July 15, 2007

Focus on O&M (July 2007)

NUCLEAR PLANT OVERHAUL

Hydro-demolition speeds reactor dome entry

Replacements of the steam generators (SGs) of nuclear power plants have become commonplace since the first one was completed 25 years ago. According to the Nuclear Energy Institute, a total of 132 SG replacements have been performed at 32 plant sites, including 11 in 2006. One of those projects—to replace four generators at Tennessee Valley Authority's (TVA's) Watts Bar Nuclear Plant in Tennessee—used a novel technique to make a penetration in the containment dome for moving the SGs in and out. The decision shaved several days from an outage and thus kept TVA from losing millions of dollars of revenue.

 

Most SG replacement projects place the replacement SG into the containment dome through a penetration in the containment building wall. A typical penetration might be made by cutting a 30-square-foot opening through perhaps 3.5 feet of reinforced concrete and then through a steel liner that is typically ¾-inch thick. This may be typical practice, but it's not necessarily a cost- or time-efficient way to make a penetration.

To shorten the time needed to replace its four SGs, managers of the Watts Bar plant decided to try a unique but unproven hydro-demolition process to penetrate the containment. The project's general contractor, Bechtel Power Corp., hired North American Services Group (NASG) and gave the company one year to complete the design, engineering, and testing of a system comprising a hydro-demolition robot and a water containment and treatment facility.

According to Joe Fogarty, senior project manager for NASG, one of the first tasks his team tackled was creating a mockup of the target containment so that a superstructure could be designed to sit atop it. The unusual design of the Watts Bar reactor precluded penetrating its side.

Once the custom hydro-demolition system was ready, NASG used its 20,000-psi water jet to cut two 1,500-ft² openings in the containment dome. The holes were made just large enough to allow Bechtel to remove and replace four 450-ton steam generators using one of the world's largest cranes.

The demolition process began with the installation of a debris barrier system inside the 2-foot-thick concrete dome roof. Next came the installation of a special fixture (Figure 1) designed to keep the water jet perpendicular to the dome roof at all times, regardless of location. The water jet then was used to cut a 30- to 36-inch-wide path about 8 inches deep around the 45 x 22-foot circumference of each concrete section to be removed. The depth of the cut was chosen to leave the steel reinforcing bar (rebar) structure intact.

 

 

NASG also designed and built two other custom fixtures. One attached to the concrete slabs to facilitate their removal. The other held a 200,000-lb "plug" in place while rebar was cut loose from the dome roof (Figure 2). The hydro-demolition phase of the project was completed in five days, three days ahead of schedule.

2. What lies beneath. An 8-inch-deep cut through the concrete reveals the rebar. Courtesy: North American Services Group

 

The concrete slabs removed from the top of the containment building weighed over 100 tons apiece. They would challenge any crane, especially at the Watts Bar project because of the height and boom reach required for this project (Figure 3).

3. Pull the plug. The 100-ton concrete plug was lifted by a crane after the rebar was cut away. Courtesy: North American Services Group

 

Not taking any chances, TVA reserved one of the world's largest mobile cranes from Mammoet (www.mammoet.com), perhaps the world leader in turnkey heavy-lift crane rentals. The crane made its way by ship and truck from the Netherlands to Tennessee as more than 100 tractor-trailer loads of individual parts that were then assembled on-site. According to Fogarty, it more than earned its keep, doing double duty on SGs as well as concrete slabs and plugs (Figure 4).

4. World's largest. A containerized crane was shipped from the Netherlands to handle the very heavy lifting. Courtesy: North American Services Group

 

In addition to the concrete plugs, approximately 480 cubic feet of concrete rubble were removed. By the time the job was done, the water jet had used about 900,000 gallons of water from the plant fire protection system.

The water-concrete slurry that the hydro-demolition left behind was removed by a high-suction vacuum system sited at the base of the building. The rubble was screened for radiation and periodically transported off-site for disposal in a local landfill. The project's wastewater was also monitored, treated, and drained per the plant's NPDES (National Pollutant Discharge Elimination System) permit requirements. The steel liners originally cut from the containment vessel after the concrete sections were removed were rewelded back in place after the new SGs were installed.

Contributed by North American Services Group (www.naisinc.com).

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