Well, it’s been a long time since my last Health Physics class (decades, in fact), but I think there are some fallacies in your statement, @PhiNotPi, well meaning as it is.
Alpha and Beta particle radiation in general do not pose a high risk to a moderately protected individual. However, they can become internalized into the body through breathing, eating or drinking, or through breaks in the skin. So complete skin, eye and mucous membrane coverage, air and water filtration and no skin breaks are the important shielding for these forms of radiation. If absorbed into the body these particles can be very dangerous. Fortunately they are low-energy and heavy, so they don’t travel far and are usually only present (to most humans) in the form of radioactive dust.
Neutron radiation is unlikely except during direct exposure to fissioning radiation, and no one is walking into that.
Gamma radiation is a wave form of radiation (similar to X-ray), but “a solid wall of cement 100 yards thick” will stop a lot more than half of the gamma radiation one may be exposed to. If that were the case, Earth’s atmosphere would not be effective at shielding us from cosmic gamma radiation, and it is. The general rule of protection from gamma radiation is Time, Distance and Shielding. Limit the time exposure, increase the distance from the source of the radiation, and increase the shielding. Lead, concrete and earth are effective shields, but not 100%.
I don’t recall the inverse square law (and don’t care to look it up right now), but simply increasing one’s distance from the source of a gamma radiation source reduces the intensity of the exposure by the square of the distance. So a 1000 millirem per hour exposure at 1 foot (a high level of exposure) would be reduced to 10 millirem per hour at a distance of ten feet (1000 รท 10 squared). That’s still not an insignificant exposure, but it’s nowhere near as dangerous. The NRC guidelines are based on an exposure of 2 mr / hr or less as “uncontrolled”, meaning anyone can work or pass through that radiation field without monitoring or controls of any kind.
So your hundred feet of concrete all by itself would reduce an exposure by a factor of 10,000. Still, not “nothing”, but ‘reduced by that much’. (And concrete itself is slightly radioactive in any case.)