Fallout is
the residual radiation hazard from
a nuclear explosion
and is named from the fact that it "falls out" of the atmosphere in to
which it is spread during the explosion. It commonly refers to the
radioactive dust created when a nuclear weapon
explodes, although it can also
refer to nuclear accidents. This
radioactive dust is a kind of radioactive contamination.
Source of fallout
A nuclear explosion
vaporizes any material within the fireball, including the ground if it
is nearby and this is combined with residual ionizing radiation to
produce fallout. The sources of this residual ionizing radiation are: Fission Products.
These are intermediate weight isotopes which are
formed when a heavy uranium or plutonium nucleus is split in a fission
reaction. There are over 300 different fission products that may result
from a fission reaction. Many of these are radioactive with widely
differing half-lives. Some are very short, i.e., fractions of a second,
while a few are long enough that the materials can be a hazard for
months or years. Their principal mode of decay is by the emission of
beta and gamma radiation. Approximately 60 g of fission products are
formed per kiloton of yield. The estimated activity of this quantity of
fission products 1 minute after detonation is 1.1 ZBq, equal to that of
30 Gg of radium, in equilibrium with its decay products.
Unfissioned
Nuclear Material. Nuclear weapons
are relatively inefficient in their use of fissionable material, and
much of the uranium and plutonium is dispersed by the explosion without
undergoing fission. Such unfissioned nuclear material decays by the
emission of alpha particles and is of relatively minor importance.
Neutron-Induced
Activity. If atomic nuclei capture neutrons when
exposed to a flux of neutron radiation, they will, as a rule, become
radioactive (neutron-induced activity) and then decay by emission of
beta and gamma radiation over an extended period of time. Neutrons
emitted as part of the initial nuclear radiation will cause activation
of the weapon residues. In addition, atoms of environmental material,
such as soil, air, and water, may be activated, depending on their
composition and distance from the burst. For example, a small area
around ground zero may become hazardous as a result of exposure of the
minerals in the soil to initial neutron radiation. This is due
principally to neutron capture by sodium (Na), manganese, aluminum, and
silicon in the soil. This is a negligible hazard because of the limited
area involved.
Worldwide Fallout
After an
air burst the fission products,
unfissioned nuclear material,
and weapon residues which have been vaporized by the heat of the
fireball will condense into a fine suspension of very small particles
10 nm to 20 µm in diameter. These particles may be quickly drawn
up into the stratosphere, particularly if the explosive yield exceeds
10 kt. They will then be dispersed by atmospheric winds and will
gradually settle to the earth's surface after weeks, months, and even
years as worldwide fallout.
The
radiobiological hazard of worldwide fallout is essentially a
long-term one due to the potential accumulation of long-lived
radioisotopes, such as strontium-90 and caesium-137, in the body as a
result of ingestion of foods incorporating these radioactive materials.
This hazard is much less serious than those which are associated with
local fallout and, therefore, is not discussed at length here. Local
fallout is of much greater immediate operational concern.
Local fallout
In a land
or water surface burst, large
amounts of earth or water will
be vaporized by the heat of the fireball and drawn up into the
radioactive cloud. This material will become radioactive when it
condenses, with fission products and other radiocontaminants that have
become neutron-activated.
There will be
large amounts of particles of less than 100 nm to several
millimeters in diameter generated in a surface burst in addition to the
very fine particles which contribute to worldwide fallout. The larger
particles will not rise into the stratosphere and consequently will
settle to earth within about 24 hours as local fallout.
Severe local
fallout contamination can extend far beyond the blast and
thermal effects, particularly in the case of high yield surface
detonations. The ground track of fallout from an explosion is a long,
thin fuzzy ellipse downwind of the explosion. It may be hundreds of
kilometers long, and up to 50 km (30 miles) wide from a single
explosion.
Whenever
individuals remain in a radiologically contaminated area, such
contamination will lead to an immediate external radiation exposure as
well as a possible later internal hazard due to inhalation and
ingestion of radiocontaminants.
Factors affecting
fallout
Location
In cases of water
surface (and shallow underwater) bursts, the
particles tend to be rather lighter and smaller and so produce less
local fallout but will extend over a greater area. The particles
contain mostly sea salts with some water; these can have a cloud
seeding affect causing local rainout and areas of high local fallout.
Fallout from seawater is unusually dangerous because it is difficult to
remove by washing.
For subsurface
bursts, there is an additional phenomenon present called
"base surge." The base surge is a cloud that rolls outward from the
bottom of the column produced by a subsurface explosion. For underwater
bursts the visible surge is, in effect, a cloud of liquid (water)
droplets with the property of flowing almost as if it were a
homogeneous fluid. After the water evaporates, an invisible base surge
of small radioactive particles may persist.
For subsurface
land bursts, the surge is made up of small solid
particles, but it still behaves like a fluid. A soil earth medium
favors base surge formation in an underground burst.
Meteorological
Meteorological
conditions will greatly
influence fallout, particularly
local fallout. Atmospheric winds are able to distribute fallout over
large areas. For example, as a result of a surface burst of a 15 Mt
thermonuclear device at Bikini Atoll on March 1, 1954, a roughly
cigar-shaped area of the Pacific extending over 500 km downwind and
varying in width to a maximum of 100 km was severely contaminated.
