Induced fission is a nuclear reaction in which a heavy atom’s nucleus is split apart after being struck by an external particle, typically a neutron. The splitting releases roughly 200 million electron volts (MeV) of energy per event, about a million times more energy than a single chemical reaction like burning coal. It is the fundamental process behind both nuclear power plants and nuclear weapons.
How Induced Fission Works
The classic example involves uranium-235, an atom with 92 protons and 143 neutrons packed into its nucleus. That arrangement is inherently somewhat unstable. When a free neutron collides with and is absorbed by a uranium-235 nucleus, the added energy destabilizes it further. The nucleus begins to deform, stretching into an elongated shape much like a wobbling water droplet (physicists actually call this the “liquid drop model”). Within a fraction of a second, the forces holding the nucleus together can no longer compete with the electrostatic repulsion pushing its protons apart, and the nucleus tears into two smaller fragments.
Those fragments are mid-sized atoms, not a clean split down the middle. A typical fission event produces one lighter fragment (in the mass range of krypton or molybdenum) and one heavier fragment (in the range of cesium or neodymium). Along with these two daughter nuclei, each fission releases two or three free neutrons and a burst of energy carried away as kinetic energy of the fragments, gamma radiation, and the motion of those newly freed neutrons.
What Makes It Different From Spontaneous Fission
Some very heavy nuclei can split on their own without any outside trigger. This is spontaneous fission, and it happens through a quantum mechanical process called tunneling. It is extraordinarily rare compared to other forms of radioactive decay. Uranium-235, for instance, has a half-life for spontaneous fission of about 1.8 × 1017 years, roughly a hundred million times longer than its half-life for ordinary alpha decay (7.1 × 108 years). In practical terms, spontaneous fission almost never happens in a given sample of uranium-235.
Induced fission, by contrast, is deliberate. You fire a neutron at a fissile nucleus and reliably trigger the split. Neutrons are especially good at this job because they carry no electric charge, so they aren’t repelled by the positively charged nucleus the way a proton or another charged particle would be. Fission can also be triggered by high-energy photons (gamma rays), but neutron-induced fission is far more practical and is the basis of all reactor and weapons technology.
Which Atoms Can Undergo Induced Fission
Not every element splits when hit by a neutron. The atoms that do are called fissile, and only three isotopes matter in practice: uranium-235, plutonium-239, and uranium-233. Of these, only uranium-235 exists in nature, making up just 0.7% of all natural uranium. The other 99.3% is uranium-238, which does not fission easily with slow neutrons. Plutonium-239 and uranium-233 are manufactured in reactors by bombarding “fertile” materials (uranium-238 and thorium-232, respectively) with neutrons, which transforms them through a series of radioactive decays into fissile fuel.
The distinction between fissile and fertile is important. Fissile isotopes split when struck by slow, low-energy neutrons. Fertile isotopes absorb neutrons and eventually become fissile through transmutation, but they don’t sustain a chain reaction on their own.
The Chain Reaction
The two or three neutrons released by each fission event are what make nuclear energy possible. If at least one of those neutrons goes on to strike another fissile nucleus and trigger another fission, the process becomes self-sustaining. This is a chain reaction.
Whether a chain reaction actually takes hold depends on accumulating enough fissile material in one place, a threshold known as critical mass. For a bare sphere of highly enriched uranium-235 with no surrounding reflector, the critical mass is in the range of tens of kilograms. Below that amount, too many neutrons escape from the surface without hitting another nucleus, and the reaction fizzles. At exactly the critical point, the system is “critical,” meaning each fission event leads on average to exactly one more fission event. The neutron population stays constant and energy output is steady.
Physicists describe this balance with a number called the multiplication factor, written as k. When k equals 1, the reactor is critical and running at a constant power level. Below 1 (subcritical), the reaction is dying out. Above 1 (supercritical), the neutron population grows with each generation, and power output increases rapidly.
Controlling the Reaction in a Reactor
A nuclear power plant keeps k at or very close to 1 using two main tools: a moderator and control rods.
The moderator is a material, usually ordinary water, that slows neutrons down after they are released. Fast-moving neutrons fresh from a fission event are less likely to be absorbed by another uranium-235 nucleus. Slowing them to “thermal” speeds dramatically increases the probability of triggering another fission. Some reactor designs use graphite as a moderator instead of water.
Control rods are made of neutron-absorbing materials like boron. They slide into the spaces between fuel rods inside the reactor core. Pushing them deeper into the core absorbs more neutrons and slows the reaction. Pulling them out allows more neutrons to reach fuel nuclei and speeds the reaction up. In a working power reactor, only one neutron per fission event is permitted to go on and cause another fission. The remaining neutrons are absorbed by control rods or lost to the environment. This careful balance is what separates a controlled energy source from an uncontrolled explosion.
Energy Released Per Fission Event
A single fission of uranium-235 releases about 200 MeV of energy. That number sounds abstract, but it becomes striking in comparison: a single chemical reaction, like one molecule of coal burning, releases roughly one electron volt. Fission produces about 200 million times more energy per event. This enormous energy density is why a nuclear power plant can generate electricity for a city using a relatively small amount of fuel, and why the discovery of fission immediately raised both hopes for energy production and fears about weapons.
Most of that 200 MeV shows up as kinetic energy of the two fission fragments, which fly apart at high speed. Their motion heats the surrounding fuel and coolant, and that heat is what ultimately boils water to drive turbines in a power plant. A smaller portion of the energy is carried by the released neutrons, gamma rays, and the radioactive decay of the fission products over time.
How Induced Fission Was Discovered
The discovery came in December 1938, when German chemists Otto Hahn and Fritz Strassmann were bombarding uranium with neutrons in their Berlin laboratory. They found barium among the reaction products, an element far too light to result from any known nuclear process at the time. The result was baffling until Lise Meitner, Hahn’s former colleague who had fled Nazi Germany for Sweden, and her nephew Otto Frisch worked through the physics. Their calculations showed that the uranium nucleus had split in two, releasing an enormous amount of energy. Frisch borrowed the term “fission” from biology, where it describes cell division, and the name stuck.
Remarkably, Enrico Fermi had unknowingly produced fission four years earlier, in 1934, while conducting neutron bombardment experiments in Rome. He simply hadn’t recognized what was happening. Within a few years of Hahn and Strassmann’s confirmed discovery, the race was on to harness the chain reaction, leading directly to the Manhattan Project and, eventually, to the first nuclear power plants.