The separation of a concrete wall at Progress Energy Inc.'s Crystal River nuclear powerplant in northwest Florida has “fundamentally changed the way the [nuclear power] industry analyzes post-tensioned, pre-stressed concrete structures,” according to the utility’s Oct. 10 filing with the Florida Public Service Commission. The PSC is probing the incident and must approve the North Carolina-based utility's request to have ratepayers cover repair costs.
Raleigh-based Progress Energy is seeking to recover expenses related to the delamination of a wall at Crystal River’s nuclear unit No. 3, a problem first identified in October 2009 and again in March 2010 as the company was replacing steam generators in the unit. The utility says the cost to repair the 42-in.-thick containment structure would range from $900 million to $1.3 billion. Fuel to replace the power that the nuclear plant would have generated will add an additional $1 billion.
In its filing, Progress Energy says that, because there was no way to predict the delamination of the wall based on standard engineering practices and analyses, its actions were reasonable and prudent.
“Nothing the company could have done, based on what management knew or should have known at the time, would have prevented the delamination and subsequent extended outage,” says the filing. “As [the utility’s] subsequent analysis showed, the delamination could not have been predicted using industry standard models and experience at the time, and was the product of unique circumstances, which neither [the firm] nor its contractors could have foreseen prior to the event.”
Progress Energy says, “Failure of the industry standard engineering modeling analyses and calculations to predict the [unit's] wall delamination was the programmatic root cause of the [failure.]”
But other parties who have filed to intervene in the PSC review and a recent story in The Times of St. Petersburg, Fla., claim that the utility’s hubris in trying to manage the complicated project itself led to the problems. Progress Energy has said its decision to self-manage the job would save about $15 million.
Progress Energy said it could not comment on the ongoing case, nor would Chicago engineering firm Sargent & Lundy, which was hired to conduct an engineering analysis of the proposed work. Independent concrete experts contacted by ENR also declined to comment.
Greg Selby, director of non-destructive evaluation at the Electric Power Research Institute, Washington, D.C., says his team was contacted after the delamination to review the method of analyzing the damage. He says that while the problem was definitely a surprise to Progress Energy, concrete degradation in powerplants is a growing concern in the nuclear power industry as facilities age. Crystal River unit No. 3 became operational in 1977.
Before the work on Crystal River unit No. 3, temporary construction openings had been made in 11 similar nuclear containment buildings at other U.S. facilities to replace steam generators. Progress Energy said it spent about five years examining options to replace its aging steam generators before deciding to cut a hole in the containment wall.
Crystal River unit No. 3 is one of 32 pressurized water reactors in the U.S. that used post-tensioned, pre-stressed nuclear containment facilities. There are 144 vertical tendons and 282 horizontal tendons around the unit. The tendons are 7 millimeters in diameter, according to a root-cause analysis performed by Performance Improvement International (PII), Oceanside, Calif.
Progress Energy and its contractors began removing a 27-ft by 25-ft rectangle in the containment wall in September 2009. Twenty-seven tendons were de-tensioned as workers began removing the portion of the wall. A day after the last tendon was de-tensioned, workers identified a delamination measuring about 10 in. from the exterior of the containment wall.
After the original delamination was repaired, this past March engineers identified a second one in another area of the structure, about 9 in. from the exterior of the containment. Progress Energy has since hired outside engineering experts to analyze possible repair options. The utility also decided to bring in contractors to remove and replace much of the concrete in the containment wall, excluding the section that was replaced during the initial 2009 repair. Their identities were not disclosed.
The root-cause analysis conducted by PII highlighted seven factors that caused the delamination: tendon stresses, radial stresses, design for stress concentration factors, concrete strength properties, aggregates, de-tensioning sequence and scope, and removal of concrete.
“In conclusion, PII has determined that the immediate cause of the delamination event was the redistribution of stresses as a result of the containment opening activities resulting in additional stress beyond original containment design,” the firm’s analysis says. “The condition exceeded the fracture capacity of the concrete and resulted in cracking along the high-stress plane connecting the horizontal tendons. As the cracks propagated and joined, delamination occurred over a wide area."