Aug. 16--The release of tritium underneath the Kewaunee nuclear plant doesn't pose a health risk because the radioactive substance hasn't been found in drinking water, federal nuclear regulators said.

The radioactive isotope of hydrogen was found in four groundwater samples taken from narrow shafts underneath the nuclear plant, located in the Kewaunee County Town of Carlton, the Nuclear Regulatory Commission said in a report.

The Nuclear Regulatory Commission and Dominion Resources Inc., which owns Kewaunee, stressed that no unsafe levels of tritium have been detected at monitoring wells near the plant or outside the plant's boundary.

Kewaunee is one of 10 plants around the country where tritium leaks have been found. The nuclear industry is stepping up testing for tritium after a series of leaks at several plants. Testing at Exelon Corp.'s Braidwood plant in Illinois detected tritium in a nearby homeowner's well.

State and local authorities in Kewaunee and Manitowoc counties were informed last week of the discovery, which Dominion revealed on Friday.

Tritium is released naturally in the Earth's upper atmosphere. It is also released as a byproduct of power production by nuclear reactors. While it's a low-level source of radiation, people who drink water that contains high levels of tritium are at higher risk of developing cancer, and pregnant women drinking tritium-tainted water are at higher risk of their babies developing abnormalities, according to the Environmental Protection Agency.

The EPA allows up to 20,000 picocuries per liter of tritium in drinking water. In one of the four shafts measured beneath the Kewaunee reactor basement, tritium was measured at 103,000 picocuries per liter, the NRC said.

"They've found a small amount of groundwater seeped into these shafts, and when they collected and measured these samples last week, they found three of them with relatively low levels of tritium and one case that was above the EPA drinking water level," said NRC spokesman Jan Strasma.

Dominion detected the tritium while conducting tests as part of a voluntary program launched by the Nuclear Energy Institute, an industry trade group. The tests were conducted in shafts that are generally used to measure whether the plant's buildings have settled.

"This is not a threat to anyone's health," said Richard Zuercher, a spokesman for Dominion, stressing that the water in which tritium was found is not drinking water.

"This is directly under the station," he said. "Right now we have a team of individuals up there who are evaluating this, trying to determine where the possible source of this tritium might be."

Strasma said possible sources of the leak include the reactor's spent fuel pool, a leaking underground pipe, or a spill of water that drained into an area where the shafts are located.

Zuercher said there wouldn't be any health risk posed by the amount of tritium found at Kewaunee.

"If you were to drink a cup of water that contained the highest level, that would be the same as the naturally occurring radiation you would receive by eating one banana," he said.

Nuclear opponent concerned

Bonnie Urfer of Nukewatch, a Luck, Wis.-based group that is critical of nuclear power, said she thinks the concerns are being minimized by both the energy company and the NRC.

"Groundwater is not stationary, it doesn't stay in one place, and they still don't know where this leak is coming from," she said. "They can't know at this point how it's going to affect the environment or drinking water in the area."

Tests are conducted weekly of wells near the plant, and "we have seen no tritium in any of those tests," said Amy Wergin, public health nurse manager at the Manitowoc County Health Department.

More and more nuclear plants across the country are disclosing findings about tritium as a result of an industrywide response to leaks at several nuclear plants.

On Tuesday, Edison International, owner of the shuttered San Onofre nuclear plant north of San Diego, told the NRC it had detected tritium beneath the reactor.

Including Kewaunee and San Onofre, 10 nuclear plants have announced tritium findings, with four leaks disclosed in the past week, David Lochbaum, a nuclear engineer with the Union of Concerned Scientists, told Bloomberg News.

Southern Co. told regulators last week of tritium at its Hatch site in Georgia, and Xcel Energy Inc. reported a low level of the substance at the Prairie Island reactor in Minnesota on Aug. 8, said Lochbaum. Xcel, based in Minneapolis, serves western Wisconsin through its Eau Claire utility.

Lochbaum said that, by some estimates, "about a fourth" of the 65 U.S. nuclear plant sites will find some level of tritium on site.

"The final total is not in," he said. The reason for the increase in tritium-leak reports is because "people are now looking, and the reporting threshold is lower."

Urfer said the leak is a cause for concern, given Kewaunee's location on the shore of Lake Michigan.

"They don't know how many of these reactors are leaking," Urfer said. "For a reactor that's sitting on Lake Michigan, one of the best freshwater supplies in the entire world, it should be a serious issue."

The Point Beach nuclear plant, a few miles south of Kewaunee, is planning to participate in the "groundwater reassessments" taking place, said plant spokeswoman Sara Cassidy.

Both Kewaunee and Point Beach do regular testing and have been in compliance with federal standards that permit tritium to be released at low levels, officials said.

In 1975, Point Beach, which is owned by Milwaukee-based Wisconsin Energy Corp., experienced a low-level radioactive release, which included tritium. The area was cleaned up to levels significantly below the EPA's drinking water limit.

Bloomberg News contributed to this report.