Unprotected Overpower Transient Analysis of CEFR Core Using the SAS4A-SASSYS-1 Code

In order to investigate China Experimental Fast Reactor (CEFR) core performance during unprotected transient overpower (UTOP) accidents, postulated control rod withdrawal accidents have been simulated and analyzed using the SAS4A/SASSYS-1 code. Researchers at China Institute for Atomic Energy (CIAE) performed the SAS4A safety analyses of CEFR in collaboration with Argonne National Laboratory (ANL) researchers. This collaboration is carried out within the framework of the Fast Reactor Technology Working Group of the U.S. – China Bilateral Civil Nuclear Energy Cooperation. Assumptions were made for the UTOP accidents that lead to relatively large reactivity insertions. The change of reactor reactivity and power as well as the fuel, cladding, and coolant temperatures during the transient process were analyzed in order to characterize the accident sequence. Accident consequences were studied with a focus on fuel and cladding melting and relocation behavior. The effects of different reactivity feedback mechanisms on the reactor power history during the transients were analyzed. The results showed that the negative reactivity feedback due to coolant temperature change and coolant voiding played an important role in counteracting the effects of the inserted positive reactivity. The simulation demonstrated the inherent safety characteristics of the CEFR core, which exhibits negative reactivity feedbacks during UTOP transients. (author)