Generation IV reactor
Generation IV reactors (Gen IV) are a set of theoretical nuclear reactor designs currently being researched. Most of these designs are generally not expected to be available for commercial construction before 2030. Current reactors in operation around the world are generally considered second- or third-generation systems, with most of the first-generation systems having been retired some time ago. Generation V reactors refer to reactors that may be possible but are not yet considered feasible, and are not actively being developed.
Reactor types 
Many reactor types were considered initially; however, the list was downsized to focus on the most promising technologies and those that could most likely meet the goals of the Gen IV initiative. Three systems are nominally thermal reactors and three are fast reactors. The Very High Temperature Reactor (VHTR) is also being researched for potentially providing high quality process heat for hydrogen production. The fast reactors offer the possibility of burning actinides to further reduce waste and of being able to "breed more fuel" than they consume. These systems offer significant advances in sustainability, safety and reliability, economics, proliferation resistance and physical protection.
Thermal reactors 
Very-high-temperature reactor (VHTR) 
The very high temperature reactor concept uses a graphite-moderated core with a once-through uranium fuel cycle, using helium or molten salt as the coolant. This reactor design envisions an outlet temperature of 1,000 °C. The reactor core can be either a prismatic-block or a pebble bed reactor design. The high temperatures enable applications such as process heat or hydrogen production via the thermochemical iodine-sulfur process. It would also be passively safe.
The planned construction of the first VHTR, the South African PBMR (pebble bed modular reactor), lost government funding in February, 2010. A pronounced increase of costs and concerns about possible unexpected technical problems had discouraged potential investors and customers.
Supercritical-water-cooled reactor (SCWR) 
The supercritical water reactor (SCWR) is a concept that uses supercritical water as the working fluid. SCWRs are basically light water reactors (LWR) operating at higher pressure and temperatures with a direct, once-through cycle. As most commonly envisioned, it would operate on a direct cycle, much like a boiling water reactor (BWR), but since it uses supercritical water (not to be confused with critical mass) as the working fluid, would have only one phase present, like the pressurized water reactor (PWR). It could operate at much higher temperatures than both current PWRs and BWRs.
Supercritical water-cooled reactors (SCWRs) are promising advanced nuclear systems because of their high thermal efficiency (i.e., about 45% vs. about 33% efficiency for current LWRs) and considerable plant simplification.
The main mission of the SCWR is generation of low-cost electricity. It is built upon two proven technologies, LWRs, which are the most commonly deployed power generating reactors in the world, and supercritical fossil fuel fired boilers, a large number of which are also in use around the world. The SCWR concept is being investigated by 32 organizations in 13 countries.
Molten-salt reactor (MSR) 
A molten salt reactor is a type of nuclear reactor where the primary coolant, or even the fuel itself is a molten salt mixture. There have been many designs put forward for this type of reactor and a few prototypes built. The early concepts and many current ones rely on nuclear fuel dissolved in the molten fluoride salt as uranium tetrafluoride (UF4) or thorium tetrafluoride (ThF4), the fluid would reach criticality by flowing into a graphite core which would also serve as the moderator. Many current concepts rely on fuel that is dispersed in a graphite matrix with the molten salt providing low pressure, high temperature cooling.
The liquid fluoride thorium reactor (acronym LFTR; spoken as lifter) is a thermal breeder molten salt reactor which uses the thorium fuel cycle in a fluoride-based molten salt fuel to achieve high operating temperatures at atmospheric pressure. It has recently been the subject of a renewed interest worldwide.
Fast reactors 
Gas-cooled fast reactor (GFR) 
The gas-cooled fast reactor (GFR) system features a fast-neutron spectrum and closed fuel cycle for efficient conversion of fertile uranium and management of actinides. The reactor is helium-cooled, with an outlet temperature of 850 °C and using a direct Brayton cycle gas turbine for high thermal efficiency. Several fuel forms are being considered for their potential to operate at very high temperatures and to ensure an excellent retention of fission products: composite ceramic fuel, advanced fuel particles, or ceramic clad elements of actinide compounds. Core configurations are being considered based on pin- or plate-based fuel assemblies or prismatic blocks.
Sodium-cooled fast reactor (SFR) 
The goals are to increase the efficiency of uranium usage by breeding plutonium and eliminating the need for transuranic isotopes ever to leave the site. The reactor design uses an unmoderated core running on fast neutrons, designed to allow any transuranic isotope to be consumed (and in some cases used as fuel). In addition to the benefits of removing the long half-life transuranics from the waste cycle, the SFR fuel expands when the reactor overheats, and the chain reaction automatically slows down. In this manner, it is passively safe.
