How does a nuclear bomb work? Fission and fusion, simply

A nuclear bomb works by releasing the energy stored in the nuclei of atoms, where a conventional bomb does it with the chemical bonds between them. In an atomic bomb, heavy nuclei of uranium or plutonium split apart in a runaway chain reaction, a process called fission. A hydrogen bomb uses a fission explosion to drive fusion, joining light nuclei together. Either way, kilogram for kilogram, the release is millions of times greater than any chemical explosive's.

In short

• Fission splits a heavy nucleus into lighter ones and frees neutrons that split more nuclei: a chain reaction.
• A hydrogen bomb adds fusion, which lets its power reach thousands of times that of the bombs used in 1945.
• The effects are blast, heat and radiation, including fallout that can spread far from the target.

This note explains the physics at the level of a textbook. It doesn't describe how weapons are designed or built.

What happens when an atom splits?

The nucleus at the center of an atom is held together by the strong nuclear force, which binds its protons and neutrons far more tightly than chemical bonds hold atoms together. Very heavy nuclei, such as uranium-235, are only just stable. If one absorbs a stray neutron, it can wobble and split into two lighter nuclei, giving off two or three new neutrons and a burst of energy, about 200 million electronvolts per split.

Where does the energy come from? The pieces together weigh slightly less than the original nucleus, and the missing mass has become energy, by Einstein's E = mc². A chemical reaction releases a few electronvolts per bond broken or made; TNT exploding releases about 10 electronvolts per molecule. Splitting one nucleus releases tens of millions of times more energy than any chemical reaction between a few atoms, which is the whole difference between a nuclear and a conventional bomb.

Fission was discovered in Berlin in December 1938 by Otto Hahn and Fritz Strassmann, and explained weeks later by Lise Meitner and Otto Frisch. Physicists saw at once that the new neutrons could cause more splits.

What is a chain reaction?

Each split frees neutrons, and each neutron can split another nucleus. If, on average, more than one neutron from each split goes on to cause another, the number of splits grows every step, or generation:

Generation	Splits, if each causes two more
1	1
10	about 500
20	about 500,000
40	about 550 billion
80	about 600 trillion billion (6 × 10²³)

Doubling turns a single split into an enormous number within a few dozen steps, and each step takes far less than a millionth of a second. Most of the energy comes out in the last few generations, so the whole release is over almost instantly, before the material can blow itself apart.

A worked number shows the scale. The standard reference The Effects of Nuclear Weapons, published by the U.S. government in 1977 and online at Fourmilab, gives the complete fission of about 57 grams of uranium or plutonium as equal to 1 kiloton of TNT. So:

1. 1 kiloton of TNT releases 4.18 × 10¹² joules.
2. Fissioning 1 kilogram would release 1,000 ÷ 57 ≈ 17.5 kilotons, or about 7.3 × 10¹³ joules.
3. Exploding 1 kilogram of TNT releases about 4.2 × 10⁶ joules.
4. Divide: fission releases about 17 million times as much energy per kilogram.

The same book notes that only part of the material in a weapon actually fissions before it disperses. The bomb that destroyed Hiroshima released about 13 kilotons. A nuclear power plant uses the same fission reaction, but holds it at a steady rate, with each split causing on average exactly one more, and its fuel cannot explode like a bomb.

How does a hydrogen bomb add fusion?

Fusion is the reverse process: very light nuclei, such as the hydrogen isotopes deuterium and tritium, join to form helium and release energy. It powers the Sun, and it is described in what nuclear fusion is. Fusion only happens at temperatures of many millions of degrees and under huge pressure. On Earth, the only way to reach those conditions in a weapon is a fission explosion.

So a hydrogen or thermonuclear bomb uses a fission explosion to set off fusion, and the fusion in turn releases fast neutrons that can split more heavy nuclei. The result has no natural upper limit on size. The first such test, by the United States on 1 November 1952, released about 10 megatons, roughly 800 times the Hiroshima bomb. The largest ever, a Soviet test in October 1961, released about 50 megatons.

