Power Increase Limits to Prevent Pellet-Cladding Interaction : Calculation of Strain- and Fission Gas Release Margins

Abstract

With a growing share of intermittent electrical energy sources in the national grid of Sweden, areview of the safety of using nuclear power for flexible operation is necessary. Today nuclearpower is used as base load power and when examining challenges of flexible power operation,the risk for Pellet-Cladding Interaction (PCI), stands out. Vattenfall Nuclear Fuel lack a methodof calculating the PCI risk which this study will develop. It does this by mapping the risk for PCIthrough calculating power increase margins, in terms of ΔLHGR for full and flexible poweroperation. The results show that cladding strain for all simulated power levels is the limitingfactor. The time at which fuel remain at reduced power also has a large impact and the speed atwhich the PCI margin deteriorates has been calculated. Data from Westinghouse show that theresults compare relatively well for similar experimental set ups with some systemicconservatism. The automated library of code can be easily used to calculate the PCI risk for fuelin the future. The risk for PCI when returning to fuel power, per a Westinghouse conferencepaper, significantly reduces and should be investigated with the use of provided code. Possiblelimits with this study, however unquantified, are the use of 100 W/cm transient speed and theuse of full power axial profiles for reduced power operation in simulations