NUCLEAR MAINTENANCE
Safer, "virtual" reactor walkdowns
In an industry where safety comes first, Atomic Energy of Canada Ltd. (AECL) has found a way to minimize a major concern of nuclear plant shutdowns: exposing inspection personnel to radiation. With the help of a laser-based device called the FARO Laser Scanner LS (Figure 1), AECL reduces the risk of radiation exposure by creating a highly realistic 3-D image of the actual reactor and using it to guide required shutdown inspections.
1. Virtual imaging. This laser scanner can produce realistic 3-D "blueprints" of in-accessible or hazardous locations—such as the interior of a nuclear reactor vault. Courtesy: FARO Technologies Inc.
The scanner recreates the interior of the reactor vault as a virtual environment, similar to a three-dimensional blueprint—everything is proportional and as detailed as a high-resolution photograph captured on silver-halide film (Figure 2). Using the scanner reduces from a dozen to one or two the number of engineers required to enter the reactor during shutdowns to measure and image components and structures. The fewer the participants, the lower the inspection team's collective radiation exposure.
2. Keep a safe distance. Using a computer-generated image of the inside of a reactor reduces total worker radiation exposure. Courtesy: FARO Technologies Inc.
Based in Mississauga, Ontario, AECL plays a unique role in the global nuclear power industry by serving as a facilitator for plant operators worldwide. Established in 1952, AECL is the designer and builder of CANDU (CANada Deuterium Uranium) reactors, including the 700-MWe CANDU 6—one of the world's top performers—and the new ACR-1000, a 1,200-MW-class unit. AECL's more than 4,000 employees offer specialized technologies, R&D support, and cutting-edge nuclear services (design and engineering, construction and waste management, decommissioning) to all nuclear utilities, not just CANDU users.
Walkdown worries
The inside of a typical nuclear reactor is cavernous, up to 135 feet in diameter. The reactor core and support systems within it are equally impressive, comprising multiple tiers of equipment, catwalks, and platforms and a labyrinth of plumbing and control conduits. Once a year, reactors are shut down (and inspected with a fine-tooth comb), allowing the entire plant to be maintained. Shutdown procedures include machine alignment and balancing, refurbishment of blades and seals, the rewinding of motors and the replacement of bearings and valves, and—of course—comprehensive tests of all operational and safety systems.
Unlike a fossil-fueled power plant or an industrial plant, nuclear plants are built to specifications that are perhaps the tightest of any ever written. For instance, where a 7-inch gate valve is called for, not just any 7-inch gate valve will do; it must be the precise one called for in the plant specs, because that's one that has proven to work. Substitute parts, or jury-rigged components that might suffice in another industry, are prohibited. This rigor is partially to prevent unexpected shutdowns, but primarily to prevent safety-related surprises.
During a so-called "walkdown" inspection of a reactor, engineers audit all the components, equipment, and key dimensions of its vault to verify that the reactor within it continues to meet as-designed specs and that no nonstandard components have been substituted for approved ones. Each valve, breaker box, motor, pump, doorway, and seal must be identified—and verified as correct—during the walkdown.
In the past, reactor walkdowns were labor-intensive tasks. Engineers in protective clothing had to clamber through the vault to measure critical dimensions such as the width of access doors and photograph everything needing positive identification. Although the engineers were trained to minimize their exposure to radiation, they wore badges or electronic dosimeters to monitor how much radiation they received during their time in the vault. Later, armed with their notes and a copy of the plant specifications, the engineers verified that all components and critical dimensions remained correct for the application.
Unfortunately, most physical walkdowns took so long that they put the health of the inspecting engineers at some risk.
Digital speed, film resolution
AECL discovered that the FARO Laser Scanner LS enabled a whole new paradigm for conducting walkdowns—one that limits radiation exposure for personnel.
