Correction appended.

The Nuclear Regulatory Commission flagged crucial safety problems at the nation's reactors last year but also tolerated known issues that could have led to disastrous accidents, the Union of Concerned Scientists said today.

The Massachusetts-based environmental group's second annual report gauges the NRC's performance in 2011 and how the agency responded to problems including leaky roofs, equipment failure, floods and earthquakes. The commission reported 15 "near misses."

After analyzing those reviews, UCS commended the commission for finding glaring safety problems. But the group said nuclear power plants continue to experience problems with safety-related equipment and worker errors that increase the risk of damage to reactor cores.

The NRC too often recognized problems but misdiagnosed them or left them unresolved, which could have led to significant events or increased their severity, the group said.

On-site NRC inspectors focused on singular issues, such as a broken device or an erroneous test result, instead of evaluating why an owner failed to fix a problem before NRC inspectors found it.

"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, the author of the report and director of the group'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."

UCS said that while none of the problems caused harm to plant employees or the public, their frequency of more than one per month is high for a mature industry.

The group said that the NRC needs to step up its fire and seismic protections, and that stronger oversight could have prevented the "near misses."

"The fact that U.S. 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," Lochbaum said. "Someday, their luck may run out."

Click here to read the report.

Correction: The report issued today analyzed the NRC's actions for 2011. An earlier version of the story referenced 2010.