The Effect of Time Delays in Fuel Pin Failure on LOF-TOP Transient Calculations for a Commercial-Sized LMFBR

Various loss-of-flow cases have been calculated for a commercial-sized liquid-metal fast breeder reactor. Particular attention has been paid to the development of loss-of-flow-driven transient-overpower (LOF-TOP) conditions. In such conditions, it is crucial to consider when an initial cladding breach might occur in LOF-TOP pins and over what length of time the initial cladding breach might extend in fuel pins failing under burst pressure. This study shows that the neutronic energy deposition in transient calculations including LOF-TOP pin failures can increase substantially compared to a calculation excluding such LOF-TOP failures in two ways. First, there will be an increase if there is no extension of an initial cladding failure in LOF-TOP pins or if there is a relatively long delay in the extension. Secondly, when, in applying a fuel melt fraction criterion for pin failure, the same melt fraction is specified for failure extension as for initial failure, which implies a certain delay time for failure extension, there will be an increase in the energy deposition compared to the case without any LOF-TOP failures only when the specified fuel melt fraction becomes very large. However, even in the case with the largest failure melt fraction, there will be no increase inmore » energy deposition when a rapid enough failure extension is assumed. These calculations make a number of very conservative assumptions. The purpose of the study is not to provide a best estimate of accident conditions but to show how quickly an initial cladding breach must extend in such conservative calculations if it is to limit the increase in neutronic energy deposition.« less