Repairing a cracked containment wall will force Progress Energy to keep the Crystal River nuclear plant offline longer than anticipated.
Progress Energy shut down the plant on Sept. 26 for a major maintenance project that was expected to last only into December.
But on Friday the utility told the Nuclear Regulatory Commission that it now plans a major repair: removing and replacing part of the containment wall, which has developed a gap below the surface.
"It is clear that the repairs will require us to extend our outage," Progress Energy spokeswoman Jessica Lambert said.
But no one can say yet how long those repairs will take, how much they will cost or whether ratepayers could see their bills rise as a result.
While the plant is shut down, Progress Energy expects to draw on its reserve generating capacity to meet customers' needs without interruption, Lambert said. The utility has other plants, some fired by natural gas, that it can bring online at times of peak demand.
News of the extended closure at Crystal River came after Progress Energy executives spent three hours briefing Nuclear Regulatory Commission officials on the defect, which workers discovered about six weeks ago.
The gap is known as a delamination, which means part of the wall has separated into two layers. The gap between those two layers is about 9 inches below the outer surface of the wall, which is 42 inches thick.
The void was discovered as workers cut a 25- by 27-foot hole in the containment wall so two huge steam generators could be removed and replaced. The utility cut the hole because the reactor building's regular equipment hatch is too small for the generators, which weigh 550 tons each.
The plant was not operating at the time, and there was no release of radiation.
Tests have since determined that the gap inside the wall is up to 2 inches wide. It is shaped like an hourglass and extends up to about 30 feet from where the hole was cut for the generators.
Tests have not found similar gaps in any other part of the containment building.
A high-ranking NRC official said the gap was worrisome because other nuclear plants around the country have cut into their containment walls 26 times before, but no one has ever seen a separation like this.
"It raises our interest and our concerns," said Eric Leeds, director of the NRC's Office of Nuclear Reactor Regulation.
The NRC pays close attention any time there's a potential weakness in any of the barriers designed to prevent a radiation leak, Leeds said.
The containment building wall is engineered to hold in a sudden buildup of heat, pressure or steam inside the reactor building. Just inside the wall is one of those barriers, a steel-plate liner three-eighths of an inch thick.
Farther in is the protective barrier consisting of the reactor vessel and coolant piping. The innermost barrier consists of the cladding, the material enclosing the fuel.
Progress Energy recognizes the seriousness of the matter and has brought in experts from around the country to help with the investigation, said James Scarola, the utility's senior vice president and chief nuclear officer.
The company's focus, Scarola told the NRC, is on bringing the highest safety standards to the investigation of the gap and the repair of the plant. It will not put the Crystal River plant back in service until those safety standards are met, he said.
So far, Progress Energy has rejected the idea of attempting repairs by using metal anchors, cement-like grout or epoxy resin to secure the separated layer of concrete in place.
That means the delaminated part of the containment wall will have to be removed, Progress Energy representatives told the NRC. The interior steel reinforcements must be replaced and perhaps enhanced. And a replacement section must be poured.
But much of the engineering of those repairs must wait on an investigation to pin down the cause of the separation.
A specialist hired by Progress Energy said the investigation of how the gap formed started with a list of 74 potential causes. The list has since been narrowed to 27.
"We haven't reached any conclusion yet," said Chong Chiu of Performance Improvement International, a California-based company that specializes in helping companies determine the root causes of problems in nuclear power plants.
One possibility being considered is whether the loads on different parts of the wall shifted as steel tendons inside the wall were de-tensioned to allow the hole to be cut. Tests have shown that the gap stopped when it reached areas where there was extra reinforcement in the wall.
But it takes a painstaking process either to rule out a possible cause or to determine that it might have played a role.
"The process is very comprehensive, very tedious," Chiu said. "Concrete is a very complex material. In order to be right on the money, we want to make sure that all the data is right on the money."
Richard Danielson can be reached at Danielson@sptimes.com or (813) 226-3403.