Not sure if really stupid, or don't know how to use google.
http://gizmodo.com/why-can-people-live-in-hiroshima-and-nagasaki-now-but-1451250877
.rerf.or.jp/general/qa_e/qa12.html
Are Hiroshima and Nagasaki still radioactive?
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The practical answer is, "No."
There are two ways residual radioactivity is produced from an atomic blast. The first is due to fallout of the
fission products or the nuclear material itself--uranium or plutonium (uranium was used for the Hiroshima bomb whereas plutonium was used for the Nagasaki bomb)--that contaminate the ground. Similar ground contamination occurred as a consequence of the Chernobyl accident, but on a much larger scale (
click here for more-detailed explanation). The second way residual radioactivity is produced is by neutron irradiation of soil or buildings (neutron activation), causing non-radioactive materials to become radioactive.
Fallout. The Hiroshima and Nagasaki bombs exploded at altitudes of 600 meters and 503 meters, respectively, then formed huge fireballs that rose with the ascending air currents. About 10% of the nuclear material in the bombs underwent fission; the remaining 90% rose in the stratosphere with the fireball.
Subsequently, the material cooled down and some of it started to fall with rain (black rain) in the Hiroshima and Nagasaki areas, but probably most of the remaining uranium or plutonium was dispersed widely in the atmosphere. Because of the wind, the rain did not fall directly on the hypocenters but rather in the west region (Koi, Takasu area) of Hiroshima and the eastern region (Nishiyama area) of Nagasaki.
The maximum estimates of dose due to fallout are 0.006-0.02 Gy in Hiroshima and 0.12-0.24 Gy in Nagasaki.* The corresponding doses at the hypocenters are believed to be only about 1/6 of these values.
Nowadays, the radioactivity is so miniscule that it is difficult to distinguish from trace amounts (including plutonium) of radioactivity caused by worldwide fallout from atmospheric (as opposed to underground) atomic-bomb tests that were conducted around the world in past decades, particularly in the 1950s and 1960s.