Nuclear explosion

A nuclear explosion is an explosion that occurs as a result of the rapid release of energy of nuclear fission, nuclear fusion, or a device combining both.

A nuclear explosion relies on a runaway nuclear chain reaction, either through fission (the process of splitting heavy atoms like Uranium-235 or Plutonium-239) or fusion (the process of fusing light atoms like Hydrogen isotopes to form heavier elements), converting a tiny amount of mass into immense energy within a fraction of a second.

Fusion-based (thermonuclear) weapons are capable of producing significantly higher explosive yields than pure fission weapons.

Fission

A neutron strikes the nucleus, the core of an atom, of a heavy, unstable atom (like Uranium-235(²³⁵U) or Plutonium-239(²³⁹Pu)). The nucleus then absorbs the neutron, becomes unstable, and splits into smaller nuclei, releasing tremendous amounts of energy and more neutrons. The newly released neutrons strike other nearby fissile nuclei, causing them to split, releasing more energy and neutrons in a rapid, exponential chain reaction. The fissile material must then be brought together to achieve "critical mass", allowing the chain reaction to sustain itself and become uncontrolled. This then causes a nuclear explosion to take place.

Fusion

A small fission bomb (as described above) detonates first. The immense heat and pressure from the fission explosion create the necessary conditions (millions of degrees) for light nuclei (like deuterium and tritium) to fuse into heavier ones, like helium, releasing even more energy, neutrons, and heat in a much more powerful explosion. The released neutrons can then be induced further fission in a surrounding uranium tamper, creating a fission-fusion-fission cycle for maximum explosive yield. A reaction purely based upon nuclear fusion remains a hypothetical device.