Discovery: uranium splits
In December 1938 Berlin chemists Otto Hahn and Fritz Strassmann bombarded uranium with neutrons and unexpectedly found much lighter barium. Physicist Lise Meitner, who had fled the Nazis to Sweden, and her nephew Otto Frisch interpreted the result as the nucleus splitting in two and borrowed the biological term "fission." They calculated that the fragments weigh slightly less than the original nucleus and that the missing mass becomes energy according to Einstein's E=mc². Hahn alone received the 1944 Nobel Prize in Chemistry, and Meitner's omission is often cited as one of science's great injustices; element 109, meitnerium, now honours her on the periodic table.
Chain reactions and critical mass
Each uranium-235 fission releases about 200 MeV of energy and on average two or three neutrons. If those neutrons split further nuclei, the reaction spreads exponentially in a chain reaction. Sustaining it requires a minimum amount of fuel in one place, the critical mass. In 1942 Enrico Fermi's team brought the first reactor, Chicago Pile-1, to criticality beneath the stands of a University of Chicago sports field, and in 1945 Manhattan Project bombs were dropped on Hiroshima and Nagasaki. Fully fissioning 1 kg of uranium releases energy comparable to burning roughly 2,500–3,000 tonnes of coal.
Nuclear power and its challenges
A nuclear power plant keeps the chain reaction steady with control rods and a moderator, using the heat to boil water and drive steam turbines. Most reactors are light-water designs using uranium enriched to 3–5%, and nuclear energy supplies about 9–10% of the world's electricity. Because it emits almost no carbon dioxide while operating, it has regained attention as a tool for carbon neutrality, but accident risks shown at Chernobyl in 1986 and Fukushima in 2011, and the disposal of spent fuel that stays radioactive for tens of thousands of years, remain unresolved. Fission products decay with their own half-lives. Nuclear fusion, which joins light nuclei, promises abundant fuel and less long-lived waste but is not yet commercial.
FAQ
- What's the difference between fission and fusion?
- Fission splits heavy nuclei; fusion joins light ones such as hydrogen. Both release energy from lost mass, but only fission is used in commercial power plants today.
- Can a nuclear power plant explode like an atomic bomb?
- No. Reactor fuel is enriched to only 3–5%, too low for a nuclear explosion. A cooling failure can, however, cause a meltdown and hydrogen explosions that release radioactive material.
Sources reviewed: 2026-10-26
References
- Hahn, O., & Strassmann, F. (1939). Über den Nachweis und das Verhalten der bei der Bestrahlung des Urans mittels Neutronen entstehenden Erdalkalimetalle. Naturwissenschaften, 27, 11–15.
- Meitner, L., & Frisch, O. R. (1939). Disintegration of uranium by neutrons: a new type of nuclear reaction. Nature, 143, 239–240.
- World Nuclear Association. (2024). Physics of Uranium and Nuclear Energy.
- International Atomic Energy Agency. (2024). Nuclear Power Reactors in the World. IAEA.