UTOP accident analysis and mitigation measures of lead-cooled fast reactor considering coupling effect with SCO2 Brayton cycle

Abstract

The lead-cooled fast reactor (LFR) coupled with SCO2 Brayton cycle is an advanced nuclear power generation system which has high efficiency and high compactness. The purpose of this study is to investigate the impact of coupling effect between LFR and SCO2 cycle on the safety analysis of unprotected transient overpower (UTOP) accident, and to propose mitigation measures to UTOP accident with the aid of SCO2 cycle. Firstly, the dynamic simulation model of a 100 MW small modular natural circulation LFR coupled with SCO2 Brayton cycle was developed using APROS software, and was verified through the steady-state and transient simulations. Secondly, the UTOP accident of the LFR coupled with SCO2 Brayton cycle was analyzed, including a comparative case which simplified SCO2 cycle as a fixed boundary in the analysis. Finally, two measures to mitigate the UTOP accident by charging more SCO2 to the circulation loop from the storage tank and by maintaining constant SCO2 temperature at the inlet of in-reactor heat exchangers (IRHXs) were proposed. The results indicate that the conventional method of simplifying power generation system as the fixed boundary underestimated the severity of accident. Specifically, the maximum fuel cladding temperature was about 40 °C higher when considering the coupling effect of reactor and SCO2 cycle than that of assuming SCO2 cycle as the fixed boundary, which greatly exceeded the safety limit. Moreover, within the scope of present UTOP accident conditions, the measures proposed in this study could mitigate the severity of accident. The maximum cladding temperature was reduced by 15–23 °C in the condition of maintaining constant SCO2 temperature at IRHX inlet, and was reduced by 5–7 °C in the condition of charging more SCO2 to the circulation loop, compared with the condition not applying any measures.