Answer to Question #279 Submitted to "Ask the Experts"

Q

How is radiation measured around nuclear plants? What do the results tell us?

A

Radiation in the environment of nuclear power plants is generally measured by examining the pathways of human exposure to effluent releases. Effluent releases are possible by both the air and water pathways. Gamma-emitting fission products in air can be measured directly using pressurized ion chambers and thermoluminescent dosimeters. Airborne fission products can also be detected using high-volume particulate samplers. Airborne fission product releases which may accumulate on the ground can be of concern if there is a pathway to the human food chain. Therefore, samples are taken of food products such as garden vegetables. Airborne radioactive iodine could also accumulate on pasture grasses, so milk sampling is also conducted.

Nuclear power plants which are on bodies of water are required to check for radioactivity in all nearby lakes, ponds, and streams. Water samples are also taken from the effluent stream of the plant. Other water samples are taken from public water supplies close by. Since bioaccumulation can occur in fish, samples of representative species are caught and analyzed as well. Although not a human pathway, bottom sediments can be a sensitive indicator of releases into water. The vast majority of the measurements taken serve to document natural background radiation. Background radiation, which is never zero, should be measured and documented carefully so that if there is ever a significant release from a nuclear power plant, its effects can be distinguished from background. Unfortunately, this kind of information was not readily available during the event at Three Mile Island in 1979.

The amount of radioactivity that power plants are allowed to release is strictly controlled by the utility and heavily regulated by the Nuclear Regulatory Commission (NRC); therefore it is quite rare to measure anything above background in the environment. Additional information and general guidance on setting up an environmental monitoring program can be found in NRC's Regulatory Guide 4.1 "Programs for Monitoring Radioactivity in the Environs of Nuclear Power Plants."

Rich Allen, CHP