How Nuclear Reactions Work: Fission and Fusion Explained

Inside the sun, 600 million tons of hydrogen are being converted into helium every second. This fusion releases energy that has sustained life on Earth for 4.5 billion years. Meanwhile, in 94 nuclear power plants across the United States, uranium atoms are being split apart — fission — providing about 20% of American electricity with no carbon emissions.

Both processes tap into the same fundamental store of energy: the binding energy of the atomic nucleus. Accessing it requires understanding forces that act at scales a billion times smaller than a human hair, and the results are among the most powerful phenomena in nature.

Key takeaways

• Nuclear reactions release energy by converting small amounts of mass to energy via E=mc²
• Fission splits heavy nuclei (uranium, plutonium); fusion joins light nuclei (hydrogen isotopes)
• The strong nuclear force holds nuclei together — nuclear energy comes from rearranging this binding energy
• Fusion powers stars and holds promise for clean energy; fission powers nuclear reactors today

The Nucleus and the Strong Force

Every atom has a nucleus at its center, made of protons and neutrons (collectively called nucleons). Protons carry positive electric charge and strongly repel each other. How, then, does the nucleus hold together?

The answer is the strong nuclear force — one of the four fundamental forces of nature. It is the most powerful force we know of, about 100 times stronger than electromagnetism. But it has a crucial limitation: it only acts over extremely short distances, about 1 femtometer (10^-15 meters). At this range, it overwhelms the electromagnetic repulsion and binds nucleons together tightly.

The energy stored in this arrangement is called nuclear binding energy. To understand nuclear reactions, you need one key idea: iron is special.

If you plot the binding energy per nucleon against atomic mass, you get a curve that peaks at iron (element 26). Nuclei lighter than iron release energy when they fuse together to approach iron's mass. Nuclei heavier than iron release energy when they split to approach iron's mass. Iron is the bottom of the nuclear energy valley — everything else can release energy by moving toward it.

Nuclear Fission: Splitting the Atom

Fission occurs when a heavy nucleus — uranium-235 or plutonium-239 are the practical choices — absorbs a neutron and becomes unstable. The nucleus wobbles, elongates, and splits into two smaller nuclei plus two or three additional neutrons, releasing energy.

The energy comes from E=mc². The products weigh slightly less than the original uranium plus neutron. That tiny mass difference — about 0.1% of the total — is converted entirely to energy. The numbers are staggering: one kilogram of uranium-235 undergoing complete fission releases about as much energy as 20,000 tons of TNT.

The neutrons released by each fission event can trigger fission in nearby uranium atoms, which release more neutrons, which trigger more fissions — a chain reaction. In a nuclear weapon, this proceeds uncontrolled and exponentially, releasing enormous energy in microseconds. In a reactor, neutron-absorbing control rods and moderators regulate the rate, sustaining a controlled, steady chain reaction.

Nuclear reactors use fission heat to produce steam, which drives turbines to generate electricity. The basic thermodynamics are the same as a coal plant — just with a different heat source. The profound difference is fuel density and emissions: nuclear fuel contains about a million times more energy per kilogram than coal, and fission produces no carbon dioxide.

The main challenges are radioactive waste (fission products remain dangerously radioactive for thousands of years) and the risk of uncontrolled reactions, as demonstrated at Chernobyl (1986) and Fukushima (2011). Modern reactor designs address these risks in various ways, including passive safety systems that shut down without any operator intervention.

Nuclear Fusion: Powering the Stars

Fusion is the mirror image of fission. Instead of splitting heavy nuclei, you force light nuclei together — specifically hydrogen isotopes called deuterium and tritium — into helium, releasing a neutron and a burst of energy.

The energy release per unit mass is about four times higher than fission. And the fuel is essentially inexhaustible: deuterium is extracted from seawater, and tritium can be bred from lithium, which is abundant. A fusion reactor would produce short-lived radioactivity compared to fission waste, and the reaction cannot run away — if conditions aren't perfect, it simply stops. The fuel for a fusion reactor could power a city for years while fitting in a swimming pool.

The problem is ignition. Forcing two positively charged nuclei close enough for the strong force to take over requires overcoming their electromagnetic repulsion. This requires temperatures of 100 million degrees Celsius — hotter than the core of the sun. At these temperatures, matter exists as plasma, and containing plasma is extraordinarily difficult.

