Effectiveness of Cr-Coated Zr-Alloy Clad in Delaying Fuel Degradation for a PWR During a Station Blackout Event

Since the accident at Fukushima, one major goal of reactor safety research has been the development of accident tolerant technologies that can mitigate or delay fuel degradation during a beyond-design-basis accident. One major effort has been focused on the development of coatings for light water reactor fuel cladding. Chromium-coated zirconium-alloy clad is one of the leading options. In this work, the MELCOR systems code (version 1.8.6 User-Defined Generalized Coating) is used to evaluate the performance of Cr-coated Zr-alloy clad as compared to Zr-alloy clad and APMT FeCrAl-coated Zr-alloy clad for a pressurized water reactor (i.e., Surry) for a station blackout (SBO) accident scenario. Our focus is primarily on the accident progression behavior depending on oxidation kinetics and the assumed failure criterion for the coated cladding material. Our simulation and comparison indicate that the presence of the coating material can significantly reduce the initial rate of hydrogen generation and delay the time when hydrogen generation becomes significant. This decrease in the rate of oxidation and delay in timing can provide additional coping time for compensatory operator actions. We also note that the effect of extended auxiliary feedwater system operation (long-term SBO) can increase this additional coping time in combination with Cr-coated Zr-alloy, but it is limited by other primary system failures (e.g., hot-leg creep rupture) that will occur driven by core decay heat and independent of coated cladding effects. Finally, we observe that while the initial suppression of hydrogen generation for Cr-coated Zr-alloy clad compared to Zr-alloy is notable, the overall amount of hydrogen produced is similar since hydrogen can also be produced through competing oxidation of stainless steel components during the accident progression. Our future work is focused on the uncertainty analysis of the oxidation rate data, coating failure criteria, and severe accident modeling limitations in order to better quantify accident tolerant fuel clad benefits.