Fallout shelter

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A fallout shelter is a civil defense measure used to reduce casualties in a nuclear war. Fallout is radioactive dust created when a nuclear weapon explodes or some other event causes a thermonuclear explosion. The explosion vaporizes any material within the fireball, including the ground if it is nearby. When this material condenses in the cloud, it forms dust and light sandy material that resembles ground pumice. This highly irradiated material then falls to earth, subjecting anything it touches to radiation. A fallout shelter is designed to allow its occupants to avoid this fallout while it is still dangerous.

Nature of Fallout

Initial radiation from fallout can exceed 30,000 rads/hr immediately downwind of a groundburst. A cumulative does of 450rad-hours is fatal to half of a population of humans. There have been no documented cases of survival beyond 600 rad-hrs. Most people become ill after an exposure to 100 rad-hrs or more. The fetuses of pregnant women are vulnerable and may miscarry, especially in the first trimester. Human biology resists mutation from large radiation exposure: grossly mutated fetuses usually miscarry. Civilian dose rates in peace-time range from .01 to .003 rad-hrs/year.

Fallout radiation falls off exponentially (quickly) with time. Most areas become safe for travel and decontamination after three to five weeks.

The ground track of fallout from an explosion is a long, thin fuzzy ellipse downwind of the explosion. It may be hundreds of kilometers long, and up to 50Km (30mi) wide from a single explosion. Rain can cause fallout to settle more quickly. This means that a rainstorm can be a very bad place just downwind of a nuclear war.

The most dangerous emissions from fallout are gamma rays, which travel in straight lines, like ordinary light. The fallout particles emit the invisible, deadly gamma rays in the same way that a light bulb emits light. Gamma rays are invisible, and cannot be seen, smelt, or felt, even at very dangerous intensities. Special equipment is required to detect and measure gamma rays.

The gamma rays do not contaminate people or objects. Fallout particles contaminate people or objects, and since they resemble sand, they can be brushed off, or washed off. The particles should be removed from the shelter, or shielded. Emergency drinking water can be adequately cleaned by filtering contaminated water through more than 25cm (10 in) of dirt. Food in sealed packages is not poisoned by fallout. Stored grain and exposed fruit can be cleaned and peeled. Vehicles are usually washed down with fire-hoses, into drains with removable filters, or deep trenches. Ground is usually decontaminated by bulldozing the fallout into deep, narrow trenches, and then back-filling the trenches.


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, a successful fallout shelter need not filter fine dust from air. The fine dust both emits relatively little radiation (because the intensity of the radiation increases as the cube of the particle size), and does not settle to the earth, where the fallout shelter is.

The required shielding can be accomplished with 10 halving-thicknesses of any material. Shields that reduce gamma ray intensity by 50% include 1cm (0.4 inches) of lead, 6cm (2.4 inches) of concrete, 9cm (3.6 inches) of packed dirt or 150m (500 ft) of air. When multiple thicknesses are built, the shielding multiplies. Thus, ten halving-thicknesses of packed dirt, 90cm (3ft) reduce gamma rays by a factor of 1024, which is 1/2 to the tenth power.

Usually, an expedient purpose-built fallout shelter is a trench, with a strong roof buried by ~1M (3ft) of dirt. The two ends of the trench have ramps or entrances at right angles to the trench, so that gamma rays cannot enter. 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.

Earth is an excellent thermal insulator, and over several weeks of inhabitation, a shelter will be completely warmed by body heat. Without good ventilation, the inhabitants are liekly to suffer heat prostration. The simplest form of effective fan is a wide, heavy frame with flaps that fits one door and can be swung from hinges on the ceiling. The flaps open in one direction,a nd close in the other, pumping air. Attach a rope, and take turns swinging it. (This is a Kearney Air Pump, or KAP, named after the inventor.) Any exposure to fine dust is far less hazardous than exposure to the gamma from the large popcorn 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 1.5Km (1mi).

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. They should sleep in a shelter for several months. Evacuation at three weeks is very practical.

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 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, available on-line at http://www.oism.org/nwss/. Inexpensive kits are available from http://www.ki4u.com.


Substantial numbers of fallout shelters were built in the 1950's in both the Eastern and Western blocs, though not in the U.S. 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, but the initial blast of a nuclear attack may have rendered these basements either buried under many tons of rubble and thus impossible to leave, or removed their upper framework and thus left the basements unprotected.

Sweden, however, 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 weeks after a nuclear attack.

Interest in fallout shelters has largely faded after the perceived threat of global nuclear war has receded since the end of the Cold War.