The sun does it with gravity — its mass is so enormous that gravity compresses the core to fusion conditions. On Earth, we use magnetic fields. The tokamak design, pioneered in the Soviet Union in the 1960s, uses a doughnut-shaped magnetic cage to confine plasma. The ITER project in southern France is the largest tokamak ever built, a 35-nation collaboration aiming to demonstrate that fusion produces more energy than is put in.

In December 2022, the National Ignition Facility in California achieved a landmark: a laser-driven fusion experiment that produced more energy than the lasers delivered to the target. This was the first "ignition" — scientific breakeven — ever achieved. Engineering breakeven (more energy out than the total electrical input to the facility) remains a goal for future machines.

How They Compare

Fission and fusion differ in key practical ways beyond fuel type. Fission produces long-lived radioactive waste requiring geological-timescale storage. Fusion's main radioactive product is the reactor vessel itself, activated by neutrons, with a much shorter hazardous lifetime. Fission can in principle run away (though modern reactors are designed to prevent this). Fusion cannot sustain itself without constant engineering — the plasma would instantly cool if the machine failed.

Both processes produce zero carbon dioxide during operation. A lifecycle analysis including construction and fuel production gives nuclear fission one of the lowest carbon footprints of any energy source — lower than solar or wind when manufacturing is included.

The sun and other stars demonstrate that fusion is the dominant energy process in the universe. We are literally powered by a fusion reactor 150 million kilometers away. Bringing fusion to Earth would represent a civilization-scale energy breakthrough.

Radioactivity: The Aftermath

When uranium fissions, it produces a diverse cocktail of lighter nuclei, most of which are radioactive. These unstable isotopes shed energy over time — through alpha decay (ejecting helium nuclei), beta decay (converting neutrons to protons with electron emission), or gamma decay (emitting high-energy photons). The rate of decay is characterized by the half-life — the time for half a sample to decay.

Some fission products have half-lives of seconds and are harmless within minutes. Others, like cesium-137 (half-life 30 years) and iodine-131 (8 days), pose serious health risks. And some, like technetium-99 (half-life 211,000 years), require very long-term containment.

Interestingly, the same radioactivity that makes waste management challenging also has life-saving applications. Iodine-131 is used to treat thyroid cancer. Technetium-99m (a different metastable form) is the most widely used radioisotope in nuclear medicine, used in millions of imaging procedures annually.

FAQ

Why is uranium used for fission rather than other elements? Uranium-235 and plutonium-239 have a special property: they undergo fission when struck by slow (thermal) neutrons. Most heavy elements require fast, high-energy neutrons. Uranium-235's cross-section for thermal neutron capture is very large, making it practical for controlled chain reactions in reactors.

Can a nuclear reactor explode like a bomb? No. A bomb requires weapons-grade uranium or plutonium enriched to over 90% fissile material, in a precisely engineered geometry to achieve a supercritical mass almost instantaneously. Reactor fuel is enriched to only 3-5% and is not in a geometry that can sustain an exponential chain reaction. What can happen — as at Chernobyl — is a steam explosion or chemical explosion that disperses radioactive material, which is dangerous but physically distinct from a nuclear detonation.

How close is commercial fusion power? The common joke has been "30 years away for the last 70 years." But genuine progress is being made. ITER aims to demonstrate Q=10 (ten times energy out vs. energy in for the plasma) by the 2030s. Private companies like Commonwealth Fusion Systems and TAE Technologies are pursuing more compact designs with newer high-temperature superconducting magnets. Commercial fusion is now more plausibly 20-30 years away than perpetually receding.

What is a hydrogen bomb? A hydrogen bomb (thermonuclear weapon) uses a fission bomb as a trigger to achieve the extreme temperature and pressure needed to initiate fusion of hydrogen isotopes. The fusion stage can release far more energy than a pure fission bomb. The Tsar Bomba, detonated by the Soviet Union in 1961, was a hydrogen bomb with a yield of 50 megatons — about 3,300 times the Hiroshima bomb.

What happens to nuclear waste? Spent nuclear fuel is initially stored in cooling pools at reactor sites, where it remains for several years while short-lived isotopes decay. It then transitions to dry cask storage — sealed metal containers. The long-term plan in most countries is deep geological repositories — stable rock formations at depths of several hundred meters. Finland's Onkalo repository is the world's first permanent geological repository, currently under construction.

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Read next:

• Nuclear Fusion Energy Explained — The Power of the Stars
• How Does Electricity Actually Work?
• Nuclear Fission Explained — How Nuclear Power Works