Preliminary prediction for survival time of fuel rod under critical heat flux

Abstract In commercial water-cooled reactor safety analysis, the fuel rod is assumed to fail and release its fission gas inventory when it reaches critical heat flux (CHF). For this reason, CHF serves as a regulatory limit for all water-cooled reactors. However, according to existing in-pile experimental data, this assumption is conservative and may over-constrain the reactor core performance. With the advent of accident tolerant fuels (ATFs), particularly their ability to structurally withstand higher temperatures, the true margin to fuel failure after reaching CHF should be re-visited. To this end, preliminary fuel performance during CHF and Post-CHF is simulated and fuel rod survival time based on different cladding failure criteria is formulated. The post-CHF fuel performance is governed by cladding temperature, oxidation, pellet-clad mechanical interaction (PCMI) and hydride formation. The analysis includes impact of cladding temperature, fuel burnup and presence of Cr-coating as a near term ATF concept. The fuel performance analysis for fresh fuel implies that the cladding survival times of few days are possible and the cladding oxidation and plastic strain limit are the likely failure modes. For burned fuel, PCMI and hydride formation are the likely limiting failure modes that could decrease the survival time to less than a day. The addition of Cr coating reduces hydride formation and improves high temperature creep performance of the cladding and it can potentially prolong the survival times to several days. The estimated large survival times motivate further investigations, particularly, expansion of available data in areas of cladding mechanical performance and hydrogen pickup in the temperature range relevant to post-CHF film boiling regime to better inform the fuel performance predictions.