Patent Number: 046831088
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an apparatus and method for locking screws so as to capture them and prevent their loosening. The invention has particular application to the remote underwater replacement and locking of the screws, such as those which secure the baffle plates to the former plates or the former plates to the core barrel in the internal region of a nuclear reactor core. 2. Description of the Prior Art In a nuclear reactor the core assembly is disposed within a cylindrical core barrel and, more particularly, is confined within a zig-zag array of baffle plates which are spaced inwardly from the core barrel side wall. The space between the baffle plates and the core barrel side wall is closed by a plurality of vertically spaced-apart former plates. The baffle plates are secured to the former plates by a plurality of locking screw assemblies. Typically, the screws are received through complementary bores in the baffle plate and threadedly engaged with the former plate, the screw head being recessed in a counterbore portion of the screw bore. The screw heads are generally lock welded in place on the baffle plate. Occasionally, the baffle plate screws become distressed in use and must be removed and replaced. Such replacement is complicated by the fact that the baffle plates are now underwater in an irradiated environment. Therefore, remote replacement of the screws is essential. Because the work is done underwater, the replacement screws cannot be readily lock welded in place, as were the original screws. Furthermore, the environment does not permit extensive machining of the baffle plates. Nevertheless, it is desirable that the replacement bolts be locked in place with a so-called "Class A locking device", i.e., one which not only prevents loosening of the screw, but also captures the screw head and prevents it from becoming a loose part in the event that the screw shank should break. At present, no satisfactory technique exists for locking replacement screws in place with a Class A locking device. Prior locking arrangements for screw or bolt assemblies are known, but these prior arrangements include a portion to be folded up against a nut or screw head after tightening of the screw or bolt. Such arrangements require lateral access to the locking device to effect the folding or bending operation. Therefore, these arrangements would not be suitable for use in applications where the screw head is recessed in the counterbore. One type of screw locking technique which has been considered involves the machining of an annular groove around the counterbore of the screw bore, leaving a thin web of baffle plate material separating the groove from the counterbore portion of the screw bore. A staking tool would then be inserted in the groove to deform the web material into a notch or recess in the screw head. Such an approach has several disadvantages. First of all, it involves significant machining of the surface of the baffle plate, which frequently becomes embrittled due to reactor core exposure. Furthermore, if a mistake is made in the machining operation, it would be very difficult to redo or repair. SUMMARY OF THE INVENTION It is a general object of the invention to provide an improved locking screw assembly which avoids the disadvantages of prior assemblies while affording additional structural and operating advantages. An important object of the invention is the provision of a locking screw assembly which is of relatively simple construction and which leads itself to remote underwater application. Another important object of the invention is the provision of a locking screw assembly of the type set forth, which results in a Class A locking of the screw head. Yet another object of the invention is the provision of a locking screw assembly of the type set forth which is applicable to securing together two structures in the internal region of a nuclear reactor core, which assembly requires only minimal machining and which requires no machining of the exposed irradiated surfaces of the structures. Still another object of the invention is the provision of a method for installing the locking screw assembly of the type set forth. In connection with the foregoing object, it is another object of the invention to provide a method of the type set forth which can easily be repaired or corrected in the event a mistake is made. In connection with the foregoing objects, another object of the invention is the provision of apparatus for performing the method. Certain ones of these and other objects of the invention are attained by providing a locking screw assembly for securing together first and second structures in the internal region of a nuclear reactor core, wherein the first structure has a screw bore therethrough with a counterbore portion, the assembly comprising: a lateral recess formed in the counterbore portion, an elongated screw receivable in the screw bore for threaded engagement with the second structure, the screw having an enlarged shoulder flange dimensioned for seating in the counterbore portion and a drive head projecting axially from the shoulder flange and having a lateral width substantially less than that of the counterbore portion, and a locking member disposed in the counterbore portion against the shoulder flange and around the drive head for engagement therewith to prevent rotation with respect thereto, the locking member having a lateral projection engaged in the recess to prevent movement of the locking member with respect to the first structure. The invention consists of certain novel features and a combination of parts hereinafter fully described, illustrated in the accompanying drawings, and particularly pointed out in the appended claims, it being understood that various changes in the details may be made without departing from the spirit, or sacrificing any of the advantages of the present invention.