Nuclear reactor

From top, left to right

1. Chicago Pile-1, the first nuclear reactor
2. Shippingport Atomic Power Station, the first peacetime reactor
3. HTR-10, a prototype to the first Generation IV reactor, HTR-PM
4. The Convair NB-36H, the first aircraft to test an onboard reactor
5. Operation Sea Orbit, the first nuclear-powered circumnavigation
6. The Chernobyl sarcophagus, built to contain the effects of the 1986 disaster

A nuclear reactor is a device used to initiate and control a fission nuclear chain reaction. They are used for commercial electricity, marine propulsion, weapons production and research. When a fissile nucleus, usually uranium-235 or plutonium-239, absorbs a neutron, it splits into lighter nuclei, releasing energy, gamma radiation, and free neutrons, which can induce further fission in a self-sustaining chain reaction. Reactors stabilize this with systems of active and passive control, varying the presence of neutron absorbers and moderators in the core, maintaining criticality with delayed neutrons. Fuel efficiency is exceptionally high;low-enriched uranium has an energy density 120,000 times higher than coal.^[1][2]

Following the discovery of nuclear fission in 1938, many countries initiated military nuclear research programs. Early subcritical "atomic piles" sought to allow research on fission and neutronics. The American Manhattan Project made the vast majority of early breakthroughs. In 1942, the first artificial^{[note 1]} critical nuclear reactor, Chicago Pile-1, was built at the University of Chicago, by a team led by Enrico Fermi.^[4] From 1944, with the goal of weapons-grade plutonium production for fission bombs, the first large-scale reactors were operated at the American Hanford Site. The pressurized water reactor design, used in over 70% of current commercial reactors, was developed by the US Navy for submarine propulsion, beginning with the S1W in 1953.^[5] In 1954, nuclear grid electricity production began with the Soviet Obninsk AM-1 reactor.^[6]

Heat from nuclear fission is passed to a working fluid coolant. In commercial reactors, this drives turbines connected to electrical generator shafts. The heat can also be used for district heating, and industrial applications including desalination and hydrogen production. Some reactors are used to produce isotopes for medical and industrial use.

Reactors pose a nuclear proliferation risk as they can be configured to produce plutonium and tritium for nuclear weapons. Spent fuel can be reprocessed, reducing nuclear waste and recovering some reactor-usable MOX fuel. Reprocessing is used in Europe and Asia, but due to proliferation concern, the United States does not engage in or encourage reprocessing.^[7]

Reactor accidents have been caused by combinations of design and operator failure. The International Nuclear Event Scale classifies Levels 1 to 7 of radioactive material released to the environment. The 1979 Three Mile Island accident, at Level 5, and the 1986 Chernobyl disaster and 2011 Fukushima disaster, both at Level 7, all had major effects on the nuclear industry and anti-nuclear movement.

As of 2025^[update], there are 417 commercial reactors, 226 research reactors, and over 160 ships were powered with over 200 marine propulsion reactors in operation globally.^{[8][9][10][11]} Commercial reactors provide 9% of the global electricity supply,^[12] compared to 30% from renewables,^[13] together comprising low-carbon electricity.

The US Department of Energy classes reactors into generations, with the majority of the global fleet being Generation II reactors constructed from the 1960s to 1990s, and Generation IV reactors currently in development. Reactors can also be grouped by the choices of coolant and moderator. Almost 90% of global nuclear energy comes from pressurized water reactors and boiling water reactors, which use water as a coolant and moderator.^[5] Other designs include heavy water reactors, gas-cooled reactors, and fast breeder reactors, variously optimizing efficiency, safety, and fuel type, enrichment, and burnup. Small modular reactors are also an area of current development.

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