ICONE19-43796 DEVELOPMENT OF AZORES CODE ANALYZING SEVERE ACCIDENT SEQUENCES FOR A TYPICAL FBR PLANT

Abstract

Japan Nuclear Energy Safety organization (JNES) has been developing severe accident analysis codes in order to apply to the seismic probabilistic safety assessment (PSA) for a typical fast breeder reactor (FBR) plant. The AZORES code analyzes the severe accident phenomena in the reactor containment that reactor coolant (sodium) and molten core debris are released from the primary cooling system boundary, and the discharge rate to the environment of fission products (FP). This report summarized results analyzed using the AZORES code for a PLOHS (loss of decay heat removal function) accident sequence with the actual plant system about the containment vessel bypass scenario (CVBP). The coolant temperature of the primary system and the secondary system in the PLOHS sequence increases with the almost same temperature, and the creep damage to the reactor coolant boundary will become remarkable when coolant temperature exceeds about 1,100 K. In the CVBP scenario, the path from the primary system to environment is formed when an intermediate heat exchanger is ruptured due to creep and the boundary of the secondary system is failed. Then, the reactor vessel (RV) is failed and sodium in the primary coolant system releases into the reactor vessel room (RV room). Sodium of high temperature which fell in the RV room damages the floor liner, and generates hydrogen due to a reaction with concrete. In addition the reactor core is exposed into atmosphere and the core temperature increases with decay heat and then volatile FP and non-volatile FP are released to the environment through the secondary system from the primary system. The CVBP scenario was analyzed with the AZORES code, and the penetration failure of the RV with the melting debris occurred about 118 hours of accident initiation after about 29 hours long time from the core uncover dut to using the loop model. The analysis result of CVBP scenario showed that the oxygen concentration in the CV dome was reached to zero at about 50 hours after the accident initiation due to using sodium oxidation model. And FPs released to environment was calculated as ratios to the initial core inventory. The release ratios of CVBP scenario were estimated to be 3x10^<-1> for the rare gas (Xe), 1x10^<-3> for the volatile FP (Cs), 2x10^<-4> for the non-volatile FP (Ce) and 1x10^<-4> for the aerosol of NaI. The analysis with the AZORES clarified quantitatively both release ratio to the environment of FP in CVBP scenario and containment failure scenario for a PLOHS sequence of a typical FBR plant.