CORDOVA, Ill. — Critics questioning whether it’s safe to boost energy output at aging nuclear plants are pointing to an incident four years ago at Exelon’s Cordova plant in northwestern Illinois, according to a published report.

Vibration in the steam system at the plant caused large cracks in heavy metal plates and threw pieces of steel into a steam pipe and the bottom of a reactor, the Chicago Tribune reported on Sunday.

Some experts see the Cordova experience as a warning about what could happen at other aging plants as the nuclear industry gets approval to run plants harder, longer and hotter.

Others, however, say the Nuclear Regulatory Commission is doing a good job reviewing and granting industry requests to increase output through so-called “uprates.” Since 1998, almost 20 percent of the nation’s reactors have received or are waiting for NRC approval of extended power uprates to improve their equipment and make more electricity.

There has been little public debate on the issue, but federal regulators and nuclear experts say safety considerations are thoroughly reviewed before applications are approved.

Chicago-based Exelon will spend at least $160 million to repair vibration issues at Cordova and milder cracking at the Dresden, Ill., plant 60 miles southwest of Chicago.

In 2002, Exelon increased output by nearly 18 percent at the Cordova plant. Components shook vibration monitors from their mounts and insulation from steam lines.

“The plant literally began shaking itself apart at the higher power level,” said nuclear energy safety expert David Lochbaum of the Union of Concerned Scientists. He said if metal fragments had lodged in a safety valve radioactive steam could have escaped.

Regulators concluded the incident was unlikely to cause an accident, the newspaper reported. Exelon said it believes it solved the problem this spring.

Craig Nesbit, a spokesman for Exelon’s nuclear operations, told The Associated Press Sunday that vibration problems at the Cordova plant did occur and needed to be resolved. He said safety concerns of industry critics spark a useful discussion that ultimately will increase the public’s comfort with nuclear power.

“Federal inspectors are on site 24 hours a day and these plants would not be allowed to operate if they’re not safe,” he said.

Without building new nuclear plants, power companies are squeezing more from aging reactors as they take advantage of deregulation to profit from increased production, industry analysts said.

“The incentives are in place to push people and machinery harder,” said Mark Sadeghian, a Morningstar utility analyst. “Everyone is doing it.”

Exelon runs 11 nuclear plants, which include six in Illinois. The company is the largest nuclear plant operator in the nation.

An Exelon executive said the industry wants to protect its assets and revenues, which he said is a big incentive for avoiding accidents.

“It is not to our benefit to run any of these billion-dollar assets into the dirt,” said Christopher Crane, president of Exelon’s nuclear division. “We are confident of our safety margins.”

Also on Sunday, Gov. Rod Blagojevich signed legislation requiring plants to report to the state releases of radioactive contaminants into soil, surface water and ground water.

The law was passed in response to tritium leaks at Exelon plants in Braidwood, Dresden and Byron, Ill.

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