The centre of a nuclear explosion reaches a temperature of several million degrees centigrade. And the resulting heat flash vaporizes all human tissue. In Hiroshima, within a radius of half a mile, the only remains of most of the people caught in the open were their shadows burnt into stone.

Beyond this area all people in the open will be killed by the heat and blast waves. People inside buildings or otherwise shielded will be indirectly killed by the effects of blast and heat as buildings collapse and all inflammable materials burst into flames. The immediate death rate will be over 90%.

The many individual fires combine into a firestorm as all the oxygen is consumed. As the heat rises, air is drawn in at or near ground level and creates hurricane force winds which perpetuate the fire as the fresh oxygen is burnt.

In underground shelters those who survive the initial heat flash will die as all the oxygen is sucked out of the atmosphere.

As the distance from the area of total destruction increases there will be an increasing percentage of immediate survivors. Most of these, however, will suffer from non-survivable burns, will be blinded, bleeding from glass splinters and sufferering massive internal injuries. Many will be trapped in collapsed and burning buildings. Even those with survivable injuries will die because medical services will have been destroyed.

Shadow of ladder and workman caused by the flash from the nuclear explosion

The death rate among the seriously injured will be close to 100%. Survival rates among the potential survivors will depend on the extent of rescue and medical services that can be brought in from outside. Many of the medical services needed, such as specialist burns units, will be in strictly limited supply. The sheer scale of the casualties would overwhelm any state's medical resources even in peacetime.

The best most casualties could hope for would be to die in as little pain as possible.

Many survivors, whether injured or not, will be affected within a matter of days by radioactive fall-out.

The amount and extent of fall-out will vary depending on the size of the bomb and whether it was an air or ground burst. The area covered by fall-out will be determined by wind speed and direction. The heavier particles of radioactive material will fall in the immediate or close vicinity. Finer particles will be wind blown over longer distances before they descend. Very fine particles may be blown very long distances before they combine with water vapour and fall as radioactive rain.

In the aftermath of the Chernobyl nuclear power explosion and fire in the Ukraine in 1986, radioactive rain fell over the next few days in a wide arc across Northern Europe, including Scandinavia and then Scotland, North Wales and Cumbria. North Wales is over 1000 miles from Chernobyl.

The effects of exposure to high levels of radioactive fall-out (radiation sickness) include hair loss, bleeding from the mouth and gums, internal bleeding and haemorrhagic diarrhoea, gangrenous ulcers, vomiting, fever, delirium and terminal coma. There is no effective treatment and death follows in a matter of days.

At lower levels of exposure, while there is an increasing chance of at least short term survival, the death rate remains high. Even where long term survival is probable, pregnant women are likely to miscarry or give birth to babies with a range of disabilities.

Healing from injuries will be slow and damage to the immune system is likely.

Radiation-induced cancers will affect many survivors, often twenty or more years later. Birth abnormalities and leukaemia rates in the children of exposed survivors will increase. Because of the long period between exposure and eventual cancer it is difficult to attribute a particular cancer to a particular cause.

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