Patent Number: 046559914
Section: description

DETAILED DESCRIPTION OF THE INVENTION With reference now to the drawings, FIG. 1 illustrates a linear matrix array 82 of helical springs 90, such as may be used in a nuclear fuel assembly. The fuel assembly typically includes a matrix array of fuel rods wherein each fuel rod comprises an elongate tube containing a fissionable fuel material. Each tube is sealed at opposite ends by means of end plugs, e.g. as shown in U.S. Pat. No. 4,022,661, which engage corresponding tie plates. Helical springs 90 are fitted around shanks 92 of upper end plugs 88 to allow expansion of the fuel rods and to insure firm seating of the latter. In row B--B of matrix array 82, shank 92' of end plug 88' is shown with its helical spring missing. The location of shank 92' within array 82 is such as to be inaccessible to visual inspection without removing the upper tie plate. Probe 10, which constitutes the subject matter of the present invention, includes an elongate arm 12 formed of a rigid material such that the arm will not warp and lose its linearity with repeated use. Arm 12 has a substantially rectangular cross section, having a width defined by first and second parallel sides 16 and 18 enabling it to be inserted between adjacent rows of helical springs 90 and a height less than the height of the springs. The length of arm 12 is sufficient to probe the full length of a row of helical springs, e.g. the length of rows A--A and B--B in linear matrix array 82. Arm 12 includes a forward end 24 and a holding end 26. A rectangular aperture 28 is formed near the forward end of the arm. The height of aperture 28 is defined by surfaces 30 and 32, and its length by surfaces 34 and 36. A pivot pin 42 is transversely positioned in aperture 28, parallel to sides 16 and 18, and with opposite pin ends embedded in surfaces 30 and 32. First and second substantially identical pawls 48 and 50 are mounted in superposed relationship on pivot pin 42 within aperture 28. Each pawl is capable of rotational movement about pivot pin 42, between a retracted and an extended pawl position and each is constrained from rotating beyond substantially a ninety degree arc with respect to arm 12. Each pawl includes first and second pawl ends 52 and 54 respectively, a forward pawl surface 60 which has a rounded surface portion near pawl end 54, and a flat rear pawl surface 62. The angle of surfaces 60 and 62 relative to each other is such as to define a surface discontinuity at pawl end 54. In FIG. 1, the respective pawl portions are designated with letter subscripts to permit separate reference to each. The dimensions of the aperture and the pawls are such that the pawls are completely contained in the aperture in their retracted positions e.g. as shown by pawl 48 in FIG. 1. Both pawls are notched near their respective first ends 52 to receive a torsion spring 68 which is coaxially disposed on pivot pin 42. Spring 68 applies a bias to each pawl, urging end 54a of pawl 48 out of aperture 28 beyond side 18 of arm 12 and urging end 54b of pawl 50 out beyond side 16. A handle 74 is affixed to arm 12 near holding end 26 by means of suitable fasteners 76 and 78 such as screws or rivets and provides a hand grip for the manual operation of probe 10. Handle surface 80 provides a stop against outer structure 94 of the fuel bundle in which the fuel rods are located. This prevents the insertion of probe 10 into linear matrix array 82 to a point beyond the last row of elements C--C in array 82. In operation, in order to detect the absence of any helical springs missing in rows A--A and B--B of the array, probe 10 is inserted horizontally into the space between these rows by way of its forward end 24. As the probe is advanced into linear matrix array 82, contact between pawl surfaces 60b and 60a and the helical springs in rows B--B and A--A, urges pawls 48 and 50 toward arm 12 and thus toward their retracted positions within aperture 28, against the force of the torsion spring. Probe 10 is advanced into array 82 until handle surface 80 abuts outer structure 94. The length of arm 12 is chosen so that the absence of a helical spring in row C--C will allow the appropriate pawl to pivot to its extended position, but not so long as to allow the pawls to assume their extended position when helical springs 90a and 90b are present. The withdrawal of probe 10 proceeds smoothly by virtue of the rounded surface portions of surfaces 60a and 60b near pawl ends 54a and 54b respectively, which make sliding contact with springs 90 in both directions of probe movement. Thus, as probe 10 is withdrawn from the space between rows A--A and B--B, helical springs 90 on the shanks of end plugs 88 continue to urge pawls 48 and 50 toward their respective retracted positions. Hence, as long as all springs 90 are present, the pawls are prevented from assuming their respective extended positions. Testing of the array for absent springs 90 continues by insertion of the probe between subsequent row pairs, until the matrix has been completely checked out. If a helical spring 90 is missing, as indicated in FIG. 2 with respect to shank 92' in row B--B, pawl 50 will pivot to its extended position at the location in question, as urged by torsion spring 68. Upon attempted withdrawal of the probe, the extended pawl becomes lodged against springless shank 92'. Since the pawl is constrained from pivoting more than 90.degree. and the construction of the fuel bundle prevents lifting the probe out, the probe is effectively locked against withdrawal from the array. To retrieve the probe, the upper tie plate of the fuel bundle must be removed. The position of the locked-in probe then indicates to the operator the location of the missing spring, which is replaced before reassembly. The present invention is not limited to use in a linear matrix array. With a suitably curved arm, the probe may also be used where the elements tested for are arranged in curved rows. Also, the probe may carry only a single pawl for use in ascertaining the absence of an element in a single row of elements. It will also be clear that the use of the probe with linear matrix array 82 is not limited to testing rows of elements parallel to rows A--A and B--B, but that it may also be used along rows parallel to row C--C. In a broader sense, the present invention is not limited to testing for the absence of helical springs, but it is applicable to detect the absence of an element from any suitable array of dimensionally similar elements which are regularly spaced from each other. Further, the invention is not limited to the particular embodiment illustrated and described. Numerous variations, changes, modifications, substitutions and equivalents will now occur to those skilled in the art, all falling within the true spirit and scope of the invention. Accordingly, the invention is intended to be limited only by the scope of the appended claims.