Fall-out

radioactive material deposited on the ground from the atmosphere. The source of the radioactive substances is nuclear explosions or escape from nuclear reactors whether through failure to adequately filter coolants or an accident. Radioactive particles settle on material in the air and, in the case of a nuclear explosion, large quantities of dust, soil etc. are sucked up into the central plume of fire and smoke to be deposited hundreds of kilometres from the source, having first risen high in the atmosphere.

Taken from Dictionary of Science