Radioactive tritium, a carcinogen discovered in potentially dangerous levels in groundwater at a nuclear plant in the New England state of Vermont, has now tainted at least 27 of the nation's 104 nuclear reactors _ raising concerns about how it is escaping from the aging nuclear plants.

The leaks _ many from deteriorating underground pipes _ come as the nuclear industry is seeking and obtaining federal license renewals, casting itself as a clean-green alternative to power plants that burn fossil fuels.

Tritium, found in nature in tiny amounts and a product of nuclear fission, has been linked to cancer if ingested, inhaled or absorbed through the skin in large amounts.

The Nuclear Regulatory Commission said Monday that new tests at a monitoring well on the site of the Vermont Yankee plant registered 70,500 picocuries per liter, more than three times the federal safety standard of 20,000 picocuries per liter.

That is the highest reading yet at the Vermont Yankee plant, where the original discovery last month drew sharp criticism by Gov. Jim Douglas and others. Officials of the New Orleans-based Entergy Corp., which owns the plant in Vernon in Vermont's southeast corner, have admitted misleading state regulators and lawmakers by saying the plant did not have the kind of underground pipes that could leak tritium into groundwater.

``What has happened at Vermont Yankee is a breach of trust that cannot be tolerated,'' said Douglas, who until now has been a strong supporter of the state's lone nuclear plant.

Vermont Yankee has said no tritium has been found in area drinking water supplies or in the Connecticut River and that earlier, lesser tritium levels discovered last month were of no health concern.

``The existence of tritium in such low levels does not present a risk to public health or safety whatsoever,'' plant spokesman Robert Williams said in an e-mail Monday.

President Barack Obama, in his State of the Union address last week, called for ``building a new generation of safe, clean nuclear power plants in this country.'' His 2011 budget request to Congress on Monday called for $54 billion in additional loan guarantees for nuclear power.

The 104 nuclear reactors operating in 31 states provide only 20 percent of the nation's electricity. But they are responsible for 70 percent of the power from non-greenhouse gas producing sources, including wind, solar and hydroelectric dams.

Vermont Yankee is just the latest of dozens of U.S. nuclear plants, many built in the 1960s and '70s, to be found with leaking tritium.

The Braidwood nuclear station in Illinois was found in the 1990s to be leaking millions of gallons of tritium-laced water, some of which contaminated residential water wells. Plant owner Exelon Corp. ended up paying for a new municipal water system.

After Braidwood, the nuclear industry stepped up voluntary checking for tritium in groundwater at plants around the country, testing that revealed the Vermont Yankee problem, plant officials said.

In New Jersey last year, tritium was reported leaking a second time from the Oyster Creek plant in Ocean County, just days after Exelon won NRC approval for a 20-year license extension there. The Pilgrim plant in Massachusetts, like Vermont Yankee, owned by Entergy, reported low levels of tritium on the ground in 2007. The Vermont leak has prompted a Plymouth-area citizens group to demand more test wells at the Massachusetts plant.

NRC spokesman Neil Sheehan says leaks have occurred at least 27 of the nation's 104 commercial reactors at 65 plant sites. He said the list likely does not include every plant where tritium has leaked.

The leaks have several causes; underground pipes corroding and the leaking of spent fuel storage pools are the most common. The source of the leak or leaks at Vermont Yankee has not been found; at Oyster Creek, corroded underground pipes were implicated.

Many radiological health scientists agree with the Environmental Protection Agency that tritium, like other radioactive isotopes, can cause cancer.