Patent Number: 047724480
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates generally to a support pin system which is a non-welded mechanical system and, more particularly, to a novel support pin system which may include a novel locking nut and/or a novel locking nut retainer, for utilization especially in connection with nuclear reactor control rod guide tubes to fixedly secure the control rod guide tubes by means of their annular flanged portions to the upper surface of the upper core plate of the reactor by means of such a nonwelded mechanical system. 2. Description of the Prior Art Support pin systems for fastening a first structural member to a second structural member by means of a support pin and nut are utilized in applications requiring a non-welded mechanical system, such as the support pin system for fastening a nuclear reactor control rod guide tube flange to a nuclear reactor upper core plate whereby control rod guide tubes are secured in the upper reactor internals between the upper core plate and an upper support plate. Within a nuclear reactor, as is well known, the upper boundary of the reactor core is defined by means of an upper core plate. The upper ends of the nuclear reactor core fuel assemblies are connected to the under surface of the upper core plate, as shown, for example, in U.S. Pat. No. 4,173,153 to Obermeyer et al, the disclosure of which is herein incorporated by reference. Each fuel assembly has fuel rods and has defined therein numerous locations within which nuclear reactor control rods are capable of being disposed for regulating the power output of the fuel assemblies and the reactor core. Protection for the control rods is provided, with respect to the crosscurrents of the flowing nuclear reactor core coolant, through means of guide tubes which are interposed between and fixedly connected to, the upper surface of the upper core plate and an upper support plate which is disposed above and vertically spaced from the upper core plate. The guide tubes are provided with annular flanges at the lower ends thereof and guide tube support pins fixedly interconnect the guide tube flanges to the upper core plate. The vertically disposed guide tube support pins have split-leaf lower portions which are disposed within and frictionally engage suitable bores within the upper core plate, and upper bolt portions which pass through through-bores provided in the guide tube flange and threadedly engage suitable, internally threaded nuts. Counter-bored portions of the guide tube flange are generally retained between an annular shoulder portion of the support pin shank and the mated nut. In order to prevent the retrograde rotation of the nut relative to the support pin whereby the nut may possibly become disengaged from the threaded portion of the guide tube support pin, a dowel pin is conventionally passed through the nut and the exposed ends welded to support pin locking tabs. While the aforenoted conventional locking system is entirely satisfactory when the various components of the nuclear reactor system are initially installed prior to commencement of plant operations, once plant operations have commenced, and the support pins require repair or replacement, for example, due to failures as a result of stress corrosion cracking problems, repair or replacement of the aforenoted welded-type dowel pin locking system cannot be simply effectuated in view of the fact that such welding operations must now be performed remotely in order to protect maintenance personnel from radiation exposure within the irradiated environment, and the operations must also be performed in a spacially restricted or confined underwater environment. Further, welding to secure the nut to the support pin, whether performed directly or indirectly by means of the aforementioned dowel pin and tab system, is disadvantageous. The welding heat adversely affects the pre-load of the nut torqued onto the support pin, which preload provides resistance to flow-induced vibration. Further, structural material adjacent to the welded area may be adversely affected, the welding heat tending to promote subsequent corrosion and stress cracking of the affected materials. A dual crimp locking system has been disclosed in the copending, commonly-assigned, U.S. patent application Ser. No. 576,645 by J. T. Land et al, filed on Feb. 3rd, 1984, the disclosure of which is herein incorporated by reference. A support pin having a split-leaf base section, an externally threaded, upper bolt portion and a top end portion provided with vertical grooves is disclosed in which the grooved top end protrudes through a securing nut threaded onto the upper bolt portion. The nut is provided with vertical splines and the locking system is secured by means of a stepped tubular cap which is crimped into place around both the top end portion of the support pin and the securing nut whereby retrograde rotation between the support pin and nut is positively prevented. Such a fastening arrangement, however, depends for its efficacy on the structural integrity of the crimped cap, i.e., the stepped tubular cap, as well as that of the support pin. The structural configuration of these support pins, however, subjects them to both high shank preload stresses and to high bending loads and stress corrosion cracking and shearing, especially of the shank portion above the annular shoulder, has been observed, especially for the heretofor preferred Inconel-750 material. The affected nuclear reactor guide tube is thus left without a properly secured support. Moreover, the longevity of the nuclear reactor as a whole may be seriously compromised by dislodged parts which may be propelled through the system by the swiftly flowing nuclear reactor core coolant and damage other power plant components, such as the steam generator. When the shanks of conventional support pins shear under the combined influence of the high preload and high bending stresses, the upper bolt portion of the support pin, along with the attached nut and/or crimped cap, may subsequently dislodge under the influence of the coolant flow. The nut, crimped cap or dowel pin, etc., may also disengage. The split leaf base section and/or one or more of the leaves may disengage, all with deleterious result. Further, all presently-known guide tube support pin and locking systems have one or more of the above recited disadvantages. Accordingly, it is an object of the present invention to provide a new and improved support pin system, which is a non-welded mechanical system, and is useful especially in connection with nuclear reactor control rod guide tubes. It is another object of the present invention to provide a locking nut retainer for positively retaining a nut in position around an elongate threaded element when the retainer is positioned within and accommodated by a structural member having the elongate threaded element disposed therein, such as for positively retaining a nut