Radioactive tritium has leaked into groundwater from the Catawba Nuclear Station on Lake Wylie, Duke Energy told federal regulators Wednesday.
One well at the Catawba plant had a tritium concentration twice as high as the federal government says is safe in drinking water.
Duke says the contamination poses no threat to the public because it is confined within the plant's boundaries.
The S.C. Department of Health and Environmental Control will sample water from about two dozen residential wells near the plant, spokesman Thom Berry said. Testing will occur in the Bethel community of York County. The department learned of the leaks late Tuesday or Wednesday.
"We want to know whether any tritium is in the groundwater used by wells outside of the plant's boundary," DHEC's Patrick Walker said.
Tritium occurs naturally and as a byproduct of nuclear plants. It emits a weak form of radiation, but people exposed to it may face increased risks of cancer or pass on genetic abnormalities.
It also can foreshadow the eventual flow of more toxic radioactive materials in groundwater, said David Lochbaum, a nuclear safety expert with the Union of Concerned Scientists in Washington.
At least six other nuclear plants, none in the Carolinas, have reported tritium leaks in recent years. The Nuclear Regulatory Commission says the leaks posed no threats to public health but revised inspection procedures to ferret out potential leaks.
Nuclear reactors produce tritium from the use of a chemical, boron, to help control the chain reaction that produces heat. Boron also is added to the water in which fuel cools after it has been used in a reactor.
Under an industry initiative, Duke spokesman Valerie Patterson said, Catawba installed 30 new wells to monitor groundwater at the plant. One of those wells -- not used for drinking water purposes -- detected the concentration Duke reported Wednesday. It was the only one of 30 test wells to show tritium levels above the EPA's standard of 20,000 picocuries per liter.
"We don't know the source; that is part of the investigation," Patterson said.
Other nuclear plants have traced leaks to spent fuel pools and to valves.
"We have no reason to believe, based on other testing of other wells, that they have elevated levels," Patterson said.
Tritium contamination has been a concern in South Carolina and across the country recently because of leaks at other nuclear plants and from Barnwell County's low-level nuclear waste landfill. The Barnwell site takes nuclear refuse from atomic power plants.
The Catawba plant has had at least three leaks of radioactive material since 1992, according to a report Lochbaum compiled. Most nuclear plants have had some sort of leak since their inception, he said.
In South Carolina, Lochbaum noted nearly 50 "groundwater events," including leaks, at the state's four nuclear plant sites.
In North Carolina, Duke's McGuire nuclear plant on Lake Norman has installed 41 wells and will add nine. It was unclear Wednesday whether reportable levels of tritium had been detected there.
The Oconee plant in northwestern South Carolina will install 28 wells later this year, Patterson said.
Sammy Fretwell of The (Columbia) State contributed to this report.