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A fallout shelter is an enclosed space specially designed to protect occupants from radioactive debris or fallout resulting from a nuclear explosion. Many such shelters were constructed as civil defense measures during the Cold War. After a nuclear explosion, matter vaporized in the resulting fireball is exposed to neutrons from the explosion, absorbs them, and becomes radioactive. When this material condenses in the cloud, it forms dust and light sandy material that resembles ground pumice. The fallout emits both beta particles and gamma rays. Much of this highly radioactive material then falls to earth, subjecting anything within the line of sight to radiation, a significant hazard. A fallout shelter is designed to allow its occupants to avoid exposure to harmful fallout until radioactivity has decayed to a safer level.
Additional recommended knowledge
The former Soviet Union and other Eastern Bloc countries often designed their underground mass-transit and subway tunnels to serve as bomb and fallout shelters in the event of an attack.
Interest in fallout shelters has largely dropped, as the perceived threat of global nuclear war reduced after the end of the Cold War. In Switzerland, most residential shelters are no longer stocked with the food and water required for prolonged habitation and a large number have been converted by the owners to other uses (e.g. wine cellars, ski rooms, gyms). However, a renewed interest has been seen since 2001. These shelters also provide a safe haven from natural disasters such as tornadoes and hurricanes, although Switzerland is not subject to such natural phenomena.
In popular culture, fallout shelters feature prominently in the Robert A. Heinlein novel Farnham's Freehold (Heinlein built a fairly extensive shelter near his home in Colorado Springs in 1963), Pulling Through by Dean Ing, A Canticle for Leibowitz by Walter M. Miller and the David Brin novel Earth. The idealized fallout shelter can be seen in the motion picture Blast from the Past. Also noted would be Interplay's Fallout and Black Isle Studios Fallout 2.
Details of shelter construction
A basic fallout shelter consists of shields that reduce gamma ray exposure by a factor of 1000. Since the most dangerous fallout has the consistency of sand or finely ground pumice, it is easily ingested into the soft tissues of the body by the respiratory system unless it is filtered from the air inside the shelter.
Usually, an expedient purpose-built fallout shelter is a trench, with a strong roof buried by ~1 m (3 ft) of dirt. The two ends of the trench have ramps or entrances at right angles to the trench, so that gamma rays cannot enter (they behave like invisible light). To make the overburden waterproof (in case of rain), a plastic sheet should be buried a few inches below the surface and held down with rocks or bricks.
Dry earth is a reasonably good thermal insulator, and over several weeks of habitation, a shelter will become fairly comfortable. The simplest form of effective fan to cool a shelter is a wide, heavy frame with flaps that swings in the shelter's doorway and can be swung from hinges on the ceiling. The flaps open in one direction and close in the other, pumping air. Attach a rope, and take turns swinging it. (This is a Kearny Air Pump, or KAP, named after the inventor.) Any exposure to fine dust is far less hazardous than exposure to the gamma from the fallout outside the shelter. Dust fine enough to pass the entrance will probably pass through the shelter.
Effective public shelters can be the middle floors of some tall buildings or parking structures, or below ground level in most buildings with more than 10 floors. The thickness of the upper floors must form an effective shield, and the windows of the sheltered area must not view fallout-covered ground that is closer than 1.5 km (1 mi). Inhabitants should plan to remain sheltered for at least two weeks, then work outside for gradually increasing amounts of time, to four hours a day at three weeks. The normal work is to sweep or wash fallout into shallow trenches to decontaminate the area. They should sleep in a shelter for several months. Evacuation at three weeks is recommended by official authorities.
A battery-powered radio is very helpful to get reports of fallout patterns and clearance. In many countries (including the U.S.) civilian radio stations have emergency generators with enough fuel to operate for extended periods without commercial electricity. It is possible to construct an electrometer-type radiation meter called the Kearny Fallout Meter from plans with just a coffee can or pail, gypsum board, monofilament fishing line, and aluminum foil. Plans are in the reference "Nuclear War Survival Skills" by Cresson Kearny.
If available, inhabitants should take potassium iodide at the rate of 130 mg/day per adult (65 mg/day per child) as an additional measure to protect the human thyroid gland from the uptake of dangerous radioactive iodine, a component of most fallout and reactor waste. (for more info, including storage, and use of an inexpensive saturated solution, see potassium iodide)
Protection offered by the solid walls and roof of a structure
The fallout from either a weapon or an accident is a complex mixture of many radioisotopes. For weapons fallout the photon energy is assumed to be the same as the gamma rays from 60Co. Data collected after the Chernobyl accident can serve in a simulation of fallout shelter efficacy, reconstructing the contribution of different radioisotopes to the radiation dose over time. The simulation detailed below assumes that no chemical separation occurred during the transport of radioactivity to the site where the fallout fell (this in real life is not true), and that no decontamination or removal of fallout (e.g. weathering) occurs.
