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How Does a Nuclear Bomb Work?

PhysicsHistory

Quick Answer

Nuclear weapons release enormous energy through uncontrolled nuclear fission (splitting heavy atoms like uranium-235 or plutonium-239) or a combination of fission and fusion (thermonuclear/hydrogen bombs), converting a small amount of mass directly into energy via E=mc2.

The Full Story

In a fission bomb ("atomic bomb"), a subcritical mass of fissile material (uranium-235 or plutonium-239) is rapidly compressed to supercritical density using conventional explosives. At criticality, a chain reaction begins: each fission event releases 2-3 neutrons that split additional atoms, doubling the number of fissions every ~10 nanoseconds. Within microseconds, trillions of trillions of atoms split, releasing energy equivalent to thousands of tonnes of TNT (kilotonnes). The Hiroshima bomb ("Little Boy") used a gun-type design firing one uranium subcritical piece into another; Nagasaki ("Fat Man") used implosion of a plutonium sphere. Thermonuclear (hydrogen) bombs add a second stage: the fission explosion compresses and heats a hydrogen isotope fuel (deuterium/tritium), triggering fusion — the same process powering the Sun. This can yield megatonnes of TNT equivalent. The largest ever detonated, the Soviet "Tsar Bomba" (1961), yielded 50 megatonnes — 3,300 times Hiroshima.

Key Facts

1.The Hiroshima bomb ("Little Boy") had a yield of about 15 kilotonnes of TNT equivalent; only about 1.7% of its uranium actually fissioned.
2.The Tsar Bomba, detonated by the Soviet Union on 30 October 1961, had a yield of 50 megatonnes — the largest human-made explosion in history, with a mushroom cloud 67 km high.
3.As of 2024, nine countries possess nuclear weapons with a combined arsenal of approximately 12,500 warheads — down from a peak of about 70,300 in 1986.

YouTube Angle

Suggested video title for this topic:

"How Nuclear Weapons Work — Fission, Fusion, and the Physics of Destruction"