Nuclear Weapons

Nuclear detonations are the most devastating of all weapons of mass destruction [WMD]. They generate an array of deadly effects, including blast, thermal pulse, neutrons, x- and gamma-rays, radiation, electromagnetic pulse (EMP), and ionization of the upper atmosphere.

Depending upon the environment in which the nuclear device is detonated, blast effects are manifested as ground shock, water shock — “blueout,” cratering, followed by large amounts of dust and radioactive fallout.

The energy of a nuclear explosion has three forms: blast, thermal radiation, and nuclear radiation. The distribution of energy among these three forms will depend on the yield of the weapon, the location of the burst, and the characteristics of the environment.

For a low-altitude atmospheric detonation of a moderate sized weapon in the kiloton (equivalent to a thousand tons of high explosive) range, the energy is distributed roughly as follows: 50% as blast; 35% as thermal radiation (made up of a wide range of the electromagnetic spectrum, including infrared, visible, and ultraviolet light and some soft x-ray emitted at the time of the explosion), and 15% as nuclear radiation (including 5% as initial ionizing radiation consisting chiefly of neutrons and gamma rays emitted within the first minute after detonation, and 10% as residual nuclear radiation. Residual nuclear radiation is the hazard in fallout.

Because of the tremendous amounts of energy liberated in a nuclear detonation, temperatures of several million degrees centigrade develop in the immediate area of the detonation (this in contrast to the few thousand degrees of a conventional explosion).

In an atmospheric detonation, this electromagnetic radiation is absorbed within a few meters of the point of detonation by the surrounding atmosphere, heating it to extremely high temperatures and forming a brilliantly hot sphere of air and gaseous weapon residues, the so-called fireball.

Immediately upon formation, the fireball begins to grow rapidly and rise like a hot air balloon. Within a millisecond after detonation, the diameter of the fireball from a 1 megaton (Mt) air burst is 150 m. This increases to a maximum of 2200 m within 10 seconds, at which time the fireball is also rising at the rate of 100 m/sec. The initial rapid expansion of the fireball severely compresses the surrounding atmosphere, producing a powerful blast wave.

As it expands toward its maximum diameter, the fireball cools, and after about a minute its temperature has decreased to such an extent that it no longer emits significant amounts of thermal radiation. The combination of the upward movement and the cooling of the fireball gives rise to the formation of the characteristic mushroom-shaped cloud. As the fireball cools, the vaporized materials in it condense to form a cloud of solid particles.

Following an air burst, condensed droplets of water give it a typical white cloud like appearance. In the case of a surface burst, this cloud will also contain large quantities of dirt and other debris that are vaporized when the fireball touches the earth's surface or are sucked up by the strong updrafts afterwards, giving the cloud a dirty brown appearance. The dirt and debris become contaminated with the radioisotopes generated by the explosion or activated by neutron radiation, and fall to earth as fallout.

The relative effects of blast, heat, and nuclear radiation will largely be determined by the altitude at which the weapon is detonated.