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% (1/2) 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, a practical fallout shield is ten halving-thicknesses of packed dirt. This reduces gamma rays by a factor of 1024, which is 1/2 multiplied by itself ten times. This multiplies out to 90cm (3ft) of dirt.
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 (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.
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 likely to suffer heat prostration.
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 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 KI4U. Combined with an effective fallout shelter plan, one should also consider the use of Potassium Iodide (KI) tablets 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.
Substantial numbers of fallout shelters were built in the 1950s 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. However, a renewed interest has been seen since terrorism has struck on American soil. These shelters also provide a safe haven from natural disasters such as tornadoes and hurricanes.
See also: BunkerDetails of improvised fallout shelters
History