Patent Number: 056028857
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS As shown in FIG. 1, a finished fuel rod 22 generally comprises a hollow zirconium alloy tube 26, filled with uranium pellets and closed by an end plug 27 welded to tube 26 along a circumferential girth weld 30. The fuel rod 22 preferably is provided with a surface finish 33 for protecting at least an upstream end 34 of fuel rod 22 from fretting damage caused by loose debris captured from circulating coolant in a reactor by a fuel rod support grid (not shown). This surface treatment 33 can comprise a layer of oxidized zirconium or some other protective coating or treatment. However, such a coating or treatment obscures the character and quality of girth weld 30. Therefore, the invention provides an automated procedure and apparatus for automatically inspecting girth welds, preferably prior to application of any protective coating or treatment. Preferably, inspection is accomplished as a step in a production method having a number of substantially automated steps such as forming and cutting the tube, mechanically affixing and welding the end plug, loading the tube with nuclear fuel pellets (not shown), application of the protective coating or oxidation treatment, etc. Such an automated production technique is disclosed, for example, in U.S. Pat. No. 4,857,260 --Schoenig, Jr. et al., the disclosure of which is hereby incorporated. End plug 27 as shown in FIG. 2 has a plug portion 42 dimensioned to fit snugly within the end of tube 26, and a body portion 44 having a diameter equal to the outside diameter of tube 26. Body portion 44 forms a shoulder 48 with plug portion 44, which shoulder 48 abuts against the axial end of tube 26 prior to welding. The girth weld 30 extends circumferentially around the tube/plug abutment, in a plane perpendicular to a longitudinal axis 52 defined by tube 26. Girth weld 30 must be continuous around the circumference, for sealing tube 26 hermetically and for providing a durable assembly that can withstand extremes of temperature cycling over its useful life. Additionally, girth weld 30 must be precise, for properly attaching end plug 27 so as to form a continuous extension of tube 26. The girth weld is made in known manner, for example, using an electric arc welding technique in an inert atmosphere, e.g., of helium or argon. According to the invention, girth weld 30 is inspected automatically, using an optical inspection method and apparatus that rely on variations in reflectance that are characteristic of weld defects vs. a lack of such variations characteristic of a continuous weld that lacks defects. The defects can be, for example, gaps or skips in the weld that extend wholly or partly around the circumference, blow holes that appear at the surface or the like. A weld missing all or part of its extension is potentially dangerous, and is preferably detected promptly after the welding procedure by which the weld was attempted. A properly formed weld is about one eighth inch (3.2 mm) wide, measured parallel to the longitudinal axis of the tube. A properly formed weld extends precisely to the outside diameter of the tube such that the junction of the tube to the plug appears the same as a continuous metal surface extending over the area of the weld. Referring to FIG. 3, the welded tube and plug assembly is fed to an inspection station 55, e.g., being advanced axially using a pneumatic slide cylinder 56, and engaged in a rotational fitting 58, which can have a pneumatic clamp for engaging and rotating the assembly. Girth weld 30 is disposed in the view field of a line scan camera 62 directed at an inspection zone 63 encompassing weld 30. Tube 26 and weld 30 preferably are illuminated at inspection zone 63 to enhance the level of reflectance over that produced by any ambient light, as generally shown by bulb 64 in FIG. 3. The tube and weld are engaged by a pneumatic tube grip 68 drivable by a rotational drive 69. Tube 26 and plug 27 welded thereto are rotated about longitudinal axis 52 for at least one full revolution, and preferably by over one revolution, e.g., 1.1 revolutions or about 400.degree., while simultaneously collecting reflectance data using line scan camera 62. Line scan camera 62 has a field of view that encompasses a longitudinal span along tube 26 that bridges over the area of weld 30 and extends slightly beyond weld 30, both onto the surface of tube 26 and the surface of end plug 27. Line scan camera 62 can include a linear array of charge coupled devices (CCD sensors) on which an image of the fuel rod is focused by appropriate optics in a known manner. The charge coupled devices are periodically reset and otherwise accumulate charge during illumination as a function of the detected light level incident on each such element. The charge or voltage signals from each element can be shifted to an output of camera 62 after a predetermined brief interval sufficient to achieve contrast between discrete locations or "pixels" (picture elements) of relatively higher or lower reflectance of light from source of illumination 64. The signal line carrying the voltages indicative of reflectance levels is coupled to a digitizer 72 as shown in FIG. 4, which provides pixel data representing the respective voltage numerically, and therefore the gray scale reflectance level at each