A nuclear explosion occurs as a result of the rapid release of energy from an uncontrolled nuclear reaction. The driving reaction may be nuclear fission, nuclear fusion or a multistage cascading combination of the two, though to date all fusion based weapons have used a fission device to initiate fusion, and a pure fusion weapon remains a hypothetical device.
Atmospheric nuclear explosions are associated with "mushroom clouds" although mushroom clouds can occur with large chemical explosions and it is possible to have an air burst nuclear explosion without these clouds. Atmospheric nuclear explosions produce large amounts of radiation and radioactive debris. In 1963, all nuclear and many non-nuclear states signed the Limited Test Ban Treaty, pledging to refrain from testing nuclear weapons in the atmosphere, underwater, or in outer space. The treaty permitted underground tests.
The primary application to date has been military (i.e. nuclear weapons). However, there are other potential applications, which have not yet been explored, or have been considered all but abandoned. They include:
- Nuclear pulse propulsion, including using a nuclear explosion as asteroid deflection strategy.
- An unsafe prototype of fusion power; see PACER
- Peaceful nuclear explosions
A nuclear explosion (nuclear detonation) has occurred on Earth twice using a nuclear weapon during war (during World War II, the atomic bombings of Hiroshima and Nagasaki), about 2,000 times during testing of nuclear weapons, and about 27 times in the U.S. and 156 in the U.S.S.R. in a series of peaceful nuclear explosions.
The energy released from a nuclear weapon comes in four primary categories:
Blast—40-60% of total energy
Thermal radiation—30-50% of total energy
Ionizing radiation—5% of total energy
Residual radiation—5-10% of total energy