Nuclear fission is the splitting of a heavy atom’s nucleus into two smaller nuclei, releasing a massive burst of energy in the process. A single uranium-235 atom splitting releases about 215 million electron volts of energy, roughly ten million times more energy than burning a single molecule of fossil fuel. This reaction powers every commercial nuclear reactor on the planet and produces isotopes used in medicine, industry, and research.
How Fission Works
The nucleus of a uranium-235 atom contains 92 protons and 143 neutrons. That arrangement is somewhat unstable, which makes it vulnerable to outside disruption. When a free neutron strikes the uranium-235 nucleus, the nucleus absorbs it, becomes excited, and quickly breaks apart into two smaller nuclei (called fission products) plus two or three additional neutrons. The combined mass of everything produced is slightly less than the mass of the original atom and the incoming neutron. That missing mass has been converted directly into energy, following Einstein’s famous equation E=mc².
Most of that energy, about 168 MeV out of the total 215, shows up as the raw kinetic energy of the two fission fragments flying apart at tremendous speed. The rest comes out as gamma radiation, the energy of the released neutrons, and the radioactive decay of the fission products over time. In a reactor, all that kinetic energy converts to heat, which boils water, drives turbines, and generates electricity.
The Chain Reaction
What makes fission so powerful as an energy source is that each splitting atom releases two or three fresh neutrons, and each of those neutrons can strike another uranium-235 atom and split it too. This self-sustaining cycle is a chain reaction. If exactly one neutron from each fission event goes on to cause another fission, the reaction holds steady. If more than one does, the reaction accelerates. If fewer than one does, it dies out.
Reactor operators control this balance precisely. They use control rods made of neutron-absorbing materials that can be inserted or withdrawn from the reactor core, soaking up more or fewer neutrons as needed. The physical arrangement of fuel rods and the concentration of uranium-235 in the fuel also limit how many neutrons find a target. In a power reactor, the goal is always a stable, controlled chain reaction producing a constant output of heat.
Fissile and Fertile Fuels
Not every heavy atom splits easily. Atoms that reliably fission when struck by slow-moving neutrons are called “fissile.” The main fissile isotopes used in the nuclear industry are uranium-235, uranium-233, plutonium-239, and plutonium-241. Of these, only uranium-235 exists in usable quantities in nature, and even then it makes up just 0.72% of natural uranium by weight. That low concentration is why uranium must go through an enrichment process before it can serve as reactor fuel.
The remaining 99%-plus of natural uranium is uranium-238, which doesn’t fission easily on its own but is considered “fertile.” When uranium-238 captures a neutron inside a reactor, it gradually transforms into plutonium-239, which is fissile. Similarly, thorium-232 can absorb a neutron and eventually become uranium-233. Power reactors contain both fissile and fertile materials. As fissile atoms are consumed, fertile atoms partially replenish them, extending the useful life of the fuel.
What Fission Produces
When a uranium-235 atom splits, the two fragment nuclei are radioactive isotopes of lighter elements. Hundreds of different isotopes can result, but two of the most significant are strontium-90 and cesium-137. Both have half-lives around 30 years (28.8 years for strontium-90, 30 years for cesium-137), meaning they remain hazardous for centuries and are a primary concern in nuclear waste management. Other fission products have half-lives ranging from fractions of a second to millions of years.
Some of these byproducts are genuinely useful. Over 30 million patients worldwide receive diagnostic medical imaging each year using technetium-99m, a radioactive tracer. Technetium-99m comes from molybdenum-99, which is itself a fission product. To produce it, targets containing uranium are placed inside a research reactor for about a week of neutron bombardment. The targets are then dissolved and the molybdenum-99 is chemically extracted. It’s one of the clearest examples of fission serving a purpose beyond electricity generation.
How Fission Was Discovered
In December 1938, German radiochemists Otto Hahn and Fritz Strassmann were bombarding uranium with neutrons in their Berlin laboratory, expecting the uranium nuclei to change only slightly. Instead, they found the uranium had broken into roughly equal pieces, producing radioactive barium isotopes and other much lighter fragments. The products weighed less than the original uranium nucleus. Lise Meitner, Hahn’s former colleague who had fled Nazi Germany to Sweden, worked with her nephew Otto Frisch to explain what had happened. Their calculations showed that the missing mass had been converted into an enormous amount of energy, confirming a fundamentally new type of nuclear reaction. Frisch named it “fission,” borrowing from the biological term for cell division.
Fission in the Global Energy Mix
Today, over 400 nuclear reactors operate worldwide, all of them powered by fission. These plants collectively generate a significant share of the world’s electricity and represent the largest source of low-carbon baseload power in many countries. France, for instance, gets roughly 70% of its electricity from nuclear fission, while the global average hovers around 10%.
Nuclear plants run continuously for months at a time and aren’t affected by weather, which makes them a reliable complement to variable sources like wind and solar. The tradeoff is the long-lived radioactive waste they produce and the high upfront cost of building new plants, both of which remain central to the ongoing debate about nuclear energy’s role in decarbonizing the grid.
How Fission Differs From Fusion
Fission splits heavy atoms apart. Fusion does the opposite: it forces light atoms together. In the sun, hydrogen atoms fuse into helium under extreme pressure and temperature, releasing several times more energy per reaction than fission. Fusion also doesn’t produce the highly radioactive fission products that make nuclear waste so difficult to manage.
The catch is that sustaining fusion on Earth requires recreating conditions similar to the interior of a star, with temperatures exceeding 100 million degrees. Scientists have achieved fusion in laboratory settings, but no one has yet built a fusion reactor that produces more energy than it consumes over a sustained period. Fission, by contrast, has been generating commercial electricity since the 1950s. For now, every operating nuclear power plant in the world runs on fission.