1 with fallout, about 4 to destroy a massive city the size of new york with the initial inferno fireball.
Effects of a Nuclear Explosion Damage caused by nuclear explosions can vary greatly, depending on 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. Whatever the factors, though, the explosion will release several distinct forms of energy. One form is the explosive blast. Other forms are direct nuclear radiation and thermal radiation. And then there’s radioactive fallout — not exactly energy released by the explosion, but still a destructive result.
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The first effect of a nuclear explosion in the air is an intense flash of light, as quick as a lightning flash but a thousand times as bright. It is accompanied by a powerful pulse of heat radiation, sufficient to set fire to light combustible material out to a distance of fourteen km., and to paint or wood at half that distance.
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The estimates for a city of one million or two million struck by a single one-megaton bomb are that around one third of the inhabitants would be killed instantly or fatally injured, one third seriously injured, and the rest uninjured or only slightly injured.
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