There are a number of small and medium nuclear reactors that are in funded development.
Toshiba has designs for a micro nuclear reactor that generates 200 kw for 40 years
The 200 kilowatt Toshiba designed reactor is engineered to be fail-safe and totally automatic and will not overheat. Unlike traditional nuclear reactors the new micro reactor uses no control rods to initiate the reaction. The new revolutionary technology uses reservoirs of liquid lithium-6, an isotope that is effective at absorbing neutrons. The Lithium-6 reservoirs are connected to a vertical tube that fits into the reactor core. The whole whole process is self sustaining and can last for up to 40 years, producing electricity for only 5 cents per kilowatt hour, about half the cost of grid energy. It has dimensions of 20 feet by 6 feet.
Toshiba expects to install the first reactor in Japan in 2008 and to begin marketing the new system in Europe and America in 2009.
This reactor is a small-scale design developed by Toshiba Corporation in cooperation with Japan's Central Research Institute of Electric Power Industry (CRIEPI) and funded by the Japan Atomic Energy Research Institute (JAERI) [unified with the Japan Atomic Energy Agency in 2005]
It is the 5 MWt, 200 kWe Rapid-L, using lithium-6 (a liquid neutron poison) as a control medium. It would have 2700 fuel pins of 40-50% enriched uranium nitride with 2600°C melting point integrated into a disposable cartridge. The reactivity control system is passive, using lithium expansion modules (LEM) which give burnup compensation, partial load operation as well as negative reactivity feedback. As the reactor temperature rises, the lithium expands into the core, displacing an inert gas. Other kinds of lithium modules, also integrated into the fuel cartridge, shut down and start up the nuclear reactor. Cooling is by molten sodium, and with the LEM control system, reactor power is proportional to primary coolant flow rate. Refuelling would be every 10 years in an inert gas environment. Operation would require no skill, due to the inherent safety design features. The whole plant would be about 6.5 meters high and 2 meters in diameter.
This information is from Hore-Lacy, Ian (Lead Author); Cutler J. Cleveland (Topic Editor). 2006. "Small nuclear power reactors." In: Encyclopedia of Earth. Eds. Cutler J. Cleveland (Washington, D.C.: Environmental Information Coalition, National Council for Science and the Environment). [Published in the Encyclopedia of Earth September 4, 2006; Retrieved December 19, 2007].
A 2001 article from the New Scientist discussed the Rapid-L
The Rapid-L reactor was conceived as a powerhouse for colonies on the Moon. Unlike normal nuclear reactors, the Rapid-L has no control rods to regulate the reaction. Instead, it uses reservoirs of molten lithium-6 - an isotope that is effective at absorbing neutrons. The reservoirs are connected to a vertical tube that runs through the reactor core.
During normal operation the tube contains an inert gas. But as the temperature of the reactor rises, the liquid lithium expands, compressing the inert gas and entering the core to absorb neutrons and slow down the reaction.
The lithium acts as a liquid control rod. And unlike solid control rods, which have to be inserted mechanically, the liquid expands naturally when the core gets warm.
The Rapid-L uses the same principle to start up and close down the reaction. The reactor would be cooled by molten sodium and run at about 530 °C. Mitsuru Kambe's, head of the research team at Japan's Central Research Institute of Electrical Power Industry (CRIEPI), main concern now is to test the fail-safe system's long-term durability.
How the lithium expands to control the reaction as heat rises.
The Rapid-L is not the same as the toshiba 4s reactor which has 50 times higher generation capacity.
Toshiba 4S reactor would generate 10 MW.
The actual reactor would be located in sealed, cylindrical vault 30 m (98 ft) underground, while the building above ground would be 22 x 16 x 11 m (72 × 52.5 x 36 ft) in size. The 4S 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. This means that the reactor is depressurized, as water at this temperature would run at thousands of pounds per square inch.
The reactor is expected to provide electric energy for between 5 and 13 cents/kWh in Galena, Alaska, which factors in only operating costs. On paper, it has been determined that the reactor could run for 30 years without being refueled.
More details on the Toshiba 4S
The encyclopedia of earth article is largely the same as this Australian Uranium Association article. Either the same author or perhaps the encyclopedia copied the nuclear association article.
UPDATE NOTE: The japanese reactor has conceptual similarities to the uranium hydride reactor The Japanese reactor is using uranium nitride. The first could be completed in 2012 and a good design would use 50% of the uranium which is 25 to 70 times more efficient with uranium fuel than existing reactors.
The Fuji Molten salt reactor is a type of reactor which would have almost no long lived nuclear waste. It seems to be under development by a consortium of Japan and Russian groups. It would ideally run with thorium instead of uranium. The mini version is still 8 year away from completion under assumptions of completing a proposed schedule.
870 page report on the status small nuclear reactors without onsite refueling
A 17 page policy framework for micro nuclear reactors
IAEA nuclear technology report (102 pages) from 2004
IAEA nuclear technology report (135 pages) from 2006
IAEA nuclear technology report (141 pages) from 2007