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BackgroundCore fuel salt emergency drain system designed for fuel salt drain and afterheat removal, provides a safe shutdown mode for a molten salt reactor (MSR). It has important significance to evaluate reliability of the system for the safety of MSRs.PurposeThis study aims to quantitatively analyze the failure probabilty of the system and identify the pivotal factors that affecting the system failures, and provide suggestions for optimization of the system in engineering application.MethodsFirst of all, the fault tree analysis was employed to model the reliability of the core fuel salt drain system of MSRE through RiskSpectrum software. Then, the minimum cut sets and importance analysis was adopted to identify the most important basic event in fault tree of the system. Finally, two optimization methods, i.e., reduce the use of welds in bayonet cooling thimbles, and use different types of valves to isolate cooling gas flow of freeze valve, were proposed.ResultsThe results show that failure probability of the system is 5.62×10-4, and the identified pivotal factors affecting the system failures are welds leakage failures of thimbles and common cause failure of two groups valves of freeze valve. The optimization methods based on results of fault tree analysis can significantly reduce the system failure probability.ConclusionsThis study provides reference value for design and engineering application of the core fuel salt emergency drain system for MSRs.

BackgroundIn the reactivity initiated accidents under low power operating conditions of molten salt reactor (MSR), different reactivity insertion rates will trigger different emergency shutdown signals. At the same time, the initial conditions such as the initial reactor power and the temperature coefficients of reactivity affect the accident process and cause differences in accident consequences.PurposeThe study aims to conduct a sensitivity analysis of the impact of the reactivity insertion rate, the initial reactor power, and the reactivity temperature coefficient on transient consequences.MethodsFirst of all, 7 reactivity insertion rate conditions were selected and simulated through RELAP5-TMSR.Then, 25 combinations of the initial reactor power and the temperature coefficients of reactivity were assumed as initial conditions. Finally, the effects of these three parameters on the consequences of the accident were discussed separately by using local sensitivity analysis method.ResultsThe insertion rate that causes a concurrent trigger of the high outlet temperature and the high-power shutdown signal leads to the most unfavorable consequence. The difference between the peak temperatures of the fuel salt and structural materials and their respective initial values under the worst reactivity insertion rate condition is negatively correlated with initial power. However, the temperature difference of each parameter caused by different initial power does not exceed 3 ℃. The difference between the peak temperatures decrease first and then increases with the increase of the temperature coefficients of reactivity, but the maximum difference does not exceed 0.5 ℃.ConclusionsUnder low power operating conditions of MSR, the consequences of reactivity introduced events are highly sensitive to the reactivity insertion rate and low sensitivity to the initial power and temperature coefficients of reactivity.