Answer to Question #8192 Submitted to "Ask the Experts"

Category: Radiation Basics — Neutrons

The following question was answered by an expert in the appropriate field:

If the half-life of neutrons is 15 minutes, would it not be safe to assume that in less than three hours all (or 99.9999999 percent) all of the neutrons would have decayed after the explosion of a nuclear bomb or even (more so) a nuclear (reactor) accident? If so, how and why do buildings, water, soil, etc., remain radioactive for months, and how and why can the food chain, in some instances, be dangerous (radioactive) to humans?

In actuality, the free neutrons produced in the fission process that takes place in a nuclear explosive device and in an operating nuclear reactor do not survive long enough to undergo natural decay. When a nuclear bomb explodes, the neutrons are produced virtually instantaneously and move rapidly through the air and other materials. Most of the neutrons are born with considerable kinetic energy with typical velocities on the order of 10 percent of the speed of light. The neutrons are able to penetrate atoms and interact with the nuclei of those atoms and lose some energy in the process. Some of these interactions of the fast-moving neutrons may result in disappearance of the neutrons as they induce nuclear reactions; many of the neutrons do not disappear, however, and lose energy and slow down through interactions. These slower neutrons are more easily captured by nuclei and ultimately get captured and disappear. Even a slow neutron, in thermal equilibrium with its surroundings, moves rapidly, more than 2,000 meters per second at typical ambient temperatures. From birth to disappearance of a neutron, the time required is a small fraction of a second so that free neutrons are not around long enough that we would expect to observe neutron decay with the 15-minute half-life that you note.

The neutrons that are captured by nuclei produce new nuclei that are often radioactive; these are referred to as neutron activation products. It is these radioactive nuclei that represent the neutron-induced radioactivity that is present months and years after a nuclear explosion since some of the radioactive species that are produced may have relatively long half-lives. These radioactive species may appear in building structures, soil, etc., and some could get incorporated into foodstuffs, depending on usage of activated areas following an explosion. However, following a nuclear explosion, the detrimental contamination of areas by radioactive materials is much more heavily influenced by the deposition of radioactive fission products that result from fallout that occurs after the bomb burst than by the neutron activation products. These radioactive fission products come from the splitting of the heavy nuclei, such as uranium or plutonium, to yield the lighter fission fragments, many of which are radioactive with half-lives that cover a wide range, from less than a minute to many years.

When atmospheric nuclear bomb testing was prevalent, the presence of these fission products was often detectable around the globe as these radioactive species were carried large distances in the upper atmospheric air currents and eventually found their way back into the lower atmosphere and to the surface of the earth. When the Chernobyl nuclear reactor underwent its excursion in 1986, many of the same kinds of these fission products that had been produced in the reactor during its operation became airborne and were measured in the air at many locations far removed from the reactor site. Fallout of these products heavily contaminated land areas close to the reactor site, and some of these areas remain markedly contaminated today.

In closing, then, it is true that some radioactive products do result from neutron activation in the cases of both nuclear bombs and in operating nuclear reactors. Much more radioactivity is produced, however, during the uncontrolled fission process that occurs when a nuclear bomb explodes and during the controlled process of nuclear fission that takes place in a reactor.

I hope this resolves your questions.

George Chabot, PhD, CHP

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