Thermal-hydraulic performance and safety assessment of an LBE-cooled reactor under steady-state and unprotected transients

Understanding the thermal–hydraulic safety and transient behavior of Gen-IV LBE-cooled fast reactors are crucial for advancing nuclear safety standards. The thermal–hydraulic performance of an LBE-cooled reactor, SPARK-NC, was analyzed using the subchannel analysis code LOONG-SACOS under steady-state natural circulation conditions, focusing on temperature distribution, velocity, and density in the hottest assembly. Results revealed a peak coolant velocity of 0.296 m/sec and a maximum coolant temperature of 471 °C, with the fuel centerline temperature remaining below 2000 °C safety threshold. This underscores the ability of SPARK-NC reactor design to maintain safe and efficient performance by regulating temperatures and flow rates within specified limits during steady-state natural circulation. In the subsequent phase, a transient analysis was conducted using LOONG-SARAX and DAISY-PK codes to evaluate the safety of SPARK-NC reactor under dynamic conditions, encompassing Unprotected Transient Overpower (UTOP), Unprotected Control Rod withdrawal (UCRW) and Scram-drop transient events across various core states. The study investigated UTOP transients by introducing positive external reactivity and evaluating the inherent reactor feedback behavior. The reactivity was increased incrementally to attain maximum reactivity while ensuring the integrity of both fuel and cladding. The results indicated that upon inserting external reactivity of 1.0$, there was an initial rapid power surge followed by stabilization, indicating that both fuel and cladding maintained integrity within the predefined failure thresholds. Additionally, analysis of UCRW transients enabled risk assessment during control rod maneuvers across various positions, wherein the withdrawal of control rod C₆ resulted in a total reactivity insertion of 0.94$, stabilizing at a normalized power level of 4.35. Finally, the scram-drop transient demonstrated the rapid shutdown capability of the reactor, promptly transitioning it to a secure state, ensuring effective post-insertion temperature control as feedback reactivity stabilizes at 0.24$, which highlights the robust inherent safety of the SPARK-NC reactor.