What Is Nuclear Fission? Splitting the Atom and What Comes Out
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In December 1938 two chemists in Berlin bombarded uranium with neutrons and found barium in the residue, an element about half the weight of uranium, which made no sense unless the uranium nucleus had split in two. Lise Meitner, their former colleague, worked out over Christmas in Sweden that it had, and that the two halves flew apart with an energy of about 200 million electron volts, a hundred million times the energy of a chemical reaction. Within seven years that calculation had become a bomb, and within twenty a source of electricity.
Why a nucleus can split
A nucleus is held together by the strong force, which binds protons and neutrons to their immediate neighbours, against the electric repulsion of the protons, which pushes on every proton in the nucleus at once. In small nuclei the binding wins comfortably. As nuclei grow, the electric repulsion grows faster than the binding, and by the time a nucleus has ninety-odd protons it is barely stable, like a drop of water that has grown too large to hold its shape. Uranium-235, with 92 protons and 143 neutrons, is such a drop. A neutron striking it adds enough energy to set it wobbling, the wobble stretches it into a dumbbell, and electric repulsion tears the two ends apart.
The two fragments, typically something like barium and krypton, are lighter together than the original nucleus by about a tenth of a percent, and that missing mass is the energy, by Einstein's equation, which is why so much comes from so little. Almost all of it is carried as the motion of the fragments, which stop within a fraction of a millimetre and turn it into heat.
The chain reaction
The crucial fact, found within weeks of the discovery, was that each fission also released two or three fresh neutrons. If at least one of those goes on to split another nucleus, the reaction sustains itself; if more than one does, it grows, doubling in a fraction of a second. Leo Szilard had imagined a neutron chain reaction in 1933 while waiting at a London traffic light, and he understood in 1939 what uranium meant. The three conditions for a chain reaction:
- •Fuel: enough uranium-235 or plutonium-239, isotopes that split easily; natural uranium is only 0.7 percent U-235 and must be enriched to about 4 percent for a reactor or 90 percent for a weapon
- •Neutrons at the right speed: fast neutrons tend to fly past, so a reactor slows them with a moderator, usually water or graphite, which makes them far more likely to be captured
- •Enough material together: below a critical mass too many neutrons escape through the surface before finding a nucleus, and the reaction dies
Taming it
A reactor holds the chain reaction at exactly self-sustaining, with each fission causing on average one more. It does that with control rods of a neutron-absorbing material such as boron or cadmium, pushed in to slow the reaction and drawn out to speed it, and with the help of a small fraction of neutrons that are released seconds rather than microseconds after fission, which gives the operators and the automatic systems time to act. The first reactor, built by Enrico Fermi under a football stand in Chicago in December 1942, was a pile of graphite blocks and uranium with cadmium rods, and it ran for four and a half minutes at half a watt.
A power reactor is the same thing at a gigawatt. The heat of the fragments boils water, the steam drives a turbine, and the rest is a conventional power station. A kilogram of reactor fuel releases roughly as much energy as a hundred tonnes or more of coal, and a kilogram of pure uranium-235, fully split, as much as about three million kilograms, with no carbon dioxide, and the fuel is the smallest part of the cost. What remains afterwards is the problem: the fission fragments are intensely radioactive, some for centuries and the transuranic elements bred in the fuel for tens of thousands of years, and no country has yet opened a permanent repository for them, though Finland is close.
The bomb
A weapon is a chain reaction allowed to run away. Two pieces of highly enriched uranium, each below critical mass, are slammed together, or a sphere of plutonium is crushed by explosives to a density at which it goes critical, and the reaction runs through some eighty doublings in less than a microsecond before the material blows itself apart. The Hiroshima bomb fissioned less than a kilogram of its uranium; that was enough to destroy the city. Fission weapons are also the trigger of hydrogen bombs, whose main energy comes from fusion, the joining of light nuclei, which is the process that powers the Sun and that fission's discoverers were not looking for.
Fission in the world
About 440 reactors in over thirty countries supply roughly a tenth of the world's electricity, and France gets about two thirds of its power from them. The accidents at Three Mile Island in 1979, Chernobyl in 1986 and Fukushima in 2011 shaped public attitudes more than the routine record, and countries have gone in opposite directions, Germany closing its last reactors in 2023 while China builds more than anyone. Fission also occurs in nature: at Oklo in Gabon, two billion years ago, a rich uranium deposit ran as a natural reactor for hundreds of thousands of years, moderated by groundwater, and left behind the same fragments a reactor makes today.
Why a reactor cannot explode like a bomb
A common fear is that a nuclear power station could go off like an atomic bomb. The physics rules this out. A bomb needs uranium enriched to around ninety percent uranium-235, assembled into a supercritical mass so quickly and held together so tightly that most of the chain reaction happens before the material flies apart. Reactor fuel is only a few percent uranium-235, spread through fuel rods among coolant and structural material. If the reaction speeds up, the fuel heats, expands and melts, and the arrangement falls apart long before anything like a nuclear explosion is possible.
That does not make reactors harmless, and the real accidents show what can go wrong. At Chernobyl, a design flaw and a badly run test let the power surge; the result was a steam explosion and a graphite fire that scattered radioactive material, not a nuclear blast. At Fukushima the reactors had shut down, but the fuel kept producing heat from radioactive decay, and when the tsunami knocked out the cooling pumps the cores melted and hydrogen gas exploded. The danger of a reactor is heat and the release of radioactive fragments, which is why cooling and containment dominate safety design.
Lise Meitner and the missing Nobel
Meitner had worked with Otto Hahn in Berlin for three decades, she as the physicist and he as the chemist, until the annexation of Austria in 1938 brought the Austrian-born scientist of Jewish descent under Nazi law and she fled to Sweden. Hahn wrote to her about the baffling barium, and on a winter walk with her nephew Otto Frisch, also a physicist, she worked out the explanation. Frisch borrowed the word fission from biology, where it describes a cell dividing in two, and in January 1939 he confirmed the idea in Copenhagen by detecting the large bursts of ionisation the fragments produced.
The Nobel Prize in Chemistry for 1944 went to Hahn alone. Meitner, nominated many times over her career, never received one, and historians who later studied the committee's records concluded that the chemists judging the prize underrated the physics needed to interpret the experiment, and that her exile, which cut her off from her old laboratory, made her part easier to overlook. Recognition came later. In 1966 she shared the American Enrico Fermi Award with Hahn and Fritz Strassmann, and in 1997 element 109 was named meitnerium in her honour, while a proposal to name element 105 after Hahn was dropped.
A reactor that nature built
In 1972 French scientists analysing uranium ore from Oklo in Gabon noticed something strange: the ore contained slightly less of the fissile isotope uranium-235 than uranium does everywhere else on Earth. The explanation turned out to be astonishing. About two billion years ago, natural nuclear chain reactions had run in the deposit, consuming some of the uranium-235.
At that time uranium-235 made up a larger share of natural uranium, enough for a reaction to sustain itself when groundwater seeped into the ore and slowed the neutrons. The reactions seem to have switched on and off as the water boiled away and returned, running intermittently for hundreds of thousands of years. Oklo remains the only known natural fission reactor, and it has been studied for clues about storing nuclear waste.
The takeaway
Nuclear fission is the splitting of a heavy, barely stable nucleus such as uranium-235 into two lighter fragments when a neutron disturbs it, releasing about 200 million electron volts and two or three more neutrons, which can split further nuclei in a chain reaction. A reactor holds that chain exactly self-sustaining with a moderator and control rods and turns the heat into electricity; a bomb lets it run away. It was discovered in 1938 and has powered stations, submarines and weapons since the 1940s.