discrete location on the surface of the fuel rod represented by a pixel, extending along longitudinal axis 52 of the fuel rod by a distance equal to the span of line scan camera 62. This pixel data is loaded into a digital memory 76 and analyzed, for example by a processor 74. The fuel rod can be rotated relatively slowly, for example at about one revolution per second, and/or line scan camera 62 can be operated at a high scan rate, to develop a large number of pixel line scans along lines parallel to axis 52 at regular angular positions around the axis to map a cylindrical span including weld 30. Preferably, each line scan is triggered by a pulse from a shaft angle encoder at regular angular steps, such that the speed and/or regularity of rotational speed are not critical. A matrix 82 of reflectance data is collected, as shown by the cells G.sub.i,j in FIG. 5, a portion of one column 84 thereof also being shown in FIG. 6. Matrix 82 includes numerical reflectance data for discrete pixels at and adjacent to girth weld 30, in rows 86 of pixels along a longitudinal length of the tube and columns 84 of pixels along adjacent circumferences of the tube within said longitudinal length. It is possible to operate the motor 69 driving rotation of the fuel rod at a constant speed and to collect line scans at a regular frequency, thereby collecting matrix 82 of pixel data. Alternatively, motor 69 can be a stepping motor driven under control of processor 74 as shown in FIG. 4, with the processor triggering operation of digitizer 72, for positively positioning the fuel rod at regular angular intervals for recordation of line scans. As another alternative as discussed above, the fuel rod and/or driving motor can be coupled to a shaft angle encoder (not shown) that produces a pulse signal as a function of rotation of the fuel rod, which pulse signal triggers digitizer 72 and/or line scan camera 62 to record a line of pixel data. According to one embodiment, the tube rotation speed and scan rate are selected to achieve a single pixel resolution of about 0.0005 by 0.0005 inches (0.003 by 0.003 mm). For a fuel rod of about 2 cm (0.8 inch) outside diameter (i.e., 6.3 cm circumference), and a scan length of about 0.7 cm longitudinally, matrix 82 of pixel data is about 21K by 2K pixels. Each pixel can be encoded, for example, to eight bit gray scale resolution, or .+-.0.04% of full scale. The matrix 82 of numeric pixel reflectance data is analyzed by processor 74 or by similar numeric processors such as discrete adders, comparators or the like, to determine the quality and integrity of girth weld 30. The inspection apparatus as described can be embodied with a Fairchild CAM 1500R or CAM 1830 line scan camera, either of which records a single line of 2,048 sensor cells or pixels per scan. Appropriate optics are provided for focusing the image of the fuel rod surface on the sensor cells. According to an inventive aspect, the pixel data is analyzed in columns or circumferential slices on the fuel rod, in a manner that serves to emphasize or highlight defects and to de-emphasize reflectance variations between adjacent individual pixels. For this purpose, a minimum defect size is chosen, defined by the span of a predetermined number of adjacent pixels at the resolution of operation. For example, the minimum defect size can be chosen in the above example as 0.002 inches (0.05 mm), which is approximately the size of four adjacent pixels. The local average of the pixel data within this span of pixels is summed or averaged, and the result is compared to a stored or selected maximum and/or minimum value to assess whether a defect is present. This operation highlights defects that are at least as large as the minimum defect size. Preferably, the minimum defect size is presettable to a different value in the X and Y directions (i.e., longitudinally of the tube across the width of the weld, and circumferentially around the tube). For example, the device can be more sensitive to the width of defects (along a longitudinal span of the tube), for example having a setpoint limit of three adjacent pixels of high reflectivity, and less sensitive to defects extending circumferentially around the tube, for example with the setpoint in that direction being ten adjacent pixels of high reflectivity. Accordingly, the defect testing is profiled in the X and Y directions to respond more strongly to particular kinds of defects. More particularly, in analyzing the pixel data in matrix 82 (see FIGS. 5 and 6), proceeding for successive columns X.sub.1 through X.sub.m, and for each pixel value G.sub.i,j in each such column, each pixel value is compared to the average of the respective values of the pixels in the matrix. Any pixels exceeding the average reflectance, for example by 75% or more, are flagged. The adjacent flagged pixels are counted in X and Y directions and the count is compared to the respective maximums. It is also possible to employ a running average of pixel values such as a column-wise local average that spans the minimum defect size number of pixels "n". This average is EQU G.sub.i,j =.sub.0.sup.3 .SIGMA.G.sub.i,j+(n) Whereas the resulting sum or average is taken over the minimum expected defect size at and adjacent each pixel, variations that span at least this size show strongly in the sum or average. On the other