Patent Number: 054188237
Section: description

DETAILED DESCRIPTION OF THE DRAWINGS In accordance with an exemplary embodiment of the present invention, the eddy-current probe arrangement and technique used to obtain impedance measurements from the outside of a cladding tube employs a pair of wire coil windings electrically interconnected to form a differential coil pair, as illustrated in FIG. 1. The differential coil pair are electrically interconnected into a impedance bridge and oscillator circuit arrangement that forms conventional electronic impedance measuring equipment (not shown). One of the coils (10) is used as the eddy-current generating "probe" in the testing of cladding tubes by passing the specimen cladding tube (11) undergoing test through the central opening (12) in the coil windings. The remaining coil (14) is contained in a separate magnetically shielded housing (indicated by dotted line 16) and surrounds a short exemplary segment (18) of a fuel rod cladding tube that is used as a "dummy" impedance load. Coils 10 and 14 are preferably constructed such that the diameter of central opening 12 is on the order of a few mils larger than the outside diameter of the nominal type of tube anticipated being tested. It is desired to match probe coil inner diameter as closely as possible with cladding tube outer diameter to minimize any "fill" errors (analogous to "lift-off" errors in conventional eddy-current surface probes) in the measured impedance. Accordingly, different specimen tube types may require a correspondingly different coil-pair size. Dummy impedance load tube segment 18 is maintained within the coil throughout the testing process and functions primarily as a nominal load impedance necessary to maintain the impedance bridge circuit (not shown) in a near-balanced condition. Accordingly, a dummy load segment is chosen which has dimensional and physical properties (e.g., electrical conductivity and magnetic permeability) that are substantially similar to the particular type of fuel cladding tube to be tested. Since the two coils are connected in a bridge circuit arrangement, any difference in electrical impedance between the two coils creates a measurable impedance imbalance which is indicative of a difference in the respective physical properties (e.g., a dimensional or compositional difference) of the dummy load cladding segment and the specimen cladding tube under test. Ideally, if the dummy load cladding tube segment is compositionally identical to the type of specimen fuel rod cladding tube undergoing a test, any impedance imbalance measured in the bridge circuit coils is caused predominately by dimensional differences between the dummy impedance load tube segment and the fuel rod cladding tube. To quantify the above dimensional difference, a carefully constructed "calibration standard" cladding tube is utilized to correlate measured impedance differences to actual physical dimensional differences. In an exemplary embodiment of the invention, primary calibration standard cladding tube 20 consists of sixteen distinct segments 22 as shown in FIG. 2. These segments are joined together to form a single long calibration standard tube. Each segment has approximately the same length as the dummy load reference segment (e.g., two inches) and is preferably not much larger than the length of coils 12 and 14. In addition, each segment 22 has the same liner thickness but different outside and inside diameters which are known to a high precision. Table 1 below lists the sixteen different segment numbers of the primary calibration standard along with exemplary outside (OD.sub.i) and inside (ID.sub.j) diameters for each segment. TABLE 1 ______________________________________ SEGMENT NO. OD ID ______________________________________ 1 .4425 .3790 2 .4400 .3790 3 .4375 .3790 4 .4350 .3790 5 .4425 .3815 6 .4400 .3815 7 .4375 .3815 8 .4350 .3815 9 .4425 .3840 10 .4400 .3840 11 .4375 .3840 12 .4350 .3840 13 .4425 .3865 14 .4400 .3865 15 .4375 .3865 16 .4350 .3865 ______________________________________ Table 2 below illustrates exemplary preferred inside and outside diameter combinations for each numbered segment of the primary calibration standard in the form of a dimensional matrix. Table 2 below illustrates exemplary preferred inside and outside diameter combinations for each numbered segment of the primary calibration standard in the form of a dimensional matrix. TABLE 2 ______________________________________ 1) OD.sub.1 /ID.sub.1 2) OD.sub.2 /ID.sub.1 3) OD.sub.3 /ID.sub.1 4) OD.sub.4 /ID.sub.1 5) OD.sub.1 /ID.sub.2 6) OD.sub.2 /ID.sub.2 7) OD.sub.3 /ID.sub.2 8) OD.sub.4 /ID.sub.2 9) OD.sub.1 /ID.sub.3 10) OD.sub.2 /ID.sub.3 11) OD.sub.3 /ID.sub.3 12) OD.sub.4 /ID.sub.3 13) OD.sub.1 /ID.sub.4 14) OD.sub.2 /ID.sub.4 15) OD.sub.3 /ID.sub.4 16) OD.sub.4 /ID.sub.4 ______________________________________ where, OD = outside diameter, ID = inside diameter, OD.sub.1 &gt; OD.sub.2 &gt; OD.sub.3 &gt; OD.sub.4, and ID.sub.1 &lt; ID.sub.2 &lt; ID.sub.3 &lt; ID.sub.4 Upon passing the primary calibration standard through the eddy-current probe coil, dimensional differences between the segments in the calibration standard and the dummy load reference tube segment will cause a unique measurable impedance imbalance for each of the sixteen segments. These impedance difference measurements are correlatable to the known dimensions of the primary calibration standard segments. Mathematically speaking, since electrical impedance consists both of real and imaginary components, namely the inductive reactance and the resistance, it may be represented diagrammatically by a point plotted in the real/imaginary complex plane. Consequently, a conventional eddy-current impedance measurement instrument having a cathode ray tube (CRT) display arrangement, which is configured to depict impedance values using horizontal and vertical axes of the display to represent the complex impedance plane, may be used to display the unique impedances of each of the sixteen segments of the calibration standard. A CRT display of measured impedances for all of the segments of calibration standard 20 in this manner forms an "impedance matrix", as illustrated in FIG. 3. The different impedance values for the segments appear as illuminated spots on the CRT display screen (represented in FIG. 3 by rectangular spots 32). For example, each displayed impedance corresponds to a particular inside and outside diameter combination listed in TABLE 2. Since the inside and outside diameter dimensions of a tube affect its measured impedance, a CRT display in the impedance plane of a particular cladding tube under test can be utilized to determine the inside and outside diameter of the tube by comparison (either visually or electronically) of its measured impedance with the previously measured impedances of the calibration standard. More specifically, the cathode ray tube display in a convention eddy-current instrument can be configured to display measured input voltages that are calibrated with respect to indicia on the face of the display in either or both of the horizontal and vertical directions. Consequently, the display of an impedance matrix can be oriented (e.g., rotated) so that horizontal axis voltage indicia correspond to differences in tube outside diameter and vertical axis voltage indicia correspond to differences in tube inside diameter or vice versa. In accordance with a preferred embodiment of the present invention, the unit difference in vertical positions of each displayed impedance spot 32 is calibrated to the unit difference in inside diameters between segments of the calibration standard. For example, referring again to FIG. 3, E.sub.1 represents a difference in CRT vertical display voltage produced by a difference in the measured impedances of two separate primary calibration standard segments. The impedance matrix, however, is purposely oriented on the display such that E.sub.1 also directly corresponds solely to a difference in inside diameters of the calibration segments. During the testing of cladding tubes, when a specimen tube is inserted into the circumferential measuring coil, the resulting detected impedance measurement will fall somewhere within the range of impedance values forming the calibration standard impedance matrix. Referring again to FIG. 3, circular mark 34 represents the impedance value of an exemplary specimen cladding tube measured at some arbitrary point along its length. Since each of the displayed impedance values, 32, making up the primary calibration standard impedance matrix are correlated to known outside and inside tube diameters of segments in the primary calibration standard, the outside and inside diameter of a specimen cladding tube can be uniquely determined for any desired point along its length from its impedance relative to the known calibration standard impedance values. However, this approach to determining diameter dimensional information is only accurate to the extent that the specimen cladding tube liner thickness is identical to the liner thickness of the calibration segments in the primary calibration standard. For example, due to relative differences in electrical resistivity of the Zircaloy-2 tube metal and the pure zirconium liner (72 micro-ohm-centimeters vs. 40 micro-ohm-centimeters, respectively) the overall measured conductance of induced eddy-current (and hence the impedance) is dependant on the relative proportions of Zircaloy-2 and zirconium in the cladding tube. Accordingly, if the cladding liner thickness is less than that of the calibration standard segments, the overall conductance (and correspondingly the overall impedance) will be less than that of the calibration standard and, therefore, will produce an erroneous measurement indicating an inside diameter greater than the actual inside diameter of the cladding tube. Conversely, a cladding tube liner thickness greater than that used in the calibration standard segments will cause an erroneous measurement indicating an inside diameter less than the actual value. However, in accordance with the present invention, if liner thickness can be separately calibrated to measured impedances and the "actual" cladding tube inside diameter determined precisely via ultrasonic (or other) techniques, then the difference between the actual inside diameter and the inside diameter as determined electromagnetically can be used to deduce the thickness of a cladding tube liner. Thus, in accordance with a preferred embodiment of the present invention, a second calibration standard cladding tube (not shown) is used to calibrate the effect that various liner thicknesses have on the overall measured impedance. Accordingly, the second calibration standard consists of multiple tube segments (e.g., seven in the presently preferred embodiment) having various known liner thicknesses but all having the same inside and outside tube diameters. FIG. 4 shows a representation of a CRT display of exemplary measured impedances (spots 32 and 42) for the two different calibration standards. The second calibration standard produces a vertical array of spots 42 on the CRT display distinct from spots 32 comprising the impedance matrix of the primary calibration standard. Spots 42, represented in FIG. 4 by seven small circles, indicate the separate measured impedances for each of the known different liner thicknesses of the seven segments of the second calibration standard. Since a separation in the displayed impedances of the second calibration standard correlates to liner thickness only, the unit difference in vertical positions of each displayed impedance spot 42 is, thus, calibrated to the unit difference in liner thicknesses. For example, referring again to FIG. 4, E.sub.2 represents a difference in CRT vertical display voltage produced by a difference in the measured impedances of two separate segments of the second calibration standard. The measured impedances of the second calibration standard, however, are oriented on the display such that E.sub.2 also corresponds to a difference in liner thicknesses of the calibration segments. In accordance with a method of the present invention, the actual liner thickness, T.sub.b, for a particular fuel rod cladding tube undergoing test, can be calculated using the known thickness of the liner in the sixteen segments of the first calibration standard, the difference between ultrasonic and electromagnetic measured values for inside diameter, and the impedance values (represented as voltages) from the two calibration standards, by using the following formula: EQU T.sub.b =T.sub.std +E.sub.1 /E.sub.2 .times.(ID.sub.ut -ID.sub.ec) (Equ. 1) where, T.sub.b =Liner (or "barrier") thickness PA1 T.sub.std =Thickness of liner in the segments of the primary calibration standard PA1 E.sub.1 =Change in vertical display voltage per unit change in inside diameter as measured by electromagnetic subsystem PA1 E.sub.2 =Change in vertical display voltage per unit change in liner thickness as measured by electromagnetic subsystem PA1 ID.sub.ut =Inside diameter determined ultrasonically, and PA1 ID.sub.ec =Inside diameter determined by electromagnetic subsystem. Referring now to FIG. 5, a block diagram illustrates the combined eddy-current and ultrasonic testing system in accordance with an exemplary embodiment of the present invention. Arithmetic unit (or computer) 501, obtains measurement information separately from both ultrasonic subsystem 503 and electromagnetic subsystem 502. Electromagnetic measuring subsystem 502 includes differential coil pair eddy-current impedance probe arrangement 508 (depicted in FIG. 1) connected to a data acquisition system (DAS) consisting of a conventional electrical impedance measurement bridge with associated signal processing and A/D (analog-to-digital) conversion circuits 510. A display device, for example, CRT 512, may also be connected to the DAS. DAS electromagnetic signal processing circuitry 510 may also include a computer or microprocessor programmed for performing conventional signal analysis and measurement interpolation calculations. For example, inputs to DAS 500 are interpolated from the correlation of "horizontal" voltage to outside diameter and "vertical" voltage to inside diameter using conventional LaGrainge interpolation calculations by