How risky is the new era of nuclear power?

— -- Nearly two years ago, the Nuclear Regulatory Commission gave the operator of the Indian Point nuclear plant a year to add backup power supplies to the plant's emergency warning sirens. Entergy paid a $130,000 government fine in April — but still hasn't done the work at the plant 24 miles north of New York City.

At the Peach Bottom nuclear plant south of Harrisburg, Pa., security guards often took 15-minute "power naps," according to a letter from a former security manager to the NRC last March. The NRC began investigating after CBS News aired video of the dozing guards in early September.

Neither of the incidents amounted to an "immediate" safety risk, the NRC says. But they — and hundreds of other seemingly minor episodes at nuclear power plants in recent years — are drawing increased scrutiny as the USA prepares to launch a new generation of nuclear reactors.

Power companies are beginning to file applications to build up to 32 nuclear plants over the next 20 years, the first since the 1979 accident at the Three Mile Island plant in Pennsylvania halted plans for new reactors and led to sweeping changes in safety regulations. It's partly a reflection of how, amid concerns about climate change, communities have become more open to nuclear power as a cleaner alternative to pollution-belching coal-fired plants.

Critics and advocates of nuclear power generally agree that improvements in equipment and employee training have helped to make nuclear plants safer since the partial meltdown of a reactor at Three Mile Island.

Watchdog groups, however, say that unless nuclear safety and security improve, the USA's expansion of its nuclear power industry — which now involves 104 reactors that supply about 20% of the nation's electricity — could pose risks to nearby communities.

"Serious safety problems" plague U.S. nuclear plants because the NRC isn't adequately enforcing its standards and has cut back on inspections, according to a report released Tuesday by the Union of Concerned Scientists (UCS), a nuclear safety watchdog group.

The report also says that even though security at nuclear plants was increased after the Sept. 11 terrorist attacks, reactors still aren't sufficiently protected against terrorist threats such as hijacked jets, and new reactors aren't being designed to be significantly safer than existing ones. Increasing the number of reactors without creating "unacceptably high safety and security risks" could be difficult, the report concludes.

There has been no meltdown of a reactor in the USA since the incident at Three Mile Island, which led to no deaths or identifiable injuries from radiation exposure but resulted in the release of some radiation from the plant.

However, since 1979, U.S. nuclear plants have had to shut down 46 times for a year or more, in most cases to fix equipment problems that accumulated over time and that regulators should have ordered repaired earlier, according to the UCS, which compiled the data from the NRC and other research. And the number of equipment failings that increase the risk of an accident is up since 2001, compared with the previous five-year period, NRC figures show.

The UCS says incidents such as occasional failures of pumps that cool the nuclear reactor core in an emergency eventually could prove disastrous if they coincide with other low-probability events, such as coolant leakages from the core.

"The track record on existing reactors leaves much to be desired, and until you fix that problem, it's going to carry over to new reactors," says David Lochbaum, director of UCS' nuclear safety project.