Abstract
The 4S is a sodium-cooled small fast reactor designed to supply energy and hot water on isolated islands or in remote locations. Small reactors are often said to have disadvantages in terms of economies of scale. This problem can be overcome by making the structure simple, doing away with the need for maintenance, and designing a core that requires no refueling during reactor lifetime. Taking into account the nature of current demand from the economy and the market, there are two options for the electrical output of the 4S reactor: 10MWe and 50MWe. The core of the 10MWe 4S reactor (4S-10M) has a 30-year lifetime without refueling. Metallic fuel is employed. Burn-up reactivity loss of the 4S is regulated by neutron reflectors which surround the core. All temperature reactivity coefficients including void reactivity are kept negative during core life-time. The 4S-10M is a tall pool-type reactor. It has an intermediate heat exchanger (IHX) in the annulus space inside the reactor vessel (R/V). There are two primary electro-magnetic pumps (EM pumps) in serial under the IHX, and an air flow path on the surface of the guard vessel (G/V) as a decay heat removal system (RVACS; Reactor vessel auxiliary cooling system). The secondary sodium heated in the shell part of the IHX flows to the steam generator (SG) via the tubes of the air cooler of the intermediate reactor auxiliary cooling system (IRACS). The decay heat removal systems of 4S-10M consist of a RVACS and IRACS. Both are passive systems. To clarify the safety margin of the 4S-10M, it is important to confirm the characteristics of the two decay heat removal systems during the plant transient. The multi-dimensional thermal-hydraulic effect in the R/V could be important under such natural circulating conditions of this sort. CERES is a multi-dimensional plant dynamics simulation code for LMRs (Liquid Metal Reactors) developed by the CRIEPI (Central Research Institute of Electric Power Industry). The CERES code can handle almost all the components needed for evaluation of LMR safety. Sodium, water, lead-bismuth eutectic and lead can be handled as the coolants. The plenum in the R/V is modeled by the R-Z 2 dimension in the CERES code. The accidents of a protected loss of flow (PLOF) caused by a total station black out (TBO) and a loss of flow without scram (ULOF) were selected as the typical accidents that provide an accurate indication of the characteristics of the cooling system. The calculation clarified the thermal-hydraulic characteristics of the 4S-10M. The safety margins of the 4S were evaluated by the results of the calculations satisfied the criteria of the temperature and CDF value criteria.