using DAS processing circuitry 510. Alternatively, this correlation may be performed from digitized measurement data by computer 501. Ultrasonic subsystem 503 basically consists of conventional ultrasonic testing equipment 505 connected either directly or indirectly, through conventional a/d conversion and signal processing circuits 507, to computer 501. Ultrasonic testing equipment 505 may be any conventional system or apparatus for accurately measuring the wall thickness of a metal tube through ultrasonic techniques, such as, for example, the apparatus disclosed in commonly assigned U.S. Pat. No. 5,063,780, issued Nov. 12, 1991 to Landry. Digitized impedance data acquired from electromagnetic subsystem 503 and digitized diameter measurement data from ultrasonic measurement subsystem 502 is stored in memory 504 and subsequently retrieved by computer 501 to calculate thickness, T.sub.b, of a cladding tube liner in accordance with Equ 1 above. Computed liner thickness data may then be stored in memory 504 or printed or displayed via output devices 506. Referring now to FIG. 6, a schematic flowchart is shown that illustrates steps of an exemplary program executed by an arithmetic unit or computer 501 for acquiring measurement data and calculating liner thickness in accordance with a preferred embodiment of the invention. Steps S1 and S2 of the flowchart provide for the setting up of reference impedance values in memory 504 from the two calibration standards for subsequent use in computing Equ 1. Steps S3 through S8 provide for determining liner thickness from both impedance and ultrasonic data acquired from the specimen under test. More specifically, in Step 1, a known value of liner thickness, T.sub.std, for the primary calibration standard is input via conventional input device 506 and stored in memory 504. In addition, impedance values from primary calibration standard 20 are obtained from electromagnetic measurement subsystem 502 and also stored in memory 504. In step S2, impedance values from the second calibration standard are, likewise, measured and stored. (step S2 may be performed prior to step S1 or as part of the same step). In step S3, the input data from the eddy-current subsystem (502) is correlated (for example, by performing LaGrainge interpolations) to the measured values of outer diameter and inner diameter from the primary standard and the known liner thicknesses from the secondary standard. Next in step S4, a specimen is tested electromagnetically using subsystem 508 and the measured impedance is stored in memory 504. If desired, impedances of the calibration standards and the test specimen may also be displayed as measured during these steps via CRT 512. In step S5, conventional eddy-current analysis and computational techniques are performed to interpolate/calculate an inside diameter value for the tested specimen based on its measured impedance and the stored reference impedance values of the primary calibration standards. This value, ID.sub.ec, is then stored in memory 504. Next, in step S6, the inside diameter of the same specimen is tested ultrasonically using subsystem 503 and the resulting diameter data, ID.sub.ut, is also stored in memory 504. (Step S5 may be performed prior to step S3 instead, if desired). In step S7, the liner thickness, T.sub.b, is calculated according to Equ 1 from the stored impedance and ultrasonic data. This information is then stored in memory 504 and/or provided to a printer or other output devices 506, as indicated in step S8. Finally, in step S9, measuring of other cladding tube specimens to determine liner thickness is conducted in the same manner or else the testing process is terminated. While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims. A person of ordinary skill in the art should appreciate that the requisite correlated reference impedance values could be obtained by utilizing a single "combined" calibration standard having the calibrated segments of both the primary and second calibration standard tubes instead of using two separate calibration tubes. Moreover, a person of ordinary skill in the art should realize that various utilizations of the invention may not require the use of all sixteen impedance references of the primary standard or all seven impedance references of the second calibration standard and that the calibration of reference impedance values would not necessarily have to correspond to any particular CRT display axis or orientation. In addition, although the presently preferred embodiment utilizes ultrasonic techniques for acquiring the inside diameter of a specimen, it is envisioned that other systems or means for acquiring "actual" inside diameter information could also be used without departing from the scope of the invention.