Limitations in the capabilities of the underwater closed circuit television system used for a remote repair operation in the pressure vessel at the Palo Verde station led to a move to colour CCD cameras which are now used extensively at the Arizona plant.
The use of underwater CCD-based video systems have proven a good alternative to the station’s radiation-tolerant tube type video systems. Compared to the old systems, which required large envelope deployment structures, the new CCD-based modules are small, light weight, easily manipulated. This technology provides high magnification capabilities, produces higher quality viewing, has reduced maintenance expenditures and can be rapidly deployed, reducing the time and cost of outages.
Combined with various in-house mounting and deployment mechanisms, the CCD based systems have become the workhorse of Palo Verde underwater video capabilities.
The catalyst for the change in viewing approach followed an incident at unit 2 in 1996. During the 6th refuelling outage, the first assembly to be removed was found to be bound and unable to be dislodged from the core support plate. Investigations also found a nearby fuel assembly to be damaged. The root cause was identified as a deformed upper guide structure (UGS) guide tube, a problem which unknowingly occurred during the previous outage.
The inspection and repair used the old underwater closed circuit television systems to view the various robotic activities. This work highlighted various deficiencies in the tube system, which required time consuming manipulations to provide adequate viewing; furthermore, the lengthy operation led to the camera receiving substantial doses leading to poor quality viewing.
Generic implications of the UGS problem mandated visual examination of the guide tubes and other inspections at the other two Palo Verde units. This provided the motivation for an alternative approach.
In the approach used in subsequent UGS guide tube inspections two CCD camera systems were set at opposite corners of the inspection area for general area viewing and a robotic crawler to travel under and around the specific locations. Visual Inspection Technologies (VIT) provided the waterproof high-resolution colour CCD cameras with 24:1 zoom lens and integrated pan & tilt, their Ca-Zoom PTZ III system, and their ROVVER crawler which provided visual mobility. The Ca-Zoom unit is lightweight (7.5 pounds) and compact with an envelope of approximately 5 in2 by 9 in.
ISI with CCDs
The next refuelling outage for unit 2 required the completion of the first 10-year in-service inspection of the reactor vessel. To access the reactor vessel inside surfaces, the core barrel assembly had to be removed, an operation which was estimated to involve a total dose of 7 rem. As this was unacceptable, the superior image quality and ease of handling identified during the UGS inspections led to the establishment of a system of VIT Ca- Zoom cameras for core barrel removal and installation remote monitoring.
For the core barrel removal which took place during the 10-year in service inspection, operators needed 14 cameras, 7 VIT Ca-Zoom cameras were placed underwater and 7 general purpose cameras above the water line. Four air cameras were mounted to provide directional and general area obstruction viewing to the lift director. One camera was mounted on the polar crane trolley providing a direct overhead view and two additional cameras provided viewing of the hydraset load cell readout and laser evaluation indicator. The overhead view was provided to the polar crane operator as well as the command station.
Underwater cameras were strategically placed to provide viewing of all four alignment keys at the reactor vessel and the alignment guide pins above the vessel. At the core barrel storage stand, cameras were placed to allow viewing at all angles as the core barrel assemble was lowered into or removed from the stand. To provide assurance of video imaging, two radiation tolerant camera systems were positioned at the flange and storage stand.
All cameras and command station equipment were in place and operationally checked prior to initial flooding of the refuelling pool. During removal of the upper guide structure, personnel were stationed at the core barrel command station to verify views were adequate of the reactor vessel flange interface. Once the UGS was removed, a complete, realistic dry run of the core barrel removal was remotely performed. With no changes required, the removal proceeded.
Video feeds were sent to a remote command station. Eight monitors were positioned to provide a representative orientation with the respective camera position relative to the core barrel.
Actual core barrel removal time, from loading the polar crane to unloading, was approximately 1.5 hours. 14 people were involved inside the containment and received a cumulative exposure of 28 mR for the entire evolution. Core barrel reinstallation times and exposure were consistent, totalling a cumulative 32 mR.
Expediency and remote handling capability was directly attributable to the exceptional image quality and viewing configuration of the underwater VIT cameras. Time savings for the evolution was estimated at four hours. Exposure reduction through utilisation of remote capabilities was greater than 6 man-Rem.
Other plant uses
The quality of the images and the success of the inspection and removal operations has led to the development of other uses. Palo Verde requires visual verification of proper core support plate seating during fuel assembly loading into the reactor vessel. This has been historically accomplished utilising a radiation tolerant system with pan, tilt and lighting peripherals installed on the core support plate. Picture quality would be initially adequate however degraded rapidly as fuel was loaded approaching the system. The large operating envelope also necessitated withdrawal of the system with a significant number of fuel assemblies remaining to be loaded. This system was replaced with a VIT Ca-Zoom camera and in-house fabricated mount located on the upper flange of the core shroud. Removal of the camera from the radiation field produced by the fuel and core shroud allowed utilisation throughout the entire reload with very little image degradation.
The camera systems have been utilised for irradiated fuel examinations, control element assembly alignment verification, foreign material identification, reactor pump bowl in-service inspection, and general area underwater monitoring. Fabrication of various mounting and deployment mechanisms has provided ease of applicability.