Patent Number: 048287822
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring first to FIG. 1, a typical BWR fuel assembly is shown, designated generally by numeral 1. The essential components of a BWR fuel assembly are: an array of fuel rods 2, an upper tie plate (not shown), a lower tie plate 4 and a fuel channel slip 6, and a number of fuel rod spacers 8. The fuel rods 2 are supported in a square array by the lower and upper tie plates. The fuel rod array ("the fuel bundle") typically comprises either a 7.times.7, 8.times.8, or a 9.times.9 square. For purposes of example, the fuel bundle will hereafter be assumed to comprise an 8.times.8 array. The fuel channel slip 6 fits around the fuel bundle to form the fuel assembly. The entire assembly typically weighs about 700 pounds and has a 5.33 inch outside dimension. The upper tie plate is a stainless steel, machined gridwork casting. The casting maintains the regular arrangement of fuel rods within a fuel rod assembly. The casting has welded to it a lifting bail 10 used for movement of the assembly. The lower tie plate 4 is also a stainless steel casting that provides grid holes for the fuel rod end plugs. Coolant flow is directed through the holes in the nosepiece into the lower tie plate grid 4, which distributes the flow to the fuel bundle. The fuel rod spacers 8 maintain even lateral spacing of the fuel rods 2, and suppresses fuel rod vibration. Each spacer 8 is a lattice with finger springs that press laterally against the walls of the fuel rods. Referring now to FIG. 2, a bottom view of the BWR fuel assembly is shown. A nosepiece 12 with an inverted tripod 14 extends downward from the fuel assembly 1. Looking up into the fuel assembly, the lower tie plate 4 can be seen, comprising a grid having a plurality of first apertures 16 for housing the end plugs 18 of each fuel rod 2, and a plurality of slightly larger second apertures 20 which allow coolant water to flow between the fuel rods during operation. In the exemplary 8.times.8 fuel assembly, there are 64 first apertures 16 and 49 second apertures 20. In accordance with the present invention, an ultrasonic probe is inserted up through the tripod 14 and successively inserted into any of the second apertures 20. While the probe could be inserted into all 49 second apertures it is actually only necessary to access 16 of the second apertures (depicted in FIG. 3 with small circles and identified by reference numeral 20A) since the probe can be rotated in each aperture 20A to examine each of the fuel rods in the surrounding first apertures 16. As is evident from FIG. 2, many of the apertures are obstructed by the nosepiece cover and the presence of the inverted tripod. Accordingly, in the present invention, a flexible probe passes through a single pivot point in or slightly below the tripod to facilitate access to each of the apertures 20. Referring now to FIG. 3, a simplified representation of the lower tie plate 4 is portrayed, showing the location of each of the 64 fuel rods in the exemplary 8.times.8 array of the BWR fuel assembly. As can be seen from FIG. 3, only 16 of the fuel rods are substantially visible through the nosepiece from a straight bottom view of the fuel assembly. The inverted tripod 14 forms three tridents I, II and III with respect to the exposed fuel rods. As is evident from FIG. 3, tridents I and II are symmetrical with respect to the fuel rods, while trident III is non-symmetric. The present inventor has found that if the probe passes through a single pivot point 22 in or slightly below trident III, the non-symmetric trident, each of the fuel rods can be readily accessed by passing the probe into any of the 49 second apertures 20, or preferably only the 16 apertures 20A (if the probe is rotated). Moreover, the present inventor has found that if this single pivot point 22 is located substantially centrally in or slightly below trident III, the access to each of the apertures 20 is made most accessible. Multiple single pivot points, one in each trident, can also be provided to allow multiple inspections to occur in parallel. In order to rotate the probe about the pivot point, many different embodiments of the present invention are possible, three of which are described herein. In a first embodiment, shown in FIG. 4, a fixed ball joint 24 is located at the pivot point 22. The ball joint 24 has an aperture 26 through which the probe 28 passes, and the probe is pivoted at its lower end by means of an x-y scanning bridge (not shown in FIG. 4). Alternatively, in a second possible embodiment of the invention, the probe is seated at its lower end in a two-stage goniometric cradle which, when rotated, pivots the probe about the single pivot point. In a still further embodiment of the invention, the probe is seated in a goniometric cradle disposed on a rotational table, which, when rotated, causes the probe to pivot about the single pivot point. In either of these last two embodiments, the center of rotation of the cradle must be located external to the body of the cradle, preferably about 9 inches up. In order to access the apertures 20 from a single pivot point, the probe 28 of the present invention has a flexible midsection 30 which allows it to bend. The probe is provided with a bullet-shaped nose 32 to guide the probe into each opening, and to protect the probe as it contacts the edge of each aperture 20. In the preferred embodiment, a Krautkrammer transducer serves as the active UT portion of the probe 28 in the preferred embodiment, although any appropriate commercially available ultrasonic device may be used. Probe 28 may utilize one transducer which both sends and receives the ultrasonic signal, or separate transmit and receive transducers may be provided. The probe operates by sending out ultrasonic signals which vibrate the outer shell of the fuel rod; the presence of water inside the fuel rod will affect the vibration of the outer shell of the fuel rod, but more significantly, will dampen the amplitude of the reflected ultrasonic signal. To test the fuel rods, the probe is rotated until a maximum signal is received. In the preferred embodiment of the invention, the return ultrasonic signal is integrated, and a threshold detector is used to determine whether the signal is of appropriate strength, indicating no water in the particular fuel rod. The amplitude of the return signal must be greater than a prescribed threshold value for a fuel rod to pass. The probe is then rotated 90.degree. to test the next fuel rod, and so on around the entire 360.degree.. Alternately, the probe may be rotated continuously through the entire 360.degree., and the rods' return signal captured on the fly. Next, the probe is brought down beneath the aperture 20, pivoted about the single pivot point, and reinserted into the next aperture, and so on until all the fuel rods of the assembly have been tested. The novel arrangement of the present invention allows each fuel rod to be examined individually and therefore isolates a leakage problem to a particular fuel rod. The arrangement of the present invention is also extremely accurate, requiring a reinspection rate of only 0.05%, as compared to a typical reinspection rate of 1.5% for the previously-described vacuum sipping techniques. Although the present invention has been described in connection with a plurality of preferred embodiments thereof, many other variations and modifications will now become apparent to those skilled in the art. For example, the novel positioning design of the invention can be used to position other tools in the nosepiece, at the bottom of the fuel rods, or in the flow paths between the fuel rods to perform other inspection or maintenance tasks (e.g., visual inspection, machining and polishing operations, welding operation, debris retrieval tasks, rod to rod gap measurements, crud sampling and removal, fuel rod dimensioning, eddy current inspection transducers, Electro Magnetic Acoustic Transducer inspection, rod reactivity measurements, serial number verification, inspection of lower fuel spacers, rod vibration, oxide measurements, flow channel blockage analysis and temperature measurements). It is preferred, therefore, that the present invention be limited not by the specific disclosure herein, but only by the appended claims.