59.2 Trauma from heat, blast and radiation

Atomic bombs release energy in four forms. Like any chemical explosion, they cause: (1) heat and (2) blast, which produce the burns, fractures, and crush injuries described on other pages. (3) Radiation in the form of neutrons, X-rays, gamma rays, and alpha and beta particles. This radiation takes two forms: (a) the initial radiation, mostly neutrons and gamma rays, which ends within a minute of the explosion, and falls off with distance according to the inverse square law, and (b) the fall out radiation emitted by a mixture of radioactive isotopes, some with a half life of centuries. This fallout occurs in two forms, immediate and delayed, and is described below. (4) There is also an intense pulse of electromagnetic energy at the moment of the explosion capable of destroying communications and all electromagnetically stored data over a huge area.

The heat and blast from bombs of one megaton and larger kill almost everyone who might possibly have survived the initial radiation. As bombs get smaller, their radiation becomes more important than their blast. This is the principle of the small, neutron bomb, which kills by radiation without causing much heat or blast.

A 10 megaton bomb exploded at a height of 2000 metres (an airburst) produces an intensely hot luminous fireball, and a blast wave which travels at supersonic speed. It produces no crater, and therefore little fallout.

The same bomb exploded on the surface (a groundburst) makes a crater nearly a kilometre wide. In doing so it makes thousands of tons of earth radioactive by irradiating them with neutrons, and draws them up into the air as the ’mushroom cloud’ in Fig. 59-2. This huge quantity of radioactive material then descends to the earth as fallout in two forms; (1) Immediate fall out which occurs in the first few days, and is comparatively localized, the larger particles falling nearby, and the finer ones progressively further downwind. The pattern of this fallout is determined by the speed and direction of the prevailing wind. The large particles are deposited close to the site of the explosion as a fine visible radioactive sand. (2) Delayed fallout in the form of much smaller particles. These reach the upper atmosphere and descend only slowly to be added to the natural background radiation of the biosphere.

Assuming that the ozone layer and the biosphere itself survive a major nuclear exchange (and there is good reason to think that they might not), it is the delayed fallout which matters to countries which are not directly attacked, and particularly to us in the Third World. This fallout is ultimately distributed round the earth, although fortunately for us, most of it falls in the hemisphere in which it is released.

The heat and blast from a one megaton bomb destroys everything within about 14 km. Within this area heat causes severe burns and ignites anything combustible. Innumerable fires are started, and if the weather conditions are right, these produce a firestorm, like that at Hiroshima which destroyed 12 sq km of the city. A powerful vertical upstream of hot air draws cool air in from the periphery until everything which can burn has burnt. In doing so it creates temperatures of 1,000C, that melt glass and metal. The Dresden firestorm, caused by conventional incendiary bombs during the second world war, killed 100,000 helpless people, many of them women and children, in one night.