Nuclear fallout

Nuclear fallout, or simply fallout, is the residual radioactive material propelled into the upper atmosphere following a nuclear blast or a nuclear reaction conducted in an unshielded facility, so called because it "falls out" of the sky after the explosion and the shock wave have passed. It commonly refers to the radioactive dust and ash created when a nuclear weapon explodes, but such dust can also originate from a damaged nuclear plant. Fallout may take the form of black rain (rain darkened by particulates).

This radioactive dust, consisting of material either directly vaporized by a nuclear blast or charged by exposure, is a highly dangerous kind of radioactive contamination.

Types of fallout

An air burst (that is, a nuclear detonation far above the surface) can eventually produce worldwide fallout. A ground burst can produce possibly much more severe, local fallout.

Global fallout

After an air burst, fission products, un-fissioned nuclear material, and weapon residues vaporized by the heat of the fireball condense into a fine suspension of small particles 10 nm to 20 µm in diameter. These particles may be quickly drawn up into the stratosphere, particularly if the explosive yield exceeds 10 kt.