In general, nuclear fallout is the residual radioactive material from a nuclear blast that “falls out” of the sky after an atmospheric explosion. Fallout can also refer to nuclear reactor accidents, although a nuclear reactor does not explode like a nuclear weapon. The isotopic signature of fallout from nuclear blast is very different from the fallout from a serious power reactor accident.
In case of radiation doses from fallout, we consider the residual radioactive material from nuclear tests (not from reactor accidents) that were performed particularly in the two periods from 1954 to 1958 and from 1961 to 1962. According to the UNSCEAR, about 502 atmospheric tests, with a total fission and fusion yield of 440 Mt, were conducted.
Special Reference: Sources and effects of ionizing radiation, Annex B. UNSCEAR. New York, 2010. ISBN: 978-92-1-142274-0.
Fallout from a nuclear tests consist of fission fragments and neutron activation products. When a blast takes place on the ground or in the atmosphere near the ground, large amounts of activation products are formed also from surface materials. The fallout is particularly significant in the neighborhood of the test site, since the larger particles and most of debris lands on the ground (local fallout). But smaller particles may remain aloft in the upper atmosphere for years. These particles can be therefore distributed nearly uniformly around the world, and contribute to so called global fallout. Equivalent doses from global fallout dropped from about 130 μSv/year in 1963 to about 10 μSv/year in recent years.