reactors

A nuclear reactor is a device used to initiate and control a fission nuclear chain reaction. Nuclear reactors are used at nuclear power plants for electricity generation and in nuclear marine propulsion. When a fissile nucleus like 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. The process is carefully controlled using control rods and neutron moderators to regulate the number of neutrons that continue the reaction, ensuring the reactor operates safely, although inherent control by means of delayed neutrons also plays an important role in reactor output control. The efficiency of energy conversion in nuclear reactors is significantly higher compared to conventional fossil fuel plants; a kilo of uranium-235, if all the atoms were split, would release about three million times more energy than a kilo of coal.
Nuclear reactors have their origins in the World War II Allied Manhattan Project. The world's first artificial nuclear reactor, Chicago Pile-1, achieved criticality on 2 December 1942. Early reactor designs sought to produce weapons-grade plutonium for fission bombs, later incorporating grid electricity production in addition. In 1957, Shippingport Atomic Power Station became the first reactor dedicated to peaceful use; in Russia, in 1954, the first small nuclear reactor APS-1 OBNINSK reached criticality. Other countries followed suit.
Heat from nuclear fission is passed to a working fluid coolant (water or gas), which in turn runs through turbines. In commercial reactors, turbines drive 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, as well as tritium gas used in boosted fission weapons. Reactor spent fuel can be reprocessed to yield up to 25% more nuclear fuel, which can be used in reactors again. Reprocessing also drastically reduces the volumes of nuclear waste, and is practiced in Europe, Russia, India and Japan. Due to initial concerns of proliferation risks, the United States has no reprocessing capability.
Reactors are also used in nuclear propulsion of vehicles. Nuclear marine propulsion of ships and submarines is largely restricted to naval use. Reactors have also been tested for nuclear aircraft propulsion and spacecraft propulsion.
Reactor safety is maintained through various systems that control the rate of fission. The insertion of control rods, which absorb neutrons, can rapidly decrease the reactor's output, while other systems automatically shut down the reactor in the event of unsafe conditions. The buildup of neutron-absorbing fission products like xenon-135 can influence reactor behavior, requiring careful management to prevent issues such as the iodine pit, which can complicate reactor restarts. There have been two reactor accidents classed as an International Nuclear Event Scale Level 7 "major accident": the 1986 Chernobyl disaster and 2011 Fukushima disaster.
As of 2022, the International Atomic Energy Agency reported there are 422 nuclear power reactors and 223 nuclear research reactors in operation around the world. 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 it as a coolant and moderator. 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. These reactors play a crucial role in generating large amounts of electricity with low carbon emissions, contributing significantly to the global energy mix.

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