Generation IV reactor
Generation IV 'reactors are nuclear reactor design technologies that are envisioned as successors of generation III reactors. The Generation IV International Forum' – an international organization that coordinates the development of generation IV reactors – specifically selected six reactor technologies as candidates for generation IV reactors. The designs target improved safety, sustainability, efficiency, and cost. The World Nuclear Association in 2015 suggested that some might enter commercial operation before 2030.
No precise definition of a Generation IV reactor exists. The term refers to nuclear reactor technologies under development as of approximately 2000, and whose designs were intended to represent 'the future shape of nuclear energy', at least at that time. The six designs selected were: the gas-cooled fast reactor, the lead-cooled fast reactor, the molten salt reactor, the sodium-cooled fast reactor, the supercritical-water-cooled reactor and the very high-temperature reactor.
The sodium fast reactor has received the greatest share of funding that supports demonstration facilities. Moir and Teller consider the molten-salt reactor, a less developed technology, as potentially having the greatest inherent safety of the six models. The very-high-temperature reactor designs operate at much higher temperatures than prior generations. This allows for high temperature electrolysis or the sulfur–iodine cycle for the efficient production of hydrogen and the synthesis of carbon-neutral fuels.
The majority of reactors in operation around the world are considered second generation and third generation reactor systems, as the majority of the first generation systems have been retired. China was the first country to operate a demonstration generation-IV reactor, the HTR-PM in Shidaowan, Shandong, which is a pebble-bed type high-temperature gas-cooled reactor. It was connected to the grid in December 2023, making it the world's first Gen IV reactor to enter commercial operation. In 2024, it was reported that China would also build the world's first thorium molten salt nuclear power station, scheduled to be operational by 2029.
Generation IV International Forum
The Generation IV International Forum is an international organization with its stated goal being "the development of concepts for one or more Generation IV systems that can be licensed, constructed, and operated in a manner that will provide a competitively priced and reliable supply of energy... while satisfactorily addressing nuclear safety, waste, proliferation and public perception concerns." It coordinates the development of GEN IV technologies. It has been instrumental in coordinating research into the six types of Generation IV reactors, and in defining the scope and meaning of the term itself.As of 2021, active members include: Australia, Canada, China, the European Atomic Energy Community, France, Japan, Russia, South Africa, South Korea, Switzerland, the United Kingdom and the United States. Non-active members include Argentina and Brazil.
The Forum was initiated in January 2000 by the Office of Nuclear Energy of the U.S. Department of Energy's "as a co-operative international endeavour seeking to develop the research necessary to test the feasibility and performance of fourth generation nuclear systems, and to make them available for industrial deployment by 2030."
In November 2013, a brief overview of the reactor designs and activities by each forum member was made available. An update of the technology roadmap which details R&D objectives for the next decade was published in January 2014.
In May 2019, Terrestrial Energy, the Canadian developer of a molten salt reactor, became the first private company to join GIF.
At the forum's October 2021 meeting, the members agreed to create a task force on non-electric applications of nuclear heat, including district and industrial heat applications, desalination and large-scale hydrogen production.
Timelines
The GIF Forum has introduced development timelines for each of the six systems. Research and development is divided into three phases:- Viability: test basic concepts under relevant conditions; identify and resolve all "potential technical show-stoppers";
- Performance: verify and optimise "engineering-scale processes, phenomena and materials capabilities" under prototypical conditions;
- Demonstration: complete and license the detailed design and carry out construction and operation of prototype or demonstration systems.
Reactor types
Many reactor types were considered initially; the list was then refined to focus on the most promising technologies. Three systems are nominally thermal reactors and three are fast reactors. The very high temperature reactor potentially can provide high quality process heat. Fast reactors offer the possibility of burning actinides to further reduce waste and can breed more fuel than they consume. These systems offer significant advances in sustainability, safety and reliability, economics, proliferation resistance, and physical protection.Thermal reactors
A thermal reactor is a nuclear reactor that uses slow or thermal neutrons. A neutron moderator is used to slow the neutrons emitted by fission to make them more likely to be captured by the fuel.Very-high-temperature reactor (VHTR)
The very-high-temperature reactor uses a graphite-moderated core with a once-through uranium fuel cycle, using helium or molten salt. 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 sulfur-iodine cycle process.In 2012, as part of its next generation nuclear plant competition, Idaho National Laboratory approved a design similar to Areva's prismatic block Antares reactor to be deployed as a prototype by 2021.
