Question 11. Are Hiroshima and Nagasaki still radioactive?
No. There are two ways radioactivity is produced from an atomic blast. The first is due to fallout of the fission products or the nuclear material itself, i.e., uranium or plutonium that contaminate the ground. (The same ground contamination occurred as a consequence of the Chernobyl accident.) The Hiroshima and Nagasaki bombs exploded at 500 to 600 m of altitude, then formed huge fireballs that rose with ascending air currents. Subsequently, the material cooled down and started to fall with rain. Because of the wind, the rain did not fall directly on the hypocenter but rather in the northwest region (Koi, Takasu area) of Hiroshima and the eastern region (Nishiyama area) of Nagasaki. Nowadays, the radioactivity is so miniscule that it is difficult to distinguish from trace amounts of radioactivity caused by atmospheric atomic-bomb tests.
Another way radioactivity is produced is by neutron irradiation of soil or buildings. (Neutrons comprise 10% or less of A-bomb radiation; nonradioactive materials become radioactive after absorbing neutrons. In contrast, gamma rays--which comprise the majority of A-bomb radiation--do not cause ground materials to become radioactive.) However, most of the radioactivity decayed very quickly so that it now takes months to measure the radioactivity using highly sensitive equipment. Despite miniscule levels, these measurements are currently utilized to estimate neutron doses released from the bombs.
In both cases, the residual radioactivity is far less than the dose received from background radiation; hence, there are no detectable effects on human health.
