Most of the radiation hazard from nuclear bursts comes from short-lived
radionuclides external to the body; these are generally confined to the
locality downwind of the weapon burst point. This radiation hazard
comes from radioactive fission fragments with half-lives of seconds to
a few months, and from soil and other materials in the vicinity of the
burst made radioactive by the intense neutron flux of the fission and
fusion reactions.
It has been estimated that a weapon with a fission yield of 1 million
tons TNT equivalent power (1 megaton) exploded at ground level in a 15
miles-per-hour wind would produce fallout in an ellipse extending
hundreds of miles downwind from the burst point. At a distance of
20-25 miles downwind, a lethal radiation dose (600 rads) would be
accumulated by a person who did not find shelter within 25 minutes
after the time the fallout began. At a distance of 40-45 miles, a
person would have at most 3 hours after the fallout began to find
shelter. Considerably smaller radiation doses will make people
seriously ill. Thus, the survival prospects of persons immediately
downwind of the burst point would be slim unless they could be
sheltered or evacuated.
It has been estimated that an attack on U.S. population centers by 100
weapons of one-megaton fission yield would kill up to 20 percent of the
population immediately through blast, heat, ground shock and instant
radiation effects (neutrons and gamma rays); an attack with 1,000 such
weapons would destroy immediately almost half the U.S. population.
These figures do not include additional deaths from fires, lack of
medical attention, starvation, or the lethal fallout showering to the
ground downwind of the burst points of the weapons.
Most of the bomb-produced radionuclides decay rapidly. Even so, beyond
the blast radius of the exploding weapons there would be areas ("hot
spots") the survivors could not enter because of radioactive
contamination from long-lived radioactive isotopes like strontium-90 or
cesium-137, which can be concentrated through the food chain and
incorporated into the body. The damage caused would be internal, with
the injurious effects appearing over many years. For the survivors of
a nuclear war, this lingering radiation hazard could represent a grave
threat for as long as 1 to 5 years after the attack.