Patent Number: 059129344
Section: summary

FIELD OF THE INVENTION The present invention relates to inspection systems for use in underwater environments. More specifically, though not exclusively, the present invention relates to the inspection and/or identification of nuclear fuel rod assemblies and other surfaces which are submerged in a hazardous underwater environment, such as a reactor core or spent fuel storage pool in nuclear power generating plants. BACKGROUND OF THE INVENTION Facilities that generate power by nuclear fission often use water for containment of the reaction rate and provide a certain amount of radiation shielding of the fuel in the reactor core (in pressured water and boiling water-type reactors), and to store spent fuel cells. A typical pressure water reactor (PWR) can have on the order of 280 fuel cells in its reactor core. A boiling water reactor (BWR) can have 500 fuel cells in use and in storage. A spent fuel pool can have as many as 5,000 units at any given time. Strict governmental regulation requires that the specific location of each fuel cell in a facility be known at all times. Failure to comply, even in the case of a minor variation, can result in substantial fines. The operation and maintenance of nuclear facilities requires that the fuel cells contained in the reactor be periodically inspected and replaced. In both PWR and BWR facilities, the reactors are typically refueled every twelve to eighteen months. The refueling process removes spent fuel to the spent fuel pool, relocates existing fuel cells within the reactor core, and inserts new fuel cells. This process involves much more than simply replacing spent cells with new cells, since a significant "shuffling" of the fuel cells already within the reactor must occur in order to balance the radiation level across the reactor core to maintain uniformity. In order to comply with the strict governmental regulations, as well as to enable identification of fuel cells to determine their appropriate position within the core, means must be provided for in situ identification of each individual fuel cell with minimum exposure of personnel to dangerous radiation levels. This requires that the fuel cells be identified while still submerged within the water in the reactor containment or spent fuel pools. This presents a significant problem, however, since the depth of the water, at least in the containment pool, is on the order of 60 feet. The industry standard is for the fuel cells to be identified with an alphanumeric serial number which is stamped, milled or engraved on the top surface of the fuel cell, or on a bail extending partially across the top of the unit. Typical depths for new characters can be 0.5 to 1.0 mm. Thus, in order to identify a particular fuel cell, it is essential that the serial number be both visible and intelligible. On new fuel cells, the contrast between the background surface and the characters in the serial number is good, and the serial number can easily be read using a common video camera, such as a camcorder. However, this contrast degrades over time due to the enhanced corrosion and/or oxidation of the surface that is caused by the radiation, and sediment buildup on the surfaces of the fuel cell, with the characters becoming as shallow as 0.07 mm. A number of identification systems have been developed which are intended to allow the identification of fuel cells. For example, U.S. Pat. No. 4,960,984 ('984 patent) which issued to Goldenfield for an invention entitled "Method and Apparatus for Reading Lased Bar Codes on Shiny-Finished Fuel Rod Cladding Tubes," teaches the identification of nuclear fuel rod tubes that are encoded with a bar code by scanning the bar code with a bar code reader. This method requires marking a bar code on the fuel cells in addition to the industry standard alphanumeric code, which may be possible for newly manufactured cells, but could prove very difficult for used fuel cells (spent or in use). Further, the system and method of the '984 patent does not allow identification of a fuel rod tube while the tube is submerged within the reactor pool. Instead, a sophisticated machine is described which receives the fuel rod tube, and directs a laser beam towards the bar code etched in the tube. This laser beam is reflected back to the beam source for decoding to yield the identification information for the fuel cell. While this system may be capable of accurately identifying the fuel cell tube using a pre-existing engraved bar code, it is incapable of obtaining such identification while the fuel cell is submerged within the reactor pool. Another fuel cell identification system is briefly disclosed in U.S. Pat. No. 5,490,185 ('185 patent), issued to Dent, et al., for an invention entitled "System for Automatic Refueling of a Nuclear Reactor," which includes the use of an optical scanner to identify the bar code or alphanumeric code. While this system includes video capability for identification of the fuel cell while still submerged within the reactor pool, the video feature is described simply as a remotely operated video camera which is attached to the fuel handling equipment. Without special considerations for contrast enhancement, corrosion on the outside surface of the nuclear fuel cells and the buildup of corrosive materials on the surface of the nuclear fuel cell will inhibit the video determination of any identifying markings. Yet another identification system is disclosed in U.S. Pat. No. 5,089,213 ('213 patent), issued to Omote, et al., entitled "Nuclear Fuel Assembly Identification Code Reader." The '213 patent discloses the use of a combination of a camera and an ultrasonic wave sensor for what is described as a more reliable process for identification of the fuel cell. The device disclosed in the '213 patent combines the data from the camera and the acoustic device to provide the identification, requiring a relatively complex processing program. The two different reading techniques complement each other in an effort to overcome the inadequacies of each system individually. Thus, a significant loss in contrast in the characters would require the system to rely almost exclusively on the acoustic component of the system. The combination of the processing requirements and the multiple independent detection components would make this system relatively complex and expensive In light of the above-stated inadequacies of the prior art, it would be desirable to provide a system and method for in situ underwater inspection of the nuclear fuel cells in reactor pools and spent fuel pools that is capable of reading existing industry-standard identification characters in which the system is capable of reading the characters in spite of the inevitable degradation of the characters caused by corrosion. It is to such a system that the system and method disclosed herein is directed. SUMMARY OF THE INVENTION It is an object of the present invention to provide an identification system which is capable of in situ identification of a nuclear fuel cell while the cell is submerged within the reactor pool. It is a further object of the present invention to provide an identification system which generates a superior image for use in identifying a submerged nuclear fuel cell with a high degree of certainty. It is still another object of the present invention to provide a nuclear fuel cell identification system which is capable of identifying the proper placement and height of the nuclear fuel cells as