Bow in Experimental Breeder Reactor II Reflector Subassemblies

Abstract

A series of investigations was performed to identify the mechanisms responsible for severe fuel handling difficulties in the Experimental Breeder Reactor II. A combination of swelling-induced bow of inner row reflectors and relaxed thermal bow of outer row reflectors was found to be responsible for the observed problems. Substantial thermal bowing arose from the existence of reverse temperature gradients in the outer regions of the reflector, which were caused by the presence of high-temperature high-burnup uranium blanket subassemblies at the reflector-blanket interface. Subassembly rotation was found to be a successful strategy to alleviate swelling bow, but not relaxed thermal bow. Long-term trends of change in reactivity parameters were correlated with the development of the observed bowing distribution. This experience indicates that large fast breeder reactors, with core assemblies that have long residence times and that operate for long periods of time between fuel handling, must be designed with care to avoid complications due to subassembly bowing.