Coal, Nuclear, Reactors

Robotics Reduces Exposure at Palo Verde Nuclear Reactor

Issue 6 and Volume 104.

PALO VERDE NUCLEAR generating station, 55 miles west of Phoenix, Arizona, is operated by Arizona Public Service Company (APSC). In additional to operating the plant, APSC also owns 29.1 percent of Palo Verde. In the spring of 1996, Palo Verde commenced refueling of Unit 2.

A major component of the reactor is the core support barrel assembly. The assembly consists of:

  • The core support barrel
  • Lower support structure and instrument assembly
  • The core shroud assembly
  • Barrel snubber lugs and
  • Alignment keys

Without any fuel, the assembly weighs approximately 286,000 pounds. The core support structure supports 241 fuel assemblies.

When the plant commenced re-fueling the first problem they encountered was a seized core support plate, which they were unable to dislodge. Further investigation of the problem revealed a damaged fuel assembly adjacent to the support plate. Because of this, the lower assembly grids and lower end fittings had expanded. It was determined that the root cause of the problem was a deformed upper guide structure (UGS) which probably occurred during the previous fuel refueling outage.

After reviewing the options, Palo Verde decided to use a robotic manipulator to repair the damaged UGS guide. The manipulator used a variety of tube gauging probes and electric discharge machining heads. Using the manipulator, the plant was able to inspect and remotely machine the guide tube to its original design tolerances.

In addition to the manipulator, underwater radiation tolerant closed circuit television systems were used to monitor the repairs and machining of the guide tube. To obtain adequate viewing of the work area required frequent repositioning of the equipment. Because of the need to frequently reposition the closed circuit monitoring equipment, the time for repair was slow.

Dose rates at the UGS guide tube inspection camera locations were in the region of 500 R/hr, which resulted in poor quality viewing. Additionally, the close proximity of the UGS guide tube region to the camera also caused some equipment problems.

Since the UGS problem could be generic, it became mandatory that the utility visually inspect the UGS guide tubes in the other Palo Verde nuclear reactors. These inspections were scheduled for the next reactor refueling outages.

Remote Monitoring Equipment

Visual Technologies, Inc. of New Jersey provided a waterproof high-resolution color camera with a 24:1 zoom lens and integrated pan and tilt. The compact camera unit weighs only 7.5 pounds and measures 5 inches by 5 inches by 9 inches. Manipulation and deployment of the camera is accomplished through a single multi-conductor cable.

Click here to enlarge image

In addition to supplying the camera and its equipment, Visual Technologies also supplied a robotic crawler mounted with a color CCD pan tilt zoom camera system (Figure 1). Similar to the high-resolution color camera, the robotic crawler was operated through a single multi-conductor cable.

Inspection of the UGS guide tube required the use of two high-resolution cameras located at opposite corners of the inspection area. This allowed the operators to monitor the area being inspected and the movement of the robotic crawler within the reactor.

After removing the USG, a detailed examination of the storage stand was carried out by using the crawler’s camera. During this operation, the camera was positioned approximately 6 feet above the storage stand. Although the camera was 6 feet away from the USG, the Palo Verde staff reported that the quality of the images was good.

The visual inspection verified no damage to the guide tube and no inside or outside damage to the tubes. Visual inspection also showed little or no misalignment of the guide tubes with respect to adjacent guide tubes. Because the cameras were able to give the operators a good view of the area around the UGS, maneuvering of the robot by the operators was relative trouble free. This also provided the operators a visible means for the proper seating of the UGS. The guide tube inspection took approximately 30 minutes.

10 Year In-service Inspection

In addition to refueling, the reactor vessel was due its 10-year in-service inspection. To do this, the 32-foot long core barrel assembly had to be removed from the reactor vessel. Because of the successful application of robots and remote cameras used in the repair and inspection of the USGs, the utility decided to use the robotic crawler and cameras for inspecting the reactor vessel.

After removing the core barrel assembly, it was positioned above the refueling pool. In that position, 14 feet of the barrel assembly was above the water while the hot leg penetrations of the core barrel assembly were below water. To monitor the core remotely required the use of 14 cameras, seven in the water and seven above the water.

The reactor core lift director used the four above-water cameras for general viewing of the reactor core area and its location. A fifth camera was mounted on the polar crane trolley. Two other cameras were used to view the “hydraset” load cell readout and laser elevation indicator.

Underwater cameras were strategically placed which allowed the operators to view the reactor vessel’s four alignment keys and the alignment guide pins above the vessel. Cameras were also placed on the core barrel storage stand for observation of the core barrel assembly as it was lowered into and out of the stand. Video signals from the cameras were sent to a remote command station located on the lower part of the containment area.

After installation and before refilling the refuel pool, all of the cameras and command station equipment were checked to make sure they were operating correctly. During the removal of the upper guide structure, personnel were stationed at the core barrel’s command station to verify that the views of the interface of the flange of the reactor vessel were adequate.

The actual core barrel removal time, from loading the polar crane to unloading, was approximately 1-1/2 hours. Fourteen personnel worked inside the containment vessel during this process.

Remote controlled devices, for inspection and handling, has been utilized by nuclear power plants for many years. Although the equipment is expensive to purchase, their use in nuclear power plant reactors have the benefits of reducing operator exposure and equipment downtime.