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The energy released from a nuclear weapon comes in four primary categories:
The amount of energy released in each form depends on the design of the weapon, and the environment in which it is detonated. The residual radiation of fallout is a delayed release of energy, while the other three forms of energy release are immediate.
The dominant effects of a nuclear weapon (the blast and thermal radiation) are the same physical damage mechanisms as conventional explosives. The primary difference is that nuclear weapons are capable of releasing much larger amounts of energy at once. Most of the damage caused by a nuclear weapon is not directly related to the nuclear process of energy release, but would be present for any explosion of the same magnitude.
The damage done by each of the three initial forms of energy release differs with the size of the weapon. Thermal radiation drops off the slowest with distance, so the larger the weapon the more important this effect becomes. Ionizing radiation is strongly absorbed by air, so it is only dangerous by itself for smaller weapons. Blast damage falls off more quickly than thermal radiation but more slowly than ionizing radiation.
When a nuclear weapon explodes, the bomb's material comes to an equilibrium temperature in about a microsecond. At this time about 75% of the energy is emitted as primary thermal radiation, mostly soft X-rays. Almost all of the rest of the energy is kinetic energy in rapidly-moving weapon debris. The interaction of the x-rays and debris with the surroundings determines how much energy is produced as blast and how much as light. In general, the denser the medium around the bomb, the more it will absorb, and the more powerful the shockwave will be.
When a nuclear detonation occurs in air near sea-level, most of the soft X-rays in the primary thermal radiation are absorbed within a few feet. Some energy is re-radiated in the ultraviolet, visible light and infrared, but most of the energy heats a spherical volume of air. This forms the fireball.
In a burst at high altitudes, where the air density is low, the soft X-rays travel long distances before they are absorbed. The energy is so diluted that the blast wave may be half as strong or less. The rest of the energy is dissipated as a more powerful thermal pulse .
The bombing of Hiroshima delivered the force of 12,000 tons of TNT, leveling buildings and killing over 100,000.
The explosive yield of a nuclear weapon is expressed in the equivalent mass of trinitrotoluene (TNT):
Compare Oklahoma City bombing: 0.002 kt.
The yield per ton is for current US weapons 600 kt to 2.2 Mt, and it was e.g. for the Davy Crocket 40 kt and the Tsar Bomba 2 Mt.
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