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A nuclear reactor is a device used to sustain a controlled fission nuclear chain reaction.

Heat from nuclear fission is passed to a working fluid coolant. In 1942, the first artificial critical nuclear reactor, Chicago Pile-1, was built by the Metallurgical Laboratory. The pressurized water reactor design, used in about 70% of commercial reactors, was developed for US Navy submarine propulsion, beginning with S1W in 1953. The first commercial nuclear power station was Calder Hall at Sellafield, England, which started operation in 1956. Spent fuel can be reprocessed, potentially reducing nuclear waste and recovering reactor-usable fuel. This also poses a nuclear proliferation risk via production of plutonium for nuclear weapons. Other designs include gas-cooled, fast-spectrum, breeder, heavy-water, molten-salt, and small modular, each of which improves safety, efficiency, cost, fuel type, enrichment, or burnup, and the now-obsolete Light water graphite reactor.

The phrases "nuclear pile" and "atomic reactor" were also common. The term "nuclear reactor" is predominantly used to refer to nuclear fission reactors. It can also refer to a nuclear fusion reactor, of which only net negative power systems have been constructed. Radioisotope thermoelectric generators and radioisotope heater units, while deriving power from nuclear decay reactions, are not referred to as nuclear reactors as they do not induce reactions.

Just as conventional thermal power stations generate electricity by harnessing the thermal energy released from burning fossil fuels, nuclear reactors convert the energy released by controlled nuclear fission into thermal energy for further conversion to mechanical or electrical forms.

This is known as a nuclear chain reaction.

A nuclear reactor coolant – usually water but sometimes a gas or a liquid metal (like liquid sodium or lead) or molten salt – is circulated past the reactor core to absorb the heat that it generates. However, in some reactors the water for the steam turbines is boiled directly by the reactor core – for example, the boiling water reactor.

The first nuclear power plant built for civil purposes was the AM-1 Obninsk Nuclear Power Plant, launched on 27 June 1954 in the Soviet Union. It was built after the F-1 (nuclear reactor), which was the first reactor to go critical in Europe, and was also built by the Soviet Union.

These constitute the great majority of operational nuclear reactors: as of 2014, 93% of the world's nuclear reactors are water-cooled, providing about 95% of the world's total nuclear generation capacity.

This is a thermal neutron reactor design, the newest of which are the Russian VVER-1200, Japanese Advanced Pressurized Water Reactor, American AP1000, Chinese Hualong Pressurized Reactor and the Franco-German European Pressurized Reactor.

This is a thermal-neutron reactor design, the newest of which are the Advanced Boiling Water Reactor and the Economic Simplified Boiling Water Reactor.

One prototype, the Molten-Salt Reactor Experiment, was built to confirm the feasibility of the Liquid fluoride thorium reactor, a thermal spectrum reactor which would breed fissile uranium-233 fuel from thorium.

The former include the advanced boiling water reactor (ABWR), two of which are now operating with others under construction, and the planned passively safe Economic Simplified Boiling Water Reactor (ESBWR) and AP1000 units (see Nuclear Power 2010 Program). The Clean and Environmentally Safe Advanced Reactor (CAESAR) is a nuclear reactor concept that uses steam as a moderator – this design is in development.

A fission fragment reactor is a nuclear reactor that generates electricity by decelerating an ion beam of fission byproducts instead of using nuclear reactions to generate heat.

Many of these fuel rods are used in each nuclear reactor.

After being discharged from the reactor, spent nuclear fuel is transferred to the on-site spent fuel pool.

The conditions at that place and time allowed a natural nuclear fission to occur with circumstances that are similar to the conditions in a constructed nuclear reactor.

Quick Facts

• The term "nuclear reactor" is predominantly used to refer to nuclear fission reactors.
• This is a thermal neutron reactor design, the newest of which are the Russian VVER-1200, Japanese Advanced Pressurized Water Reactor, American AP1000, Chinese Hualong Pressurized Reactor and the Franco-German European Pressurized Reactor.
• Many of these fuel rods are used in each nuclear reactor.
• A fission fragment reactor is a nuclear reactor that generates electricity by decelerating an ion beam of fission byproducts instead of using nuclear reactions to generate heat.
• In 1942, the first artificial critical nuclear reactor, Chicago Pile-1, was built by the Metallurgical Laboratory.

Source material: Wikipedia - "Nuclear reactor". Adapted and summarized for DiscoverScroll. Original contributors are credited through the linked Wikipedia article. Read original on Wikipedia. CC BY-SA 4.0. Changes were made from the original.