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Lecture 17

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Author: Grant Mathews

Radioactive Fallout

  1. Residual Radiation
    When a nuclear bomb is detonated near the earth's surface, large amounts of earth or water enter the rising fireball and mix with the radioactive debris of the explosion at an early stage.  As the violence of the explosion subsides, the contaminated particles and droplets gradually descend to earth.  This phenomenon is referred to as ''fallout.''  The same name also applies to the particles themselves as  they reach the ground.  This fallout is the main source of residual radioactivity after a nuclear explosion.
  2. Delayed Radiation
    Fission of U-235 or Pt-239 generates a whole range of long-lived radioactive isotopes in the medium mass range (atomic mass between 80 and 160 amu).  There are about 40 ways to fission which produce about 80 radioactive species.  These isotopes also produce new radioactive isotopes by subsequent decay processes.
  3. Isotopes of special importance
    The most dangerous isotopes are I-131, Sr-89, Sr-90, and Cs-137, due to both their abundance in fallout and their biological affinity.
    1. I-131 is a beta and gamma emitter with a half-life of 8.07 d (specific activity 124,000 Ci/g).  It constitutes some 2% of fission-produced isotopes. Iodine is readily absorbed by the body and concentrated in one small gland, the thyroid.
    2. Sr-90 is a beta emitter with a half-life of 28.1 years (specific activity of 141 Ci/g).  SR-89 is has a half-life of 52 d (specific activity 28,200 Ci/g). Both Sr isotopes constitute about 3% of total fission isotopes: 190 curies of 90Sr and 3.8x104 curies of Sr-89 per kiloton. Due to their chemical resemblance to calcium these isotopes are absorbed and stored in bones. Sr-89 is an important hazard for a year or two after an explosion, but Sr-90 remains a hazard for centuries. Actually most of the injury from Sr-90 is due to its daughter isotope Y-90 which has a half-life of only 64.2 h, so it decays as fast as it is formed.
    3. Cs-137 is a beta and gamma emitter with a half-life of 30.0 y (specific activity 87 Ci/g).  It comprises some 3-3.5% of total fission products. It is the primary long-term gamma emitter hazard from fallout, and remains a hazard for centuries.
  4. Short distance radioactive fall out
  5. The fission of 57 grams of material produces 3 x 10^23 atoms of fission products (two for each atom of fissionable material). One minute after the explosion this mass is undergoing decays at a rate of 1021 disintegrations/sec (3 x 10^10 curies). It is estimated that if these products were spread over 1 km^2, then at a height of 1 m above the ground one hour after the explosion the radiation intensity would be 7500 rad/hr.


Map of nuclear fallout distribution after a potential nuclear attack on the United States.  Source: FEMA

Military Personnel Tests

  1. To get realistic results in atomic exercises, the military suggested that the men should be "stressed."
  2. The military wanted the men to be placed closer to the atomic blasts to learn how to conduct atomic warfare on a future nuclear battlefield. To do that, the DOD would have to take control of personnel away from the AEC. By early 1953, the Pentagon had succeeded. When a nuclear test involved battlefield maneuvers, field commanders would be responsible for the placement of their men near atomic detonations.

High Altitude Distribution

Material is ejected into the troposphere and the lower stratosphere.  The material is then re-distributed over polar (3-12 months) or equatorial regions (8-24 months).  Fall-out removal times (defined in terms of half-life) ranges from 10 to 24 months depending upon seasonal conditions, most rapid during spring, slow in summer.

Fallout Patterns 

Distribution of radioactivity is determined by high altitude wind & atmospheric conditions beyond the control of test conducting personnel. Depending upon the test yield and wind velocity (15 mph) radioactive fallout is spread within hours over large distances without awareness of the local population.


  1. Operation Castle 1953 - Bravo Test
    The Bravo test created the worst radiological disaster in US history. Due to a failure to postpone the test following unfavorable changes in the weather, combined with an unexpectedly high yield and the failure to conduct pre-test evacuations as a precaution, Marshallese Islanders on Rongerik, Rongelap, Ailinginae, and Utirik atolls were blanketed with the fallout plume. They were evacuated on March 3, 1953 but 64 Marshalls received doses of 175 rad. In addition, the Japanese fishing vessel Lucky Dragon was also heavily contaminated, with the 23 crewmen receiving exposures of 300 rad.  One of them later died.
  2. Atmospheric tests 1945-1963
    1. Atmospheric bomb tests at the Nevada test site caused the production of large amounts of long-lived radioactivity in the atmosphere which was distributed by high altitude winds over the USA and Canada and even world wide. 
    2. As shown in a study of the National Cancer Institute NCI 1997 internal exposures to radioiodine I-131 from fallout was the most serious health risk of continental nuclear testing.
    3. Radioiodine concentrates in milk were consumed by grazing cattle, and concentrates in the human thyroid glands were found after contaminated milk was ingested.
    4. The NCI study estimates that the average American alive at the time received a 2 rad thyroid radiation exposure, with some people receiving up to 300 rads. It has been estimated that from 380 million person-rads of total exposure roughly 120,000 extra cases of thyroid cancer can be expected to develop, resulting in ~6,000 deaths.
  3. Plumbbob 1957
    The Plumbbob test series released ~ 58,300 kCi  of radioiodine (I-131) into the atmosphere. This was more than twice as much as any other continental test series. This produced total civilian radiation exposure amounting to 120 million person-rads of thyroid tissue exposure (about 32% of all exposure due to continental nuclear tests). This has been estimated to cause about 38,000 cases of thyroid cancer, leading to some 1,900 deaths.


  1. The probability of a grand scale nuclear attack has declined with the demise of the Soviet Union.  China has not emerged as a comparable nuclear power, but its arsenal is growing.  If such an attack did occur, it would be disastrous for the population.  There would be an immediate high death toll and extreme high rate of heat and radiation induced injuries and health problems.  This, coupled with an insufficient medical support system, would lead to a complete collapse of civilian infrastructure.  However, presently, this has a low probability of taking place.
  2. The probability of a small scale nuclear attack (terrorist incident) depends on the availability of fissionable material. The most likely source would be the former Central Asian Soviet Republics which had maintained a considerable stockpile.


  1. How would most people die from a 1 MT explosion? 
    • Radioactive Fallout
  2. How did most people die from the 20 kT Hiroshima/Nagasaki explosions?
    • Burns/Heat Wave
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