2018 | AFC ACCOMPLISHMENTS 76 Elucidating cladding behavior under unmitigated large-break loss-of- coolant accident conditions using the BISON fuel performance code Collaborators: Ryan Sweet, Andy Nelson, Kurt Terrani, and Brian Wirth The main goal of identifying an alternative cladding material to Zircaloy for Light Water Reactor (LWR) fuel systems is to improve the reactor safety during high-temperature transient conditions. In order to provide insight into how FeCrAl cladding will perform compared to Zircaloy in this environment, fuel performance code capabilities can be extended, provided that thorough constitutive models are developed and implemented.The target of this analysis is the cladding behavior in the high-temperature environment sustained during a potential large-break loss-of-coolant (LBLOCA) accident. In this analysis, conditions from a mitigated LBLOCA are extended to compare the beyond design basis accident response of FeCrAl as compared to standard Zircaloy cladding. During a loss-of-coolant accident with Zircaloy cladding, as the reactor loses its capability to cool the fuel, fuel rod temperatures begin to increase. Eventually, if this increase in temperatures is unmitigated, then the cladding will begin to‘balloon’, or deform outwards, due to the pressure differential between the interior of the fuel rod and the pressure remaining in the reactor pressure vessel. If this deformation persists, cladding burst can occur during these high-temperature conditions from a combination of thermal creep and Coupled BISON & TRACE modeling efforts improve understanding of cladding behavior during large break loss of coolant accidents and informs design of transient testing. plasticity. If the temperature continues to increase further, the exothermic oxidation reaction of the Zircaloy cladding may become autocatalytic. This produces large amounts of heat, consumes the cladding, and produces hydrogen gas.Within this study, the cladding performance for both FeCrAl and Zircaloy cladding under these conditions are targeted. Project Description: Traditionally, separate fuel performance codes are utilized to simulate the fuel rod behavior under steady-state and transient conditions either in conjunction with each other, or to provide a stand-alone analysis.Transitioning the state of the fuel rod from normal operation into the transient environment is necessary to evaluate the unique condition of the fuel at a variety of fuel burnups.To accomplish this, these simulation conditions contain long- term steady-state reactor operation before transitioning into the accident scenario.This allows the state of the integral fuel rod at different specified burnups to be incorporated into the transient analysis. To accurately simulate the cladding behavior under a loss-of-coolant accident, several key phenomena must be incorporated, including: chemical and phase changes of the cladding alloy (such as oxidation), high- temperature constitutive behavior,