The 4S concept is a small modular SFR with the good safety features and a very long operation period without refueling. It has been proposed by the Toshiba Corporation (Tsuboi et al., 2009, 2012) in cooperation with CRIEPI (Central Research Institute of Power Industries). The features of the 4S are summarized in Table 19.8. A schematic view of the reactor concept is shown in Figure 19.6 and its major specifications are summarized in Table 19.9. The 4S concept is characterized by the significantly simplified system design with nonrefueling, low maintenance requirements and high safety features; it is suitable for supplying energy to remote communities, mining sites, and so on.

Table 19.8 Features of a 4S reactor

Sodium-cooled fast reactor with power output of 10 or 50 MWe 30 years continuous operation without refueling

Safety design utilizing natural phenomena (automatic reactor shutdown and decay heat removal without human operation)

Reduction of maintenance introducing passive components

Enhanced safety and security by locating reactor building under ground level

Steam generator

Turbine generator

Figure 19.6 Schematic view of 4S reactor concept (taken from Tsuboi et al., 2009).

The 4S is a pool-type sodium-cooled uranium metallic fuel fast reactor, setting the core, the primary electro-magnetic pumps and the intermediate heat exchanger (IHX) inside the reactor vessel. The power output is 10 or 50 MWe. The average discharge burnup is about 35 GWd/t and the refueling interval is about 30 years. The core is narrow relative to its height (3.5 m) and is controlled by the movable six-segmented cylindrical reflector surrounding the core. The core can be shutdown by fall-down of the reflector to below the core region by the gravity. The reflector is able to shut the core down even if, for some reason, one of the six reflector segments is stuck.

As shown in Figure 19.6, the reactor vessel is located below ground level, providing substantial protection against an aircraft impact, and hence enhancing the security of the design. The containment system consists of the top dome and the guard vessel, which surrounds the rector vessel and the reflector drive equipment. The core heat

is transferred from the primary loop to the single intermediate heat transport system, and is then exchanged in the steam generator, also located below ground, to produce the steam, which drives the conventional steam turbine generator equipment in the water-steam loop. The 4S power plant can be constructed in a small site. The overall area covering the below-ground and above-ground structures is about 50 m long by 30 m wide.

The 4S has some passive safety features, such as the redundant residual heat removal system using only the natural circulation and the metallic core with the negative reactivity coefficients. It has two independent and redundant residual heat removal systems. They consist of the intermediate reactor auxiliary cooling system (IRACS), which removes decay heat using the air cooler installed in the intermediate heat transport system, and the reactor vessel auxiliary cooling system (RVACS), which removes heat transferred radially from the sodium coolant to the annulus between the guard vessel and a cylindrical steel heat collector located inside the cylindrical underground concrete wall.