After the Chernobyl Unit 4 accident in 1986, there was little new reactor installation. In 2000, the Generation IV reactor development was proposed by the US DOE (Department of Energy). This stimulated R&D activity in Japan, including that on SMRs. At this time, the economic aspects were considered to be the most important point in the development, and hence R&D was aimed at overcoming the scale demerit to achieve the same level of the construction cost (per kWe) and the electricity generation cost (per kWh) as for the existing large LWRs. In order to accomplish high economic performance, the design simplification of the system is considered to be the most effective approach. Optimizing the simplification, three SMR design concepts were proposed and developed in Japan based on the LWR concept. They are IMR (integrated modular water reactor), CCR (compact containment water reactor) and DMS (double MS (modular simplified and medium small) reactor) (Okubo, 2011). In addition to these, other SMR concepts based on the HTGR (high-temperature gas-cooled reactor) were also proposed, utilizing their excellent characteristic of the higher thermal efficiency in HTGR than in LWRs. One of them is GTHTR 300 (Gas Turbine High Temperature Reactor 300). Furthermore, an SMR concept based on the SFR (sodium-cooled fast reactor) concept was also proposed, especially for local use in isolated area conditions, such as in Alaska. This is named as the 4S (super-safe, small and simple). In the following sections, these five SMR concepts are presented briefly.