Transient characteristics analysis of lead-based reactor from forced circulation to natural circulation

Highlights: • A RELAP5 model for a 10MWth lead-based reactor (CLEAR-I) was built to study the transient characteristics from forced circulation to natural circulation. • The effects of the five factors on the transient characteristics were analyzed separately. • The optimized control strategy taking into account the combined effects of three factors can increase the maximum initial power level from 45%FP to 75%FP. -- Abstract: Natural circulation and forced circulation are two types of normal operation conditions that usually appeared in nuclear power systems. However, it should be noted that the reactor safety and reliability can be jeopardized due to significant fluctuation of the mass flow rate through the lead-based reactor core during the conversion from forced to natural circulation. Therefore, a model of 10MW_th lead-based reactor based on RELAP5 mod4.0 is developed to analyze the effects of the five factors on the transient characteristics, including the power control system, the initial steady-state power level, the human intervention, shutting down each main pump one by one and the bypass of the main pump separately. The results show that using power control system can obviously reduce the transition time and realize the rapid response of the reactor during the conversion. Moreover, a higher initial power level results in a larger power fluctuation, a larger maximum value of cladding peak temperature and a stronger natural circulation. As a result, the maximum initial power level can be obtained according to the cladding temperature limitation (823 K). Especially, the optimized control strategy taking into account the combined effects of three factors, namely human intervention, shutting down each main pump one by one as well as the bypass of the main pump, can increase the maximum initial power level from 45%FP to 75%FP.