How risky is the new era of nuclear power?

"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.

The NRC and the Nuclear Energy Institute (NEI), the industry's trade group, say just one incident since Three Mile Island — a water leak at the Davis-Besse plant in Ohio in 2002 — has come close to threatening communities near any plant.

The NRC says that in the episode involving the sleeping guards at Peach Bottom, it didn't act sooner because it couldn't substantiate the claims with Exelon exc, the plant's operator. At Indian Point, Entergy etr says its plan to install backup power for the sirens has been delayed by technical hurdles and the need to get permits from dozens of towns, counties and state offices.

A 'reliable fleet of reactors'

Nuclear reactors generate heat that produces electricity when uranium atoms split. In the reactor core, uranium is kept in water to prevent it from overheating, melting down and releasing radiation.

A meltdown by itself typically would not be disastrous because the reactor sits in a concrete containment structure to prevent radiation from escaping.

However, a meltdown could cause a buildup of temperature and pressure that ruptures the containment building. A massive release of radioactive gas into a surrounding community could destroy or damage human cells and cause death or cancer.

That's what happened at the Chernobyl nuclear plant in the former Soviet Union in 1986. The world's worst nuclear plant disaster involved a meltdown and an explosion that killed 56 people. At least an additional 4,000 are projected to die from cancer because of exposure to radiation.

In the accident at Three Mile Island seven years earlier, water cooling the core in one of the plant's two reactors leaked through a partly open valve. The valve was closed enough to prevent an alarm from sounding. Half the core melted, but the containment building stopped all but a small amount of radiation from seeping into the environment.

The incident led the U.S. government to require upgrades in piping, valves and other equipment at all nuclear plants, and NRC inspections were increased.

Today, "The U.S. operates not only the biggest but probably the safest and most reliable fleet of reactors," says NEI Senior Vice President Marvin Fertel.

UCS' Lochbaum counters that the 46 reactor shutdowns during the past three decades indicate there has been a buildup of multiple problems that regulators should have caught sooner.

In 1995, for example, Public Service Electric & Gas had to close its Salem plant in New Jersey for three years until 43 equipment problems were fixed, including a broken fan that kept safety gear from overheating.

A Government Accountability Office report said the NRC knew about 38 of the flaws — in two cases for more than six years — and that its "lack of more aggressive action" compounded the plant's problems.

Plants inspected less frequently

In the most serious episode involving a U.S. nuclear plant since Three Mile Island, the Davis-Besse plant in Ohio was shut down from 2002 to 2004 after the NRC failed to spot what it acknowledges were early signs of trouble.

An acid leak through the reactor vessel's lid left a quarter-inch-thick steel veneer, according to NRC reports. Because emergency pumps also were faulty, core-cooling water leaking through the ruptured lid could have led to a meltdown.

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