hand, variations that span only one or more pixels of a smaller number than the predetermined number "n" show less strongly in the results due to the effect of translating the raw pixel data into running sums or averages spanning the predetermined number of adjacent pixels. For translating the raw pixel data as described, it is possible to add the pixel values and divide by the number of pixels added (i.e., by the predetermined number), to obtain the actual average value using the same scale as the raw data. This is particularly convenient if the number of pixels is of a binary increment (e.g., four or eight), whereby the sum is simply shifted or the least significant bits are ignored. Alternatively, translating the pixel data can simply produce the sum of the values of the local pixels in the respective column, which is a form of averaging, albeit producing translated pixel values with a change of scale. For pixels at the edges of the matrix which are assured of being clear of the weld, the average or sum can be taken only up to that pixel that is within the predetermined number of the edge of the matrix. Alternatively, the average can be taken over the number of pixels remaining between the subject pixel and the edge (however, this effectively causes the minimum defect size to be smaller at the edges of the matrix, which is not desirable). The reflectance value for the pixels is compared to at least one of a maximum and minimum reflectance standard preferably based on the average reflectance value over the matrix, and adjacent pixels having reflectances out of range are counted and compared to setpoints preferably defining an X-Y profiled standard, for reaching an accept/reject decision on the tube based on a result of said comparing. Additional calculations can be made besides the comparison of individual pixel values to a preset proportion of the average value. The average reflectance value for all the pixels can be compared to a setpoint. The standard deviation can be required to be within specifications. It is also possible to calculate and record additional variables such as the maximum and minimum reflectance values, the average and standard deviation of the pixel values for each local group (e.g., line scan) and/or for the overall matrix, etc. These data are useful for adjusting the maximum and/or minimum reflectance specifications used to analyze the fuel rod welds. For example, if the source of illumination dims over time, this will be reflected by the overall average reflectance value. By adjusting the specifications as a function of the average, the specifications can be corrected over time to correspond to the variation in illumination. Preferably, the digitizer, processor and memory functions of the apparatus are accomplished using image processing hardware having high throughput and the capability of processing steps such as local sums or averages, accumulation of overall averages and the like embodied in hardware. For example, the Max Video family of modular VME boards available from Data Cube, Inc. can be applied according to the invention for managing image acquisition from line scan camera 62, image storage, pixel processing including edge sensitivity, and other features such as the generation of statistical histograms and the like. This hardware is based on bus data transfers operable at a rate of 10 million pixel operations per second, which is easily capable of operation at the speed required to effect image collection and analysis as described, within the time available for inspection preferably less than or equal to the time required to produce the weld to be inspected. Accordingly, the analysis of the data can be accomplished substantially contemporaneously with its collection. The line scan image that is repetitively collected and digitized from the output of the line scan camera encompasses a longitudinal portion of the tube including the girth weld. It is preferred to analyze the matrix pixel data through the individual line scans (i.e., counting adjacent high reflectivity pixels in scan rows) and aligning and counting adjacent high reflectivity pixels in the same position in successive scan lines. Thus, defects in the weld are detectable by reflectance variations widthwise along rows (longitudinally on the fuel rod), and also circumferentially around the tube. A detected reflectance variation over a first setpoint number of pixels (in an X or longitudinal direction) may be wide enough to be interpreted as a rejectable gap. Even if not wide enough to be rejectable, the defect may extend around the tube (in a Y or circumferential direction) sufficiently to form a rejectable gap as well. Moreover, the setpoint for the number of adjacent high reflectance pixels in the two profile directions can be related to one another, such that if a detect gap is wider then a shorter circumference setpoint is permitted, and vice-versa. The invention having been disclosed in connection with the foregoing variations and examples, additional variations will now be apparent to persons skilled in the art. The invention is not intended to be limited to the variations specifically mentioned, and accordingly reference should be made to the appended claims rather than the foregoing discussion of preferred examples, to assess the scope of the invention in which exclusive rights are claimed.