Frequently Asked Questions
|12 |Are Hiroshima and Nagasaki still
|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
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.
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.
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