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Federal regulators knew when they renewed the Vermont Yankee nuclear plant’s license last month that electrical cables serving key plant safety systems remained submerged in water for extended periods of time, Nuclear Regulatory Commission (NRC) documents show.

Cables underwater has a whole new meaning now following the nuclear disaster in Japan, where tsunami flooding knocked cooling systems out of service, causing reactors to overheat at the Fukushima Dai-ichi nuclear station.

An NRC report in December said 23 reactors around the country had electrical cable failures between 1988 and 2004, with nine more instances since 2007 of cables improperly submerged in water.

“Because these cables are not designed or qualified for submerged or moist environments, the possibility that more than one cable could fail has increased,” the report said. “This failure could disable safety-related accident mitigation systems.”

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The agency’s documents show it has been concerned about submerged electrical cables at U.S. nuclear plants for years. The December report said it would not require any changes by the industry.

Raymond Shadis, a technical adviser for the New England Coalition watchdog group, said allowing cables not designed for underwater use to remain underwater violated a key NRC rule issued in the early 1970s, around the time Vermont Yankee went into operation.

NRC spokesman Neil Sheehan said the submerged cable issue had come up in several license renewal reviews at nuclear plants around the country. Vermont Yankee’s owner, New Orleans-based Entergy Corp., agreed to inspect manholes for water accumulation at least once a year as a condition of its license renewal, he said.

The coalition said it would file an enforcement petition this week, asking the NRC to follow its own rules on the submerged cables.

“If (an enforcement) petition is filed regarding the issue of submerged electrical cables at Vermont Yankee, we will review it using our clearly defined process for doing so,” Sheehan said.

Vermont Yankee spokesman Larry Smith said Entergy would have no comment until officials see the coalition’s petition.

The coalition cited NRC papers that listed nine plants with cables improperly submerged in water since 2007. Those were at the Monticello nuclear plant in Minnesota; the Fermi plant in Michigan; the Point Beach plant in Wisconsin; the Beaver Valley, Three Mile Island and Peach Bottom plants in Pennsylvania; the Wolf Creek station in Kansas; the Callaway plant in Missouri; and Vermont Yankee.

Shadis said the NRC rule on underwater use was in a report on the Fukushima disaster, saying nuclear plant components “important to safety (must) be designed to withstand the effects of natural phenomena such as earthquakes, tornadoes, hurricanes, floods, tsunami … without loss of capability to perform their safety functions.”

The agency’s December report said it had found during a May Vermont Yankee inspection that Entergy had “allowed the continuous submergence of safety-related cables that were not designed or qualified for continuous submergence and failed to demonstrate that the cables would remain operable.”

It said the problem was of “very low safety significance” because the cables at Vermont Yankee had not actually failed to operate. But it also said “an increased potential exists” for a failure of “accident-mitigating system cables if they are subjected to the same environment and degradation mechanism for which they are not designed.”

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