Nuclear Reactor

A nuclear reactor is a device used to sustain a controlled nuclear fission chain reaction, harnessing the energy released when a fissile nucleus such as uranium-235 or plutonium-239 absorbs a neutron, splits, and releases further neutrons that go on to trigger additional fissions; control rods made of neutron-absorbing material regulate the rate of the reaction to keep it steady rather than runaway. The first artificial reactor, Chicago Pile-1, achieved criticality on December 2, 1942, at the University of Chicago under the wartime Manhattan Project, the first human-made self-sustaining nuclear chain reaction. Commercial power reactors today are dominated by the pressurized water reactor design, which accounts for roughly 63 percent of the world fleet, followed by the boiling water reactor at about 18 percent, alongside gas-cooled, heavy-water and fast-spectrum designs. As of 2025 about 417 commercial reactors operate worldwide, supplying roughly 9 percent of global electricity generation, and the technology also supports naval propulsion, medical isotope production and research.