An evaluation of passive safety features of the Japanese prototype LMFBR Monju

Japan Nuclear Energy Safety Organization, JNES, has developed a system dynamics analysis code NALAP-II, in order to apply in the safety regulation of liquid-metal-cooled fast breeder reactor (LMFBR). In this study, the heat removal by the coolant natural circulation (NC), which is one of passive safety features of LMFBR, was examined using the code. This paper presents the model verification of the decay heat removal system and the result of trial calculation to the Japanese prototype LMFBR MONJU. In the MONJU plant, the decay heat is removed normally by three loops of the secondary heat transport system (SHTS) coupled with the intermediate reactor auxiliary cooling system (IRACS) as shown in Fig.1. To enable the cooling by NC, the air cooler (AC) of MONJU is installed in a position where the heat-transfer center is higher than that of the intermediate heat exchanger (IHX). Verification analyses of the IRACS model of NALAP-II have been carried out, by using the data of a 'natural convection test' conducted as a part of MONJU's performance tests. This test was conducted adding the heat generated by the pump operation in the primary heat transport system (PHTS) instead of the reactor power. The test was started by tripping SHTS pony-motored pump and sodium began to flow by the natural convective force through the air cooling system (ACS) of the IRACS. Figure 2 presents the analytical results of the SHTS transient comparing with the test results. In this test, about 2% of the rated SHTS flow rate was kept by the NC resulting from the balance between the heat input at IHX and the heat removal at IRACS. The calculated results of SHTS flow rate and sodium temperature during NC showed a good agreement with the test results. Then, using the verified NALAP-II, an evaluation of heat removal by NC of MONJU IRACS after the trip at the rated power operation was performed. The result showed that even if only one loop operation of IRACS removes the decay heat resulting in maintaining the PHTS boundary temperature lower than 660 degree-C. Thus, any plural loops operation of IRACS will surely remove the decay heat enough to make the boundary temperature less than a limit to keep the integrity. Taking into account of the NC heat removal of IRACS, the core damage frequency (CDF) caused by PLOHS decreases by two orders. The entire CDF of MONJU becomes to a low level of 10-7/yr [1]. Reference: [1] 'Probabilistic safety assessment of the fast breeder reactor; level-1 PSA', INS/ M03-39, JNES, 2003. (authors)