Answer to Question #2304 Submitted to "Ask the Experts"

Q

Kindly explain the various reasons we conduct soil radioactivity research.

A

When talking about "soil radioactivity research," there are several situations to consider. First, one can study the naturally occurring radioactive materials (NORM) in undisturbed situations. These include radionuclides with long half-lives, uranium-238, thorium-232, and potassium-40, as well as radionuclides with shorter lives, hydrogen-3, carbon-14, and berylium-7. Of these radionuclides the ones that emit gamma rays are often studied to determine the external gamma radiation dose to the public in a particular location. Since uranium-238 decays through a dozen different levels on its way to stable lead-206, including radon-222, it is often studied for another reason. High levels of uranium in the soil can indicate an increased risk from higher levels of radon in homes. Some of these radionuclides are also studied to find locations of high enough concentrations that can be economically mined for the minerals. Also carbon-14 is often studied to determine the age of artifacts through radiocarbon dating, and other radionuclides are used as tracers in environmental studies. Surveys of NORM background are often conducted to characterize a site before it is used for an activity that involves radiation. For example, Australia is mapping the natural background radiation around all of the seaports where nuclear powered ships dock so that they know the "before" situation in case of an accident. Also, raw building materials need to be characterized so that they don't cause radiation problems with subsequent use, for example, using monazite sands, which can be rich in thorium, in buildings. The next situation is where naturally occurring radioactive materials have been concentrated by the actions of mankind. These materials are referred to as technically enhanced NORM or TENORM. They occur in many industrial situations, including petroleum recovery, energy production, mineral processing, water and sewage treatment, and agricultural product fabrication and can contaminate the soils around these sites. The activities of man can concentrate naturally occurring radionuclides in amounts large enough to be of concern as a radiation risk. For example, the use of uranium mill tailings as fill around building foundations can cause a radon problem. Regulatory authorities are only now beginning to control these sources of radiation.

Finally, there are several situations where artificially produced radionuclides end up in the soil and research is conducted to determine the locations and levels of these radionuclides. The impact of the nuclear explosions at Hiroshima and Nagasaki has been studied extensively, with some of the information coming from soil studies. Another situation was the atmospheric testing of nuclear weapons in the 1950s and 1960s that produced radioactive fallout. Radionuclides of concern from these events include cesium-137, strontium-90, and various transuranic elements, including isotopes of uranium, plutonium, and americium. Since these radionuclides were dispersed around the globe by atmospheric transport after the tests, the concentrations were never very high except immediately downwind of the test sites and most radionuclides have decayed to undetectable levels.

Another more recent event was the Chernobyl nuclear accident in 1986. The explosion and subsequent fire vaporized the core, releasing a large part of the radionuclide fission product inventory of the reactor. Because of prevailing meteorological conditions, most of the radionuclides were deposited on the ground generally northeast of the reactor site in several European and Former Soviet Union countries, but were measured around the Northern Hemisphere. Much research has been conducted subsequently to determine the location and concentration of these radionuclides and their impact on humans and the human food chain.

Other situations include releases from nuclear weapons facilities, mostly in the United States and in Russia. Extensive soil research has been done around sites such as the Rocky Flats facility in Colorado, the Hanford Reservation in Washington State, and the Mayak Chemical Combine near Chelyabinsk in Russia. Accidental and deliberate releases from these sites contaminated significant areas around these sites. Research is currently being done to characterize the hazards at these sites.

Bob Fairchild, CHP, PhD