in position around a support pin for fastening a nuclear reactor control rod guide tube flange to a nuclear reactor upper core plate. It is yet another object of the present invention to provide a locking nut for positively preventing relative rotation of the nut when threadedly engaged to an elongate threaded element, which locking nut may be used in the above support pin system and/or may be used in conjunction with the above locking nut retainer. SUMMARY OF THE INVENTION The foregoing and other objectives are achieved in accordance with the present invention through the provision of a new and improved support pin system for fastening a first structural member having a through-bore defined therethrough to a second structural member having a bore defined therein, which system includes a support pin having a longitudinal axis, a first pin portion and a second pin portion, the first pin portion being disposed within and passing through the through-bore and having an externally threaded section, and the second pin portion being fixedly secured within the bore and having a solid body section and a split-leaf base section, the solid body section having an outer diameter which is accommodated by the bore by a close clearance fit, the split-leaf base section having a split intermediate section which extends from the solid body section and has an outer diameter which is less than the outer diameter of the solid body section, and a split end section which extends from the split intermediate section and biasingly engages at least a portion of the wall of the bore, whereby the support pin is fixedly secured within the second structural member by a frictional fit and whereby loads applied transversely to the longitudinal axis of the support pin are reacted substantially in pure shear by the second pin portion substantially through the solid body section; and nut means having an internally threaded section which threadedly engages the externally threaded section of the first pin portion and cooperates therewith, whereby the first structural member is retained between the nut means and the solid body section of the support pin. The first pin portion of the support pin system may further include an end section positioned remotely from the second pin section and having a plurality of recesses, preferably longitudinal recesses, provided on the external surface thereof. When the nut means is a locking nut and further comprises a crimpable cylindrical section, which crimpable cylindrical section extends from and is integrally connected to the internally threaded section and crimpingly engages at least one, but preferably at least two, most preferably at least four, of the plurality of recesses of the support pin in use, relative rotation between the locking nut and the support pin is positively prevented. The support pin system may further encompass a locking nut retainer which includes a split cylindrical wall portion which is crimpable and has an axial slot defined therein; and tabe means which extends radially from the wall portion and is positioned along the external surface thereof. The locking nut retainer is positioned around at least a portion of the nut means. When the first structural member is provided with a counter-bore having an annular recess radially defined in the wall thereof, the counter-bore accommodates at least the portion of the wall portion of the locking nut retainer which includes the tab means, the tab means is positioned within the annular recess, and the locking nut retainer is positively retained in the first structural member. Further, when the nut means further comprises a crimp receiving section which has at least one recess provided in the external surface thereof, at least a portion of the wall portion of the locking nut retainer crimpingly engages the at least one recess and the nut means is positively retained in position around the support pin and relative rotation therebetween is positively prevented. The nut means may be the aforementioned locking nut. In the above described support pin system, the first structural member may be a nuclear reactor control rod guide tube flange and the second structural member may be a nuclear reactor upper core plate. The foregoing and other objectives may be further achieved in accordance with the present invention through the provision of a locking nut retainer. A locking nut retainer, adapted to be positioned within and accommodated by a structural member, such as a nuclear reactor upper core plate, having an elongate threaded element disposed therein for positively retaining a nut in position around the elongate threaded element, includes a split cylindrical wall portion which is crimpable and has an axial slot defined therein whereby the locking nut retainer is rendered substantially resiliently compressible along the radial axis thereof for insertion thereof during use. The wall portion is adapted to be positioned around the external periphery of the nut and to crimpingly engage at least a portion of the periphery of the nut in use. The locking nut retainer also includes tab means extending radially from the wall portion along the external surface thereof. The structural member is provided with a bore for accommodating the locking nut retainer and the elongate threaded element, and the bore has an annular recess defined therein for accommodating the tab means. Thus, the locking nut retainer is positively retained in the structural member and the nut is positively retained around the elongate threaded element when crimpingly engaged by the locking nut retainer. In a preferred locking nut retainer embodiment, the tab means is a pair of tabs positioned opposite one another, equidistant from the axial slot of the wall portion, and the nut has a plurality of splines and a plurality of spline grooves positioned along the external surface thereof, the splines being alternatingly associated with the spline grooves. The wall portion of the locking nut retainer crimpingly engages at least two spline grooves in use. Most preferably, two spline grooves are crimpingly engaged in use, the engaged spline grooves being spaced 144.degree. apart. BRIEF DESCRIPTION OF THE DRAWINGS Various other objects, features, and attendant advantages of the present invention will be more fully appreciated as the same becomes better understood from the following detailed description when considered in connection with the accompanying drawing, in which: FIG. 1 is an elevational side view, partly in cross-section, of a support pin system according to the present invention, which includes a vertically disposed support pin, a locking nut and a locking nut retainer; FIG. 2 is a plan top view of a portion of the annular flange of a nuclear reactor control rod guide tube including the support pin system of FIG. 1; and FIG. 3 is a plan top view of a locking nut retainer according to the present invention and as included in the support pin system of FIG. 1.