Fallout shelter

A fallout shelter is a civil defense measure used to reduce casualties in a nuclear war. Fallout is dust created when a nuclear weapon explodes. The explosion vaporizes the bomb and earth or other material within the fireball. When this material condenses in the cloud, it forms dust and light sandy material that resembles ground pumice. Fallout is initially highly radioactive. The most dangerous emission from fallout is gamma rays which travel in straight lines, like ordinary light.

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 humans. The radiation falls off exponentially (quickly) with time. Most areas become safe for travel and decontamination after three to five weeks. Fallout is distributed in a wide, usually thin ellipse downwind of the explosion. Rain can cause fallout to settle more quickly.

Details

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 thicknesses of any material that reduce the radiation by half. Shields that reduce gammay 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.

Inhabitants should plan to remain in the shelter 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 possible.

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.

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.

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. Inexpensive kits are available from www.ki4u.com.

History

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.