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 undergoing fission releases about 215 million electron volts of energy, roughly 50 million times more energy than burning one atom of carbon. This is the reaction that powers every commercial nuclear reactor on Earth.
What Happens Inside the Atom
A uranium-235 nucleus contains 92 protons and 143 neutrons. That arrangement is somewhat unstable. When a slow-moving neutron strikes the nucleus and is absorbed, the extra energy destabilizes it enough to make it split apart into two smaller nuclei almost instantly. Those two fragments fly apart at tremendous speed, and their kinetic energy accounts for the bulk of the heat a reactor produces. Two or three free neutrons are also ejected, along with bursts of gamma radiation.
The reason this releases energy comes down to how tightly the particles inside a nucleus are bound together. Mid-sized nuclei (around 60 protons and neutrons total) are the most tightly bound and therefore the most stable. Heavy nuclei like uranium sit on a less stable part of the curve. When they split into two mid-sized fragments, the resulting nuclei are more tightly bound than the original, and the difference in binding energy is converted into heat, radiation, and the kinetic energy of the fragments and neutrons.
The Chain Reaction
The two or three neutrons released by each fission event are what make a sustained chain reaction possible. If at least one of those neutrons strikes another uranium-235 nucleus and causes it to split, the process continues. If each fission event triggers exactly one subsequent fission on average, the reaction rate holds steady. This balanced state is called criticality, and it’s the target operating condition for a power reactor.
Engineers describe the chain reaction using a number called the multiplication factor, or k. When k equals 1, the system is critical: the neutron population stays constant and power output is stable. When k drops below 1, the reaction winds down (subcritical). When k climbs above 1, the neutron population grows exponentially (supercritical). A nuclear weapon relies on a rapid, uncontrolled supercritical state. A reactor, by contrast, is designed to hover right at k = 1, with tiny adjustments made continuously.
What Keeps It Under Control
Reactors stay controllable largely because of a quirk of fission physics: not all neutrons are released at the instant of splitting. About 99.3 to 99.5 percent are “prompt” neutrons, emitted immediately. But a small fraction, roughly 0.65 percent, are delayed. These delayed neutrons come from the radioactive decay of fission fragments and emerge seconds to minutes after the original split. That tiny delay enormously extends the average time between neutron generations, giving operators and automated systems enough time to adjust power levels. Without delayed neutrons, any increase in reactivity would cause an uncontrollable, near-instantaneous power spike.
Control rods are the primary tool for fine-tuning the chain reaction. These are rods made of materials that absorb neutrons extremely well, with boron-10 being one of the most effective. The rods slide in and out of the reactor core. Push them in, and they absorb more neutrons, slowing the reaction. Pull them out, and more neutrons survive to cause fission, increasing power. In an emergency, the rods can be fully inserted in seconds to shut the reactor down entirely.
The other critical component is the moderator, typically ordinary water. Neutrons produced by fission are moving extremely fast. At those high speeds, they’re less likely to be absorbed by uranium-235 nuclei. The moderator slows them down to what physicists call “thermal” speeds, where their probability of triggering another fission event increases dramatically. This is why most commercial reactors are called “thermal reactors”: they rely on slowed-down, thermal neutrons.
Fast Reactors and Different Fuels
Not all reactors slow neutrons down. Fast reactors skip the moderator and use the high-energy neutrons directly. Instead of water, they use liquid metals like sodium or lead as a coolant, which removes heat without slowing the neutrons. Fast neutrons are more efficient at splitting certain heavy elements that thermal neutrons struggle with, including long-lived radioactive waste. They can also convert uranium-238, which makes up about 95 percent of the fuel in conventional reactors and is otherwise not fissile, into plutonium-239, which is.
Plutonium-239 is the other major fission fuel besides uranium-235. It behaves similarly but produces more neutrons per split: about 2.9 on average in a thermal reactor compared to roughly 2.5 for uranium-235. In a fast reactor, plutonium-239 produces even more, around four neutrons per fission. This higher neutron yield is what allows fast reactors to “breed” new fuel from otherwise unusable uranium, potentially stretching the world’s nuclear fuel supply dramatically.
Where the Energy Goes
Of the roughly 215 MeV released per fission event, the largest share (about 168 MeV) is the kinetic energy of the two fission fragments as they fly apart. These fragments slam into surrounding atoms and convert their motion into heat, which is ultimately what boils the water that spins a turbine to generate electricity. Another 5 MeV goes into the kinetic energy of the released neutrons. Gamma rays, both immediate and delayed, contribute another 10 to 19 MeV. The radioactive decay of fission products adds roughly 27 MeV more in the form of beta particles, gamma rays, and neutrinos over the hours and days after the split.
This is why a spent nuclear fuel rod remains intensely hot and radioactive long after it’s removed from the reactor. The fission fragments continue to decay, releasing energy as they transform into more stable elements.
What Fission Produces
When a uranium-235 atom splits, the two fragment nuclei are almost never equal in size. The most common products cluster around two mass ranges: roughly 90 and roughly 137 nucleons. This means isotopes like strontium-90 and cesium-137 are among the most frequently produced fission byproducts. Both are significant because they’re radioactive with half-lives around 30 years, long enough to require careful waste management but short enough to be intensely radioactive.
The full list of fission products spans much of the periodic table. It includes noble gases like krypton-85 and xenon-133, halogens like iodine-131, and trace amounts of tritium (a radioactive form of hydrogen). Managing these byproducts, particularly the long-lived ones, is one of the central challenges of nuclear power.
Spontaneous Fission
Fission doesn’t always require an incoming neutron. Very heavy elements with atomic mass numbers above 92 can undergo spontaneous fission, splitting on their own through a quantum mechanical process called tunneling. The nucleus essentially has a small probability of “tunneling” through its own energy barrier without any outside trigger. This happens naturally in thorium-232, uranium-235, and uranium-238, though it’s extremely rare compared to their other forms of radioactive decay. Spontaneous fission is more of a curiosity than a practical energy source, but it does provide the initial “seed” neutrons that help start up some reactor designs.