The rapid release of energy in an explosion creates a shock wave of overpressure. Very close to the centre of a nuclear explosion, overpressure is equivalent to several thousand pounds per square inch (psi). This is hundreds of times greater than the pressure in a pressure cooker.
The overpressure crushes objects. Human lungs are crushed at about 30 psi overpressure. Brick houses are destroyed at about 10-15 psi overpressure. The blast also generates high velocity winds which can turn humans or objects into missiles. At 15 - 20 psi the winds can fling a person at several hundred kilometres per hour. The velocity of winds accompanying different levels of overpressure is listed in table [ ]
Thermal radiation includes light and heat. Nuclear weapons release a huge amount of energy as light (utlraviolet, visible and infrared). This light is so intense that it can make sand explode, blind people many miles away, burn shadows into concrete, burn skin and ignite flammable materials at large distances.
The heat at the centre of the explosion (epicentre) is so intense as to vapourise most materials. The thermal radiation creates a fireball which expands rapidly outwards consuming oxygen and, combined with the blast effect, creating near total destruction for some distance from the epicentre.
Initial nuclear radiation
Initial radiation consists mostly of gamma rays and neutrons which are generated in the nuclear reaction and can enter the human body directly. In general a radiation dose of 400 rads will be lethal to 50% or more of the exposed population. These deaths occur within about 6 - 7 weeks.
Fallout consists of large numbers of particles, from the earth, buildings and other ground objects, which are irradiated by the explosion, mixing with the radioactive products of the explosion itself and then being distributed over a wide area by wind. An example of a fallout pattern is given in the table.
There are a large number of radioactive products from a nuclear explosion. These lodge in different parts of the body (table [ ]). The radioactive products emit different types of radiation - alpha particles, beta particles or gamma rays, which differ in the degree of damage they can do. Alpha particles do the most damage once inside the body, but do little damage outside as they cannot penetrate skin.
There is no known way of nuetralising a radioactive substance, apart from sending it through a nuclear reactor. Thus, radioactive products are dangerous until they have decayed to such an extent that they no longer emit significant amounts of radiation. This time is usually considered to be 10 times the half-life.
The effects of fallout persist for hundreds of years, although the relative risk reduces over time.
Extent of damage
The extent of damage depends on the size of the nuclear weapon, the terrain and the height at which it is detonated. Nuclear weapons detonated at ground level generate more fallout as a result of the large amount of ground material which is irradiated by the explosion and thrown in the air, but the effects of thermal radiation and radioactive waves is less than in an air blast.
The nuclear weapon detonated in Hiroshima was about 12kt, i.e. the equivalent of 12,000 tons of TNT. The combined effects of blast, and radiation killed about 300,000 people. Current nuclear weapons range in size from 1 kt to over 1000 kt. Most are about 100kt, i.e. about 10 times the force of the Hiroshima bomb.