How Nuclear Fission Works: Splitting Atoms and Releasing Energy

The Energy Hidden in the Nucleus

Every atom is made of a nucleus—a dense core of protons and neutrons bound together by the strong nuclear force—surrounded by a cloud of electrons. The strong force is, as its name implies, extraordinarily powerful at short range, easily overcoming the electrostatic repulsion that would otherwise push the positively charged protons apart. The energy stored in nuclear binding is immense: releasing even a tiny fraction of a nucleus's mass as energy, following Einstein's famous equation E = mc², yields energy millions of times greater than any chemical reaction involving the same number of atoms.

Nuclear fission is the process by which a heavy atomic nucleus—typically uranium-235 or plutonium-239—absorbs a neutron, becomes unstable, and splits into two smaller nuclei (called fission fragments), releasing two or three additional neutrons and a large amount of energy, primarily as kinetic energy of the fragments and as gamma radiation. This seemingly simple process, first achieved artificially in 1938 by the German chemists Otto Hahn and Fritz Strassmann (with theoretical interpretation by Lise Meitner and Otto Frisch), launched the nuclear age and introduced an energy source of almost unimaginable power.

The Physics of Fission

Not every atomic nucleus can undergo fission. Only nuclei that are large enough—heavy enough—that the disruptive electrostatic repulsion between their protons begins to rival the cohesive strong force are candidates for fission. In practice, naturally occurring fissile and fissionable nuclei include uranium-235 (U-235), uranium-238 (U-238), and thorium-232 (Th-232). Of these, U-235 is uniquely valuable because it undergoes fission readily when struck by a slow (thermal) neutron. U-238 and Th-232 require fast neutrons or high-energy particles to undergo fission, or must be converted into fissile materials (plutonium-239 and uranium-233, respectively) by neutron capture in a reactor.

When a U-235 nucleus absorbs a thermal neutron, it momentarily becomes U-236 in a highly excited state. The nucleus oscillates, elongates, and within about 10⁻¹⁴ seconds, the electrostatic repulsion between the two forming lobes overcomes the strong force and the nucleus splits. The most common fission products are nuclei with mass numbers between 90 and 140—pairs such as barium-141 and krypton-92, or strontium-94 and xenon-140. The specific fragments produced vary statistically across an ensemble of fission events; no single pair always results. Two to three fast neutrons are also released per fission event, along with gamma rays, beta particles (from subsequent radioactive decay of the fission products), and heat.

The total mass of the fission products plus the released neutrons is slightly less than the mass of the original U-235 nucleus plus the absorbed neutron. This mass defect—typically about 0.1% of the original mass—is converted to energy according to E = mc². The energy released per fission event is approximately 200 MeV (megaelectronvolts). To put this in perspective: burning one carbon atom in a chemical reaction releases about 4 eV of energy. Nuclear fission releases fifty million times more energy per atom than combustion. A kilogram of U-235 undergoing complete fission releases roughly 80 terajoules of energy—equivalent to the energy in about 20,000 tonnes of TNT.

The Chain Reaction

The released neutrons from each fission event can go on to induce fission in other nearby U-235 nuclei, which release more neutrons, which induce more fissions—a chain reaction. Whether this chain reaction sustains itself, grows, or dies out depends on a quantity called k, the neutron multiplication factor: the average number of neutrons from each fission that go on to cause another fission.

• If k < 1 (subcritical): each generation produces fewer fissions than the last; the chain reaction dies out.
• If k = 1 (critical): each generation produces the same number of fissions as the last; the chain reaction sustains itself at a constant rate.
• If k > 1 (supercritical): each generation produces more fissions than the last; the chain reaction grows exponentially.

In a nuclear reactor, the goal is to maintain k very close to 1—a sustained, controlled chain reaction that produces heat at a steady rate. In a nuclear weapon, k is made to exceed 1 as rapidly as possible, producing an explosive exponential release of energy in microseconds. The difference between a reactor and a bomb is one of control, geometry, and speed—not of fundamental physics.

Natural uranium contains only about 0.7% U-235; the rest is U-238, which does not sustain a thermal neutron chain reaction. Reactor fuel is typically enriched uranium—uranium in which the proportion of U-235 has been increased to 3–5% (for light-water reactors) or up to 20% (for research reactors). Weapons-grade uranium is enriched to over 90% U-235. Enrichment is technically demanding, requiring either gaseous diffusion or gas centrifuges to separate the isotopes of uranium hexafluoride based on their slightly different masses.

How Nuclear Reactors Work

A nuclear reactor is essentially a device for maintaining a controlled nuclear chain reaction and using the heat it generates to produce electricity. The fundamental components of a typical light-water reactor (LWR)—the most common type worldwide—are:

