RADIOACTIVE FALLOUT


Both the local and worldwide fallout hazards of nuclear explosions depend
on a variety of interacting factors: weapon design, explosive force,
altitude and latitude of detonation, time of year, and local weather
conditions.

All present nuclear weapon designs require the splitting of heavy elements
like uranium and plutonium. The energy released in this fission process is
many millions of times greater, pound for pound, than the most energetic
chemical reactions. The smaller nuclear weapon, in the low-kiloton range,
may rely solely on the energy released by the fission process, as did the
first bombs which devastated Hiroshima and Nagasaki in 1945. The larger
yield nuclear weapons derive a substantial part of their explosive force
from the fusion of heavy forms of hydrogen--deuterium and tritium. Since
there is virtually no limitation on the volume of fusion materials in a
weapon, and the materials are less costly than fissionable materials, the
fusion, "thermonuclear," or "hydrogen" bomb brought a radical increase in
the explosive power of weapons. However, the fission process is still
necessary to achieve the high temperatures and pressures needed to trigger
the hydrogen fusion reactions. Thus, all nuclear detonations produce
radioactive fragments of heavy elements fission, with the larger bursts
producing an additional radiation component from the fusion process.

The nuclear fragments of heavy-element fission which are of greatest
concern are those radioactive atoms (also called radionuclides) which decay
by emitting energetic electrons or gamma particles. (See "Radioactivity"
note.) An important characteristic here is the rate of decay. This is