prior to expansion. For the bounding calculation with sodium removed, 4.6% of the fuel vaporized when the expanded fuel occupied all of the available volume. This value was reduced to 0.9% when mixing and thermal equilibrium prior to expansion was assumed.