What does a nuclear explosion do?

For a burst in the lower atmosphere, The Effects of Nuclear Weapons gives a typical split of the energy: about 50% as blast, 35% as heat and light, and 15% as nuclear radiation, 5% in the first minute and 10% later as fallout.

Effect	Share of energy	What it does
Blast	about 50%	A shock wave and winds that flatten buildings for kilometers
Heat and light	about 35%	A flash that burns skin and starts fires far beyond the blast's worst damage
Initial radiation	about 5%	Gamma rays and neutrons in the first minute, deadly close in
Residual radiation (fallout)	about 10%	Radioactive dust and debris that can drift far downwind for days and weeks

A burst high in the air spreads blast and heat widely but makes less fallout; a burst at the ground digs up soil and makes far more. Large explosions also produce an electromagnetic pulse that can damage electronics. Some scientists argue that many weapons used at once could loft enough smoke to cool the climate, an idea called nuclear winter, whose size is still debated.

How did nuclear weapons shape history?

The Manhattan Project, the U.S. wartime program, set off the first nuclear explosion, the Trinity test, in New Mexico on 16 July 1945; it released about 21 kilotons, the U.S. Department of Energy's history records. On 6 August 1945, a uranium bomb destroyed Hiroshima, and on 9 August a plutonium bomb struck Nagasaki. The Atomic Heritage Foundation puts Hiroshima's deaths in the following four months at between 90,000 and 166,000. Why the bombs were used is the subject of why the U.S. dropped the atomic bomb.

The Soviet Union tested its first bomb in 1949, and the Cold War arms race followed. Treaties then set limits: the Partial Test Ban Treaty of 1963 ended tests in the air, sea and space for those who joined; the Nuclear Non-Proliferation Treaty, opened in 1968 and in force since 1970, has 191 member states, according to the UN Office for Disarmament Affairs. The Comprehensive Nuclear-Test-Ban Treaty of 1996 has not entered into force.

In January 2026, the world held an estimated 12,187 nuclear warheads, about 9,745 of them in military stockpiles and 4,012 deployed, according to the SIPRI Yearbook 2026. Nine countries have them. SIPRI notes that New START, the last treaty limiting U.S. and Russian strategic forces, expired in February 2026, and that the long post-Cold War fall in the total is likely to reverse.

Still argued

Whether nuclear weapons keep the peace is argued. Supporters of deterrence say the threat of unacceptable damage has helped prevent war between the major powers since 1945. Advocates of disarmament say deterrence depends on leaders never miscalculating, and point to near-misses during the Cold War and to the humanitarian effects of any use; their case led to the Treaty on the Prohibition of Nuclear Weapons, in force since 2021, which no nuclear-armed state has joined. The evidence can't settle the question, because the outcome deterrence claims to prevent is one nobody can observe.

The physics of fission took less than seven years to go from a laboratory puzzle to a weapon, one of the starkest examples of how science can change the world faster than its politics.

Check yourself

• Without looking back, explain where the energy of a split nucleus comes from.
• Why does a chain reaction release its energy so quickly?
• How does a hydrogen bomb differ from an atomic bomb, and why does it need fission first?

Sources

• The Effects of Nuclear Weapons, third edition — Samuel Glasstone and Philip J. Dolan, U.S. Department of Defense and Department of Energy, 1977
• The Trinity test, July 16, 1945 — U.S. Department of Energy, Office of Scientific and Technical Information
• Bombings of Hiroshima and Nagasaki, 1945 — Atomic Heritage Foundation, National Museum of Nuclear Science and History
• Treaty on the Non-Proliferation of Nuclear Weapons — United Nations Office for Disarmament Affairs
• Increasing focus on nuclear weapons amid heightened escalation risks: SIPRI Yearbook 2026 — Stockholm International Peace Research Institute, 8 June 2026