Explosive Disassembly of Fast Reactors

Abstract

Theoretical analysis of the explosive disassembly of fast reactors, following accidental loss of coolant and melting and gravity compaction of the fuel material,has been undertaken. A general expression for the rate of reactivity feedback dueto material disassembly in a core with more than one enrichment zone has been derived. Energy release in cylindrical geometry has been calculated with the effects due to zonal interfaces properly considered in the limit of zero acoustic speeds. The effect on the energy release of the assumption of fixed fuel density in the fuel equation-of-state has been investigated for oxide-fueled zoned cores.Fuel zoning can result in either greater or less energy release, as compared with a homogeneous core of the same fuel inventory and the same initial conditions, depending on the gradient of the density worth function at the zonal interface. In some large fast breeder reactors the gradient of the density worth function may be negative at the zonal interface. The positive pressure gradient at the interface during the disassembly phase of a transient results in inwardly directed fuel displacement at the interface and in a positive contribution to the disassembly reactivity feedback. Typically, this can increase the energy released in a disassembly transient by as much as 35%. The effect on energy release of the assumption of fixed fuel density in the fuel equation-of-state was not significant for the transients initiated in the completely molten sodium-free reactor cores studied in this investigation.