A tiny fraction of late neutrons is what makes reactors controllable
Most neutrons from splitting uranium fly out instantly, but about 0.65 percent arrive late, released by fission products over milliseconds to minutes. That small delay is the whole trick. Running a reactor where those stragglers are needed to stay critical slows everything down enough for machines and operators to keep up.
A reactor sustains a chain reaction. When a heavy nucleus such as uranium-235 or plutonium-239 absorbs a neutron, it can split into lighter fragments, releasing energy and more neutrons that go on to split other nuclei. Control rods made of neutron-absorbing material are pushed in to slow things and pulled out to speed them up. If the reaction ever sustained itself on prompt neutrons alone, a state called prompt critical, power would climb too quickly for anyone to intervene. Engineers even measure reactivity in money: bare criticality is zero dollars, prompt critical is one dollar, with cents in between.
The payoff is extraordinary energy density. A kilogram of uranium-235 releases roughly three million times the energy of a kilogram of burned coal, equal to about 2.7 million kilograms of it. That heat, carried off by water, gas, liquid metal or molten salt, raises steam for turbines just as in a coal plant. Some heat lingers even after shutdown, because fission products keep decaying.
The history moved fast. Fission was discovered in 1938, and in 1942 Chicago Pile-1 became the first artificial reactor to go critical, which is when the older term atomic pile began giving way to reactor. Large reactors at Hanford made weapons material from 1944. The pressurized water design, now used in about 70 percent of commercial reactors, grew out of US Navy submarine work starting in 1953. The Soviet Obninsk plant began producing small amounts of electricity in 1954, and Calder Hall in England opened in 1956 as the first commercial nuclear power station.
Accidents at Three Mile Island in 1979, Chernobyl in 1986 and Fukushima in 2011 shaped the industry deeply. As of 2025, 417 commercial reactors supplied about 9 percent of the world's electricity.
Source: Nuclear reactor
