In October 2005 Bechtel Power replaced the aging steam generators in an over-1,000-megawatt unit of a western state nuclear generating station. Unit shutdown commenced October 8 and was completed December 23. Here’s an account of how 3D laser scanning was used to improve project execution at this large nuclear power site.
Steam generators take the heat from the nuclear power plant reactor core and produce steam which in turn drives the plant turbines. Hot, radioactive water is pumped under high pressure through 12,580 alloy tubes which heat non-radioactive water to create steam. The steam generators, which weigh approximately 800 tons apiece, are located behind a 4-foot-thick concrete shield – the D-Ring wall.
Replacing the SGs required cutting a notch approximately 25ft by 25ft in the D-Ring wall to move the old unit out and move the new unit in. The D-Ring wall concrete is reinforced with hardened carbon steel rebar of up to 2-1/4 inches in diameter. Figure 2 shows the D-Ring wall after being notched. After the SG was replaced, the wall was rebuilt using thick steel plate to form the concrete.
3D laser scanning – optical templating
Here’s where 3D laser scanning comes in. Bechtel had to drill 72 holes in the steel plates to locate the tie-rods which connect both sides of the D-Ring wall and to hang commodities (pipes, valves, instruments, etc.). The construction challenges were to locate the holes to avoid interferences with the rebar, and to locate the commodities. According to Casey Corcoran, the project manager at the plant’s owner, drilling through the rebar is not an option, as this could compromise wall integrity.
Bechtel commissioned Optira to laser scan the notched wall, locate the rebar tie-ins which had been cut flush, and produce a 3D MicroStation model of the wall. Optira COO Greg Conklin reports that his firm scanned both sides of the wall (5-10 scans) using a Leica HDS4500 scanner after establishing control by surveying targets, know monuments and wall markings with a Trimble 5603 total station. Epic Scan‘s Carlos Velazquez provided survey control and follow-on modeling support. Two weeks later, after the wall had been notched, Optira returned to the site and scanned the notch, executing 3 high-density scans. The 3D model was provided 6 days later, and drilling of the plates commenced soon after that. Figure 5 reveals the dense packing of the rebar. The process worked – none of the 72 holes was obstructed by rebar.
Using permanent steel plates to form the concrete for the replacement wall required the Bechtel team to rethink how to tie the forms together. Bechtel civil field engineer Chuck Rice reports that experience led him to expect challenges in areas where the rebar meets at odd angles and the rebar is densely arranged. Rice says, “I don’t know another way that we could have done this and had the success that we had.” He says that without the model data based on laser scanning, the team would have been doing very well to avoid rebar obstructions in half the holes. According to Rice, the laser scanning-based work process allowed the team to drill both form walls at the same time when the steel was horizontal on cribbing in the yard, well away from the D-Ring wall. Not only did this let the drilling operation be performed more easily; it also removed the drilling operation from the critical path.
Initial resistance turned to enthusiastic support
Corcoran says the decision to deploy 3D laser scanning on this project met some initial resistance. Although cost was an issue in the beginning, the team was won over by the prospect of getting very complete 3D data as well as the opportunity to refine safe work processes. Corcoran also sees the opportunity to perform drop analysis for determining load paths and to sequence the removal of large and heavy equipment items.
This work process now has some new champions- Bechtel’s term for this process is optical templating. Rice says this process has now become standard for the projects he’s involved with.
Alignment with Department of Energy initiatives for new construction
In September 2004 the U.S. Department of Energy published a report it commissioned from MPR to evaluate advanced construction technologies that could potentially decrease the construction time of new domestic nuclear plants planned for deployment in the 2010 timeframe. The report identified thirteen technologies developed since completion of the last domestic power plant more than 10 years ago. Nine of these technologies were reported to be sufficiently mature to be used without additional research and development. Two of these mature technologies – the use of steel-plate reinforced concrete structures, and the use of positioning applications in construction (GPS and laser scanning) – were deployed on the SG replacement for this unit.
Preparation is key
Conklin says that scanning inside a nuclear facility involves significant preparation of personnel and equipment. Before access to the containment zone of a nuclear facility is granted, four days of safety training are required. Background checks and drug testing are also required. Access credentials have to be refreshed on a monthly basis.
Though safety is the highest-order priority in a nuclear power facility, time is also precious. A day of production has a value of order of $1 million or more. This high premium for lost time means that job planning is critical. Conklin says his firm provides complete equipment redundancy either onsite or within ready access.
Rates for nuclear work tend to be higher than for routine laser scanning jobs. One reason is that insurance for equipment damage is unavailable. Another is scheduling uncertainty. Conklin says tentative schedule dates can change, but firms that want this work must be prepared to respond quickly.
The SGs for another unit at this facility are scheduled to be replaced in 2007. Optira has already scanned the D-Ring wall for this unit in preparation for this work. Optira has also scanned the valve relocation project which has been completed in the first unit and scheduled to be done in two additional units. Optira has also scanned the equipment hatch in preparation for a reactor head replacement project.