By: Sharon Packer (TACDA Board Member)
The nuclear threat from North Korea has prompted many callers during the past few weeks, asking about the effects and attenuation of radiation. There is a great deal of misinformation about radiation from fallout. The following old rule of thumb for shelter design still holds true. NBC shelters should have four feet of dirt cover, or three feet of concrete cover to give a minimum PF level of 1,000 from fallout. If a rainout should occur, or if the sheltered area is within 1.5 miles of a potential primary target, the shelter will require a minimum of eight to ten feet of cover. Shelter entrances require careful engineering, as most of the radiation exposure will come from these entrance areas.
I recently reviewed a series of articles about Nuclear Weapons Effects, written by Carsten Haaland, of the Oak Ridge national Laboratory. The entire series of articles can be found in our Journal of Civil Defense published in 1990. Some of you may be fortunate enough to still possess these journal articles. I have re-typed, in part, the section on Fallout and Rainout for this current article.
FALLOUT FROM NUCLEAR DETONATIONS
Carsten M. Haaland, Oak Ridge National Laboratory
What is Fallout?
Fallout is the radioactive dust that comes back to earth as a result of a nuclear explosion at the surface of the earth, or at an altitude low enough for the fireball to engulf solid materials. Fallout dust may look like sand, ash or crystals, depending on the kind of material engulfed by the fireball. If the material engulfed is ordinary earth or sand the fallout will look like sand, but if the engulfed material contains calcium to the extent found in concrete buildings or coral, the fallout may look like ashes. Large dense particles will descend faster than very small particles. For this reason, fallout particles several hundred miles downwind from a nuclear surface burst will be very small, somewhat like particles in atmospheric pollution, and the nuclear radiation from the fallout will be greatly reduced.
The danger of fallout arises from the intense and highly penetrating nuclear radiation emitted from it, which produces a potentially lethal hazard to people in the vicinity unless they have protection. Large areas, covering hundreds to thousands of square miles, depending on the yield and number of surface detonations, can be poisoned with fallout such that radiation from the contaminated area is hazardous or lethal to an unprotected person passing through or dwelling in the area, for periods of days to weeks after the detonations.
How is Fallout Produced?
When a nuclear weapon explodes near the ground, the instantaneous release of incredible energy makes a huge pit or crater. Tons of earth in the crater are instantly changed from solids into hot gas and fine dust, by the tremendous heat and pressure from the bomb explosion. This hot gas and dust, together with vaporized materials of the bomb itself, form a giant fireball that rises like a hot-air balloon to high altitude. This material spreads out, cools, and becomes more dense as it rises. The fireball stops rising when its density reaches the same density as the atmosphere into which it has risen.
Some of the dust and heavier particles that are drawn up with the fireball form the stem of the mushroom cloud. The dust in the cap of the mushroom spreads out horizontally when the fireball stops rising, and begins to be shaped and drawn along by the winds at that altitude. This dust cloud can be carried for hundreds of miles by the upper winds. The dust falling and drifting to the earth from this moving cloud becomes the radioactive fallout with which we are concerned. Somewhat confusingly, the process itself; that is, the dust’s action of falling and drifting to the ground, is also called fallout.
The dust in the stem and in the mushroom cloud becomes radioactive mostly from the fission products created in the nuclear explosion that become stuck to part of the dust particles. The air around the particles does not become radioactive, and neither do the ground-surface materials on which they settle.
The smallest particles of fallout can be carried hundreds of miles by the wind before reaching the earth. Most of the fallout will come down to the ground within 24 hours after the detonation. Very small particles come down very slowly and may be spread over large areas of the earth’s surface in the downwind directions over time periods of many days, even weeks. This delayed fallout is sometimes called worldwide fallout, although most of the fallout comes down in the hemisphere in which it is produced (Northern or Southern). Fallout that arrives within the first day or two after the explosion poses a much greater threat to human life than does delayed fallout.
Because the rate of fall of a fallout particle depends on the size, shape and density of the particle and on the local winds (Haaland, 1989), the pattern of deposition on the ground can be highly irregular. The pattern shown in Fig. 1 resulted from measurement of radiation intensities on the ground after the nuclear test named TURK at the Nevada Test Site in 1955, a 43 kiloton tower shot (Glasstone, 1977). The pattern shown in Fig. 2 shows how an idealized fallout pattern is used to estimate fallout on the city of Phoenix, Arizona, resulting from a hypothetical ground burst of a 10 megaton nuclear weapon on Luke Air Force Base (Haaland, 1987a).