The SFR reactor concept is cooled by liquid sodium and fueled by a metallic alloy of uranium and plutonium. The fuel is contained in steel cladding with liquid sodium filling in the space between the clad elements which make up the fuel assembly. One of the design challenges of an SFR is the risks of handling sodium, which reacts explosively if it comes into contact with water. However, the use of liquid metal instead of water as coolant allows the system to work at atmospheric pressure, reducing the risk of leakage.
Lead-cooled fast reactor (LFR) 
The lead-cooled fast reactor features a fast-neutron-spectrum lead or lead/bismuth eutectic (LBE) liquid-metal-cooled reactor with a closed fuel cycle. Options include a range of plant ratings, including a "battery" of 50 to 150 MW of electricity that features a very long refueling interval, a modular system rated at 300 to 400 MW, and a large monolithic plant option at 1,200 MW. (The term battery refers to the long-life, factory-fabricated core, not to any provision for electrochemical energy conversion.) The fuel is metal or nitride-based containing fertile uranium and transuranics. The LFR is cooled by natural convection with a reactor outlet coolant temperature of 550 °C, possibly ranging up to 800 °C with advanced materials. The higher temperature enables the production of hydrogen by thermochemical processes.
Advantages and disadvantages 
Relative to current nuclear power plant technology, the claimed benefits for 4th generation reactors include:
- Nuclear waste that remains radioactive for a few centuries instead of millennia 
- 100-300 times more energy yield from the same amount of nuclear fuel 
- The ability to consume existing nuclear waste in the production of electricity
- Improved operating safety
One disadvantage of any new reactor technology is that safety risks may be greater initially as reactor operators have little experience with the new design. Nuclear engineer David Lochbaum has explained that almost all serious nuclear accidents have occurred with what was at the time the most recent technology. He argues that "the problem with new reactors and accidents is twofold: scenarios arise that are impossible to plan for in simulations; and humans make mistakes". As one director of a U.S. research laboratory put it, "fabrication, construction, operation, and maintenance of new reactors will face a steep learning curve: advanced technologies will have a heightened risk of accidents and mistakes. The technology may be proven, but people are not".
A specific risk of the sodium-cooled fast reactor is related to using metallic sodium as a coolant. In case of a breach, sodium explosively reacts with water. Fixing breaches may also prove dangerous, as the noble gas argon is also used to prevent sodium oxidation. Argon is an asphyxiant, so workers may be exposed to this additional risk. This is a pertinent problem as can be testified by the events at the Prototype Fast Breeder Reactor Monju at Tsuruga, Japan.
Participating countries 
The members of the Generation IV International Forum (GIF) are:
- Argentina  (Spanish-only web site)
- Brazil 
- Canada 
- China 
- European Union 
- France 
- Japan 
- South Korea  (Korean-only web site)
- Russia 
- South Africa 
- Switzerland 
- United Kingdom 
- United States 
The nine GIF founding members were joined by Switzerland in 2002, Euratom in 2003 and most recently by China and Russia at the end of 2006.
Australia has also shown interest in joining the GIF.
Designs under development 
- VVER-1700/393 (Super-VVER or VVER-SKD) — Supercritical-water-cooled reactor with double-inlet-core
- BREST-OD-300 (Lead-cooled fast reactor)
See also 
- South Africa to stop funding Pebble Bed nuclear reactor
- US DOE Nuclear Energy Research Advisory Committee (2002). A Technology Roadmap for Generation IV Nuclear Energy Systems. GIF-002-00.
- Stenger, Victor (12 January 2012). "LFTR: A Long-Term Energy Solution?". Huffington Post.
- "Strategies to Address Global Warming".
- "4th Generation Nuclear Power".
- Benjamin K. Sovacool. A Critical Evaluation of Nuclear Power and Renewable Electricity in Asia, Journal of Contemporary Asia, Vol. 40, No. 3, August 2010, p. 381.
- Tabuchi, Hiroko (17 June 2011). "Japan Strains to Fix a Reactor Damaged Before Quake". The New York Times.
- Commissariat à l'Énergie Atomique. "Future nuclear systems".
- Article from Idaho National Laboratory detailing some current efforts at developing Gen. IV reactors.
- Generation IV International Forum (GIF)
- U.S. Department of Energy Office of Nuclear Energy, Science and Technology
- Gen IV presentation
- Science or Fiction - Is there a Future for Nuclear? (Nov. 2007) - A publication from the Austrian Ecology Institute about 'Generation IV' and Fusion reactors.
- Gail H. Marcus (December 2011). "Nuclear Power After Fukushima". Mechanical Engineering (the magazine of ASME). Retrieved 23 January 2012. "In the wake of a severe plant accident, advanced reactor designs are getting renewed attention."