positioned within the reactor itself. It is another object of the present invention to provide a nuclear fuel cell identification system which is capable of maintaining inventory records of the nuclear fuel cells within the reactor and spent fuel pool. It is still another object of the present invention to provide a nuclear fuel cell identification system capable of automatically identifying a fuel cell identification marking using an optical character recognition scheme. It is a further object of the present invention to provide a nuclear fuel cell identification system that is capable of identifying any imperfections or surface defects in the nuclear fuel cell and any surface within the pool. It is another object of the present invention to provide a nuclear fuel cell identification system that is capable of the automated identification and processing of the nuclear fuel cells. In an exemplary embodiment, the underwater inspection system for nuclear facilities has a video camera equipped with a fiberscope which terminates to a camera lens. The fiberscope is several feet long and conveys light from the camera lens to the video camera. The camera lens may be submerged in the pool containing the nuclear fuel for a close view of a nuclear fuel cell, while the camera remains at a distance such that radiation exposure is minimized. An anchoring bracket attaches to the camera lens to support a number of illumination lamps. Each of these lamps is angled downward towards a single location such that the object within that location would be illuminated from several different angles. The present embodiment includes three such lamps spaced 120 degrees apart generally in a common plane, however, any combination and arrangement of multiple lamps that provides illumination from different directions would be appropriate. The lamps can be selectively and sequentially illuminated to produce shadows from multiple sides of the target, thereby improving the contrast between the targeted markings and the surface in which they lie by enhancing the edges of the markings. The video camera is electrically connected to a power supply and control unit which provides all power to the camera, receives the electronic video signal, and controls the illumination of the illuminators. The illuminators are sequenced on and off such that by viewing the video output of the video camera, the fuel cell image will be constantly varying between three views, each associated with the illumination of a single illuminator. The video camera creates an electronic video signal which is received by the power supply and control unit. Within the control unit the video signal is conditioned and amplified for transmission to a frame grabber which is housed in a computer system. The frame grabber digitizes the video signal thereby facilitating digital analysis of the video image. By alternating the illumination of the lamps, for example, by strobing the lamps, the nuclear fuel cell being viewed is illuminated and the corresponding image is digitized. The same process is repeated for each of the illuminating lamps, resulting in a distinct image corresponding to each lamp. Once the images have been digitized, there is an option as to whether the images are first combined, then the resultant image optimized, or whether the images are individually optimized and the optimized images are then combined. In either case, the resultant image is more clear and has considerably more resolution than a standard video image of the top of a nuclear fuel cell. The computer system, in addition to housing the frame grabber, is capable of identifying and tracking each nuclear fuel cell within the nuclear reactor. This identification and tracking is accomplished by maintaining a log and/or generating a map of the nuclear fuel cell serial numbers and corresponding locations within the nuclear reactor and spent fuel pools. Thus, it is possible to identify a nuclear fuel cell and determine the length of time the fuel cell has been in the reactor, the precise position of the fuel cell, the approximate enrichment level of the fuel cell, and any other relevant data regarding that particular fuel cell. The operation of the first exemplary embodiment consists of lowering the camera and illumination lamps into the pool and positioning them over a nuclear fuel cell for identification. Such positioning can easily be accomplished by mounting the camera lens and illuminators on the telescoping arm of the fuel handling machine. Then, when the telescoping arm extends down into the reactor pool, the fuel cells may be viewed and identified with a high degree of certainty prior to engagement by the fuel handling machine. Once the camera lens is positioned above a fuel cell, the multiple illuminators are sequentially activated to create a series of shadows on the surface of the cell. These shadows are created by emphasizing the edges of the imprinted identification numbers and characters which constitute the fuel cell's serial number. As each illuminator is activated, the shadow image is conveyed from the camera lens through the fiberscope to the video camera which is preferably at a distance sufficient to significantly reduce the radiation levels impinging upon the camera. The output from the video camera is fed into a frame grabber which digitizes the video image and stores the video information on a computer system. This illumination and digitization process is repeated for the remaining illuminators and results in a set of digitized images in the computer having distinct shadow characteristics. Once the digitization is completed, the three images are combined and optimized to yield an image of the fuel cell having an enhanced image of the identification numbers. In an alternative embodiment, a camera equipped with multiple illuminators is replaced by a laser scanning head which is positioned on the telescoping arm. This laser scanning head can use a variety of techniques for scanning and improving the resolution of a serial number image. Such techniques, for example, include the use of a distance-determining laser, an interferometric laser system, or holographic interferometry. In each such technique, the image of the serial numbers is improved over traditional video techniques. Using the distance determining laser, the entire top surface of the fuel cell is raster scanned with a laser beam, with the laser light being reflected to a photodetector, which may include a CCD detector. The laser is pulsed and precise timing analyses provides an accurate distance reading to the reflecting surface. Combining each point of reflective, the general topography of the fuel cell may be determined thereby revealing the increased depths resulting from the presence of the serial number of the fuel cell. Similar results may be achieved using a fan-shaped beam and a one- or two-dimensional CCD array which can detect variations in the reflections at discrete points. An interferometer produces an array or grid of interference lines which may be projected onto the surface of the fuel cell in order to determine the topography of its surface. A camera is then used to generate an image of the array of interference lines. Variations in the spacing between the interference lines correspond to displacements in the surface being inspected. By mapping the variations in the line spacing in the collected image, the topographic variations resulting from the characters to be read can be seen, allowing identification of the fuel cell.