Potential impact of accident tolerant fuel cladding critical heat flux characteristics on the high temperature phase of reactivity initiated accidents

The present paper details scoping RELAP5-3D simulations to assess the potential impact of boiling heat transfer coefficients and critical heat flux (CHF) on peak cladding temperature (PCT) of key accident tolerant fuel (ATF) cladding types in a design basis accident (DBA). The accident event studied is a Hot Zero Power (HZP) Reactivity-Initiated Accident (RIA) in a Pressurized Water Reactor (PWR), so the present study is focused on bubble crowding CHF. Results are presented for the sensitivity of PCT and the duration time of film boiling to the key parameters in the boiling curve: nucleate, transition, and film boiling heat transfer coefficient, and CHF. This included a generic study of perturbations in boiling heat transfer characteristics for Zircaloy-4 cladding as well as a study which focused on the key candidate ATF cladding types. For Zircaloy-4 cladding, the simulation results show that both PCT and duration of film boiling are particularly sensitive to the value of CHF. We also compared the performance of FeCrAl claddings and SiC/SiC claddings with Zircaloy-4 cladding under the same RIA scenario. A 10% CHF enhancement for FeCrAl cladding can significantly reduce the PCT of FeCrAl claddings. Lower irradiated thermal conductivity of SiC/SiC claddings does cause a high temperature gradient in the radial direction.