The damage caused by a nuclear explosion depends upon the weapon’s yield (measured in kilotons or megatons), the type of nuclear fuel used, the design of the device, whether it’s exploded in the air or at earth’s surface, the geography surrounding the target, whether it’s winter or summer, hazy or clear, night or day, windy or calm.
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With a 1 megaton surface detonation: at a distance of 7.5 miles from the center, 25% of the population will have been injured (by falling glass, thermal burns, etc.) Commercial buildings may have windows blown out and residential structures will have moderate damage.
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Check the Homeland Security site if you wish further information.