Snow and rain,
especially if they come from considerable heights, will
accelerate local fallout. Under special meteorological conditions, such
as a local rain shower that originates above the radioactive cloud,
limited areas of heavy contamination just downwind of a nuclear blast may
be formed.
Effects of fallout
A wide
range of biological changes may follow
the irradiation of
animals. These vary from rapid death following high doses of
penetrating whole-body radiation, to essentially normal lives for a
variable period of time until the development of delayed radiation
effects, in a portion of the exposed population, following low dose
exposures.
Short Term
Median
Lethal Dose (LD50): When comparing the
effects of various types
or circumstances, that dose which is lethal to 50% of a given
population is a very useful parameter. The term is usually defined for
a specific time, being limited, generally, to studies of acute
lethality. The common time periods used are 30 days or less for most
small laboratory animals and to 60 days for large animals and humans.
It should be understood that the LD50 assumes that the individuals did
not receive other injuries or medical treatment.
Initial radiation
from fallout can exceed 300 gray per hour (Gy/h)
immediately downwind of a ground burst. A cumulative dose of 4.5 Gy is
fatal to half of a population of humans. There have been no documented
cases of survival beyond 6 Gy. Most people become ill after an exposure
to 1 Gy or more. The fetuses of pregnant women are vulnerable and may
miscarry, especially in the first trimester. Human biology resists
mutation from large radiation exposure: grossly mutated fetuses usually
miscarry. Civilian dose rates in peace-time range from 30 to 100
µGy/y.
Fallout radiation
falls off ('decays') exponentially (quickly) with
time. Most areas become safe for travel and decontamination after three
to five weeks.
The most
dangerous emissions from fallout are gamma rays, which travel
in straight lines, like ordinary light. The fallout particles emit the
invisible, deadly gamma rays in the same way that a light bulb emits
light. Gamma rays are invisible, and cannot be seen, smelt, or felt.
Special equipment is required to detect and measure gamma rays.
For yields of up
to 10 kt, initial nuclear radiation is the dominant
casualty producer on the battlefield. Humans receiving an acute
incapacitation dose (30 Gy) will become performance degraded almost
immediately and ineffective within several hours. However, they will
not die until 5 to 6 days after exposure assuming they do not receive
any other injuries which make them more susceptible to the radiation
dose. Individuals receiving less than a total of 150 cGy will remain
effective. Between those two extremes, people receiving doses greater
than 150 cGy will become degraded; some will eventually die.
A dose of 530 cGy
to 830 cGy is considered lethal but not immediately
incapacitating. Personnel exposed to this amount of radiation will
become performance degraded within 2 to 3 hours, depending on how
physically demanding the tasks they must perform are, and will remain
in this degraded state at least 2 days. However, at that point they
will experience a recovery period and be effective at performing
non-demanding tasks for about 6 days, after which they will relapse
into a degraded state of performance and remain so for about 4 weeks.
At this time they will begin exhibiting radiation symptoms of
sufficient severity to render them totally ineffective. Death follows
at approximately 6 weeks after exposure.
Long Term
Late or
delayed effects of radiation occur
following a wide range of
doses and dose rates. Delayed effects may appear months to years after
irradiation and include a wide variety of effects involving almost all
tissues or organs. Some of the possible delayed consequences of
radiation injury are life shortening, carcinogenesis, cataract
formation, chronic radiodermatitis, decreased fertility, and genetic
mutations.
Tactical military
considerations
Blast and
thermal injuries, due to the use of nuclear weapons
for military action, in many cases will far outnumber radiation
injuries. However, radiation effects are considerably more complex and
varied than are blast or thermal effects and are subject to
considerable misunderstanding.
The closer to
ground an atomic bomb is detonated, the more dust and
debris is thrown into the air, resulting in greater amounts of local
fallout. From a tactical standpoint, this has the disadvantage of
hindering any occupation efforts until the fallout clears, but more
directly, the impact with the ground severely limits the destructive
force of the bomb. For these reasons, ground bursts are not usually
considered tactically advantageous, with the exception of hardened
underground targets such as missile silos or command centers such as
Cheyenne Mountain. "Salting" enemy territory with a fallout-heavy
atomic burst could be used to deny enemy access to a contaminated area
but such use is generally not considered an ethical military action.
Prevention and
Cleanup
In severe
cases of fallout contamination,
lethal doses of external
radiation may be incurred if protective or evasive measures are not
undertaken. A fallout shelter is designed to shield its occupants from
radiation. However, some radioactive contamination will probably remain
when the inhabitants eventually emerge.
Gamma rays do not
contaminate people or objects. Fallout particles
contaminate people or objects, and since they resemble sand, they can
be brushed off, or washed off. The radioactive fog from seawater is a
notable exception, being very difficult to wash off. The particles
should be removed from the shelter, or shielded. Emergency drinking
water can be adequately cleaned by filtering contaminated water through
more than 25 cm (10 in) of dirt. Food in sealed packages is not
poisoned by fallout. Stored grain and exposed fruit can be cleaned and
peeled. Vehicles are usually washed down with fire-hoses, into drains
with removable filters, or deep trenches. Ground is usually
decontaminated by bulldozing the fallout into deep, narrow trenches,
and then back-filling the trenches.
The fallout
residue can be used to analyze the source and nature of the
weapon used. Materials used in the weapon will have distinct signature,
which with proper analysis could reveal where or by whom the weapon was
made.
