Every nuclear weapon ever built falls into one of two categories: pure-fission "atomic" bombs that split heavy atoms, and "hydrogen" bombs that use a small fission stage to ignite a much larger fusion stage. The two designs differ by orders of magnitude in yield — the Hiroshima bomb released about 15 kilotons of energy, while the largest hydrogen bomb ever tested released 50 million tons. The mechanism behind that 3,300× scaling is the subject of this guide.
What "splitting an atom" actually means
A nuclear fission weapon releases energy by splitting heavy nuclei — typically uranium-235 or plutonium-239 — into lighter fragments. When a neutron strikes a U-235 nucleus, the nucleus splits, releasing roughly 200 MeV of energy and two or three new neutrons. Each new neutron can split another nucleus, producing more neutrons, in a self-sustaining chain reaction.
For the chain reaction to release usable energy, the fissile material must be assembled into a "supercritical mass" — enough material in a compact enough geometry that more neutrons trigger fissions than escape. Below this threshold (the critical mass) the reaction dies out; above it, the reaction multiplies exponentially in microseconds, releasing all of the bomb's energy before it physically blows itself apart.
The trick of an atomic bomb is to assemble a supercritical mass of fissile material extremely quickly — too quickly for the early energy release to disassemble the bomb before most of the material has fissioned.
Little Boy: gun-type uranium
The Hiroshima bomb (Little Boy) used the simplest possible design: a "gun-type" device that fired one subcritical mass of uranium-235 down a barrel into another subcritical mass. When the two pieces collided, they formed a single supercritical mass and the chain reaction began.
Gun-type assembly is slow but works for U-235, which has a low spontaneous-fission rate. Little Boy used roughly 64 kg of highly enriched uranium and yielded about 15 kilotons. The weapon was so confidently expected to work that it was never tested before being dropped.
Gun-type cannot work for plutonium-239 because Pu-239 contains trace Pu-240 that spontaneously fissions, causing premature ignition (a "fizzle") before the masses fully assemble. A different approach was needed for plutonium.
Fat Man: implosion plutonium
The Nagasaki bomb (Fat Man) used implosion: a hollow plutonium sphere surrounded by precisely shaped explosive lenses that, when detonated simultaneously, compress the plutonium inward at supersonic speeds. The compression dramatically increases density, pushing the plutonium past critical mass before predetonation can occur.
Implosion is much harder to engineer than gun-type — the explosive lenses must fire within microseconds of each other, and the shock wave must remain spherically symmetric. The Trinity test on July 16, 1945 was a proof-of-concept for the implosion design before Fat Man was used over Nagasaki on August 9. Fat Man yielded about 21 kilotons from roughly 6 kg of plutonium.
Implosion remains the standard fission-primary design in modern thermonuclear weapons, where the high yield-per-mass efficiency of plutonium matters.
The leap to hydrogen bombs
Pure-fission weapons cap out at roughly 500 kilotons because of geometry: as the fissile core fissions and heats up, it disassembles before all of the material has reacted. To go higher, you need a fundamentally different energy mechanism. Nuclear fusion — combining light nuclei — releases far more energy per unit mass than fission, but requires temperatures of roughly 100 million degrees to ignite.
A hydrogen bomb uses a small fission "primary" as a match to ignite a much larger fusion "secondary." The primary detonates first; its X-ray flux is channeled to the secondary, where it compresses and heats fusion fuel (typically lithium-deuteride) to fusion-ignition conditions. The secondary then releases its energy as fusion neutrons, with a third stage of fission in the surrounding U-238 jacket.
The Teller-Ulam design
The breakthrough that made hydrogen bombs practical was the two-stage radiation-implosion configuration, designed by Stanislaw Ulam and Edward Teller in 1951. In the Teller-Ulam design, the fission primary and fusion secondary sit in a hohlraum (radiation cavity) inside the bomb case. X-rays from the primary travel through the cavity at the speed of light and ablate the outer surface of the secondary, driving it inward by reaction force.
This radiation implosion compresses the secondary far more uniformly and powerfully than ordinary explosives could, raising it to fusion-ignition temperature. The first test of the design was Ivy Mike in November 1952 — a 10.4 megaton detonation that vaporized the entire test island. Every operational thermonuclear weapon since uses some variant of the Teller-Ulam configuration.
Boosted fission and modern weapons
Most modern fission primaries are also "boosted": a small amount of deuterium-tritium gas is injected into the plutonium core. When the primary detonates, the D-T undergoes fusion, releasing a flood of high-energy neutrons that drive a much higher fraction of the plutonium to fission. Boosting can multiply the primary's yield by 5× or more.
The result of these design improvements is that modern strategic warheads are remarkably compact. The W88, deployed on US Trident II SLBMs, packs a 475-kiloton yield into a warhead small enough to fit eight per missile. The B83, the most powerful US weapon currently in active service, yields up to 1.2 megatons.
See the full Weapons Database for individual profiles, the methodology page for the blast scaling laws, and the glossary for definitions of fission, fusion, boosted fission, and the Teller-Ulam design.