Fallout shelter

    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.

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History

During the Cold War many countries built fallout shelters for high-ranking government officials and crucial military facilities. Plans were made, however, to use existing buildings with sturdy below-ground-level basements as makeshift fallout shelters. These buildings were usually placarded with the yellow and black trefoil sign as shown in the top of this article. The initial blast of a nuclear attack might have rendered these basements either buried under many tons of rubble and thus impossible to leave, or removed their upper framework, thus leaving the basements unprotected. Switzerland built an extensive network of fallout shelters (mainly through extra hardening of government buildings such as schools) of a scale to protect and feed the entire population for two years after a nuclear attack. This nation has the highest ratio of shelter space to national population of any country. All these shelters are capable of withstanding nuclear fallout and biological or chemical (NBC) attacks but the blast-proof requirement varies depending on the size of the building. The largest buildings usually have dedicated shelters tunneled into solid rock. Similar projects have been undertaken in Finland, which requires all buildings with area over 600 m² to have a NBC shelter, and Norway, which requires all buildings with an area over 1000 m² to have a shelter[2].

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)^[1], 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.

The required shielding can be accomplished with 10 times the amount of any quantity of material capable of cutting gamma ray effects in half. Shields that reduce gamma ray intensity by 50% (1/2) include 1 cm (0.4 inch) of lead, 6 cm (2.4 inches) of concrete, 9 cm (3.6 inches) of packed dirt or 150 m (500 ft) of air. When multiple thicknesses are built, the shielding multiplies. Thus, a practical fallout shield is ten halving-thicknesses of packed dirt. This reduces gamma rays by a factor of 1024, which is 2 multiplied by itself ten times. This multiplies out to 90 cm (3 ft) of dirt.

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.

No shielding

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).

• Roofs and gutters should be cleaned to lower the dose rate in the house.

• The top inch of soil in the area near the house should be either removed or dug up and mixed with the deeper layers of soil. This reduces the dose rate as the gamma photons have to pass through the soil before they can irradiate a person.

• Nearby roads can be rinsed and washed down to remove dust and debris; the contaminated materials would collect in the sewers and gutters for easier disposal. In Kiev after the Chernobyl accident a program of road washing was used to control the spread of radioactivity.

• Windows can be bricked up, or the sill raised to reduce the hole in the shielding formed by the wall.

• Gaps in the shielding can be blocked using water cans, such as bottles of water. While water only has a density which is one tenth that of lead, it is still able to absorb gamma rays.

• Earth can be heaped up against the exposed walls of the building, this forces the gamma rays to pass through a thicker layer of shielding before entering the house.

• Nearby trees can be removed to reduce the dose due to fallout which is on the branches and leaves. It has been suggested by the US government that a fallout shelter should not be dug close to trees for this reason.

Different types of radiation emitted by fallout

Alpha

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.

Beta

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.

Gamma

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.

Weapons fallout

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.

See also

• Bunker
• Blast shelter
• Diefenbunker
• Fallout Protection
• Collective protection
• Fission product
• Survivalism
• Survival Under Atomic Attack
• Urban exploration
• Sonnenberg Tunnel

Reference and external links