Radionuclides in Groundwater

Water containing trace amounts of various radioactive materials is normally released from U.S. nuclear power plants under controlled, monitored conditions that meet conservative NRC limits to protect public health and safety. Recently, several instances of unintended, abnormal releases of radioactive liquids to the environment were identified. All available information on those releases shows no threat to the public. Materials detected to date in groundwater around nuclear power plants include Tritium and Strontium 90. The NRC is inspecting each of these events to identify the cause, verify the impact on public health and safety, and review licensee plans to remediate the event. The NRC has also established a lessons learned task force to review these incidents to determine what, if any, changes are needed to our rules and regulations.

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Tritium is a radioactive isotope of the element hydrogen. Tritium occurs naturally in the upper atmosphere when cosmic rays strike air molecules. It is also produced during nuclear weapons explosions, and commercially in nuclear reactors producing electricity. Tritium has a half-life of 12.3 years. The most common form of tritium is in water, since both radioactive tritium and non-radioactive hydrogen react with oxygen in the same way to form water. Tritium replaces one of the stable hydrogens in the water molecule, H2O, creating tritiated water, which is colorless and odorless.

Tritium has several commercial uses in self-luminescent devices, such as exit signs in buildings, aircraft dials, gauges, luminous paints, and wristwatches. It is also used in life science research and in studies investigating the safety of potential new drugs. Tritium is one of the least dangerous radioactive isotopes known. It emits very weak radiation and leaves the body relatively quick. Since tritium is almost always found as water, if ingested, it goes directly into soft tissues and organs, and is expelled from the body along with the water.

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Strontium -90

Strontium-90 (Sr-90) is a radioactive isotope that is produced in nuclear fission, the splitting of an atom's center that releases energy. It comes from three sources:

  1. fallout from above-ground explosions of nuclear weapons testing worldwide from 1963 to 1980;
  2. radioactive releases from the 1986 Chernobyl nuclear power plant accident in the Ukraine; and
  3. radioactive releases from nuclear power plants into the environment.

Approximately 99 percent of the Sr-90 in the environment comes from weapons testing fallout. Since Sr-90 takes about 29 years to lose half of its radioactivity, the fallout-based Sr-90 still remains in the environment at nominal levels. Most of the remaining one percent of Sr-90 in the environment came from the Chernobyl accident. At individual U.S. nuclear power plants, the amount of Sr-90 released is so low that it is usually at or below the minimum detectable activity of sensitive detection equipment. Radiation doses from Sr-90 to people living within 30 miles of a U.S. nuclear power plant are a tiny fraction of a percent of the annual dose an average person in this country receives from all sources.

Sr-90, if ingested, tends to mimic calcium when it is in the body and therefore becomes concentrated in calcified tissues such as bones and teeth. If ingested in quantities that produce very large doses (about a thousand times higher than what we all receive from natural radiation), Sr- 90 is known to increase the risk of bone cancer and leukemia in animals, and is presumed to do so in people. Below these doses, there is no evidence of excess cancer.

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