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5 June 2002

Q: Is there a way to stop radiation from a nuclear bomb if people are around?

A: The simple answer is: Yes, distance and shielding reduce the radiation dose to people from the detonation of a nuclear explosive.

Radiation concerns from a nuclear detonation fall in three time realms: (1) the prompt gamma and neutron radiation produced by the nuclear detonation, (2) the near-term, residual but decreasing, post-detonation radiation from the fallout and from materials that have been made radioactive by absorbing the neutrons from the nuclear detonation, and (3) the long-term, lower-level, more widely dispersed residual radiation.

Focusing on the prompt and the short-term residual radiation, distance is a very important factor. Very close to the device, blast and heat would be lethal and protection against radiation is irrelevant. Beyond the range of blast effects, radiation intensity decreases with distance and the radiation intensity at any distance can be reduced by shielding materials that are present.

The most important consideration in shielding is the mass of material between the radiation source and the target individual. In general, the radiation is reduced by certain fraction for each unit of mass that is added; thus we usually talk in terms of "reducing the intensity" rather than totally "stopping" the radiation. Any material provides shielding; in general it is the amount of mass that is important and shielding is provided by a wide variety of materials–earth, concrete, other masonry, lead, iron and other metals, water, etc. Thus, terrain and buildings between the detonation and the individual may provide incidental shielding. Shielding is also provided by designated shelters which may include interior and basement areas of massive buildings, subways, tunnels, and specially constructed bunkers.

This answer has necessarily been very general. A more specific discussion of the amount of shielding required would require a consideration of the magnitude of the radiation source as determined by the type and size of the nuclear device and the elevation above the ground at detonation and the intensity of the prompt and fallout radiation fields as determined by the distance from the point of detonation. Similarly a discussion of the effectiveness of in-place shielding would require specification of the factors just mentioned and of the type and amount of shielding present.

The question did not state whether it was in the context of a terrorism event or in the context of a nuclear war. In a terrorism event, there might not be any warning and thus the distance and shielding protection are left pretty much up to chance. In the event of a nuclear war, there is a greater possibility of planning for intervention such as evacuation and sheltering.

The longer-term residual fallout and activation present another whole set of considerations which are not covered in this answer.

Charles E. Roessler, Ph.D., CHP

Robert N. Cherry, Jr., CHP

Roessler and Cherry are radiation safety scientists. Google reveals that Cherry is, or was, a neighbor of the Director of the National Security Agency at Fort Meade, MD.

James Bamford wrote in Body of Secrets (2001) p. 573:

The moving vans, loaded with Minihan's well-traveled belongings, had barely pulled away from the handsome redbrick house on Butler Avenue when painters and cleaners arrived to spruce it up for his successor. For more than four decades this has been the official residence of the director of NSA. Located on a restricted, tree-shaded corner of Fort Meade, it is equipped with its own Secure Compartmented Information Facility (SCIF). Inside the Vault Type Room is a STU-III crypto phone connected to NSA, about three miles away, and a heavy safe in which to hold highly classified documents brought home for late-night reading.

On a wall near the kitchen is a plaque containing the names of all the NSA heads who have lived here -- every director except for the first, Lieutenant General Ralph Canine. After Minihan's departure, a new brass plate was attached to the plaque, one bearing the name of Michael V. Hayden, an Air Force lieutenant general and the fifteenth director of NSA.

Here's an eyeball of the tree-shaded SCIF: