Technology & Medicine › Atomic & space age
Nuclear energy (bombs and reactors)
first reactor December 2, 1942· Invention or discovery· Tools, energy & materials
Well documentedSolid records, physical evidence or scientific consensus.
Splitting heavy atoms releases millions of times more energy per pound than burning coal, used both in bombs and in power reactors.
See it on the Technology & Medicine map →How it works
A uranium-235 or plutonium nucleus hit by a neutron splits, releasing heat and two or three more neutrons, which can split further nuclei. In a reactor, control rods and a moderator keep this chain reaction steady; in a bomb it runs away in about a millionth of a second. Enrico Fermi's team started the first reactor, Chicago Pile-1, under the stands of a University of Chicago football field on December 2, 1942.
Why it mattered
It created weapons able to destroy cities, which shaped the Cold War, while reactors now supply about 9% of the world's electricity with little carbon.
Think of it like
Like a chain letter: each split triggers more. A reactor holds it at exactly one new split per split; a bomb lets it multiply.
How we know
Fermi's logbooks and Manhattan Project records, decades of reactor operating data, and lab measurements of fission energy that match Einstein's E = mc².
Still unknown or debated
Whether nuclear energy's benefits outweigh weapons proliferation, accident and waste risks is still argued; the answer differs from country to country.
Inside this (4)
Connections (5)
▸Marie Curie and radioactivity opened the path to Nuclear energy (bombs and reactors)Mostly agreed
The connection
Curie proposed in 1898 that radioactivity comes from inside atoms, and in 1903 Pierre Curie showed radium gives off heat steadily. That revealed vast energy stored in atoms. Her daughter Irène and son-in-law Frédéric Joliot-Curie created artificial radioactivity in 1934, and Irène's work on uranium helped lead to the discovery of fission.
When and how
Paris, 1898 (Marie and Pierre Curie: polonium and radium); Paris, 1934 (the Joliot-Curies); Berlin, 1938 (fission).
How sure are we?
The Curies' papers and the Joliot-Curies' 1934 report are on record; Hahn and Strassmann's fission work began partly as checks on Irène Joliot-Curie's uranium results.
Debates and open questions
Early researchers, Curie included, misjudged radiation's danger; her 1934 death from aplastic anemia is widely blamed on exposure, and her notebooks are still radioactive.
▸Discovery of nuclear fission led to Nuclear energy (bombs and reactors)Well documented
The connection
Fission showed that splitting one uranium atom releases about 200 million electron volts, tens of millions of times more than a chemical reaction, and frees spare neutrons. Physicists quickly realized a self-sustaining chain reaction was possible. Fermi's team achieved one in Chicago in December 1942, just four years later.
When and how
Berlin and Sweden, December 1938; neutron release measured in Paris and New York, March 1939; Chicago Pile-1, December 2, 1942.
How we know
Published 1939 papers, including the Joliot-Curie team's and Szilárd and Fermi's neutron measurements, and Fermi's reactor logbooks document each step.
Debates and open questions
Leó Szilárd imagined a neutron chain reaction in 1933 and patented it, years before fission was known; whether others would soon have found fission anyway is debated.
▸Electronic computer (Z3, Colossus, ENIAC) ran early weapons calculations for Nuclear energy (bombs and reactors)Well documented
The connection
Nuclear weapon designs need vast calculations of how shock waves and neutrons move. Manhattan Project scientists first used rooms of human 'computers' and IBM punched-card machines. ENIAC's first major job, from December 1945, ran calculations for Edward Teller's proposed hydrogen bomb, and nuclear labs stayed leading buyers of supercomputers.
When and how
Los Alamos, 1944–45 (IBM machines and human computers); ENIAC in Philadelphia, December 1945 to early 1946, run with Los Alamos physicists Nicholas Metropolis and Stanley Frankel.
How we know
Los Alamos histories and ENIAC records document the hydrogen-bomb calculation; the Monte Carlo method, later central to computing, was developed for such problems.
Debates and open questions
How much computers shaped weapons design, versus physical insight and testing, is debated; ENIAC's early hydrogen-bomb results were in fact misleadingly optimistic.
▸Nuclear energy (bombs and reactors) was turned to electricity in Nuclear power stationsWell documented
The connection
The first big reactors were built to make plutonium for bombs, and their heat was wasted. Engineers soon used such heat to boil water and drive turbines. President Eisenhower's 1953 'Atoms for Peace' speech promoted civilian power, and US submarine reactors, starting with USS Nautilus in 1955, became the model for most power plants.
When and how
EBR-I in Idaho lit bulbs with nuclear electricity in December 1951; Obninsk, USSR, 1954; Calder Hall, England, 1956; Shippingport, Pennsylvania, 1957.
How we know
Plant records and government histories trace power reactors directly to wartime reactors and the naval program led by Admiral Hyman Rickover.
Debates and open questions
Were early plants mainly for electricity or for weapons? Calder Hall also made plutonium for Britain's bombs, blurring the line.
▸Chernobyl disaster turned public opinion against Nuclear energy (bombs and reactors)Well documented
The connection
Fallout across Europe hardened public fear of nuclear power. Italy voted in 1987 to phase out its plants; other countries delayed or cancelled new reactors, and construction worldwide slowed sharply. The industry responded with stronger international safety reviews, including peer inspections run by a new association of plant operators from 1989.
When and how
Across Europe after April 1986: Sweden detected the fallout first; Italy held its referendum in 1987; Germany, after Fukushima, closed its last reactors in April 2023.
How we know
Opinion polls, referendum results and reactor construction data show the shift; new reactor construction starts worldwide fell steeply in the late 1980s.
Debates and open questions
How much Chernobyl, versus high costs, cheap oil and gas after 1986 and the 1979 Three Mile Island accident, slowed nuclear power is debated; climate concerns have revived interest.