Globally, in 2019, nuclear power supplied 2,586 terawatt hours (TWh) of electricity demand (about 10% of total global electricity generation) and was the second-largest, low-carbon power source after hydro-electricity.
Nuclear fuel (Uranium 235) contains millions of times the amount of energy available from a similar mass of chemical fuel, such as coal, diesel or gasoline, making it an excellent power source for shipping and generating electricity. It also releases no carbon dioxide (CO2) to the atmosphere.
Most reactors reduce (moderate) the speed of neutrons emitted by the fission of U235 so that they are more likely to strike another U235 nucleus with enough energy to split it, thereby releasing two or more neutrons and an enormous amount of energy from the loss of some total mass, as described by Einstein's famous equation.
The moderator is ordinary water, or heavy water or graphite that slows the neutrons that are (initially spontaneously) emitted from the tiny amount of 235-uranium (0.7%) naturally present with 238-uranium. The fission rate is controlled by the insertion of cadmium rods which rapidly absorb free neutrons. Removing them increases the rate of fission. These rods are also used to shut down the reactor.
Using heavy water as a moderator allows the reactor to use natural uranium fuel (which contains only 0.7% U235) and permits fast emergency shut downs. A molecule of heavy water has two deuterium atoms instead of the two protons of ordinary "light" H2O water. Deuterium is an hydrogen isotope containing a neutron in the nucleus in addition to the proton. The more common 'light' water moderated reactors all use uranium fuel slightly enriched to about 3.2% U235.
In January 1955, the world's first nuclear submarine, was delivered to the U.S.Navy and, in August 1958, it was the first submarine to complete, an under-ice, transit of the North Pole. A nuclear reactor allows submarines to operate underwater at speeds up to 30 mph for a virtually unlimited range. It needs to be refuelled only every 25 years.
On September 5, 1945, the first nuclear reactor outside of the United States was a heavy water moderated research reactor built near Ottawa, Canada. The first nuclear reactor in France was also moderated by heavy water and went critical near the end of 1948. France later built 25 nuclear power plants and as of 2019, 71% of French electricity was generated by nuclear power.
The Obninsk Power Plant in the USSR was the world's first nuclear power plant to generate electricity for a power grid on June 27, 1954.
Built in the Soviet Union in 1977, the Chernoble reactor was an early Generation 2 reactor with several design flaws which caused a disastrous release of radio-active isotopes in 1986. The problems were corrected in similar reactors which have operated without serious incidents for 30 years.
The 1979 accident at Three Mile Island reactors (with no fatalities), persuaded authorities in the USA to cancel more than 120 reactor proposals.
Commissioned in Japan, in 1971, the Fukushima Daiichi nuclear reactors failed, in 2011, because they were located where they were inundated by a tsunami that submerged vital cooling pumps located below floor level. Modern nuclear reactors are much safer.
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