Preliminary analysis of core heat-up for ATFs under accident conditions

Experience gained in decades of nuclear safety research and previous nuclear accidents direct to the investigation of passive safety system design and accident-tolerant fuel (ATF) system which is now becoming a hot research point in the nuclear energy field. The ATF system is aim at upgrading safety characteristics of the nuclear fuel and cladding in a reactor core where active cooling has been lost, and is preferable or comparable to the conventional UO₂-Zr fuel system when the reactor is in normal operation. By virtue of advanced materials with improved basic properties and oxidation resistance, the ATF system is placed high hopes on slowing down the accident progression, allowing wider margin of time for the mitigation measures to be effective. SiC and FeCrAl accident-tolerant claddings and FCM accident-tolerant fuel are considered in this work, as they are receiving more and more attention in China. In this paper, the improved performance of UO2-SiC, UO₂-FeCrAl, FCM-SiC and FCM-FeCrAl fuel systems on the core heat-up progress in a pressurized water reactor (PWR) is analyzed by using the best-estimate system code RELAP5 and severe accident analysis program respectively. For this purpose two accident scenarios are selected, e.g., a large break LOCA as a design base accident and an extended station blackout (SBO) severe accident, to reflect the accident-tolerance characteristic of ATFs. Obtained quantitative results show promising safety enhancement in terms of peak cladding temperature (PCT) and recovery of the core coolability for the selected ATF systems, which is beneficial for further down-selections of various kinds of ATF systems.