Making the latest news is the new design from Toshiba is the 4S. The four ‘S’s are for super, safe, small and simple. The design is such that the tendency to call sealed reactors batteries applies as well.
The technical specifications for the 4S reactor are unique in the nuclear industry. The actual reactor would be located in a sealed, cylindrical vault 30 m (98 ft) installed underground, while the building above ground would be only 22 x 16 x 11 m (72 × 52.5 x 36 ft) in size. This power plant is designed to provide 10 MW of electrical power. It’s not a big reactor when sized up to those 1800 MG proposals.
The 4S is a fast neutron reactor using neutron reflector panels around the perimeter to maintain neutron density. The reflector panels replace complicated control rods, while still keeping the ability to shut down the nuclear reaction in case of an emergency. Toshiba’s 4S utilizes liquid sodium as a coolant, allowing the reactor to operate 200 degrees hotter than by using water. Using sodium allows the reactor to be unpressurized, even though water at such temperatures would run at thousands of pounds per square inch.
The 4S reactor is expected to provide electric energy for between 5 and 13 cents/kWh, factoring in only the operating costs, to which unknown decommissioning and fuel waste processing and safe disposal costs need to be added. On paper, it has been determined that the reactor could run for 30 years without being refueled.
The Toshiba 4S Nuclear Battery is already being proposed as the power source for the remote Galena Nuclear Power Plant in Galena, Alaska.
Toshiba is the parent company for Westinghouse whose AP1000 is working through its revisions needed to get the original certification from the Nuclear Regulatory Commission back in force. Thus Toshiba has some expertise for an application attempt employed now. News is expected any day for the formal application to be submitted. Toshiba’s staff had at last report met with the NRC back on August 8, 2008, at which time the NRC’s staff met for a pre-application presentation of a Phenomena Identification and Ranking Table (PIRT) for the 4S reactor.
Still in the starting blocks is the Fuji Molten Salt Reactor (FMSR). FMSR is a Japanese design that can run on thorium or a mix of thorium and Uranium or Plutonium. The project plan is to take 6 or 7 more years to develop a miniature FMSR. The R&D is mostly related to the details of the structural material and components.
In a molten salt reactor the fuel is a molten mixture of say, lithium and beryllium fluoride salts with dissolved enriched uranium, thorium or uranium-233 fluorides. The core consists of an unclad graphite moderator arranged to allow the flow of salt at some 700°C and at low pressure. The heat is transferred to a secondary salt circuit to be transferred by a heat exchanger to water and on to steam. These are not fast reactors, but with some moderation by the graphite they are epithermal (intermediate neutron speed).
The fission products dissolve in the salt and are removed continuously in an on-line reprocessing loop and replaced with thorium-232 or uranium-238. Essentially refueling as they run. The actinides remain in the reactor until they fission or are converted to higher actinides for fission. A full-size 1000 MWe MSR breeder reactor was designed but not built. In 2002 a thorium MSR was designed in France with a fissile zone where most power would be produced and a surrounding fertile zone where most conversion of Th-232 to U-233 would occur.
The Fuji design fits the IAEA definition of a small reactor that generates less than 300 Mwe. There is worldwide interest to develop smaller units in reactor designs due partly to the high capital cost of large nuclear power reactors generating electricity via the steam cycle and in part to considerations of the public’s perceptions. Small reactors may be built independently or as modules in larger complexes, with capacity added incrementally as required. Economies of scale are provided by the larger number of reactors produced.
Also in the running is Mitsubishi Heavy Industries with an advanced High Temperature Engineering Test Reactor. Mitsubishi is going straight to developing hydrogen production technology for the multi-purpose use of the high temperature gas-cooled reactor, and the helium gas turbine direct power generation system for high efficiency power generation. Mitsubishi believes these technologies will be the key to the future commercialization of high temperature gas-cooled reactors.
Hitachi is going for Advanced Boiling Water Reactors in the smaller sizes. Already a partner with GE an others outside for the U.S., Hitachi draws on great resources. The designs are ranging from 400 MW electric on up to near 1400 MWe.
Hitachi’s designs differ in that is that the boiling water reactors (BWR) operate at a lower pressure than the pressurized water reactors (PWR). Even though the BWRs are highly pressurized (1000 – 1100 psia), they operate at only half of the pressure found in PWRs. The lower pressure in the BWRs allows the water circulating through the core to boil resulting in a mixture of water and steam surrounding the reactor core. Since steam does not carry heat away from the reactor core nearly as well as water, advanced BWRs are always monitored and attempting to maintain a certain amount of water circulating in the reactor core to prevent overheating. Not simple engineering, but this reactor’s waste stream is nearly a fast breeder reactor of plutonium if that’s the way a buyer wants to go.
The 4S looks to be coming to the U.S. market, and has started procedures for approval in the United States, the Nikkei business daily said. Fuji has a way to go in R&D. Mitsubishi’s marketing plans are still essentially inscrutable. Hitachi has a list of partners and may well see sales in Asia before or instead of the U.S. skipping the ‘gold standard’ of an NRC approval. One thing is certain, the Russians, the French and the U.S. manufactures are not alone, and if the history of Japanese manufacturing prowess is considered, some of these are going to be sales hits.