Toshiba 4S
The Toshiba 4S (Ultra super safe, Small and Simple) is a micro sodium reactor design. General description. The plant design is developed by a partnership that includes Toshiba and the Central Research Institute of Electric Power Industry (CRIEPI) of Japan. The technical specifications of the 4S reactor are unique in the nuclear industry. The actual reactor would be located in a sealed, cylindrical vault 30 m (98 ft) underground, while the building above ground would be 22×16×11 m (72×52.5×36 ft) in size. This power plant is designed to provide 10 megawatts of electrical power with a 50 MW  version available in the future. The 4S is a fast neutron sodium reactor. It uses neutron reflector panels around the perimeter to maintain neutron density. These reflector panels replace complicated control rods, yet keep the ability to shut down the nuclear reaction in case of an emergency. Additionally, the Toshiba 4S utilizes liquid sodium as a coolant, allowing the reactor to operate 200 degrees hotter than if it used water. Although water would readily boil at these temperatures, sodium remains a liquid; the sodium coolant therefore exerts very low pressure on the reactor vessel even at extremely high temperatures. The  Toshiba 4S Nuclear Battery was proposed as the power source for the Galena Nuclear Power Plant in Alaska, but the project was abandoned in 2011 and Toshiba did not proceed with an application for certification of the design.