Boilers, Reactors

Evaluating an Unexpected Crack in Containment

Issue 5 and Volume 115.

By Brian Wheeler, Associate Editor

In September 2009, Progress Energy was in the process of planning a scheduled refueling and maintenance outage at the 860 MW Crystal River Unit 3 nuclear power plant, a project that the utility completes every two years.

During the outage, Progress planned routine refuel and maintenance activities, modifications for an extended power uprate project and the replacement of the steam generators. In order to complete the steam generator replacement (SGR) project, Progress engineers had to create a 23-by-27-foot opening in the containment building some 40 feet above the equipment hatch. For this particular steam generator opening, the hole was created between buttresses 3 and 4.

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Steam Generator Replacement opening at Crystal River Unit 3. Photo courtesy Progress Energy.

To do this, the reactor was placed in cold shutdown. From September 26 to October 1, a total of 10 vertical tendons and 17 hoop tendons were detensioned. From there, concrete removal by hydro-demolition began as the first step to creating the opening. During the hydro-demolition, water was observed leaking from the exterior of the containment at various locations below the bottom of the construction opening. Also during hydro-demolition some of the concrete broke off into large pieces.

As work continued to remove the concrete from the building, the utility discovered a crack, also referred to as delamination, near the outer layer of the wall. Walls of the reactor building are about 42-inches thick, contain both horizontal and vertical tensioned steel tendons and are lined with steel plate. The crack was found about nine inches from the outer surface of the concrete.

After discovering the crack, the Nuclear Regulatory Commission (NRC) was notified and on-site team was assembled to look at four main components:

1. A root-cause analysis
2. A design-basic evaluation
3. A repair plan, and
4. An evaluation of the extent of the problem

“We had not seen this before,” said Roger Hannah, NRC Region II Sr. Public Affairs Officer. “And that is something that was verified when we first began inspections there.”

Crystal River Nuclear Generating Plant Unit 3 (CR3) is a Babcock & Wilcox pressurized water reactor that has been in operation for over 30 years, with the current license due to expire Dec. 3, 2016. An application is pending with the NRC requesting a license renewal to extend the plant’s operating life for another 20 years.

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A close up of the delamination at Unit 3. Photo courtesy Progress Energy.

The Crystal River Unit 3 containment wall was completed in 1973, followed by placing the dome concrete in 1974 and completing post tensioning in 1975. The plant went into service in 1977. The containment building is a right circular cylinder design that contains six buttresses with six bays that form the containment walls between the buttresses.

According to a 2010 report by Performance Improvement International, a consulting and training firm, this was not the first delamination project at Crystal River 3. In 1976 delamination was found in the containment dome of CR3. The delaminated concrete was removed and new concrete was placed, reinforced with meridional and hoop steel. New concrete with a nominal thickness of 12 inches was placed.

In October 2009, following discovery of the latest crack NRC sent a Special Investigation Team to Crystal River. Based on information developed through inspections and analyses from Progress, NRC concluded the crack happened after the unit was shut-down.

“There are no indications that it (the crack) was there while plant was at power previously,” said Hannah.

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Hydro-excavation work at Unit 3 completed. Photo courtesy Progress Energy.

Progress Energy’s inspection found the crack was caused during creation of the containment opening. Along with internal personnel, Progress consulted with industry peers such as Exelon and Southern Company and contacted external experts from Areva and WorleyParsons, in addition to establishing an investigation and design basis team. Progress also engaged Performance Improvement International (PII) to perform a comprehensive root cause assessment to identify the cause or causes of the delamination.

With investigation data and information after the delamination event, PII created a computer model that allowed them to start with information now known about how the CR3 containment structure behaved and work backwards in time to determine what factors contributed to the delamination event.

Using this process, PII identified seven factors that, working in conjunction, caused the delamination. These factors were:

1. Tendon stresses
2. Radial stresses
3. Design for stress concentration factors
4. Concrete strength properties
5. Aggregates
6. Detensioning sequence and scope, and
7. Removing concrete.

“The root-cause analysis concluded that a redistribution of stresses on the containment wall cause the delamination following the containment opening activities,” said Jessica Lambert, Progress Energy spokesperson. “These activities created additional stress beyond the original containment design.”

After almost a year of inspections, the Special Investigations Team in September 2010 completed their inspection and found the delamination did not pose an increase in risk to the public. But since containment is one of the fission product barriers for a nuclear plant, it was important that Progress make sure that the integrity of containment was the way it was designed prior to restarting the unit, said Hannah.

The repair approach taken by Progress Energy to remove and replace the delaminated condition included:

1. Additional detensioning of containment
2. Removing delaminated concrete
3. Installing reinforcement, including radial reinforcement through the delamination plane
4. Placing of new concrete
5. Retensioning containment, and
6. Performing post-repair confirmatory system pressure testing.

Repair Status

Progress Energy engineers, with assistance of other experts, continue work to repair the containment and exceed all design requirements for safe operation in the future, said Lambert. Progress has removed the delaminated concrete and placed new concrete within the affected section of the containment wall.

Crews had been working to re-tighten more than 200 tendons (155 horizontal and 64 vertical) that were previously loosened and reset an additional 80 vertical tendons.

However, in mid-March 2011, Progress Energy reported to the NRC indications of additional separation resulting from the work on the original containment wall. The final re-tensioning of tendons within the Crystal River Nuclear Plant reactor building was temporarily suspended while engineers investigated information from monitoring equipment at the repair site.

“We are doing a careful and systematic review of the new delamination and the options to return the plant to service,” said Vincent Dolan, president and chief executive officer of Progress Energy Florida in a press release issued after the March setback.

As of December 2010, Progress had spent about $150 million on the repair and $290 million on replacement power costs. Nuclear Electrical Insurance Limited covered $63 million of the repair and $117 million of the replacement power costs through an insurance policy with Progress.

Plans to bring the plant back online by April were delayed. The option to return the plant to service will be analyzed after the company completes a review and engineering analysis of the new delamination. Progress said that it cannot estimate a return to service date. But, upon completing repairs and necessary testing, the containment will be fully capable of meeting its design safety function, Progress told the NRC.

The NRC said that Progress has been communicating to other plants through the Institute of Nuclear Power Operations and other industry organizations, the lessons learned from this ongoing event.

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