Know the difference between a fallout and blast shelter.
3.08 Underground Fallout Shelters:
Underground fallout shelters provide much better protection than basement shelters. Four feet of
soil overhead will provide a PF greater than 1000. Good underground fallout shelter entrances
should be between 30 and 48 inches in diameter and should have a total length of 22 feet or
more. The best attenuation is reached if each leg of the L shaped entrance is 11 feet in length or
3.09 NBC Shelters:
Blast shelters should be built with arched ceilings. Flat-topped shelters will not carry the dirt load
at those depths, and could fail catastrophically under the additional load from large overpressures.
Shelters in areas near heavy blast targets should be buried at twice their diameter. This depth of
cover provides an ‘earth arching’ effect. The full earth arching effect will provide the shelter with
approximately 200 psi of overpressure protection. A 10-ft. diameter shelter should be placed into a
20-ft. deep hole. A 9-ft. diameter shelter should be placed into an 18 ft. deep hole. This level of
protection provides survivability at 1/2 mile from ground zero of a one-megaton yield ground
The concept of building shelters from corrugated steel tanks was conceived by scientists and
engineers at Oak Ridge National Laboratory (ORNL), and actually tested under blast conditions to
This type blast shelter would also provide protection from the ‘initial radiation’ which otherwise is
lethal within 1 1/2 mile range of the detonation. Shielding, however, must be placed into the
horizontal runs of the entrances to capture the neutrons from the initial radiation. The overhead
shielding for initial radiation requires 8 ft. of cover.
Outside of the 1-mile radius of the blast, initial radiation is not an issue. Residual radiation
(fallout) is easily attenuated with as little as 4 ft. of dirt cover overhead.