Nuclear fission is a nuclear reaction in which a heavy nucleus splits into two lighter nuclei, releasing a considerable amount of energy.

In the 20th and 21st century, nuclear fission was used as a power source in nuclear fission reactors, but also as a devastating weapon. Nuclear fission produces beta radiation. (Star Trek IV: The Voyage Home; Star Trek: First Contact; DS9: "Little Green Men")

The thought of nuclear fission taking place in a planet's atmosphere shocked Quark and Rom when they learned about it from Nog in 1947. (DS9: "Little Green Men") Nuclear power is the use of sustained exothermic nuclear processes to generate heat and electricity. The term includes the use of exothermic nuclear fission, nuclear decay and nuclear fusion reactions. Presently the nuclear fission of elements in the actinide series of the periodic table produce the vast majority of nuclear power in the service of humankind, with nuclear decay processes, primarily in the form of geothermal energy, and radioisotope thermoelectric generators, in niche uses making up the rest. As of 2013, attaining a net energy gain from sustained nuclear fusion reactions, excluding natural fusion power sources such as the Sun, remains an area of active physics and engineering research.

Nuclear (fission) power stations, that is excluding the contribution from naval nuclear reactors, provided about 5.7% of the world's energy and 13% of the world's electricity, in 2012.[1] In 2013, the IAEA report that there are 437 operational nuclear power reactors (although not all are producing electricity[2]),[3] in 31 countries.[4] In addition, there are approximately 140 naval vessels using nuclear propulsion in operation, powered by some 180 reactors.[5][6][7]

There is an ongoing debate about the use of nuclear energy.[8][9][10] Proponents, such as the World Nuclear Association, the IAEA and Environmentalists for Nuclear Energy contend that nuclear power is a safe sustainable energy source that reduces carbon emissions.[11] Opponents, such as Greenpeace International and NIRS, contend that nuclear power poses many threats to people and the environment.[12][13][14]

Nuclear power plant accidents include the Chernobyl disaster (1986), Fukushima Daiichi nuclear disaster (2011), and the Three Mile Island accident (1979).[15] There have also been some nuclear-powered submarine mishaps.[15][16][17] Research into safety improvements is continuing[18] and nuclear fusion, believed to be safer, may be used in the future.

As of 2012, according to the IAEA, worldwide there were 68 civil nuclear power reactors under construction in 15 countries,[3] approximately 28 of which in the Peoples Republic of China, with the most recent nuclear power reactor, as of May 2013, to be connected to the electrical grid, occurring on February 17 2013 in Hongyanhe Nuclear Power Plant China.[19]

In the USA, two new Generation III reactors are, beginning in March 2013, under construction at Vogtle, a dual construction project which marks the end of a 34 year period of stagnation in the US construction of civil nuclear power reactors. The station operator licenses of almost half the present 104 power reactors in the US, as of 2008, have been given extensions to 60 years,[20] and plans to build another dozen power reactors are under serious consideration.[21]

However, Japan's 2011 Fukushima Daiichi nuclear accident, which occurred in a reactor design from the 1960s, prompted a rethink of nuclear safety and nuclear energy policy in many countries.[22] Germany decided to close all its reactors by 2022, and Italy has banned nuclear power.[22]

Following Fukushima, in 2011 the International Energy Agency halved its estimate of additional nuclear generating capacity to be built by 2035

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