Nuclear power is the use of sustained nuclear fission to generate heat and electricity. Nuclear power plants provided about 5.7% of the world's energy and 13% of the world's electricity, in 2012. In 2013, the IAEA report that there are 437 operational nuclear power reactors (although not all are producing electricity), in 31 countries. In addition, there are approximately 140 naval vessels using nuclear propulsion in operation, powered by some 180 reactors.

There is an ongoing debate about the use of nuclear energy. Proponents, such as the World Nuclear Association, the IAEA and Environmentalists for Nuclear Energy contend that nuclear power is a sustainable energy source that reduces carbon emissions. Opponents, such as Greenpeace International and NIRS, believe that nuclear power poses many threats to people and the environment.

Nuclear power plant accidents include the Chernobyl disaster (1986), Fukushima Daiichi nuclear disaster (2011), and the Three Mile Island accident (1979). There have also been some nuclear-powered submarine mishaps. Research into safety improvements is continuing 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. In the US the licenses of almost half its reactors have been extended to 60 years, and plans to build another dozen are under serious consideration. However, Japan's 2011 Fukushima Daiichi nuclear disaster prompted a rethink of nuclear energy policy in many countries. Germany decided to close all its reactors by 2022, and Italy has banned nuclear power. Following Fukushima, the International Energy Agency halved its estimate of additional nuclear generating capacity to be built by 2035.