Patent Number: 046831088
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, there is generally designated by the numeral 10 the core barrel of a nuclear reactor. Typically, there will be disposed within the core barrel 10 a fuel core assembly (not shown) of known construction, which is confined between upper and lower core plates. The core assembly is peripherally enclosed within a baffle comprising a zig-zag array of a plurality of baffle plates 11, with each baffle plate 11 being disposed substantially perpendicular to adjacent baffle plates 11. The space between the baffle plates 11 and the core barrel 10 is closed by a plurality of vertically spaced-apart former plates 12 which are seated in recesses 13 in the outer surfaces of the baffle plates 11 (see FIGS. 4 and 5). The baffle plates 11 are fixedly secured to the former plates 12 by a plurality of screw assemblies 15. More specifically, referring to FIGS. 2, 4 and 5, each baffle plate 11 has a plurality of screw bores 16 formed therethrough, each having an enlarged-diameter counterbore portion 17 at the inner end thereof which defines a recessed annular shoulder 18. Each of the screw bores 16 is disposed in axial alignment with a counterbore portion 19a of an internally threaded bore 19 in the associated former plate 12. In the original construction of the nuclear reactor, a suitable screw (not shown) is inserted in the bore 16 and threadedly engaged in the bore 19, the screw having a screw head which is recessed in the counterbore portion 17 and there lock welded to the baffle plate 11. Referring now in particular to FIGS. 2-6, there is disclosed a locking bolt apparatus 20 in accordance with the present invention, the apparatus 20 including a detent cutting tool 22 (FIG. 2), a screw 30, a locking cup 35 and a staking tool assembly 40. The screw 30 and the locking cup 35 cooperate, after operation thereon by the staking tool assembly 40, to provide a locking screw assembly 60 in accordance with the present invention. The detent cutting tool 22 is provided at its distal end with a cutting tip 24 carrying suitable cutting members 24a. The cutting tool 22 is adapted to be rotatably mounted on a suitable support and drive mechanism (not shown) which can be lowered into the core barrel 10 for inserting the cutting tip 24 axially into the counterbore portion 17 of a screw bore 16 which has had a damaged screw removed therefrom. The cutting tool 22 is then moved radially and rotated to machine a part-spherical, radially outwardly extending lateral recess 25 in the counterbore portion 17. Any desired number of such recesses 25 may be formed. In the preferred embodiment four equiangularly spaced-apart recesses 25 are machined in the counterbore portion 17. When the machining operation is completed the screw 30 can be inserted in the screw bore 16. The screw 30 has an elongated shank 31 provided at one end with an enlarged-diameter externally threaded portion 32. The other end of the shank 31 is provided with a radially outwardly extending annular shoulder flange 33 which preferably has a diameter slightly less than that of the counterbore portion 17 of the screw bore 16. Integral with the shoulder flange 33 and projecting axially therefrom is a hexagonal drive head 34 which has a lateral width substantially less than that of the shoulder flange 33. The locking cup 35 has a circular end wall 36 provided with a hexagonal opening 37 formed centrally therethrough. Integral with the circular end wall 36 around the outer periphery thereof and projecting therefrom coaxially therewith is a cylindrical side wall 38. Preferably, the cylindrical side wall 38 has a thickness substantially less than that of the circular end wall 36 so as to facilitate deformation thereof, as will be explained more fully below. The hexagonal opening 37 is dimensioned to receive the drive head 34 therethrough in close fitting relationship, i.e., in a frictional or slight interference fit, with the circular end wall 36 disposed against the shoulder flange 33. Thus, if desired, the locking cup 35 can be preassembled with the screw 30 before the screw 30 is mounted in the screw bore 16. The screw 30 is inserted into the screw bore 16 and threadedly engaged in the bore 19 until the shoulder flange 33 seats firmly against the shoulder 18 of the counterbore portion 17 with whatever torque value is required, as illustrated in FIG. 4. When the parts are thus assembled, the locking cup 35 is completely recessed in the counterbore portion 17, but the cylindrical side wall 38 extends upwardly a distance sufficient to completely cover the lateral recesses 25. The staking tool assembly 40 includes an elongated cylindrical die block 41 having a generally cylindrical bore 42 extending axially therethrough. Formed in one end of the die block 41 is a diametrical slot 43 which bifurcates the die block 41 and separates it into a pair of legs 44. Each of the legs 44 is provided with a flat part-circular bearing surface 45. Integral with the bearing surface 45 and projecting downwardly therefrom on each of the legs 44 is a die finger 46 shaped complementary to the recesses 25, each of the die fingers 46 having a downwardly and inwardly tapered inner cam surface 47 (see FIG. 5). The staking tool assembly 40 also includes an elongated cylindrical drive bolt 50 dimensioned to slidably fit in the bore 42. The drive bolt 50 has a frustonconical tapered drive end 51 in which is formed an axial bore or recess 52. In operation, the staking tool assembly 40 is carried by a suitable drive mechanism (not shown). The die block 41 is seated against the surface of the baffle plate 11 in surrounding relationship with the counterbore portion 17 of the screw bore 16, and with the die fingers 46 extending downwardly into the counterbore portion 17 and into the locking cup 35 in positions respectively opposite the lateral recesses 25. To facilitate accurate rotational positioning of the die block 41, suitable positioning marks may be provided on the surface of the baffle plate 11. When the die block 41 has been thus positioned, the drive belt 50 is then driven axially in the direction of the arrow in FIG. 5 for driving the tapered drive end 51 into camming engagement with the cam surfaces 47 and laterally deflecting the die fingers 46 radially outwardly against the cylindrical side wall 38 of the locking cup 35, thereby deforming the side wall 38 into the recesses 25 to form a plurality of locking projections 55 (see FIG. 6) which are engaged firmly in the recesses 25. This lateral deflection of the die fingers 46 is facilitated by the bifurcation of the die block 41. The staking tool assembly 40 is then removed and there results a completed locking assembly 60 (FIG. 6). The screw 30 is firmly threadedly engaged with the former plate 12. The locking cup 35 cooperates with the drive head 34 to prevent relative rotation of those two parts, while the engagement of the projections 55 in the lateral recesses 25 prevents any movement of the locking cup 35 with respect to the baffle plate 11. Thus, it will be appreciated that the screw 30 is securely retained against loosening rotation and, furthermore, the locking cup 35 acts securely to trap the screw drive head 34 and shoulder flange 33 to prevent accidental dislodgement thereof in the event of breakage of the shank 31, thereby resulting in an effective Class A locking device. It is a significant aspect of the invention that the recesses 25 are disposed beneath the surface of the baffle plate 11, thereby avoiding machining of that surface which can be embrittled due to reactor core exposure. Furthermore, in the event that an error is made in the machining of one of the recesses 25, the error is easily corrected since the entire circumferential surface of the counterbore portion 17 remains for the formation of other recesses 25. From the foregoing, it can been seen that there has been provided an improved locking screw assembly and apparatus and method for installation thereof, which is of relatively simple and economical construction, is suitable for remote underwater application, results in a Class A locking device, is suitable for use in replacing the baffle plate bolts of a nuclear reactor core barrel without machining the exposed surface of the baffle plate, and which involves relatively low risk of damage to the baffle plate or irreparable error in the installation operation.