A fallout shelter doesn't protect you from a bomb landing on it. It protects you from what falls from the sky in the two weeks after. Much less spectacular, and much more useful.
What you're protecting against
A nuclear explosion acts in four stages, and a shelter doesn't answer each in the same way.
- The flash and the heat, within a second. Any wall stops the flash; it's the fires that follow that kill.
- The blast, a few seconds later. Near ground zero, nothing holds. Further out, it's a wind of several hundred kilometres per hour and a rain of glass.
- Initial radiation, in the first minute, mostly close to the explosion.
- Fallout, for days: dust lifted and made radioactive by a ground burst falls back over tens or hundreds of kilometres downwind. That's where most lives are saved or lost, and it's why shelters are built.
What stops radiation: mass
The gamma rays from fallout go through matter, but each layer stops part of them. This is measured in halving thickness: the thickness that cuts radiation in half.
| Material | Halves it | Cuts it 1,000 times (10 layers) |
|---|---|---|
| Steel | 1.8 cm (0.7 in) | 18 cm (7 in) |
| Concrete | 5.6 cm (2.2 in) | 56 cm (22 in) |
| Packed earth | 8.4 cm (3.3 in) | 84 cm (33 in) |
| Water | 12 cm (4.8 in) | 1.2 m (4 ft) |
| Wood | 22 cm (8.8 in) | 2.2 m (7 ft) |
What counts is the total mass between you and the dust, above and around. That's why a basement in the middle of a building, under several floors, already protects much better than a single-storey house.
Time is on your side: the 7-10 rule
Fallout radioactivity drops very fast. Every time the elapsed time is multiplied by 7, radiation is divided by 10.
- 7 hours after the explosion: ten times less than after one hour;
- 2 days (49 hours): a hundred times less;
- 2 weeks: a thousand times less.
Hence the advice: the first 24 to 48 hours matter most. And hence the stay shelters are designed for: about two weeks.
How a real shelter is built
Shelters built to standard, as in Switzerland, all share the same elements:
- a thick reinforced concrete shell, often underground, which resists the blast at a distance and stops radiation;
- a sealed blast door and an airlock, with baffles: radiation travels in straight lines, a bend in a corridor cuts it;
- filtered ventilation: a fan, often with a hand crank as backup, draws air through a dust pre-filter then an activated carbon filter, and keeps the shelter at a slight overpressure. Dust doesn't get in, even through cracks;
- blast valves on air inlets and outlets, which slam shut when the pressure wave arrives;
- water for two weeks, dry toilets, bunk beds, battery lighting, a radio;
- an emergency exit that doesn't go through the main stairs, often a tunnel or a hatch opening further away, outside the rubble zone.
Switzerland is the only country that has guaranteed by law, since 1963, a shelter place for every resident: there are more places than people. Finland, Sweden and Israel also have large networks. France has no shelter network for the public.
Improvising a shelter
US Army studies showed it: a well-made expedient shelter protects almost as well as a built one, against fallout.
- A basement: the most underground corner, away from vents. Pile everything heavy against the walls and on the floor above you: sandbags, books, water jugs, full furniture.
- A covered trench: a trench a metre wide, as deep as you are tall, covered with doors or planks then 60 to 90 cm (2 to 3 ft) of earth. That's Kearny's shelter, built in 48 hours by ordinary families during trials.
- A building: the middle of the building, halfway up or in the basement, away from the roof (where the dust settles) and away from outer walls.
Tips from people who've studied the question
- Dust is the real enemy. When you come in, take off your outer clothes, bag them away from you, wash or wipe yourself, hair included. That removes most of the contamination.
- Heat kills before radiation in a packed shelter without ventilation. Kearny designed an expedient fan, the "Kearny air pump", a simple wooden frame with plastic flaps swung by hand.
- A meter changes everything. Without a dosimeter, you come out too early or too late. Kearny published plans for an electrometer made with a tin can and aluminium foil.
- Iodine only protects the thyroid, and only against radioactive iodine. It isn't an anti-radiation pill. Take it when the authorities say so.
- Water in the pipes and toilet cisterns, and in a water heater, is protected from fallout: it can be drunk.
- Listen to the radio, battery or hand-cranked. See Radio, without the jargon.
Contaminated air and filters are in Air contaminé and Masques, filtres et tenues de protection.