Radiation from Fallout
The radioactivity from fallout decays and fades away by natural processes. The radioactive materials produced by the nuclear explosion are unstable. These materials change (or decay) into a stable condition by shooting out nuclear radiation, such as alpha, beta, and gamma rays. Gamma radiation is by far the most dangerous of the three kinds of fallout radiation, because it can penetrate the entire body and cause cell damage to all parts, to the organs, blood and bones.
A more detailed discussion of the kinds of fallout radiation and their potentially harmful effects may be found in Radiation Safety in Shelters, CPG 2-6.4, 1983, available from the Federal Emergency Management Agency, Washington, DC. The penetration of gamma radiation through matter, dose-factors for the body, comparison of fallout radiation with initial nuclear radiation, and other topics, are discussed in great technical detail in Fallout Facts for Nuclear-Battlefield Commanders (Haaland, 1989). Methods of providing protective shielding from lethal fallout contamination have been presented by Chester (1986) and Spencer (1980).
Decay of Radioactivity
Some materials decay into their stable form faster than others. Those that change fast produce intense nuclear radiation in the first few moments after a nuclear explosion. Those that decay more slowly, such as cesium-137 and strontium-90, may be responsible for measurable nuclear radiation years after the explosion. These particular radioisotopes may enter the body through the food chain and may remain for long periods in certain parts of the body. The increased radioactive emissions from these isotopes (above the normal radioactive emissions from potassium-40 which exists in our bodies) may increase the potential for various cancers.
Because many materials in the fallout cloud decay quickly, the nuclear radiation from a given quantity of fallout is most intense in the first moments after detonation and its intensity rapidly falls to lower levels. This behavior can be approximately described by a rule of thumb called the seven-ten rule. This rule applies only to fallout of the same effective age. If the fallout results from unclear detonations that all exploded within a few minutes of each other, then the effective age is the same as the actual age, the time measured from the mean time of the detonations. If the fallout is produced from detonations that are separated in time by more than a half-hour or so, then the average decay rates of the different clouds of fallout are sufficiently different. The concept of effective age must be applied to estimate the decay rate of the composite fallout. Methods have been developed for determining the effective age of composite fallout from simple measurements by a survey meter and the use of a monogram (Haaland, 1989).
The seven-ten rule states that the measured radiation intensity from a given quantity of fallout particles will decay to (1) one-tenth as much when the fallout becomes seven times older than the effective age at the time of measurement, (2) one-hundredth (1/10 x 1/10) as much when the fallout becomes forty-nine times (7 x 7) older than the effective age at the time of measurement, and so on. The unit of time can be seconds, minutes, hours, half-days, days or whatever period of time is appropriate for the situation. For instance, if the measured level of radiation is 1,000 R/hr., after 7 hours the radiation level will decay to 100 R/hr. After 7 x 7 hours (about 2 days) the radiation level will decay to 10 R/hr. After 7 x 2 days (about 2 weeks) the radiation level will decay to 1 R/hr.
If the air is humid, the nuclear explosion may start a local rain. The fireball from a low-yield nuclear detonation, less than a few hundred kilotons, may not rise above the troposphere. In this case, if it is already raining or if the explosion starts a rain shower, much of the radioactive material will come quickly to the ground as rainout. A light rainout produced low-level fallout-type radiation after the Hiroshima and Nagasaki detonations, even though the fireballs did not engulf solid materials on the ground. Radiation from rainout could be extremely intense and localized if the fireball does not rise above the rain cloud, because the fallout cloud has not had a chance to spread out as it does when carried a long way by the wind, and it has not had as much time to decay. If the rainfall is heavy, the fallout may be washed into gutters, ditches, and storm sewers, from whence it may be carried into streams and rivers. In this case the earth surrounding the ditches, sewers and streams, and the water itself will provide shielding to greatly reduce the fallout hazard to local residents. However, radioactive material, like dirt and sand particles, can collect in unpredictable locations under these circumstances to produce highly lethal concentrations. A radiation survey meter will be needed to help detect, and avoid remaining in such locations.
Fallout radiation is a potential hazard that must be considered in the event of nuclear attack. The magnitude of the area covered, the geographical shape, and the levels of radiation intensity CANNOT be precisely predicted. Protection by shelters is possible, and radiation management through the use of rate meters and dosimeters will reduce the potential risk.