Repair & maintenance: concrete

Cracks in Crystal

20 September 2011

Progress Energy has a $1 billion plan to restore the Crystal River 3 containment to its original design basis. By Will Dalrymple

To tackle a second episode of containment concrete cracking in as many years, Progress Energy has decided to completely remove and replace concrete and steel reinforcing bars in most of the walls of the containment building of the Crystal River 3 station in Florida that it operates. It anticipates the repairs alone will cost between $900 million-$1.3 billion.

Crystal River 3 shut down for a steam generator replacement outage in September 2009. At the time, contractors cut a 25-foot by 27-foot square hole in the side of the 185 foot-high, 137-foot diameter cylindrical containment building, above the equipment hatch (bay 3-4). The entire containment building that houses the 890 MWe B&W PWR is made up of six recessed bays, crisscrossed with 94 sets (of three) tendons horizontally and 144 tendons vertically. The tendon hoops are tensioned and detensioned from concrete buttresses that stand in between, and about two feet proud of the bays.

To cut the hole, contractors used water-blasting to remove the concrete in the 42-inch thick wall. The 5-inch thick horizontal and vertical tendons were removed by pulling them out of their sleeves. A grid of rebar buried 10 inches in the concrete from the exterior, tendon guide tubes, and the 3/8-inch thick containment plate, were cut with arc flame cutting tools. It was during this work that staff found the thick crack that ran up between the guide tubes.

Infrared and core bore surveys into the containment wall determined that the concrete separation, or delamination, was in the shape of a fat figure eight over nearly the entire bay centred around the steam generator replacement hole. Other bays and the dome were found to be unaffected. The engineering investigation involved contractors Performance Improvement International, MPR Associates, Areva, Worley Parsons, Wiss Janney Elstner Associates and Construction Technology Laboratories, as well as Progress Energy’s own staff and industry peers. The investigation evaluated some 75 failure modes and generated a complex 3D computer model of the containment. It determined that the number and order of de-tensioned tendons resulted in redistribution of stresses in the containment wall that exceeded tensile capacity, initiating the delamination. Removal of concrete increased the stress in the remaining concrete, contributing to the final extent of delamination.

After evaluating several alternatives, Progress Energy eventually chose to remove the concrete and replace it, in five phases: crack arrest, de-tensioning, concrete removal, concrete placement, re-tensioning and testing. The project took place from June to November 2010.

But, as the containment was being retensioned in March 2011, acoustic monitors and strain gauges indicated a second delamination in the adjacent wall, bay 5-6. Although the repaired section was not affected by the crack, the crack appears to have been initiated by the same root cause. According to the Nuclear Regulatory Commission, the delamination affected a large area of the bay.

After this, Progress Energy began work to determine whether to repair or retire Crystal River 3 on technical and economic grounds. It commissioned an internal analysis and a study from Bechtel. These were both reviewed by three independent panels (licensing, technical and constructability), which had been set up to review the documents and suggest the best repair option out of four. All three committees chose the same solution: to repeat the previous repair in the remaining five bays. SGT, the contractor who performed the first delamination repair, reviewed the solution from a high-level construction point of view. In a conference call with analysts in late June, Jeff Lyash, Progress Energy executive vice president, energy supply, said that this particular strategy had the benefits of being the least challenging technically, having the highest probability of success, drawing on standard and well-proven construction materials and processes, and having the highest chance for licensing approval.

Then Progress Energy compared this potential solution against a case of decommissioning (based on a report prepared by consultants TLG Services) and replacement with building a new combined-cycle gas turbine power plant. In the analyst call, Vincent Dolan, Progress Energy president and CEO, said that the repair decision would offer customers ‘billions of dollars in benefits’. He did not elaborate on how that calculation was made.

As of 31 May, Progress Energy had spent $214 million on repairs. It filed a status update with the Florida Public Service Commission on 27 June. Progress Energy is now preparing the plant for long-term lay-up. Detailed engineering design has begun, and preparations are beginning to put the project out to tender. No significant construction work is expected on site until 2012; the work is expected to be completed in 2014.

Progress Energy owns a majority of the station, with many municipal authorities each owning a small stake. US utility Duke Energy announced plans to acquire Progress Energy in January 2011. The plant’s original 40-year operating licence expires in 2016. In 2008, Progress Energy filed for a 20-year licence extension. “Essentially we are through that process, with the exception of containment,” a Progress Energy official said in the June conference call, adding that once the detailed design for the repair plan is complete, then it will proceed with the containment part of the application.

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