Patent Number: 050358529
Section: description

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS The present invention provides a support pin system which is a nonwelded mechanical system in which a novel support pin fastens a first structural member to a second structural member. As described herein, the first structural member is a control rod guide tube and the second structural member is an upper core plate of a nuclear reactor. A first pin portion passes through a through bore in the control rod guide tube flange and has a threaded section which mates with a locking nut. A second pin portion has a solid body section and a split-leaf base section, the split-leaf base section frictionally engaging a bore provided in the upper core plate. The solid body section is accommodated by the bore by a close clearance fit. The split-leaf base section has an intermediate section with an outer diameter which is less than the outer diameter of the solid body section. Thus, loads applied transversely to the longitudinal axis of the support pins system are reacted substantially in pure shear through the solid body section of the support pin. By reacting to the transverse loads substantially in this manner, rather than through the bending moment of the leaves of the pin, the bending loads on the pin are substantially reduced and the bending stresses are substantially relieved. