Thermal hydraulic analysis of the PWR with high uranium density accident tolerant fuels under accident transients with and without reactivity

Highlights: • The thermal hydraulic performances of high uranium density accident tolerant fuels have been studied. • PFCT and MDNBR can be obtained by using ATF UN and UN+U₃Si₂ composite fuels. • UN and UN+U₃Si₂ composite fuels have larger negative reactivity feedback relative to UO₂. • Safety margin of UN+U₃Si₂ composite may be lower than UN as result of low melting temperature of U₃Si₂. - Abstract: High uranium density accident tolerant fuel (ATF) with high economy and safety seems to be one of the most promising candidates to replace UO₂ in future reactor. The thermal hydraulic analysis of the PWR loaded with high uranium density ATF fuels needs to be carried out to demonstrate this fuel performance. The thermal hydraulic performance of UN and UN+U₃Si₂ composites were evaluated in the PWR under unprotected complete loss of coolant flow accident (CLOFA) and reactivity initiated accident (RIA). The processes of CLOFA and RIA transients with and without reactivity were carried out using system code Relap5/Mod3.4, and the thermal hydraulic behaviors of UN and UN+U₃Si₂ composite under CLOFA and RIA transients were assessed by sub-channel code COBRA-EN. The results show that UN and UN+U₃Si₂ fuels had more intensive negative reactivity feedback to further mitigate reactor power resulting in lower fuel temperature during CLOFA and RIA transients. The UN and UN+U₃Si₂ composites demonstrated better safety relative to UO₂ because of higher minimal departure from nucleate boiling ratio (MDNBR) and lower peak fuel centerline temperature (PFCT). UN+U₃Si₂ composite performed better thermal hydraulic performance when compared to UN, but the safety margin may be lower due to low melting temperature of U₃Si₂.