System code evaluation of near-term accident tolerant claddings during boiling water reactor short-term and long-term station blackout accidents

Near-term Accident Tolerant Fuel (ATF) claddings, such as iron-chromium-aluminum (FeCrAl) and chromium (Cr)-coating, are being extensively studied as candidates to replace the current zircaloy claddings. They present excellent oxidation resistance in high-temperature steam environment that has the potential to greatly reduce the hydrogen production and increase coping time in Beyond Design Basis Accident (BDBA) scenarios. In this work, since we are investigating unmitigated BDBA conditions, coping time is treated as the delay in the time to cladding melting. Three Station Blackout (SBO) scenarios for a representative U.S. operating Boiling Water Reactor (BWR) based on Peach Bottom Atomic Power Station are analysed using system thermal-hydraulics code TRACE. They are (1) short-term SBO without Reactor Core Isolation Cooling (RCIC) blackstart, (2) short-term SBO with RCIC blackstart and (3) long-term SBO. These scenarios are defined to be very similar to the accidents studied in the State-of-the-Art Reactor Consequence Analysis (SOARCA) project. TRACE code is also modified to reflect the oxidation kinetics of FeCrAl and Cr-coating. TRACE simulations only estimated marginal increase of the coping times (1-22 min) with both near-term ATF claddings. However, the hydrogen productions from FeCrAl and Cr-coating are one to two orders of magnitude less that those from Zircaloy cladding at the time of clad melting. Furthermore, the time to generate significant amount of hydrogen gas (0.5 kg) are delayed by 5-32 min. Finally, at the time of complete cladding oxidation, the hydrogen generated by Cr-coated cladding is similar to Zircaloy, while those generated by FeCrAl claddings are reduced by 37%-48% due to its thinner cladding thickness.