Answer to Question #2877 Submitted to "Ask the Experts"

I live about two miles away from a university nuclear research reactor and about 40 miles from a nuclear power plant. Growing up, I never gave it another thought. But recently, I have some concerns that I am sure you can clear up for me. I understand the analogy that radiation is like light coming from a light bulb. I am concerned about neutron radiation, which is very penetrating. I believe neutron radiation is much worse and will cause anything it comes in contact with to become radioactive, much like a neutron bomb. And anything that produces fission, i.e., nuclear power plant, research reactor, submarine, nuclear weapons, gives off neutron radiation. So when the experts say living by a nuclear power plant would only add 1 millirem extra per year of exposure to radiation, it fails to take into account that some of that is probably neutron radiation. When I receive this neutron radiation, would it make my clothes, car, and other items radioactive? Therefore, why aren't all the people who work at a nuclear power plant radioactive? And, then, why don't they then spread it all over the town and on their own possessions? And why aren't people who drive by the plant radioactive?

There are actually two questions in your submission (if I may paraphrase): (1) How does neutron radiation compare with other types of radiation in terms of the health risk, and is this accounted for in statements about radiation dose received by members of the public in the vicinity of operating nuclear reactors? and (2) Is there induced radioactivity in people and things exposed to neutron radiation in the vicinity of operating nuclear reactors and, if so, is it significant?

The answer to both questions requires some understanding of how neutrons behave in the presence of other matter. Neutrons interact¹ with matter in two ways: (1) by collisions with atomic nuclei that impart motion to the struck nucleus and slow down the neutron and (2) by becoming incorporated into an atomic nucleus (a process known as neutron capture), transforming that nucleus into another isotope. The second process is the one that can create radioactivity (also called neutron activation) in people and materials. Most neutrons escaping from nuclear reactors have a lot of kinetic energy (energy of motion), and their most likely interaction is through collisions, which imparts most of the neutron dose. These collisions don't usually result in neutron activation which, in general, is not likely to occur until a neutron has lost most of its kinetic energy.

Your perception that neutron dose is worse than beta and gamma dose is partly correct, but is addressed in limits for radiation exposure recommended by organizations like the National Council on Radiation Protection and Measurements (NCRP) and the International Commission on Radiological Protection (ICRP). It is more correct to say that the energy deposited in tissue by both alpha particles and neutrons has a greater biological effect than the same amount of energy deposited by photons or beta particles. The reason has to do with how densely the energy is deposited.² Two radiation dose units are commonly used: absorbed dose (in rad) reflects the average amount of energy deposited in tissue without regard to biological effect, but dose equivalent (in rem) does reflect the relative biological effectiveness of the radiation. It is correct³ to say that a 1 millirad neutron dose is worse (in terms of health risk) than a 1 millirad dose from gamma photons. However, a 1 millirem neutron dose is equivalent in health risk to a 1 millirem gamma photon dose. The annual public dose equivalent (expressed in millirem) around an operating nuclear reactor includes all the relevant dose contributions from neutron and gamma photon radiations. (Alpha and beta radiations don't travel outside the reactor shield.)

You are correct that materials exposed to alpha, beta, or photon (x ray and gamma) radiations do not become radioactive, but that exposure to neutrons can result in the creation of radioactive elements by neutron capture. People and materials in the vicinity of a nuclear reactor do, in fact, incorporate some induced radioactivity because of neutron exposure. Everyone is already radioactive, however, because of radioisotopes in the natural environment. Induced radioactivity from a properly operating and well-shielded nuclear reactor is so small⁴, compared with that already present naturally, that it would be difficult (if not impossible) to measure. That is why we do not consider people and materials exposed to the small radiation fields around commercial nuclear power plants and research reactors to be contaminated—the amount of additional radioactive material created by the neutrons is vanishingly small. Some of the more important neutron capture reactions that take place in common materials are discussed below, for your information.

Neutron capture in common materials (including your tissues) is most likely to occur with atoms of hydrogen, nitrogen, sodium, or sulfur. Of these, capture by hydrogen is very likely, simply because there is so much of it in water and organic materials. Neutron capture by hydrogen creates deuterium, a stable isotope of hydrogen. Neutron capture by nitrogen-14 (the most abundant isotope in atmospheric nitrogen) creates carbon-14, which is radioactive and decays slowly (half-life of 5,700 years) with the emission of a beta particle. Other common elements also capture neutrons, generally producing radioactive isotopes with short half-lives. Sodium and chlorine in tissues and phosphorus in hair capture neutrons, and the resulting radioactive products can be used to estimate doses to exposed personnel in the event of a nuclear criticality accident⁵—but only after doses tens of thousands of times greater than the 1 millirem annual public dose you mentioned. The table below shows isotopes created by neutron absorption for the elements noted above.⁶

^aSulfur-32 is unusual here because it is more likely to capture a high-energy neutron than one that has lost its energy in collisions with atomic nuclei.

Most, by far, of the radioactivity in your tissues and in common materials is from radioisotopes naturally present in the environment.⁷ The interaction of neutrons from space with nitrogen in the upper atmosphere, for instance, is the source of carbon-14 incorporated in all living tissue at easily measurable levels that can be used to date archaeological artifacts containing animal tissue, wood, or charcoal. You also are radioactive because of potassium-40, a primordial isotope which is present in natural potassium, decaying with the resulting emission of a photon, a positron, and a beta particle.

James S. Bogard, PhD, CHP
Oak Ridge National Laboratory

References:

1. Excellent discussions of neutrons and their properties, interactions with matter, and dosimetry are found in Chapters 9 and 12 of Atoms, Radiation, and Radiation Protection by James E. Turner (John Wiley & Sons, Inc., 1995). For more at this Web site about neutron interactions with matter, see Bruce Busby's response to Question #609 in Ask the Experts category "Radiation Basics-Neutrons" and George Chabot's answer to Question #1094 in "Radiation Basics-Radiation Shielding."
    
2. See Bruce Boecker's response to Question #647, and that of S. Julian Gibbs to Question #2030, at this Web site in Ask the Experts category "Radiation Effects-Radiation Modifiers" for more about relative biological effects.
    
3. This applies when speaking of low doses, where radiation-induced cancer fatality is the biological measure.
    
4. Research reactors and commercial nuclear power plants are typically shielded with materials like concrete or water having a high hydrogen content, because hydrogen is very effective both in slowing down neutrons and, then, in capturing them. The amount of neutron radiation escaping these reactors is quite low.
    
5. See C.S. Sims' response to Question #140 at this Web site in Ask the Experts category "Doses and Dose Calculations-External Dose Calculations" for more about biological neutron dosimetry.
    
6. Neutron capture is also accompanied by the prompt emission of either a gamma photon or a charged particle. Charged particles deposit all their energy quickly, and photons either deposit energy over a much longer path or else escape completely.
    
7. See Gary Kramer's response to Question #322 and Paul Frame's answer to Question #1617 in Ask the Experts category "Environmental and Background Radiation-In the Body" at this Web site for more about naturally occurring radioactivity in biological tissues.