Nuclear weapons: estimated chance of death or injury from a one megaton bomb

In the event of a one megaton nuclear air burst explosion roughly 2.5 kilometers (8,000 feet) above the earth's surface, it is estimated that the immediate mortality rate for those where the blast overpressure (i.e. the force of the blast) was 12 pounds per square inch (psi) or higher would be approximately 98 percent. For such an explosion, it is estimated that a peak overpressure of 12 psi or higher would be felt up to six kilometers (3.7 miles) from ground zero, before the pressure gradually decreases with distance, to around one psi up to 30 km (18.7 miles) away.

Other immediate effects

The blast energy makes up around 50 percent of the energy released from a nuclear explosion, 35 percent is released as thermal energy (much hotter than a traditional explosion; up to several thousand degrees Celsius), while the rest is nuclear radiation. The blast itself is strong enough to kill those nearest to ground zero, while the temperatures can instantly vaporize a person, although much of the damage is also caused by the destruction of the surrounding environment as these explosions are strong enough to wipe out several square kilometers of an urban setting. For those who are further away from ground zero, and who survive the explosion, they still have a high chance of injury from the debris launched through the air. Additionally, even those uninjured from the blast or debris are very likely to be burnt by the explosion, with all of those within the 5+ psi radius expected to suffer third degree burns, destroying most of the skin and possibly causing damage on muscular or skeletal levels. Again, this risk decreases over distance, but people within 30 km of the explosion would still be expected to suffer first and second degree burns.

Ground level explosion

The term "ground zero" is used to describe the point on the earth's surface where the explosion took place, for both explosions in the air and on (or under) the ground - however, the position of the explosion can have a significant impact on the effect of the explosion. In simple terms, the area of effect of a ground level explosion is smaller than that of an air burst explosion, as much of the impact is absorbed by the ground, but this does mean that the amount of debris thrown into the air is much higher - a ground level explosion would leave a crater at least 50 meters (175 feet) deep and 200 meters (700 feet) wide. In addition to the debris, ground level explosions also create a much greater amount of nuclear fallout, which is radioactive dust and ash that is thrown into the atmosphere, most famously in the form of a mushroom cloud. These clouds may take ten minutes to reach their full height, before being dispersed by the wind or carried back to the surface as black rain in the hours that follow - the weather conditions after the explosion can have a significant impact on the extent of the long-term impacts of radiation on the surrounding area and its population, as strong winds have the ability to widen the area of effect.