Toshiba 4S

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Toshiba’s 10-50-megawatt “4S” fast-neutron reactor

The Toshiba 4S (Super Safe, Small and Simple) is a micro nuclear reactor design.The plant design is offered 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 reactor. It uses neutron reflector panels around the perimeter to maintain neutron density. These reflector panels replace complicated control rods, yet keep the ability toshut 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 easily 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.

Currently Toshiba, together with its Westinghouse subsidiary, is in the preliminary design review stage of the Design Certification process before the USNRC.  Application for certification of the design is currently planned for 2012 when the standardized Design Certification application will be filed for the 4S. The most recent meeting with the NRC took place on August 8, 2008, at which time the NRC’s staff met with representatives of Toshiba and Westinghouse for a pre-application presentation of a Phenomena Identification and Ranking Table (PIRT) for the Toshiba 4S (Super-Safe, Small and Simple) reactor. Lawrence Livermore National Laboratory recently released an interesting study on the Toshiba 4S design, which provides an overview of the 4S design and suggests that certain goals may be easier to meet if lead is used as the coolant rather than sodium, due to lead’s high transparency to neutrons and low transparency to gamma radiation, though lead has a higher melting point than sodium does.

From The Union of Concerned Scientists:

The proponents of the Toshiba 4S design (and it’s worth noting that this design is not the next evolutionary step from a Toshiba 1S, Toshiba 2S, and Toshiba 3S series of proven designs; this design currently exists only on paper and in cyberspace) want to skip some steps in the normal development process for new technology:

“Toshiba currently is conducting engineering work to complete the reactor and plant designs. Therefore, if the first operational unit is installed at a site such as Galena it would be considered a “reference” rather than a “prototype” or “demonstration” plant. … The assumption that the 4S would be a reference plant is subject to some question by U.S. National Laboratory staff.”

The publicly available information on the Toshiba 4S sodium-cooled nuclear reactor – I find it curious that NONE of the 4S’s stands for sodium even though it’s the most distinguishing aspect of the reactor’s design – completely misrepresent actual operating experience worldwide with sodium-cooled reactors. Consider these verbatim statements from two different documents:

“Sodium cooled reactors throughout the world have been run for thousands of hours without incidents involving the reactor core.” and “Liquid metal reactors (LMRs) have been operated successfully worldwide and have been used in the United States at test facilities, with over 300 reactor years of operational experience.”

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