Radiation Types
The types of radiation that we are dealing with are Alpha particles, Beta
particles and Gamma rays.
Alpha particles are very heavy nuclear particles which have the same make up as
the nucleus of a Helium Atom. Their ability to penetrate matter is very low and they will
be stopped by a few feet of air. They offer little hazard unless the radioactive material
producing them is ingested or inhaled.
Beta particles are energetic particles with a mass and charge equal in
magnitude to that of an electron. Their ability to penetrate matter is low, and they would
usually be stopped by a layer of skin, a sheet of plastic, wood, etc.
Gamma Rays are short wavelength electromagnetic radiations of nuclear origin
which are identical to x-rays. They are able to easily penetrate low density material such
as wood. Adequate protection from gamma rays may require lead 1 or 2 inches thick or
concrete 2 or 3 feet thick.
Life of the Material
Though radioactive material may change its physical form, the radioactivity can
not be destroyed, but it decays naturally. Because of this it is difficult to apply an
actual age as such to it.
Half Life of the Material
The half life of a radionuclide is the time required for one-half of the
material to decay away. After the passing of time equal to two half lives, one-half of
what is left at the end of the first half life will have decayed away leaving one-fourth
of the original quantity. After 3 half lives, one-eighth of the original quantity will be
left and after 4 half lives one-sixteen will be left. Each radionuclide has a unique half
life.
Radionuclides which could potentially be released as the result of a nuclear
power accident include the following:
Radionuclide Half Life
Xenon 133
5.4 days
Iodine 131
8.4 days
Cesium 137
30 years
Strontium 90 27.7
years
Plutonium 239 24,390 years
Krypton 85
10.76 years
Release of Radioactive Gases
Small amounts of radioactive gases are normally released from nuclear power
plants. Under the conditions of an accident, both the amounts and kinds of radioactive
gases could increase.
Effect on Plants and Animals
Farm animals are either mammals or birds. All mammals have similar
sensitivities to radiation exposure and all mammals will incorporate radionuclides into
their tissues under a given set of situations in similar ways. The amount of radioactive
material incorporated would depend largely on the grazing habits of the animal.
Birds, on the average, are slightly less sensitive to radiation than mammals.
Radioactive material is more likely to enter and be assimilated by the body through food
and water rather than by breathing.
Radiation effects on the animals will be the same as for humans. High total
doses, such as 400 to 600 rads delivered over a short period of time, will kill or at
least severely disable the animals for a long time. Lower doses of radiation, such as the
doses to persons receiving a chest x-ray, could produce genetic effects and an increased
incidence of cancer in animals with a long life span; but the increase in the incidence of
these problems compared to the natural rate would probably be too small to measure.
Temporary sterility, drops in milk and egg production, and retarded growth may result from
high sub-lethal doses.
The ingestion of radioactive materials by farm animals should not be a problem
since radioactive material in milk, meat, and eggs would become a human health problem
before the dose to farm animals would become significant. It may be possible to take
animals off pasture to reduce their ingestion of radioactive material.
Radiation levels which would easily kill humans and farm animals should have
little or no effect on seed or on crops already in the field. Many plants require a dose
of thousands of rads to stop seed germination or growth. A dose of 4,000 to 10,000 rads is
needed to inhibit sprouting of potatoes or onions. The attached chart shows the doses
needed to affect plant growth.
Soil
Soil would not be of immediate concern but proper management procedures could
do a great deal to reduce a problem should it occur. The addition of lime to calcium-poor
soils can reduce the uptake into plants of such radionuclides as strontium 90 and
strontium 89.
Vegetables
The greatest amount of contamination reaches vegetables by direct
contamination. Rainfall carries radionuclides into cavities of the fresh leafy vegetables
and collects there. For commercially produced vegetables that have a waiting period
between harvest and market, this would not be a problem as far as some contaminants are
concerned. With vegetables grown at home, skins or outer leaves should be removed and the
remainder washed thoroughly. Canning, freezing, or other storage of vegetables would allow
decay of short-lived radionuclides.
Grain
For grains, PAG action would probably never be needed, but if so it should
include milling and polishing. Time from harvest to consumer would be an important factor
here, for this may be several months in many cases.
Milk
Lactating dairy cows currently on pasture should be removed and fed
uncontaminated stored feed and water. Silages or other feeds harvested and stored before
an accident would be acceptable. Milk should be tested by an appropriate agency and their
advice should be followed. This would likely include diversion of milk for manufacturing
purposes or withholding contaminated milk from market to allow for radioactive decay. Milk
could be frozen, concentrated, dehydrated or other methods used to allow this to occur.
