U.S. nuclear power plant operators haven’t figured out how to quickly detect leaks of radioactive water from aging pipes that snake underneath the sites — and the leaks, often undetected for years, are not going to stop, according to a new report by congressional investigators.

The report by the Government Accountability Office was released by two congressmen Tuesday in response to an Associated Press investigation that shows three-quarters of America’s 65 nuclear plant sites have leaked radioactive tritium, sometimes into groundwater.

Two senators asked the GAO, the auditing and watchdog arm of Congress, to investigate the findings of the AP series Aging Nukes, which concludes that the U.S. Nuclear Regulatory Commission and the nuclear power industry have worked closely to keep old reactors operating within safety standards by weakening them, or not enforcing the rules.

A third senator, independent Bernie Sanders of Vermont, said the AP series has raised disturbing allegations about safety at aging plants and reiterated his demand that the Vermont Yankee plant be shut.

In the report released Tuesday by Democrats Edward J. Markey of Massachusetts and Peter Welch of Vermont, the GAO concluded that while a voluntary initiative that industry recently adopted is supposed to identify leaks, the U.S. Nuclear Regulatory Commission doesn’t know how fast problems are detected.

“Absent such an assessment, we continue to believe that NRC has no assurance that the Groundwater Protection Initiative will lead to prompt detection of underground piping system leaks as nuclear power plants age,” the report’s authors concluded.

No leak is known to have reached aquifers that hold the drinking water supplies of public utilities, though tritium has contaminated residential drinking wells near at least three nuclear power plants.

Markey’s spokeswoman said his office received the GAO report in early June after requesting it in 2009 following reports of a tritium leak at the Indian Point nuclear plant north of New York City. Typically congressional offices hold reports for 30 days, but Markey released it in response AP’s tritium story, part of an ongoing investigative series.

In a written statement, he compared the ongoing nuclear crisis at Japan’s Fukushima Dai-ichi plant to the kind of meltdown he said could happen in the U.S. if a pipe that is supposed to carry water to cool a reactor’s core fails.

“There would be no warning because no one ever checks the integrity of these underground pipes,” Markey said.

The industry’s Nuclear Energy Institute cited its “underground piping integrity initiative policy,” launched voluntarily in 2009, as proof that it takes tritium leaks seriously.

“The initiative commits the industry to a series of actions to establish more frequent inspection and enhance dependability of underground piping with a goal of protecting structural integrity and preventing leaks,” the institute said in a statement.

The institute also criticized AP’s overall findings and “selective, misleading reporting in a series of new articles on U.S. nuclear power plant safety.”

Previously, the AP reported that regulators and industry have weakened safety standards for decades to keep the nation’s commercial nuclear reactors operating. While NRC officials and plant operators argue that safety margins can be eased without compromising safety, critics say these accommodations are inching the reactors closer to an accident.

In response to those findings, New Jersey’s two Democratic senators asked the GAO for a new investigation based on “the serious allegations” documented by the AP.

“It would be of grave concern to us if, in fact, aging power stations have achieved compliance with operating rules because of weakened NRC rules, rather than demonstrated compliance with existing standards,” Sens. Frank R. Lautenberg and Robert Menendez wrote.

In a Senate speech Tuesday, Sanders said the NRC and Vermont Yankee operator Entergy have ignored the will of Vermonters. The Vermont state Senate recently voted to close the plant once its license expires next year.

He also called for a GAO investigation into the safety issues raised in the AP series. “These allegations by the AP are incredibly disturbing,” Sanders said. “Safety at our nuclear plants should be the top priority at the NRC, particularly after what we saw happen in Japan. They should not answer to the nuclear industry, the NRC must answer to the public.”

Sanders said the investigation should determine whether the NRC is systematically working with industry to undermine safety standards to keep aging plants operating.

California Democrat Barbara Boxer, chairwoman of the Senate’s Environment and Public Works Committee, said she is supporting Sanders.

Late Tuesday, the NRC said it disagreed with AP’s conclusions in the stories, but welcomed the attention to nuclear plant safety the stories have generated. The agency defended its standards and approach to safety.

“The NRC never wavers from its primary mission — ensuring that the public remains safe during the civilian use of radioactive materials in the United States,” the statement said.

Addressing the main issue of the AP series regarding weakening of standards, the NRC said it “only endorses changes when they maintain acceptable levels of public safety; this can include adding or strengthening requirements.”