Underground shelter - How much fallout protection?

ABSTRACT

The purpose of this article is to show by a single, simple example how effectively ordinary earth can reduce the penetration of gamma radiation into a buried shelter. It is possible, and not beyond reasonable attainment, to have a fallout shelter so well shielded that the radiation levels inside the shelter will be less than ordinary, day-to-day radiation levels, even when the ground above is covered with an extremely heavy pile-up of radioactive falllout. If the radiation intensities at the surface above the shelter are as high as 10,000 R/hr, the radiation intensity in the shelter can be kept bellow 0.01 mR/hr (mR is an abbreviation for "milliroentgen", which is one-thousandth of one roentgen). This low radiation level is equal to the average U.S. natural background radiation level at the earth's surface. The reduction in radiation intensity from 10,000 R/hr at the surface to 0.01 mR/hr at the surface to 0.01 mR/hr in the shelter can be obtained with about 8 ft. of earth above the shelter ceiling and a well-designed extranceway