Methods exist to remove radionuclides from milk via ion-exchange resin columns; however,
to date this process has not been commercially exploited on a large scale and is available
in only a few areas. The same principles would be involved from fallout from nuclear
weapons except weapons fallout would have much higher concentrations of radioactivity
covering much greater areas.
GAMMA DOSE IN ROENTGENS TO REDUCE
CROP YIELD BY 50 PERCENT
Crops YD-50 Dose
- Peas, Broadbeans less than 1000R
- Rye, Barley, Onion 1000 to 2000R
- Wheat, Corn, Oats, Cucumber 2000 to
4000R
- Peanut, Alfalfa, Fescue, Sorghum
4000 to 6000R
- Cotton, Sugar Cane, Melons, Celery
6000 to 8000R
- Soybeans, Beets, Broccoli, Red
Clover 8000 to 12,000R
- Rice, Turnips, Sweet Potatoes,
Strawberries 12,000 to 16,000R
- Squash 16,000 to 24,000R
NOTE: Based on estimates in NATO document AC/25-WP/79. The Effects of
Radioactive Fallout on Food and Agriculture, November 1972.
Levels Dangerous to Animals
Whole body doses of 50 to 100 rads would start to become significant for farm
animals but healthy animals may show little or no effect from this dose. Levels higher
than this would begin to affect fertility, egg production, milk production, growth, etc.
The severity and duration would depend on the dose received.
Long before these doses would be reached, however, an evacuation of the human
population would be ordered. People may then have to return periodically to care for the
animals if evacuation of the animals was not feasible. If the radiation dose to these
people was unacceptable, then barring evacuation, the only choice would be to abandon or
destroy the animals. What would actually happen would depend on the specifics of the
incident or accident.
Percent Reduction of Radiation Expected from a Bank Barn or Shelter Enclosed on Three
Sides
The Emergency Management Agency does not have data specifically for farm
structures, so the numbers shown are extrapolations from other structures. They list a
building’s ability to protect against radiation from fallout by assigning it a
protection factor. The protection factor is the number equal to the dose rate outside
divided by the dose rate inside the structure. A protection factor of 10 means that the
dose rate outside the building would be 10 times higher than inside the building. A bank
barn would have a protection factor of about 20. A shelter enclosed on three sides would
offer very little protection. Its protection factor would be only about 2.
These numbers refer to protection from fallout from nuclear explosions. The
radioactive material from a nuclear power accident may not be particulate but in a gaseous
form, although accidents at nuclear power plants could produce particulate fallout under
the proper conditions. Protection factors from gaseous emissions for a bank barn would
probably be less than 2 and would be insignificant for a three-sided shelter since the gas
would readily diffuse into these structures.
Particulate Releases Reduced by Filtering
Radiation levels in a barn would more rapidly approach the outside levels with
ventilating fans running. The ventilation fans could not make radiation levels in the barn
higher than outside the barn. If the accident would involve particulate releases,
radiation levels in the barn could be kept down by filtering the ventilating air.
Time One Could Spend Evacuating Animals
The National Council on Radiation Protection and Measurements recommends that
exposures incurred by persons who must enter a hazardous area to protect facilities and
fight fires should not exceed 25 rads.
Levels Tolerated in Meat, Animals, Milk and Eggs
The radioisotopes having the greatest impact on agriculture are the
radioiodines, particularly lodine 131. These are significant because of their abundance in
the nuclear reactor, physio-chemical behavior and biological significance. Concentration
thought the pasture-to-cow-to-milk-to-child food chain further increases their
significance. Further, the population at risk from contaminated milk can be much larger
than the population affected by direct exposure.
The Food and Drug Administration has set a level of 12,000 picocuries per liter
of milk as the acceptable maximum concentration for Iodine 131. Acceptable levels for
Iodine 131 in meat, eggs, and other foods have been set, but the actual levels found would
probably be less because the concentration through this food chain is much less or
non-existent. Acceptable levels for other radioisotopes have also been set but to list all
levels for all foods can not be done here. The numbers that would be used and food that
would be involved would depend on what and how much was released.
Meat
Meat and meat products would be considered by a case-by-case evaluation
according to the PAGs.
Eggs
As eggs would contribute minor amounts of contamination to the food
chain, these would not be a problem. Furthermore, chickens are housed in
buildings that could provide shielding from most aerial contamination and their
feed likely would have been processed and stored before an accident.
