The Union of Concerned Scientists chronicled 14 "significant events" at US nuclear plants, and said many "occurred because reactor owners, and often the NRC (Nuclear Regulatory Commission), tolerated known safety problems."

These "near-misses ... exposed a variety of shortcomings, such as inadequate training, faulty maintenance, poor design, and failure to investigate problems thoroughly," said the study by UCS nuclear engineer David Lochbaum.

The report was released amid heightened concerns over the safety of nuclear reactors caused by the unfolding disaster at Japan's Fukushima plant.

The study examined 14 events that were reported by the NRC itself, only about five percent of its yearly activities, and so is far from a comprehensive survey of all the safety problems that occurred in the United States last year.

The fact that more than one incident per month was reported "is high for a mature industry," said the report.

One example occurred at New York's Indian Point nuclear plant when the "NRC discovered that the liner of a refueling cavity at Unit 2 has been leaking since at least 1993," it said.

"By allowing this reactor to continue operating with equipment that cannot perform its only safety function, the NRC is putting people living around Indian Point at elevated and undue risk," it said.

At plants in South Carolina and Arkansas "security problems prompted the NRC to conduct a special inspection. Details of the problems, their causes, and their fixes are not publicly available," the study said.

Despite the incidents, the study said that, overall, the likelihood of a severe nuclear disaster at US plants is "low".

But it noted that when problems are not swiftly fixed or properly investigated, the risk of a larger incident rises.

"Recognized but misdiagnosed or unresolved safety problems often cause significant events at nuclear power plants, or increase their severity," it said.