User Guide

where precipitation occurs. All of this fallout will thus be brought

to the ground by weather processes within months at most (usu-

ally much faster). In the megaton range, the fireball rises so high

that it enters the stratosphere. The stratosphere is dry, and no

weather processes exist there to bring fallout down quickly. Small

fallout particles will descend over a period of months or years.

Such long-delayed fallout has lost most of its hazard by the time

it comes down, and will be distributed on a global scale. As yields

increase above 100kT, progressively more and more of the total

fallout is injected into the stratosphere.

An explosion closer to the ground (close enough for the fire-

ball to touch) sucks large amounts of dirt into the fireball. The

dirt usually does not vaporize, and if it does, there is so much of

it that it forms large particles. The radioactive isotopes are

deposited on soil particles, which can fall quickly to earth. Fallout

is deposited over a time span of minutes to days, creating down-

wind contamination both nearby and thousands of kilometers

away. The most intense radiation is created by nearby fallout,

because it is more densely deposited, and because short-lived iso-

topes haven't decayed yet. Weather conditions can affect this con-

siderably of course. In particular, rainfall can "rain out" fallout to

create very intense localized concentrations. Both external expo-

sure to penetrating radiation, and internal exposure (ingestion of

radioactive material) pose serious health risks.

Explosions close to the ground that do not touch it can still

generate substantial hazards immediately below the burst point

by neutron-activation. Neutrons absorbed by the soil can generate

considerable radiation for several hours.

The megaton class weapons have been largely retired, being

replaced with much smaller yield warheads. The yield of a modern

strategic warhead is, with few exceptions, now typically in the

range of 200-750 kT. Recent work with sophisticated climate mod-

els has shown that this reduction in yield results in a much larger

proportion of the fallout being deposited in the lower atmosphere,

and a much faster and more intense deposition of fallout than had

been assumed in studies made during the sixties and seventies.

The reduction in aggregate strategic arsenal yield that occurred

when high yield weapons were retired in favor of more numerous

lower yield weapons has actually increased the fallout risk.

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