nuclear fallout

Short definition from Wiktionary

Noun

1. 1
   Residual radioactive material in the upper atmosphere which settles down to Earth over a protracted period; typically ash which has been propelled upwards by a nuclear explosion or disaster, sometimes mixed with rain or snow.
2. 2
   An event in which this occurs, generally perceived as an ecological disaster due to the distribution of irradiated material over a much larger area than the zone directly affected by the original event.