A study of safety lapses at nuclear power facilities in the United States found that owners of atomic plants too often either close an eye to problems or fail to adequately address them, a watchdog group said Tuesday.

The report by the Union of Concerned Scientists (UCS) in its study found 15 cases of safety equipment problems and security shortcomings at 13 nuclear plants last year, calling that number "high."

Titled "The NRC and Nuclear Power Plant Safety in 2011: Living on Borrowed Time," the UCS report said no plant employees or members of the public were harmed in the incidents.

But the lapses nevertheless were deemed serious enough to warrant special inspections by the US Nuclear Regulatory Commission (NRC), which is tasked with oversight of the industry and which itself had a mixed record in responding to the problems.

The science group said that in some instances the NRC did an outstanding job of addressing safety problems before they could lead to a potentially dangerous situation.

But there were also times when the federal agency did a less-than-adequate job of cracking down on nuclear plant owners, who in some cases have flouted agency regulations for decades.

"Last year's record shows that the NRC is quite capable of being an effective watchdog that protects the public and saves the nuclear industry from its worst tendencies," said Dave Lochbaum, lead author of the report and the director of UCS's Nuclear Safety Project.

"But the agency too often does not live up to its potential, and we are still finding significant problems at nuclear plants that could too easily trigger a serious accident."

The group said that lax NRC oversight has allowed some problems to fester for decades, and found that 47 nuclear reactors -- nearly half of the 104 nuclear plants operating in the United States today -- still do not comply with fire regulations established by the NRC in 1980 and amended in 2004.

It also said that there are 27 reactors with inadequate protection against earthquakes.

"The fact that US plant owners could have avoided nearly all the near-misses in 2011 if they had addressed known problems in a timely manner suggests that they and the NRC have not learned the lessons of these accidents," said Lochbaum, a nuclear engineer with 17 years of experience working at nuclear plants.

"Someday their luck may run out," he said.

The vulnerability of nuclear reactors to earthquakes was underscored after problems following a magnitude 5.8 quake that rattled the US East Coast last August, damaging two reactors at the North Anna plant in Virginia, some 12 miles (20 kilometers) from the epicenter of the temblor.