Patent Number: 056617666
Section: description

DETAILED DESCRIPTION The system of the present invention measures the bow and twist of either an unirradiated nuclear fuel assembly, or an irradiated nuclear fuel assembly which has been removed from the reactor core. This is done by using a non-contact measuring device in the form of an ultrasonic transponder and a reference measure from which the measurements are made in conjunction with a fixture which constrains the fuel assembly as if the fuel assembly was in the reactor core. By simulating the fuel assembly's actual position within the core, the effect of loading conditions which may not otherwise be present upon bow and twist in the fuel assembly can be measured. Referring to FIG. 1, the system of the present invention is shown. An irradiated fuel assembly 1 shown in schematic form is positioned under water within the fuel assembly bow and twist measuring apparatus 5 in accordance with the present invention. The details of the fuel assembly have been omitted from FIG. 1 for clarity of illustration. Although the present invention is shown and described with reference to an irradiated fuel assembly, an unirradiated or new fuel assembly may similarly be measured by the present invention. The fuel assembly bow and twist measuring system comprises a fixturing apparatus, a reference device, and an ultrasonic measurement device as more fully discussed below. The fuel assembly bow and twist measuring system is positioned underwater to provide protection from the radiation emitted from the irradiated fuel assembly. The fixturing apparatus includes a large diameter tube 10 which encloses the entire length of fuel assembly 1. Tube 10 is secured at its lower end to bottom reference plate 18 and both are secured by flanges 24 which extend downward and rest on a support surface which is depicted in FIG. 1 as the floor of a nuclear fuel assembly spent fuel pool 8 of a typical nuclear power plant (not shown). At the upper end of tube 10 is a top flange 16 which extends around the inner wall of tube 10. Also at the upper end of tube 10 is a removable top 22 which when removed, enables the placement of the fuel assembly within the fuel assembly bow twist measuring apparatus. Bottom reference plate 18 has locating holes 14 which receive the lower alignment pins 2 of fuel assembly 1. Extending from the underside of top 22 are locating pins 12 which engage alignment holes 3 in the top of fuel assembly 1. Each locating pin 12 has a longitudinal axis 12a which defines the longitudinal axis of the fixture. Each locating hole 14 similarly has a longitudinal axis 14a which also defines the longitudinal axis of the fixture. Depending upon the design of the fuel assembly, the longitudinal axis of the fixture can be either the longitudinal axis of a locating pin or the longitudinal axis of a locating hole. In some fuel assembly designs, the fuel assembly lower alignment pins and upper alignment holes are positioned to be collinear in which case the longitudinal axis of the locating pin and the longitudinal axis of the locating hole are also collinear. Locating pins 12 and locating holes 14 are positioned and oriented so as to position the fuel assembly upper tie plate 4a and lower tie plate 4b in relationship to each other as would be found when the fuel assembly is positioned in the reactofuel assembly fuel assembly 1 is placed in the fuel assembly bow twist measuring apparatus 5, fuel assembly position in the reactor core during operating conditions is thereby simulated. Although the configuration of locating pins 12 and locating holes 14 are dependent upon the particular fuel assembly design, the number and position of locating pins and holes in top 22 and bottom plate 18 can be changed to accommodate virtually any fuel assembly type and shape. Thus, while fuel assemblies for boiling water reactors generally do not have lower alignment pins and upper alignment holes, they can nonetheless be tested and measured in the apparatus of the present invention. The reference device includes small diameter reference wires 40(a, b, c, d, e, f, g, and h) which extend from top flange 16 through apertures 19 in bottom reference plate 18 and is secured at its lower end to floating plate 42. Apertures 19 are so positioned in bottom reference plate so that reference wires 40 are each parallel to the longitudinal axis 14a of locating holes 14, as well as the longitudinal axis 12a of locating pins 12 when the removable top is placed in position on the upper flange. Since the longitudinal axis of either the locating pin 12a or the locating hole 14a determines the longitudinal axis of the fixture, the reference wires are parallel to the longitudinal axis of the fixture. The weight of floating plate 42 provides constant tension in reference wires 40. The position of reference wires 40 provide a known relationship between the fuel assembly and the fixture. The number and position of reference wires 40 can be changed (together with the position and orientation of locating pins 12 and locating holes 14) to accommodate different fuel assembly designs. Extending longitudinally through tube 10 are slots 20 which provide openings for the measurement system. The slots 20 are positioned in tube 10 so as to provide a line of sight access to the reference wire 40 which is orthogonal to the fuel assembly 1 as shown in FIG. 2 and more particularly in FIG. 3. The measuring system comprises a transponder 60, a transponder holding block 62, cable 61 which connects transponder 60 to an ultrasonic flaw detector 65 which makes time of flight measurements of ultrasonic signals from transponder 60. As shown in FIG. 3, transponder 60 is positioned in a slot 20 so that it, a reference wire 40, and the specific region or detail of the fuel assembly (e.g. a fuel rod or a guide bar) are in a line of sight which is orthogonal to the detail of the fuel assembly