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Frequently Asked Questions
12
 Are Hiroshima and Nagasaki still radioactive?
12
 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 northwest region (Koi, Takasu area) of Hiroshima and the eastern region (Nishiyama area) of Nagasaki.

The maximum estimates of dose due to fallout are 0.01-0.03 Gy in Hiroshima and 0.2-0.4 Gy in Nagasaki. The corresponding doses at the hypocenters are believed to be only about 1/10 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.

Neutron activation. Neutrons comprised 10% or less of the A-bomb radiation, whereas gamma rays comprised the majority of A-bomb radiation. Neutrons cause ordinary, non-radioactive materials to become radioactive, but gamma rays do not. The bombs were detonated far above ground, so neutron induction of radioactivity on the ground did not produce the degree of contamination people associate with nuclear test sites (Nevada test site in Southwest U.S., Maralinga test site in South Australia, Bikini and Mururoa Atolls, etc.).

Past investigations suggested that the maximum cumulative dose at the hypocenter from immediately after the bombing until today is 0.8 Gy in Hiroshima and 0.3-0.4 Gy in Nagasaki. When the distance is 0.5 km or 1.0 km from the hypocenter, the estimates are about 1/10 and 1/100 of the value at the hypocenter, respectively. The induced radioactivity decayed very quickly with time. In fact, nearly 80% of the above-mentioned doses were released within a day, about 10% between days 2 and 5, and the remaining 10% from day 6 afterward. Considering the extensive fires near the hypocenters that prevented people from entering until the following day, it seems unlikely that any person received over 20% of the above-mentioned dose, i.e., 0.16 Gy in Hiroshima and 0.06-0.08 Gy in Nagasaki.