The Toshiba Super Safe Small and Simple (4S) reactor design is a sodium-cooled fast-spectrum reactor with an output of either 10 or 50 MWe. The reactor has a compact core design with steel-clad metal-alloy uranium fuel. A unique feature of
the core design is that it does not require refueling over the 30-year lifetime of the plant (10 MWe version). This is accomplished by designing for a high conversion of the fertile material in the core and by using a slowly moving reflector to compensate for fuel burn-up over the core lifetime. A 50 MWe design option is available with a 10-year refueling cycle. The U-10%Zr metal alloy fuel has an enrichment of less than 20% 235U and is clad in HT-9 alloy. The basic layout of 4S is a pool-type configuration, with the electromagnetic (EM) pumps and a single intermediate heat exchanger contained inside the primary vessel. An intermediate sodium loop delivers heat from the primary system to the external steam generator used to generate steam for the Rankine power-conversion system. The reactor containment consists of a lower nitrogen-filled steel guard vessel and an upper steel dome.

The 4S design is supported by extensive testing facilities in Japan, including the Toshiba Sodium Loop Test facility. The design is targeting the diverse and remote energy market where alternative energy sources are very expensive or difficult to sustain. A consortium including the local government in Galena, Alaska, initiated pre-application meetings with the US Nuclear Regulatory Commission in 2007. This project has not yet proceeded into formal licensing and currently Toshiba is exploring alternative first customers. Key parameters and a representative graphic for the 4S design are given in Figure 2.20. [18, 19]

Figure 2.20 4S (Japan) — Toshiba/Westinghouse © Toshiba Corporation.