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The cooling towers of the TVA Watts Bar Nuclear Plant in Spring City, Tenn.

A problem that surfaced 18 months ago at Watts Bar Nuclear Plant's still-under-construction Unit 2 reactor has resulted in the safety reviews for 500 packages of TVA-purchased parts.

The discovery that not all of the parts -- everything from bolts and fan belts to cables and electrical breakers -- had been tested adequately to assure they would meet nuclear plant safety and quality standards was first made by the Nuclear Regulatory Commission during a September 2011 Watts Bar inspection.

But just over a week ago, TVA filed an event report with the NRC stating that more than 500 packages of parts -- some already installed -- must be evaluated.

"Watts Bar Nuclear Unit 2 has determined that certain equipment components have been installed that may not meet the requirements of the commercial grade dedication process. This condition has the potential to create a substantial safety hazard had it remained uncorrected. Evaluations are being performed," TVA said in the event report.

TVA officials said no hazards have been found so far, but they acknowledge that their new parts reviews must be spread to all of the utility's six operating reactors.

That could involve "thousands more" parts packages, according to TVA spokesman Ray Golden.

"We've determined the cause of why this happened to us. And it's essentially a failure on our part to incorporate an industry update," Golden said Monday.

Work to evaluate the parts, which includes TVA and contract lab testing, has been ongoing at Watts Bar for more than a year, according to Ric Wiggall, head of nuclear engineering for Watts Bar Unit 2 construction.

"All the materials testing done to date has given us acceptable results," Wiggall said. "We've tested about 30 percent of the [Watts Bar] packages."

He said the additional work will not delay completion of the new reactor or drive up expenses.

The reactor is expected to be complete by December 2015 and cost about $4.5 billion. The reactor originally was expected to be complete in October 2012 at a cost of just under $2.5 billion.

What happened?

When work began on Watts Bar's Unit 2 in 2007, it marked the construction start of the only new reactor added to America's nuclear fleet in the past 25 years.

But the safety of reactors begins before the building, and the components for reactors have to be nuclear worthy, according to NRC spokesman Joey Ledford.

"Worthy" components have "gone through an 18-step process for quality assurance before they are certified as nuclear grade," he said.

But finding certified parts has gotten tougher.

"I think what happened is that over the years, it has gotten more difficult for some of the [nuclear plant operators] to get components, so they've gone out on the market to get them themselves," Ledford said. "What we don't know yet is how complete [TVA's] work had been [to assure the purchases were nuclear worthy], Ledford said.

Joe Williams, TVA's general manager for nuclear power engineering, said the problem for TVA was one of rules interpretation.

"We performed an investigation and concluded that the TVA experts who were in charge of the process some years back misinterpreted their procedure as fully meeting the requirements when it did not," Williams said.

He said he knows of no other utilities' nuclear plants in the country with such concerns.

At the time the NRC initially noticed the problem in 2011, inspectors gave TVA what they called a "nonstated violation" -- essentially a warning and a suggestion that TVA recheck its parts purchases.

Wiggall said the NRC came back this past December to look at TVA's progress on the matter and raised more questions that caused utility officials "to re-evaluate."

"That was when we decided we would conservatively report it [in the Jan. 3 event report]," Wiggall said. "We do not at this point believe there will be any problems. ... What we expect is that when we complete the work and there are no problems, we'll retract it."

Williams said key difference in what TVA had done versus what it now is doing "is an additional level of rigor in verification tests."

Wiggall and Golden said much of the problem is just missing paperwork, and that's what TVA's new tougher testing will provide.

"There are some parts that when you buy them they don't come with the paperwork that would come with them from a nuclear component supplier. So it's incumbent upon TVA in that case to fill that paperwork, do any testing and provide ourselves the assurance that that part would perform its intended function," Wiggall said.

"And that's common in the industry," Wiggall said.

Ledford said it is too soon to say if the NRC will take further action.

"Obviously this is going to take a lot of work on the part of TVA and the NRC to sort out," Ledford said. Our concern is that many of these noncommercial-grade components could be installed in safety-related systems."