PERRY, Ohio -- The Perry nuclear power plant is leaking tritium, a radioactive form of water with a half-life of more than 12 years.

The radioactive water has been found in groundwater at concentrations more than twice the federal drinking water limit outside of a building where the leak was discovered Monday. No other, more dangerous radioactive isotopes were found.

Plant owner FirstEnergy said the tritium has not made its way into the plant’s larger under-drain system designed to collect groundwater from under the entire site. Nor has the isotope been found in other groundwater test sites on the property or into nearby Lake Erie.

“It was found in one sample area next to the building. I know of at least five other areas that have been sampled and there have been no indications of tritium beyond that one,” said Jennifer Young, FirstEnergy nuclear spokeswoman.

“We are doing additional sampling today. Any groundwater flows into the plant’s under-drain system,” she said. “It has not left the plant boundaries.”

Perry engineers reported the problem to the Nuclear Regulatory Commission and the NRC's on-site inspectors, as well as to county and state emergency agencies about 1:40 a.m. Tuesday.

“Actions are in progress to stop the leak," the Nuclear Regulatory Commission report said. Engineers were still working late Tuesday afternoon to seal it.

Workers discovered the leak Monday in a valve on a water line that carries reactor water back to the reactor after it has run through the plant’s steam turbine and then been condensed back into water.

The leaky valve was in a pipe contained in a hallway-sized steam tunnel running from the turbine and generator building through a second, auxiliary equipment building and then back into the reactor containment building, said Young.

She could not say when the leak began. She described it as a small spray of water and steam, which cameras monitoring the tunnel picked up.

David Lochbaum, nuclear safety engineer with the Union of Concerned Scientists, which is not opposed to nuclear energy, praised the company for reporting the leak even though it appears to be minor.

"I'd not rush to the store for bottled water if I lived near the Perry nuclear plant because of this leak," he said, adding that FirstEnergy's actions exemplify the industry's zero tolerance for leaks that may appear to be insignificant.

"Leaks were often tolerated when considered insignificant. No one ever asked, yet alone answered, the question of how long it takes for an insignificant leak to become significant," he said.

Lochbaum said the nuclear industry took it upon itself to track down tritium leaks after operators of the Braidwood nuclear plant in Illinois discovered in 2005 that more than six million gallons of radioactive water had leaked into ground water over the previous six years.

"The (industry) measures are voluntary. It is commendable that FirstEnergy is doing this. I just wish the NRC had a more prominent role in protecting the environment," he said.

The NRC on Tuesday said the leak, as described by the company, does not pose a serious risk to the public.

"Based on the current information, the NRC does not have a public safety concern," said Viktoria Mitlyng, agency spokeswoman for Midwest.

"It was reported to the state of Ohio as a courtesy notification by Perry in accordance with the Nuclear Energy Institute's groundwater monitoring initiative. Such notifications are then provided to the NRC. NRC inspectors at the plant and the specialist inspectors in the NRC's Midwestern office are aware of the leak and making sure we understand the situation."