Nuclear Fallout: The Inverse Square Law, by Don Shift

What if I told you that it may be possible to survive radioactive fallout from a nuclear war without having a shelter? Bottom line up front: get inside, into an uncontaminated environment, and stay at least 10 feet away from the outside and your radiation exposure will be approximately 1/100th of the dose outside. Such a strategy, combined with the low likelihood of fallout for many Americans, could give those without basements, underground shelters, or ways of making improvised shielding a higher chance of survival.

Radiation protection is achieved by time, distance, and shielding. Shielding we all know; dense stuff blocks radiation. In time, radiation decays. After the first seven hours, radiation decays to about 1/10th its original rate. The first few hours after fallout arrives are the worst, then the radiation falls off to another tenth again after 49 hours. The seven-ten rule is explained in a lot of publications and videos.

But distance is poorly understood. Tell someone “just go to the center of your house and get under a mattress” and they’ll look at you like you’re nuts. It’s a nuclear weapon, not a tornado. Yes, that might save you from the blast effects if your house falls apart, but it won’t save you from incineration. With radiation, being in the center of a house or other building actually does help.

Note that the distance-only solution depends entirely on having a house that isn’t blasted to bits and the radiation level being fairly low outside, say 100 R/hr (Roentgen) or less. Being underground or behind concrete/masonry is always best.

The Inverse Square Law

This “one trick to survive nuclear war” is known as the inverse square law. Put plainly, radiation gets weaker the further you are from the source. A proper definition is “the radiation intensity is inversely proportional to the square of the distance.” This is true for any radiation, including the dangerous gamma radiation from nuclear fallout.

An analogy for how this works is like backing up from a shotgun. Gamma radiation is composed of particles called photons, which can be understood as like firearm projectiles. Theoretically, you could get so far away that the shot pattern just goes around you as it spreads out. Even if you do get hit, you’ll be hit fewer times than you would be, close to the muzzle.

Typically this rule is used with point sources, like the head of an x-ray machine, not a flat contaminated surface like the ground or roof. However, a collection of point sources becomes a plane source and a plane source, on the other side of a wall, becomes a linear source. Gamma radiation from nuclear fallout will obey the same laws of physics and the intensity will decline the further you retreat into the house as above. There are some caveats, of course.

Why is understanding this important? First off, because it sounds ridiculous if you don’t explain it. Radiation to many people is an unstoppable force that requires you be behind lead and concrete or underground to survive. While that’s the best case, radiation is subject to the laws of physics and understanding those laws helps people make decisions. A person who assumes they will die regardless of the actual conditions may make poor decisions that end up killing them when they might otherwise live.

Now to the caveats. This entire idea is predicated on the idea that you may be exposed to fallout, but not blast. This is possible for a lot of rural dwellers who are downwind of a target, but many miles away. An intact home will keep fallout from infiltrating to a reasonable degree. A home with broken windows or a torn open roof will admit fallout into the interior, making its protection useless.

You may be in an area near a target where you are subjected to very heavy fallout. Radiation levels may be too high to survive without a very thickly shielded shelter with an air filtration system. Areas downwind may receive multiple fallout plumes that combine and together raise the radiation intensity. Dense shielding is always preferred.

Filtering the air entering the house is a good idea, such as placing filtration material over a gable vent, but filtering a room will not help. Again, if the fallout is inside, the radiation is that much closer. An intact structure is your “bubble” keeping the radioactivity emitting particles away from you. If fallout is inside the house, you need to relocate to a secure building or construct a shielded shelter and filter the air coming into that shelter.

Radiation will be present at varying levels from all sides, including above you. Exposure is thus hemispherical. For easy math, figure on five sides; four walls and the roof. A reading taken in the center of the shelter will reflect the cumulative total from the exterior of this hemisphere. Do not be deceived that you are only taking 1 R/hr (Roentgen) at the center because a reading at an exterior wall, one foot inside, reads 100 R/hr. It’s 100 R/hr from five sides, or 5 R/hr in your central refuge, or 20 R after a day.

