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Federal regulators are stepping up oversight at the Susquehanna-1 nuclear reactor in Pennsylvania, which was already under closer supervision than most reactors in the U.S., documents show.

On top of that, Reactor Unit No. 2 at the nuclear power plant will remain shut down for an additional four to six weeks after an inspection revealed defects in some turbine blades.

The reactor at the Susquehanna Steam Electric Station has been down since April 5 for its biannual inspection and the replacement of fuel rods and some other upgrades, said PPL spokesman Joe Scopelliti.

When staff found “indications of cracks” in some of the low-pressure turbine blades, officials extended the outage to replace those blades.

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“The turbine blade inspection is a normal part of our outage inspection, but we didn’t expect to find defects in the blades,” Scopelliti said.

Each of the two reactors at the plant heat water to the point where it converts into steam. The steam then carries over to turbine generators. Each generator has a shaft with a high-pressure turbine on one end attached to three low-pressure turbines, the last of which attaches to the generator shaft.

The blades of the turbines catch the steam and cause the shaft to spin at about 1,800 revolutions per minute, which enables the generator to create electricity.

Each turbine weighs about 170 tons and has hundreds of blades. Because defects were in a few blades, most or all will undergo inspection, Scopelliti said.

Every two years, the plant shuts down one of the reactors for about a month to replace about 40 percent of the fuel rods and perform other modifications and upgrades along with an inspection, he said.

Scopelliti said PPL is also upgrading the plant’s control system to a new digital system.

Meanwhile, in a letter, the U.S. Nuclear Regulatory Commission (NRC) notified plant operator PPL Susquehanna reactor number one moved from the “regulatory response column,” which is the second lowest oversight level, to one level above — the “degraded cornerstone column.”

The higher level of oversight took effect “beginning in the first quarter of 2011,” the agency said.

In the five-column action matrix of NRC’s reactor oversight process, the degraded cornerstone column, or Column 3, goes to plants whose “cornerstone objectives [are] met with minimal reduction in safety margin,” according the NRC.

Column 1 plants require the least amount of agency oversight, while plants in Column 4, “multiple/repetitive degraded cornerstone,” receive the most NRC attention short of a mandated shutdown under the “unacceptable performance” criteria of Column 5.

NRC said Susquehanna-1 has had “greater than three unplanned” shutdowns per 7,000 hours when the reactor is critical, a condition the agency said has “crossed the Green-to-White threshold.”

Under the NRC reactor oversight process, an inspection finding gets a color indicating its safety significance. Findings with very low safety significance are “green,” while “white” findings have low to moderate safety significance.

The four unplanned shutdowns happened in 2010 on April 22, May 14, July 16 and on January 25, 2011. A steam leak was the cause of the most recent shutdown.

Susquehanna-1 will receive an “increase in the NRC’s inspection and assessment oversight” as a result of its elevation into Column 3, NRC said in the letter. Specifically, the agency said it will perform a supplemental inspection when the plant operator notifies NRC it is ready.

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