Letter: Neutron Shielding for Fallout Shelters

Hugh and Jim,
I was recently reading a book on nuclear reactors. I learned that iron can be used in concrete to provide neutron shielding. The iron slows the fast neutrons down to thermal levels that can be easily absorbed in the concrete. I also mine my own gold. As a byproduct of my mining, I have buckets of magnetite and hematite iron ore sand. The magnetite ore I sell to a local blacksmith for making steel. The hematite I’ve found no use for until now. I can use this black sand in place of silica sand in the concrete to provide neutron shielding for the temporary bomb shelter I’m building at my primary residence. Magnetite is Fe3O4 and is magnetic, and hematite is Fe2O3 and non-magnetic. Both comprise the black sand in the gold pan. So talk to your local gold miners about obtaining their black sands. – D.W.

JWR Replies: That is a useful idea, but perhaps it is more labor intensive than necessary. The main thing to remember about stopping the most energetic forms of radiation (neutron, x-ray, and gamma) is that the lack of quality of any particular type of shielding can be made up for with quantity. If in doubt, add thickness. With neutron radiation, it is water that is the best shielding. So damp soil works just fine. So does concrete, because it holds moisture for many years. Iron or steel actually stops gamma rays better than they do neutrons. My advice: Construct a deep shelter (my favorite is a dual purpose shelter and root cellar) with at least four feet of soil on all sides and overhead.  Six feet would be great. If you live in a region with sandy soil, then the soil should be kept dampened during times of high international tensions, for the best shielding.


  1. Neutrons are not a hazard except for being around operating reactors and particle accelerators. Fallout doesn’t produce neutron radiation. Also fast neutrons do not react with the body much until they are slowed down to thermal levels. The best shielding for neutrons is hydrogenous materials, i.e. water and oil which use elastic and inelastic scatter interactions to reduce neutron flux. I worked for 20 years as a radiation safety/nuclear emergency response specialist in the nuclear industry. In short don’t worry about neutron radiation, it is not a concern unless you are inside an operating nuclear facility.

  2. 1) Neutron bombs are low yield — because the neutrons that are used in a regular bomb to fission atoms are instead allowed to escape.
    The W70 Mod 3 yield was only about 1 kiloton –versus the 300 to 1000 kiloton yield of regular nukes.
    2) They are a specialized weapon with a limited range — so unless you are leading a pack of tanks in an attack, I doubt you will need to worry about any being used against you.
    PS Note that Boron is used in neutron shielding

  3. Something to consider in abruptly slowing fast neutrons is the release of X-rays due to Bremsstrahlung effect. The x-ray exposure would become problematic fairly quickly.

  4. Clarification: Neutron bombs have a limited range in the sense of not creating large fallout clouds that threaten life a hundred miles downwind. But their intense radiation does kill well beyond the range of blast and thermal pulse from a 1 kiloton yield. They were developed for use against tanks because tanks provide good shielding against blast pressure and thermal pulses.

    1. Yes, neutron bombs are a specific radiation enhanced device of low yield, designed to stop a breakthrough armored attack without mass destruction of buildings (to be used on soviet tank army in what was west Germany). Of very little use in other situations due to limited radius of effects. I doubt anyone has more than a handfull in their inventory. Bremsstrahlung only applies to charged particles, neutrons have no charge. Neutrons can cause harm 2 ways. 1-Direct exposure, only a hazard in an operating reactor or accelerator. 2-Radiation from activated materials. Some of fallout is material activated by the neutron exposure from the nuclear fissioning occuring during the blast itself. If you were close enough to get activated the blast and heat would have reduced you to your component atoms. Farther away the gamma and neutron exposure would kill you but so would the blast and thermal effects. If you were in a shelter good enough to protect you from the blast effects from a near miss it would probably protect you from the direct radiation effects. I repeat, neutron exposure is not a problem. Fallout and blast (if you are at a target) are the areas of concern. And yes boron is a good neutron absorber, used in control rods an as a chemical in reactor coolant to adjust reactivity (not to be confused with radioactivity) it is simply not needed in a shelter, blast and thermal effects are more of a danger in a target area. Move away from potential targets, have a viable fallout shelter, don’t waste resources and energy worrying about neutrons.

  5. 1) In theory, Neutron bombs could be used as anti-ballistic missile defense — instead of hitting a warhead with a bullet you hit it with a relatively large cloud of energetic neutrons . But do that in space and you blind your radars with EMP. Do it closer in and you are nuking yourself trying to hit a high speed target closing rapidly. Reason they closed Nike Hercules down.
    2) In theory, Might also try it against enemy cities to avoid that whole nuclear winter problem –but it leaves enemy resources intact and is kinda like trying to eliminate cockroaches with shotgun blasts. Plus you still end up with a lot of enemy cockroaches who have nukes and are really angry.

    1. Correction: I should have said Nike Zeus, the upgrade to the Nike Hercules. The Wiki article indicates the Air Force was a far more ruthless enemy to the Army’s ABM efforts than was the Soviets. After all, shooting down the Russian ICBMs also meant shooting down the Air Force’s IBCM budget. Pilots attack — they don’t defend. Which kinda left the rest of us holding our ..er.. thumbs.

  6. PS Robert McNamara — who poured tons of money into Vietnam — kneecapped Nike Zeus with the argument that fallout shelters would be a less expensive protection for Americans. Of course, he then failed to build the fallout shelters.

  7. Good comments on the shelter issue, Hugh and Don! I would add that unless you are within 7500 feet of a surface burst, the neutron issue is a non-starter. Less than one percent of your readers need be concerned about this. Four feet of soil over your shelter is plenty good for fallout protection. With SEVEN FEET of soil over your shelter, you will still get up to 100 REM of prompt neutrons inside the shelter, assuming the entrances are perfectly configured (a great weakness in most of the shelter designs I’ve seen). 100 REM won’t kill, but it will make you sick. Eight feet will do a good job. We like ten feet because it creates the maximum blast hardness in our steel pipe shelters (earth arching). Stacking water containers and grain (also rich in hydrogen) in your entrances will vastly improve your shielding.

    If you are in the Redoubt, 48″ of shielding on top will provide a protection factor of around 4,000PF. Will keep you from getting sick even in a rainout. Another four inches will double this….and so on. I live near a prompt nuclear target…so I am ten feet under. Thicker top cover also means a warmer shelter in the winter.
    An interesting video on Communist China’s nuclear program can be had by searching “China’s Great Underground Wall”.

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