A fallout shelter has two jobs that pull against each other: put enough mass between occupants and the fallout outside, and keep air moving without letting contaminated dust in. Mass is the shielding problem, about 60 cm of packed earth or its equivalent overhead and around, and air is the ventilation problem, which an airtight seal fails because people need to breathe.
Part of Radiation Safety
A fallout shelter is a space where people can stay for days to weeks while the dose rate outside falls, shielded from gamma radiation and kept free of dust.
Where To Put It
The parent article, Nuclear Safety, says to go underground if possible, since a basement reduces exposure by 90 percent or more; failing that, go to the centre of a large building where walls on all sides add shielding. Below ground, soil is the shield. Choose ground that cannot flood and see Storm and Fallout Shelters for site, drainage and two exits, and Underground & Earth-Sheltered Buildings and Fully Underground Structures for building one.
Mass: Put It Overhead As Well As Around
The parent says to pile earth, sandbags or concrete blocks against exterior walls, aiming for at least 60 cm of packed earth on all exposed sides and the roof. By Half-Value Layers, 60 cm of earth is 6.67 layers, about a hundredth of an unshielded beam. Equivalent masses: see Shielding Materials; for a given mass of rubble, water barrels or stacked blocks, check mass per square metre rather than the name.
- Roof first. Fallout settles on the roof and the ground, so the roof and the ground-level wall zone matter most. A thin roof on thick walls is shielded to the thin roof.
- Structure. 0.6 m of earth on a 5 x 5 m roof is 22.5-30 t (Earth Load Calculations); use Roof Structures for Earth Loading. A collapsing roof kills more reliably than fallout.
- Entrance. A straight opening lets radiation in along its line. A doorway with a turn or an offset baffle, built with earth or blocks, shields better than a straight one (recalled general practice, not from the corpus).
- Quick options. In the first hours there is no time to dig deep; the fast shelter articles, Emergency Shelter and Rapid Shelters, cover rough construction, and the mass rules here apply to anything you pile around them.
Air: Exclude Dust, Not Air
The parent says to seal openings with plastic sheeting and tape. The housing article warns that sealing a shelter tight is a mistake because people need air and carbon dioxide pools at floor level. These conflict, and the safer reading is not airtight: keep a low intake and a high exhaust, size them from the housing article’s minimum of 150 cm² per 20 m² of floor (more occupants need more), and put a cloth or fine filter on the intake to catch dust, replacing it when dirty. See Ventilation Shafts and Chimneys. That article notes natural draught is weak, a couple of pascals, and can reverse in summer, so do not rely on it for a crowded shelter. Handle a dirty filter as contaminated.
Never burn anything inside: see Carbon Monoxide Risks. Crowded people also make heat and moisture; plan for it.
Stock For The Stay
The parent’s list for a two-week stay: at least 4 litres of water per person per day, non-perishable food, sanitation containers (buckets with lids), a battery radio if available, medications and an electroscope. Water: 4 x 14 = 56 litres per person; for 4 people, 224 litres. Store the water before the event; see Water Storage. Keep a place near the entrance for outer clothing and boots, so dust stays out of the living area.
How Long To Stay
The parent advises at least 48 hours, ideally 2 weeks. The 7-10 rule says the rate is 1/100 of its 1-hour value at 49 hours and 1/1,000 at 2 weeks. By the derivation in Fallout Patterns, about three-quarters of the two-week unsheltered dose comes in the first 48 hours. Plan excursions for water or sanitation as late as possible, short, and in protective clothing; see Personal Protection.
Common Mistakes
- Making it airtight.
- Thick walls, thin roof.
- No latrine plan. Buckets with lids, emptied later.
- Leaving early to look. The first days are where the dose is.
- Keeping contaminated clothes inside.
Safety
The shelter can kill by water, bad air or collapse
Do not use a flame to test the air. If anyone is drowsy or has a headache, get fresh air at once. A shelter that floods or has a roof not rated for its soil load is worse than none.
What’s Next
- Learn next: Personal Protection — going out and coming back in
- Combine with: Storm and Fallout Shelters — the building side