Mystery Crack in the Crystal River’s Containment Building Solved

- Edited By Tom Lamar -

From the start, one question especially has concerned the federal Nuclear Regulatory Commission: Why Crystal River?

Why did a 42-inch-thick wall at that nuclear plant separate into two layers during a big maintenance project last fall? Other nuclear plants have done similar jobs 26 times around the country, but no one ever saw a crack like Crystal River's - a crack that has kept the plant off-line for 11 months and piled up nearly a quarter of a billion dollars in outage costs.

On Thursday, NRC officials said they think they know the answers to that question and others.

First, NRC inspectors said Progress Energy's analysis of the crack - known as a delamination - was thorough and supported the company's conclusions about when and how it formed. And they concluded that because the wall separated when the plant was shut down with no fuel in the reactor, there was no risk of a release of radiation.

"The delamination did not represent an increase in risk to the public," NRC senior reactor inspector Louis Lake said during a public meeting to review the agency's findings.

Moreover, they said the Crystal River plant appears to be unique.

"We looked hard to see, is there something about this delamination that would cause us concern for other plants?" said Mark Franke, an engineering branch chief for the NRC's regional office for the southeast United States. "We found that we did not have an immediate safety concern for other nuclear facilities."

So what happened?

At the Crystal River reactor building, the barrier that keeps any sudden build-up of heat, pressure or radiation from escaping is a steel liner three-eighths of an inch thick. It covers the inside of the building. It is supported by a thick concrete wall that is strengthened with hundreds of tightened vertical and horizontal steel tendons.

Last September, workers shut down the plant and began relaxing the tension in some of those tendons so they could cut a big hole in the reactor building wall to remove and replace two huge steam generators. Easing the tension caused unexpected stresses inside the concrete, and that formed the delamination.

While other nuclear plants have cut holes in their walls, only a small number had similar designs and used similar methods, officials said. And Crystal River is still unique in the way its concrete was made and the tendons were put in the wall.

"Crystal River has larger tendons (and) fewer of them," Franke said. "If you can imagine, that means that each tendon makes a bigger difference to the local stresses in the concrete when you either tension or detension it."

In another plant, with more tendons circling and reinforcing the liner like rubber bands, easing the tension in any one might not make as much of a difference.

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