A nuclear reactor is a device in which nuclear chain reactions are initiated, controlled, and sustained at a steady rate (as opposed to a nuclear bomb, in which the chain reaction occurs in a fraction of a second and is completely uncontrolled).

Nuclear reactors are used for many purposes. The most significant current use is for the generation of electrical power (see nuclear power). Research reactors are used for radioisotope production and for beamline experiments with free neutrons. Historically, the first use of nuclear reactors was the production of weapons grade plutonium for nuclear weapons. Another military use is submarine / ship propulsion (Though this involves a much smaller nuclear reactor than one used in a nuclear power plant).

Currently all commercial nuclear reactors are based on nuclear fission, and are considered problematic by some for their safety and health risks. Conversely, some consider nuclear power to be a safe and pollution-free method of generating electricity. Fusion power is an experimental technology based on nuclear fusion instead of fission. There are other devices in which nuclear reactions occur in a controlled fashion, including radioisotope thermoelectric generators and atomic batteries, which generate heat and power by exploiting passive radioactive decay, as well as Farnsworth-Hirsch fusors, in which controlled nuclear fusion is used to produce neutron radiation.

Although mankind has only tamed nuclear power recently, the first nuclear reactors were naturally occurring. Fifteen natural fission reactors have so far been found in three separate ore deposits at the Oklo mine in Gabon, West Africa. First discovered in 1972 by French physicist Francis Perrin, they are collectively known as the Oklo Fossil Reactors. These reactors ran for approximately 150 million years, averaging 100 kW of power output during that time. Also, stars rely on nuclear fusion for their output of heat, light and other radiations. The concept of a natural nuclear reactor was theorized as early as 1956 by Paul Kuroda at the University of Arkansas •

Enrico Fermi and Leó Szilárd, while both were at the University of Chicago, were the first to build a nuclear pile and demonstrate a controlled chain reaction on December 2, 1942. In 1955 they shared for the nuclear reactor.

The first nuclear reactors were used to generate plutonium for nuclear weapons. Additional reactors were used in the navy (see United States Naval reactor) to propel submarines and aircraft carriers. In the mid-1950s, both the Soviet Union and western countries were expanding their nuclear research to include non-military uses of the atom. However, as with the military program, much of the non-military work was done in secret.

On December 20, 1951, electric power from a nuclear powered generator was produced for the first time at Experimental Breeder Reactor-I (EBR-1) located near Arco, Idaho. On June 26, 1954, at 5:30 pm, the world's first nuclear power plant to generate electricity began operations at Obninsk, Kaluga Oblast, USSR. It produced 5 megawatts, enough to power 2,000 homes. ••.



The world's first commercial scale nuclear power station, Calder Hall, began generation on October 17, 1956 • Another early power reactor was the Shippingport Reactor in Pennsylvania (1957).

Even before the 1979 Three Mile Island accident, new orders for nuclear plants in the U.S. had ceased for economic reasons primarily related to greatly extended construction times. As of 2004, no new nuclear plants have been ordered in the USA since 1978 •, although that may change by 2010 (see Future of the industry below).

Unlike the Three Mile Island accident, the 1986 Chernobyl accident did not increase regulations affecting Western reactors. This was because the Chernobyl reactors were known to be an unsafe design, using the RBMK, without containment buildings and operated unsafely, and the West had little to learn from them •. There was however political fallout: Italy held a referendum the next year in 1987•, the results of which led to a shutdown of the country's four nuclear power plants.

In 1992 the Turkey Point Nuclear Generating Station was hit directly by Hurricane Andrew. Over $90 million of damage was done, largely to a water tank and to a smokestack of one of the fossil-fueled units on-site, but the containment buildings were undamaged ••.

The first organization to develop utilitarian nuclear power, the U.S. Navy, is the only organization worldwide with a totally clean record. This is perhaps because of the stringent demands of Admiral Hyman G. Rickover, who was the driving force behind nuclear marine propulsion. The U.S. Navy has operated more nuclear reactors than any other entity, other than the Soviet Navy, with no publicly known major incidents. Two U.S. nuclear submarines, USS ''Scorpion'' and ''Thresher'', have been lost at sea, though for reasons not related to their reactors, and their wrecks are situated such that the risk of nuclear pollution is considered low.