The gamma flux from outside will not be uniform. Fallout concentration will vary based on how the wind deposits the particles (think of snow drifts), the geometry and density of your home, and the exact nature of the fallout particles. The wind-sheltered parts of your home may have low radiation levels while the windward side’s reading is higher than flat ground outside. For our examples, we presume everything is uniform.

The Safest Spot

There is no place in a suburban home or building that, without shielding, is “immune” from radiation. You can only get to a safer place. You are still being impacted by the radiation, which can only be stopped by dense shielding. Using our five sides assumption, 1,000 R/hr rate outside means a 200 R accumulated dose inside after a day; enough to make anyone seriously ill and kill some.

Shielding in combination with distance can dramatically reduce the radiation you are exposed to, perhaps making the aforementioned 200 R dose into 50 R. Improvised shielding can make a lifesaving difference. Survival Blog has a great post about Protection Factors (PF) and shielding back in 2005: Radiation Protection Factors for Dummies – by L.H..

To have negligible radiation levels without shielding, you would need to be in a very large building with many dozens of feet of floor space around you and several floors above. In fact, old Civil Defense literature mentions the middle floors of big-city high-rise apartment buildings or department stores as ideal shelters for this reason.

Your home geometry matters. For instance, being on the second floor will get you further away from the radiation on the ground, but will bring you close to the radiation on the roof. You may be able to “cheat” by getting closer to the exterior if that exterior wall is made of thick brick, etc. Or, what seems like the perfect spot may not be because you forgot the wall behind you doesn’t border a bedroom, but an open breezeway.

Monitoring Equipment

A radiation meter is indispensable. If you cannot afford or find a proper Geiger counter, construct a homemade Kearney Fallout Meter (KFM). Print out the instructions from Nuclear War Survival Skills by Cresson H. Kearney (et al) before the crisis starts. Not every city or military base that gets nuked will produce fallout.

Nuclear fallout is only caused by warheads detonating at ground level. These surface bursts are used to get at hardened, buried targets like command bunkers and missile silos. Those downwind of NORAD in Colorado Springs or the Minuteman missile fields in the Midwest are at greatest risk. Fallout will be most intense the closer to the target, say within 50 miles, for 150-300 kiloton yield weapon (the most common yield believed to be in Chinese and Russian arsenals).

If you haven’t gotten out of the cities, even just to the fringe suburbs, you may be killed in the war if your city is large enough. Countervalue targeting means nuking cities: “Will you trade Chicago for Kyiv?” The civilian population is the target so living in a large metropolitan area is a bad idea. Airbursts, not surface, will target the city to maximize the destructive punch of the weapons. A weapon, like the warhead of an ICBM, that explodes in midair and whose fireball does not reach ground will produce negligible fallout.

On the plus side, all those old fallout maps dating from the 1960s bomber era mean that rural folks aren’t going to be poisoned from every single target. Southerners and those living west of the Rockies stand the best chance of experiencing little fallout. What comes after the nuclear strike is the tough part. Nuclear winter is conjectural, but even without one the aftereffects will be almost as bad.

More Than Blast and Fallout

Millions of Americans will survive the initial nuclear war. Keep in mind there are only about 1,200 deliverable American and Russian warheads, so not everything will be nuked. The true effect will be the devastation to the American economy because of the destruction. Nuclear war is more than just the bomb dropping; it’s EMP and an economic collapse all rolled into one.

The 1979 Congressional publication The Effects of Nuclear War concluded with a chilling short story, “Charlottesville: A Fictional Account of a Nuclear Attack,” about the Virginia city’s experience attempting to recover. The background is a period of intense privation and famine. For SurvivalBlog readers, that is just what many of us have prepped for, so we may stand a chance.

The takeaway I want people to have is that outside of the major cities, you will probably survive a nuclear war. Fallout shouldn’t be the danger that the glib popular belief makes it out to be and without a shelter, nuclear war isn’t an automatic death sentence.

About The Author: Don Shift is the author of seven novels and nonfiction books including Nuclear Survival in the Suburbs. (That one is nonfiction.)