The Fireball
The detonation of a nuclear weapon leads to the liberation of a large amount of energy in an extremely short period of time within a tiny amount of matter, typically in the low kilogram range. As a result, the fission products, bomb casing, and other weapon parts are raised to extremely high temperatures, higher than those in the centre of the sun. The maximum temperature attained by fission weapons residues is several tens of million degrees Celsius, which compares with a maximum of 5,000°C (or 9,000°F) in a conventional HE weapon. Because of the great heat produced by the nuclear explosion, all the materials are converted into gases. Since the gases, at the instant of explosion, are restricted to the region occupied by the original constituents in the weapon, tremendous pressures will be produced, in the region of a million times normal atmospheric pressure.
Within less than a millisecond of the detonation of the weapon, remnants radiate large amounts of energy, mainly as invisible gamma-rays, which are absorbed within a few metres in the surrounding atmosphere. This leads to the formation of an extremely hot and therefore incandescent spherical mass of air and gaseous weapon residues known as the fireball. The surface brightness decreases with time, but after about a millisecond the fireball from a 1Mt nuclear weapon would appear to an observer 50 miles away to be many times more brilliant than the sun at noon. In several of the low altitude atmospheric nuclear tests made at the Nevada Test Site, all of which were under 100 kilotons, the glare in the sky, in the early hours of the morning, was visible 400 (or more) miles away. This was not the result of line-of-sight transmission, but rather of scattering and diffraction, i.e. bending, of the light rays by particles of dust and possibly by moisture in the atmosphere. However, high-altitude bursts in the megaton range have been seen directly as far as 100 miles away.
The surface temperatures of the fireball, upon which the brightness, or luminance, depends, do not vary greatly with the total energy yield of the weapon. Consequently, the observed brightness of the fireball in an air burst is roughly the same, regardless of the size of weapon. The physical size of the fireball and its duration account for the differences in effects. Immediately after its formation, the fireball begins to grow rapidly in size, drawing in the surrounding air. This growth is accompanied by a decrease in temperature because of the consequent increase in mass. At the same time, the fireball rises, like a hot-air balloon.
Within 700 µs from the detonation, the fireball from a 1Mt weapon is about 440 feet (about 130 metres) across, and this increases to a maximum value of about a mile (about 1.7 kilometres) in 10 seconds. It is then rising at a rate of 170 mph to 240 mph (270 - 390 kph). Alter a minute, the fireball has cooled to such an extent that it is no longer luminous, it has then risen roughly 4.5 miles from the point of burst.
While the fireball is still luminous, the temperature, in the interior at least, is so high that all the weapon materials are still in the form of vapour, including the radioactive fission products fissile material that has not taken part in the detonation, and the weapon casing and other materials from the weapon.
As the fireball rises the circulation draws in more air through the bottom of the toroid, thereby cooling the cloud and dissipating the energy contained in the fireball. Quite early in the ascent of the fireball, cooling of the outside by radiation and the drag of the air through which it rises frequently bring about a change in shape. The roughly spherical form becomes a toroid (or doughnut). As it ascends, the toroid undergoes a violent, internal circulatory motion. The formation of the toroid is usually observable in the lower part of the visible cloud. As a result, the toroidal motion increases in size and cools, the vapours condense to form a cloud containing solid particles of the weapon debris, as well as water droplets derived from the air sucked into the rising fireball. .
The colour of the radioactive cloud is initially red or reddish brown, due to the presence of oxides of nitrogen, resulting from the chemical combination of oxygen and nitrogen. As the fireball cools and condensation occurs the colour of the cloud changes to white, mainly due to the water droplets as in an ordinary cloud, the oxides of nitrogen dissolve in the water droplets, forming nitrous and nitric acid.
Depending on the height of burst of the nuclear weapon and the nature of the terrain below, a strong updraft with inflowing winds, known as "afterwinds" occurs in the immediate vicinity. These winds draw in varying amounts of soil and dust particles and other loose debris.
Only a relatively small proportion of the dirt particles become contaminated with radioactivity in a low air-burst explosion. This is because the particles do not mix intimately with the weapon residues in the cloud at the time when the fission products are still vapourized and about to condense. For a ground burst, however, large quantities of soil and other debris are drawn into the cloud at early times. Good mixing then occurs during the initial phases of cloud formation and growth. Consequently, when the vaporized fission products condense they do so on the foreign matter, thus forming highly radioactive particles.
Initially the rising mass of air carries the particles upward, but after a time they begin to fall slowly under the influence of gravity at rates dependent upon their size. Consequently, a lengthening, and widening column of cloud is produced. This cloud consisting mainly of very small particles of radioactive fission products, weapon residues, water droplets, and larger particles of dirt and debris carried up by the afterwinds.
The speed with which the top of the radioactive cloud continues to ascend depends on the meteorological conditions as well as on the energy yield of the weapon. In general, the cloud will have attained a height of 3 miles in 30 seconds and 5 miles in about a minute.