Developed by FARO Technologies Inc. (Lake Mary, Fla.), the LS captures detailed images of big things. It has a range of 80 meters and can scan 360 degrees horizontally and 320 degrees vertically. Capturing 120,000 data points per second, a typical laser scan can be done in just 4.5 minutes—up to 100 times faster than most time-of-flight scans. The speed with which the LS scans is impressive, but its image resolution is even more so. After capturing a "data cloud" of about 28 million pixels per scan, the laser generates images in which minute details are razor-sharp, even at maximum range (Figure 3).
3. Industrial-strength scanner. High-resolution, photo-realistic images of large equipment can be produced in minutes. Courtesy: FARO Technologies Inc.
"Although the finished image is digital, it gives the same impression as a high-res photo," noted Mark Carney of the AECL Simulation Department, the companys primary user of the Laser Scanner LS. One customer who scanned an entire business jet reports that details as fine as rivets and tire lugs are easily distinguishable on the scans. Others use the instrument to check the trueness of walls during tunnel construction, or to recreate the scene of vehicle accidents.
Because a typical reactor walkdown requires inspection of thousands of components, AECL personnel must scan the entire vault down to the smallest details. But a little planning and teamwork make each scan session go smoothly. Before a project begins, each engineering team responsible for checking a specific portion of the reactor gives Carney a shopping list of what it needs to see in its area of interest (Figure 4). "For example, one team will want to be sure that an access door is wide enough to move transfer flasks in and out; another will want to see the serial numbers on pumps," he said.
4. Small package, big results. The small, tripod-mounted laser scanner and high-resolution digital camera create crisp, full-color overlays. Courtesy: FARO Technologies Inc.
Carney and, occasionally, one other colleague travel to the reactor site and plan the specific positions from which they will do the line-of-sight scanning. They have two objectives. "Naturally, we make sure that we can cover everything important to the inspection. But we also have to make sure that the laser setups are compatible with the technology. For instance, we avoid locations where we would be scanning very shiny or deep black areas, because they are beyond the light range of the receiver."
Once the planning is done, the scanning team goes to work, setting up the scanner on its tripod, imaging all the equipment and vault surfaces that can be seen from that position, and then moving to the next area. Data from scans are saved in the instrument's on-board computer. Carney and his cohorts have found that being thorough doesn't necessarily take a lot of time because the scans go quickly. For instance, at one reactor in Canada, it took only four days to capture every area and item on the engineers' shopping list. On some overseas jobs, it may take almost as long to fly to the site and back as it does to do the scanning.
Quilting bee
Once he has returned from a customer's plant to his suburban Toronto office, Carney begins the process of assembling, in software, the individual scans into a single image of the entire reactor vault (Figure 5). "As we shoot each scene, we mark the edges of the area in the file, and those notations become the points at which we stitch all the areas together," he explained.
5. Stitch in time. Adjacent images can be automatically stitched together into a single, large image. Courtesy: FARO Technologies Inc.
When the stitching is complete, AECL has a virtual environment in which engineers can "move about" at will. They can change directions, examine particular areas from multiple perspectives, find the location of valves, read the serial numbers on junction boxes and other hardware, and check to see that no "creative engineering" has been added to the vault since the last inspection.
The usefulness of the virtual environment isn't limited to visual inspections. Because the simulated vault is proportional in three dimensions, engineers can zoom in and measure dimensions of critical passageways or evaluate the flatness and plumbness of walls—as if they were inside the real vault.
Streamlined process
Before AECL made virtual nuclear inspections possible, reactor walkdowns required a dozen or more people to climb through a vault with tape measure, note pad, and digital camera in hand. Typically, it took thousands of person-hours in a "hot" environment to collect the required data.
Today, in-reactor time usually totals less than 100 hours per walkdown. The auditing portion of the project has been streamlined, too. Because the virtual environment is so much more accessible than the actual reactor, engineers can quickly verify that existing components and dimensions are those called for in the plant specifications.
—Contributed by FARO Technologies Inc. (www.faro.com or 800-736-0234).
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