Quick Answer
Nuclear fission is the process of splitting the nucleus of a heavy atom — such as uranium — into smaller pieces, releasing a large amount of energy. When a nucleus splits, it also releases neutrons that can split more nuclei, creating a self-sustaining chain reaction. Controlled fission powers nuclear reactors that generate electricity; an uncontrolled chain reaction is what makes a nuclear weapon so destructive. The energy comes from converting a tiny amount of mass into energy, following Einstein’s E = mc².
The splitting of the atom is one of the defining scientific discoveries of the 20th century — a source of both clean electricity and humanity’s most fearsome weapons. This guide explains the science of nuclear fission: what it is, how a chain reaction works, the difference between reactors and bombs, and how it compares to its counterpart, nuclear fusion.
What Is Nuclear Fission?
Nuclear fission is a reaction in which the nucleus of a heavy atom splits into two (or more) lighter nuclei. It typically happens when a heavy, unstable nucleus — most commonly uranium-235 or plutonium-239 — absorbs a neutron, becomes unstable, and breaks apart. The split produces lighter “daughter” nuclei, a few free neutrons, and a burst of energy.
The energy comes from a remarkable fact: the combined mass of the fragments is slightly less than the mass of the original nucleus. That missing mass has been converted into energy according to Einstein’s famous equation, E = mc². Because the speed of light squared (c²) is such an enormous number, even a tiny amount of lost mass releases a tremendous amount of energy. A single fission event releases millions of times more energy than a chemical reaction like burning, which is why nuclear fuel is so extraordinarily energy-dense.
The Chain Reaction Explained
The key to harnessing fission is the chain reaction. When a uranium nucleus splits, it releases two or three neutrons. Each of those neutrons can strike another uranium nucleus and cause it to split, releasing yet more neutrons, which split still more nuclei. If, on average, at least one neutron from each fission goes on to cause another fission, the reaction sustains itself.
This depends on having enough fissile material packed closely enough together — a quantity known as the critical mass. Below the critical mass, too many neutrons escape without causing further fissions and the reaction fizzles out. At critical mass, the reaction becomes self-sustaining. Whether that self-sustaining reaction is gentle and steady or explosive and runaway is the crucial distinction between a power plant and a weapon.
Fission in Power Plants vs Weapons
The same underlying physics powers both nuclear reactors and nuclear weapons, but they are engineered to do opposite things with the chain reaction.
Controlled vs uncontrolled reactions
In a nuclear power plant, the chain reaction is carefully controlled so that it proceeds at a slow, steady rate, releasing heat gradually. Operators use control rods, which absorb excess neutrons, to keep the reaction balanced — speeding it up or slowing it down as needed. A moderator (often water) slows the neutrons to make the reaction efficient, and the heat produced boils water into steam that turns turbines to generate electricity. Reactor fuel uses uranium enriched to only a few percent — far too low to ever explode like a bomb.
A nuclear weapon does the opposite: it is designed to make the chain reaction uncontrolled, so that an enormous number of fissions occur in a tiny fraction of a second, releasing all the energy at once in an explosion. This requires highly concentrated fissile material and a very different design. The fundamental safety point is that the low-enriched fuel in a power reactor physically cannot produce a nuclear explosion — the two applications are deliberately and profoundly different.
Fission vs Fusion
Fission has a counterpart called nuclear fusion, and the two are opposites. Fission splits heavy nuclei apart, while fusion joins light nuclei together — for example, fusing hydrogen into helium. Fusion is what powers the Sun and the stars.
- Fission: splits heavy atoms (uranium, plutonium); used in today’s nuclear power plants.
- Fusion: combines light atoms (hydrogen isotopes); powers stars and is being developed for future clean energy.
- Energy: fusion releases even more energy per unit of mass than fission.
- Difficulty: fusion requires extreme temperatures and pressures, making controlled fusion power very hard to achieve on Earth.
Fusion produces less long-lived radioactive waste and uses abundant fuel, which is why it is seen as a promising future energy source — but harnessing it for practical power has proven enormously challenging.
How a Fission Bomb Releases Its Energy
From a scientific standpoint, a fission weapon works by forcing an extremely rapid, uncontrolled chain reaction. The principle is to bring fissile material into a supercritical state so quickly that an immense number of fissions cascade through it before it blows itself apart, converting a small amount of mass into a devastating release of energy, heat, and radiation in an instant.
The result is the catastrophic blast, thermal flash, and radiation associated with nuclear weapons — and, on a global scale, the potential for long-lasting climate effects if many were used. The full planetary consequences of such an event are explored in what if all nuclear weapons detonated at once. The ongoing risk these weapons pose to civilisation is tracked symbolically by the Doomsday Clock.
Q&A
An enormous amount relative to its fuel. A single fission of a uranium-235 nucleus releases about 200 million electron-volts — millions of times more than a typical chemical reaction. Because of E = mc², the complete fission of about one kilogram of uranium could release energy comparable to thousands of tonnes of conventional explosive.
Modern nuclear power plants are designed with multiple safety systems, and nuclear energy has a strong overall safety record as a low-carbon power source. The main challenges are preventing accidents and safely managing long-lived radioactive waste. Reactor fuel cannot produce a nuclear explosion because it is only lightly enriched.
The most common fissile fuels are uranium-235 and plutonium-239. Natural uranium is mostly uranium-238, which is not easily fissile, so it is enriched to increase the proportion of uranium-235 for use in reactors. These heavy, unstable nuclei split readily when they absorb a neutron.
Fission splits heavy atomic nuclei apart, while fusion fuses light nuclei together. Fission powers today’s nuclear reactors; fusion powers the stars and is being developed as a future energy source. Fusion releases more energy per unit mass but is far harder to control on Earth.
The Bigger Question
Nuclear fission shows how the splitting of a single atom can unleash energy out of all proportion to its size — energy that lights cities, but that can also be weaponised. What would happen if the entire global arsenal of fission and fusion weapons were unleashed at once, not just in immediate destruction but in the lasting effect on the planet’s climate? That is the grave scenario examined in what if all nuclear weapons detonated at once.
The threat these weapons pose is symbolically measured by the Doomsday Clock. Explore more about the forces that could reshape civilisation on the Earth & Humanity Survival hub.
Watch the nuclear scenario to understand the global stakes of the energy locked inside the atom.