• Fuel assemblies: Bundles of fuel rods containing enriched uranium dioxide (UO₂) pellets clad in zirconium alloy, which acts as a barrier between the fuel and the coolant.
• Moderator: Light water (ordinary water, H₂O) in most reactors, heavy water (D₂O) in CANDU reactors, or graphite in some designs. The moderator slows fast neutrons released by fission down to thermal energies at which they are far more likely to cause further fission in U-235. (A fast neutron typically passes through a U-235 nucleus without being captured; a thermal neutron is captured much more readily.)
• Control rods: Rods made of neutron-absorbing materials such as boron, cadmium, or hafnium that can be inserted into or withdrawn from the reactor core. Inserting control rods absorbs neutrons, reducing k and decreasing power; withdrawing them increases k and raises power. In an emergency, all control rods are rapidly inserted (a SCRAM) to shut down the chain reaction.
• Coolant: Water (which also serves as the moderator in LWRs) circulates through the core, carrying heat away from the fuel. The heated coolant either converts to steam directly in the reactor vessel (boiling water reactor, BWR) or transfers heat to a secondary water loop via a steam generator (pressurized water reactor, PWR).
• Steam turbine and generator: The steam drives a turbine connected to an electrical generator, exactly as in a conventional fossil-fuel power plant. Nuclear reactors produce heat; everything after that is conventional thermodynamics.
• Containment structure: A reinforced concrete and steel structure surrounding the reactor to prevent the release of radioactive material in the event of an accident.

Types of Nuclear Reactors

Type	Moderator	Coolant	Notable Examples
Pressurized Water Reactor (PWR)	Light water	Pressurized light water	Most US, French, and Chinese reactors
Boiling Water Reactor (BWR)	Light water	Boiling light water	Fukushima Daiichi (Japan)
CANDU (Pressurized Heavy Water)	Heavy water	Heavy water	Canadian, Indian reactors
Gas-Cooled Reactor (AGR/MAGNOX)	Graphite	CO₂ gas	UK fleet (mostly retired)
RBMK (Soviet graphite-moderated)	Graphite	Boiling light water	Chernobyl reactor
Fast Breeder Reactor (FBR)	None	Liquid sodium	Superphénix (France, now closed)
Small Modular Reactor (SMR)	Varies	Varies	NuScale, Rolls-Royce designs (in development)

Nuclear Energy: Benefits and Challenges

Nuclear fission power offers a combination of advantages that few other energy sources can match. It produces large quantities of reliable, dispatchable electricity with virtually no carbon dioxide emissions during operation. A single nuclear power plant on a modest land footprint can supply electricity for millions of homes. France generates approximately 70% of its electricity from nuclear power and has one of the lowest per-capita carbon footprints in the developed world for electricity generation. Globally, nuclear power avoids the emission of roughly 2 billion tonnes of CO₂ per year—about 5% of annual global emissions.

The challenges are equally significant. The capital cost of constructing a nuclear power plant is enormous and has increased, not decreased, in Western countries over recent decades due to safety upgrades, regulatory requirements, and the loss of experienced construction workforces. The Vogtle Unit 3 reactor in Georgia, USA, came online in 2023 after years of delay and cost overruns that pushed the price of the project to over $35 billion for two units. Nuclear projects have fared better in South Korea and China, where experienced construction teams and streamlined regulatory frameworks have kept costs lower.

Radioactive waste disposal remains an unresolved political and technical challenge. Spent nuclear fuel contains highly radioactive fission products that must be isolated from the environment for thousands of years. No country has yet opened a permanent deep geological repository for high-level nuclear waste, though Finland's Onkalo facility is the most advanced, targeting operational status in the late 2020s. In the interim, spent fuel is stored at reactor sites in water pools (which provide shielding and cooling) and in dry cask storage.

Major nuclear accidents—Three Mile Island (1979), Chernobyl (1986), and Fukushima (2011)—have profoundly shaped public perception of nuclear energy, even though the direct death toll of nuclear power per unit of energy generated is far lower than that of fossil fuels. The health impacts of Chernobyl resulted in approximately 30–60 direct deaths from acute radiation syndrome and an estimated few thousand additional thyroid cancer cases (most treatable); the fears of widespread cancer mortality were not borne out by epidemiological data, though they remain politically potent.

The Future of Fission: Gen IV Reactors and Advanced Designs

A new generation of nuclear reactor designs—Generation IV reactors—is under development with the goals of improving safety, reducing waste, and potentially closing the nuclear fuel cycle. Advanced designs include molten salt reactors (MSRs), which use liquid fluoride salts as both fuel and coolant and are passively safe (they cannot melt down because the fuel is already liquid), and fast neutron reactors that can "breed" new fissile material from U-238 and potentially consume existing spent nuclear fuel as additional fuel, dramatically reducing the volume and longevity of nuclear waste.

Small modular reactors (SMRs) are another focus of intensive development. By standardizing and factory-manufacturing reactor components in modules of 50–300 MW electrical output (compared to 1,000–1,600 MW for conventional large reactors), SMR advocates hope to reduce construction time and capital cost while maintaining the carbon-free operating credentials of conventional nuclear power.

Nuclear fission, in parallel with the development of nuclear fusion (a fundamentally different nuclear process that remains under development), remains a contested but potentially essential component of the global clean energy transition—providing firm, low-carbon power to complement the variable output of wind and solar generation.

Conclusion

Nuclear fission is one of the most energetically potent processes accessible to human technology, releasing millions of times more energy per atom than any chemical reaction. Discovered less than ninety years ago, it has been applied to both the most destructive weapons ever built and to a major source of humanity's low-carbon electricity. The physics of fission—neutron-induced splitting of heavy nuclei, chain reactions, and the mass-energy equivalence—are elegant in their clarity. The engineering of reactors that harness this physics safely, reliably, and economically is far more complex. The decisions about whether, how, and how much to employ fission energy in the coming decades are ultimately not just technical questions but political, ethical, and economic ones that societies around the world are actively debating.