Using the data for the source term (radioactive release) from Chernobyl, and other literature data it is possible to estimate how much protection a wall of concrete will offer in the event of a Chernobyl like accident. These calculations are for a room with no windows or doors. The radioactive dust on the roof, and the windows and doors will make the estimation of the protection factor more difficult.
10 cm concrete shielding
These graphs show that thicker walls increase the protection factor. The protection factor is the ratio of the dose rate suffered by a person inside the shelter divided by the dose rate in the open. The protection factor changes as a function of time. This is because some of the short-lived isotopes such as 95Zr and 95Nb generate very high energy gamma photons, while the longer lived 137Cs have a lower photon energy.
As the wall is made thicker the average gamma photon energy for those photons which pass through the wall becomes higher. So each additional layer of concrete has a smaller effect on the dose rate.
20 and 30 cm concrete shielding
(Refer to charts, at right) As the shield becomes thicker the very high photon energy emitters such as 140Ba/140La and 95Zr/95Nb become more and more important.
Other matters and simple improvements
In the long term it is important to consider the protection which is offered by a person's home in the months and years after an event such as the Chernobyl accident. While the person's home may not be a purpose-made shelter, it can be thought of as a shelter if any action is taken to improve the degree of protection.
Measures to lower the beta dose
The main threat from beta emitters is from a hot particle which is in contact or close to the skin of the person. Also a swallowed or inhaled hot particle could cause beta burns. As it is important to avoid bringing hot particles into the shelter, one option is to remove one's outer clothing on entry.
Measures to lower the gamma dose rate
It is likely that the gamma dose rate due to the contamination brought into the shelter on the clothing of a person will be insignificant unless the shelter has very good shielding on the walls and roof (or if the person was very badly contaminated).
Different types of radiation emitted by fallout
In the vast majority of accidents and in all atomic bombs the threat due to beta and gamma emitters is far greater than that posed by the small amount of alpha emitters in the fallout. Alpha radiation can be very harmful, but only if radioactive materials are ingested or inhaled. Alpha particles can be blocked easily by a sheet of paper.
It is likely that even a light structure will give good protection against most beta emitters, but small particles of fallout can cause localised radiation injuries known as beta burns. It is thought that if a person entering a fallout shelter was to change their footwear and leave their outer clothing outside the main area then the persons inside will be protected from these beta burns. Beta rays are more penetrating than alpha rays, but internal exposure will tend to do less damage because the LET is lower.
Three centimeters of aluminum can block the beta emissions from even a high energy beta emitter such as 90Sr, while a lower energy beta emitter such as tritium or 14C will be stopped by thinner objects.
These are not charged particles, and are thus more able to pass through objects and may pose a great threat to a person in a fallout shelter. Most of the design of a fallout shelter is intended to protect against gamma rays. The rays' intensity can be reduced by dense materials such as lead, steel, concrete or packed earth.
The bulk of the radioactivity in nuclear accident fallout is more long-lived than that in weapons fallout. A good table of the nuclides such as that provided by the Korean Atomic Energy Research Institute includes the fission yields of the different nuclides. From this data it is possible to calculate the isotopic mixture in the fallout (due to fission products in bomb fallout). The mixture of radioisotopes present in used power reactor fuel can be more complex because neutron activation of fission products is possible, a good example of this is the cesium isotropic signature. In terms of activity (becquerels or curies) it is the case that the activity in a power reactor fuel one hour after shutdown tends to be more long lived because the majority of the short lived fission products will have had time to decay.
For example, imagine that some fissile material is used in a bomb, and that in 1012 fissions an equal number of 131I and 137Cs atoms are formed. Because the 131I has such a short half life when compared with the 137Cs, the activity ratio of 131I to 137Cs will be very much in favour of the 131I one hour after the fission event.
If, on the other hand, a lump of fuel in a power reactor undergoes 1012 fissions, which will generate a given amount of 131I, if the reactor was run at a constant power for one year then the majority of the 131I will have had time to decay. However the vast majority of the 137Cs atoms will not have had time to decay. So the 131I to 137Cs ratio is more in favour of 137Cs than the mixture formed.
Reference and external links
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Fallout_shelter". A list of authors is available in Wikipedia.|