In January 2016, X-energy was provided a five-year grant of up to $40 million by the United States Department of Energy to advance their reactor development. The Xe-100 is a PBMR that would generate 80 MWe, or 320 MWe in a 'four-pack'.
Since 2021, the Chinese government is operating a demonstration HTR-PM 200-MW high temperature pebble bed reactor as a successor to its HTR-10.
Molten-salt reactor (MSR)
A molten salt reactor is a type of reactor where the primary coolant or the fuel itself is a molten salt mixture. It operates at high temperature and low pressure.Molten salt can be used for thermal, epithermal and fast reactors. Since 2005 the focus has been on fast spectrum MSRs.
Other designs include integral molten salt reactors and molten chloride salt fast reactors.
Early thermal spectrum concepts and many current ones rely on uranium tetrafluoride or thorium tetrafluoride, dissolved in molten fluoride salt. The fluid reaches criticality by flowing into a core with a graphite moderator. The fuel may be dispersed in a graphite matrix. These designs are more accurately termed an epithermal reactor than a thermal reactor due to the higher average speed of the neutrons that cause the fission events.
MCSFR does away with the graphite moderator. They achieve criticality using a sufficient volume of salt and fissile material. They can consume much more of the fuel and leave only short-lived waste.
Most MSR designs are derived from the 1960s Molten-Salt Reactor Experiment. Variants include the conceptual dual fluid reactor that uses lead as a cooling medium with molten salt fuel, commonly a metal chloride, e.g. plutonium chloride, to aid in greater closed-fuel cycle capabilities. Other notable approaches include the stable salt reactor concept, which encases the molten salt in the well-established fuel rods of conventional reactors. This latter design was found to be the most competitive by consultancy firm Energy Process Development in 2015.
Another design under development is TerraPower's molten chloride fast reactor. This concept mixes the liquid natural uranium and molten chloride coolant in the reactor core, reaching very high temperatures at atmospheric pressure. In 2025, the Molten Chloride Reactor Experiment, a joint project between Idaho National Labs, Southern Company and TerraPower, achieved a major milestone, when a prototype furnace created a fuel based on denatured uranium at the rate of per batch. It further completed a Molten Salt Flow Loop Test Bed, using stainless steel containing a slurry of lithium chloride-potassium chloride salts. Slurry properties such as temperature can be adjusted while the salts circulate. Five sensors analyze factors such as surface tension, fluid density, corrosion, and heat transfer rates.
Another notable MSR feature is the possibility of a thermal spectrum nuclear waste-burner. Conventionally only fast spectrum reactors have been considered viable for utilization or reduction of the spent nuclear fuel. Thermal waste-burning was achieved by replacing a fraction of the uranium in the spent nuclear fuel with thorium. The net production rate of transuranic elements is below the consumption rate, thus reducing the nuclear storage problem, without the nuclear proliferation concerns and other technical issues associated with a fast reactor.
Supercritical-water-cooled reactor (SCWR)
The supercritical water reactor is a reduced moderation water reactor concept. Because the average speed of the fission-causing neutrons within the fuel is faster than thermal neutrons, it is more accurately termed an epithermal reactor than a thermal reactor. It uses supercritical water as the working fluid. SCWRs are basically light water reactors operating at higher pressure and temperatures with a direct, once-through heat exchange cycle. As commonly envisioned, it would operate on a direct cycle, much like a boiling water reactor. Since it uses supercritical water as the working fluid, it would have only one water phase. This makes the heat exchange method more similar to a pressurized water reactor. It could operate at much higher temperatures than both current PWRs and BWRs.Supercritical water-cooled reactors offer high thermal efficiency and considerable simplification.
The mission of the SCWR is generation of low-cost electricity. It is built upon two proven technologies, LWRs, the most commonly deployed power generating reactors, and superheated fossil fuel fired boilers, also in wide use. 32 organizations in 13 countries are investigating the concept.
SCWRs share the steam explosion and radioactive steam release hazards of BWRs and LWRs as well as the need for extremely expensive heavy duty pressure vessels, pipes, valves, and pumps. These shared problems are inherently more severe for SCWRs due to their higher temperatures.
One SCWR design under development is the VVER-1700/393 – a Russian SCWR with double-inlet-core and a breeding ratio of 0.95.