The number and severity of the leaks has been escalating, even as federal regulators extend the licenses of more and more reactors across the nation.
Tritium, which is a radioactive form of hydrogen, has leaked from at least 48 of 65 sites, according to U.S. Nuclear Regulatory Commission records reviewed as part of the AP's yearlong examination of safety issues at aging nuclear power plants. Leaks from at least 37 of those facilities contained concentrations exceeding the federal drinking water standard -- sometimes at hundreds of times the limit.
While most leaks have been found within plant boundaries, some have migrated off site. None is known to have reached public water supplies.
Any exposure to radioactivity, no matter how slight, boosts cancer risk, according to the National Academy of Sciences. Federal regulators set a limit for how much tritium is allowed in drinking water, where this contaminant poses its main health risk. The U.S. Environmental Protection Agency says tritium should measure no more than 20,000 picocuries per liter in drinking water.
The tritium leaks also have spurred doubts among independent engineers about the reliability of emergency safety systems at the 104 nuclear reactors situated on the 65 sites. That's partly because some of the leaky underground pipes carry water meant to cool a reactor in an emergency shutdown and to prevent a meltdown. Fast moving, tritium can indicate the presence of more powerful radioactive isotopes such as cesium-137 and strontium-90.
So far, federal and industry officials say, the tritium leaks pose no health or safety threat. Tony Pietrangelo, the chief nuclear officer of the industry's Nuclear Energy Institute, said impacts are "next to zero."
Leaks are prolific
There were 38 leaks from underground piping between 2000 and 2009, according to an industry document presented at a tritium conference. Nearly two-thirds of the leaks were reported over the latest five years.
For example, at the three-unit Browns Ferry complex in Alabama, a valve was mistakenly left open in a storage tank during modifications over the years. When the tank was filled in April 2010, about 1,000 gallons of tritium-laden water poured onto the ground at a concentration of 2 million picocuries per liter. In drinking water, that would be 100 times higher than the EPA health standard.
Subsurface water not only rusts underground pipes, it attacks other buried components, including electrical cables that carry signals to control operations.
A 2008 NRC staff memo reported industry data showing 83 failed cables between 21 and 30 years of service -- but only 40 within their first 10 years of service.
Under NRC rules, tiny concentrations of tritium and other contaminants routinely are released in monitored increments from nuclear plants; leaks from corroded pipes are not permitted.
The leaks sometimes go undiscovered for years, the AP found. Mistakes and defective material have contributed to some leaks. However, corrosion -- from decades of use and deterioration -- is the main cause.
Over the history of the U.S. industry, more than 400 known radioactive leaks of all kinds of substances have occurred, the activist Union of Concerned Scientists reported in September.
However, even with the best probes, it is hard to pinpoint partial cracks or damage in skinny pipes or bends. The industry tends to inspect piping when it must be dug up for some other reason. Even when leaks are detected, repairs may be postponed for up to two years with the NRC's blessing.
East Coast issues
One of the highest known tritium readings was discovered in 2002 at the Salem nuclear plant in Lower Alloways Creek Township, N.J. Tritium leaks from the spent fuel pool contaminated groundwater under the facility -- located on an island in Delaware Bay -- at a concentration of 15 million picocuries per liter. That's 750 times the EPA drinking water limit. According to NRC records, the tritium readings last year still exceeded EPA drinking water standards.
At 41-year-old Oyster Creek in southern New Jersey, the country's oldest operating reactor, the latest tritium troubles started in April 2009, a week after it was relicensed for 20 more years. That's when plant workers discovered tritium by chance in about 3,000 gallons of water that had leaked into a concrete vault housing electrical lines.
Since then, workers have found leaking tritium three more times at concentrations up to 10.8 million picocuries per liter -- 540 times the EPA's drinking water limit -- according to the New Jersey Department of Environmental Protection. None has been directly measured in drinking water, but it has been found in an aquifer and in a canal discharging into nearby Barnegat Bay, a popular spot for swimming, boating and fishing.
To Oyster Creek owner Exelon -- the country's biggest nuclear operator, with 17 units -- piping problems are just a fact of life. At a meeting with regulators in 2009, representatives of Exelon acknowledged that "100 percent verification of piping integrity is not practical," according to a copy of its presentation.
Of course, the company could dig up the pipes and check them out. But that would be costly.
"Excavations have significant impact on plant operations," the company said.
Exelon has had some major leaks. At the company's two-reactor Dresden site west of Chicago, tritium has leaked into the ground at up to 9 million picocuries per liter -- 450 times the federal limit for drinking water. Leaks from Dresden also have contaminated off-site drinking water wells, but below the EPA drinking water limit.
There's also been contamination of off-site drinking water wells near the two-unit Prairie Island plant southeast of Minneapolis, then operated by Nuclear Management Co. and now by Xcel Energy, and at Exelon's two-unit Braidwood nuclear facility, 10 miles from Dresden. The off-site tritium concentrations from both facilities also were below the EPA level.
The Prairie Island leak was found in the well of a nearby home in 1989. It was traced to a canal where radioactive waste was discharged.
Braidwood has leaked more than six million gallons of tritium-laden water in repeated leaks dating back to the 1990s -- but not publicly reported until 2005. The leaks were traced to pipes that carried limited, monitored discharges of tritium into the river.
"They weren't properly maintained, and some of them had corrosion," said Exelon spokeswoman Krista Lopykinski.
An NRC task force on tritium leaks last year dismissed the danger to public health. Instead, its report called the leaks "a challenging issue from the perspective of communications around environmental protection." The task force admitted that the rampant leaking had "impacted public confidence."
For sure, the industry also is trying to stop the leaks. For several years now, plant owners around the country have been drilling more monitoring wells and taking a more aggressive approach in replacing old piping when leaks are suspected or discovered.
But such measures have yet to stop widespread leaking.
In an initiative started last year, NRC Chairman Gregory Jaczko asked his staff to examine regulations on buried piping to evaluate if stricter standards or more inspections were needed.
The staff report, issued in June, openly acknowledged that the NRC "has not placed an emphasis on preventing" the leaks.
And they predicted even more.