to be measured. After being positioned in one of slots 20 and aimed toward the reference wire 40 and fuel assembly 1, transponder 60 in holding block 62 produces an ultrasonic signal which propagates through the water towards the fuel assembly. A portion of the ultrasonic wave is first reflected from reference wire 40 back to transponder 60 and the remaining wave which occurs later in time, is reflected from the fuel assembly detail back to transponder 60. These signals are then transmitted via cable 61 to ultrasonic flaw detector 65. The time differential between the two signals which is a measure of the distance from the reference wire to the fuel assembly is converted by ultrasonic flaw detector 65 by multiplying the time difference by the velocity of the sound wave through the propagating medium, which for irradiated fuel rods is water. This provides an accurate distance measurement from the reference wire 40 to the detail of the fuel assembly 1. Since the time of flight measurement is performed only from the reference wire to the fuel assembly feature, any error in determining the distance from the ultrasonic transponder to the reference wire is avoided. Thus, accurate positioning of the measuring device relative to a reference position as is attempted in the prior art devices, is not required by the present invention. Transponder 60 is moved from its initial position in a slot 20 to successive positions along the length of the slot where additional ultrasonic signals are produced, and reflected waves from the reference wire and the fuel assembly are received to yield distance measurements from the reference wire to the detail of the fuel assembly as a function of the height of the fuel assembly. The transponder 60 would then be moved to each of the other slots and measurements would again be taken along the height of the fuel assembly or at selected elevations. After placing the nuclear fuel assembly bow and twist measurement apparatus (with the exception of the ultrasonic flow detector 65) on a support surface such as the spent fuel pool floor 8, the apparatus is operated by removing top 22 and placing fuel assembly 1 guided by guides 28 into tube 10, and positioning the fuel assembly lower locating pins 2 in lower locating holes 14 in bottom plate 18. Top 22 would be installed onto top flange 16 and locating pins 12 would be inserted into the fuel assembly upper tie plate holes 3 to secure and position the assembly as if actually positioned within the reactor core. The ultrasonic transponder would be positioned in a slot and measurements taken. The signals received by the transponder from the reference wire and the fuel assembly are transmitted to the ultrasonic flaw detector positioned outside of the spent fuel pool where distance readings would be recorded manually or by a computer. The transponder would be moved in the slot or if desired in any of the other slots and measurements taken at selected locations along the height of the assembly. After all desired measurements were obtained, the top would be removed, and the assembly extracted. Data indicating variations in the distance from the reference wires to the fuel assembly details are a direct measure of bow. For measuring single axis assembly bow, distance variations from adjacent reference wires (e.g. 40a and 40b) on the same side of the fuel assembly will show equal displacement, e.g. x+.DELTA.x where x is the distance from the reference wire to the upper lower tie plate of the fuel assembly, while the corresponding readings from the reference wires on opposite side of the assembly (e.g. 40e and 40f) will show the same magnitude displacement but in the opposite displacement direction, e.g. x-.DELTA.x. (See FIG. 4A) Readings from the adjacent sides of the assembly will show no displacement readings when there is single axis assembly bow. Two axis bow is determined similarly with the exception that all four sides will show displacement magnitude and direction. For determining assembly twist, adjacent readings on the same side of the fuel assembly will show an opposite direction displacement. (See FIG. 4B). The displacement magnitude is a measure of the magnitude of the assembly twist. Comparison of corresponding reference data from the opposite side of the assembly will produce the same displacement magnitude but in opposite direction. An assembly with both bow and twist can be measured by a comparison of the differences in displacement magnitudes and directions from both corresponding opposite and adjacent reference measurements. One of the advantages of the present invention over the prior art is that bow and twist measurements of the fuel assembly positioned and constrained as if it was in the reactor core can be obtained. In addition, since the time of flight measurement is performed only from the reference wire to the fuel assembly feature, the distance from the ultrasonic transponder to the reference wire is irrelevant. Thus, precise positioning me the measuring device relative to a reference position as in the prior art devices, is not required by the present invention. Therefore, an additional advantage of the preset invention is that the precise positioning of the measuring device is not required thereby eliminating inaccuracies and/or the need to provide additional devices intended to correct for the improper positioning of the measuring device. A further advantage of the present invention is the use of ultrasonics for distance measurements which insures greater accuracy (.+-.0.10 inch) due to the elimination of the interaction between the measuring system and the fuel assembly. While the foregoing description and drawings represent the preferred embodiments of the present invention, it will be understood by those skilled in the art that various changes and modifications may be made therein without departing from the true spirit and scope of the present invention.