ATHLET simulation of different DHX operation responses in sodium-cooled fast reactor melting accidents

Abstract:

Sodium-cooled fast reactor (SFR) is an advanced nuclear reactor that uses liquid sodium as a coolant and relies on fast neutrons for nuclear fission, with passive safety characteristics. However, it may lead to core melting when a severe accident occurs. In order to improve the safety response performance of SFR in severe accidents, the ATHLET (analysis of thermal-hydraulics of leaks and transients) code is provided to conduct a safety analysis of the temperature of sodium pool in SFR under severe accidents in this article. Firstly, the SFR core structure was reasonably simplified and a nodalization diagram was developed based on the established model. Then, the ATHLET code was used to simulate the core melting accident of SFR, the key parameters including the decay heat of molten matter and the power of decay heat exchangers (DHX) in the cold/hot pools were input. Finally, based on the simulation results, the impact of different operating conditions of four DHXs located in the cold/hot pools on the thermal parameters of the main regions in SFR was analyzed. The results showed that the closure of DHX in the hot pool would cause backflow in the flow channel between the lower cooling pool and the lower chamber, resulting in a significant increase in the temperature of the descending section of the lower chamber. The closure of DHX in the cold pool would cause a severe increase in the temperature of the coolant in various parts of the SFR. In comparison, the closure of one DHX in the cold pool is more dangerous for SFR in severe accidents and it is necessary to ensure that both DHXs in the cold pool are in operation under severe accidents, which can timely remove the heat of core melting. In summary, the results on the role of DHX in core melting accidents in this article can provide valuable references for optimizing structural design and validating thermal hydraulic models of SFRs.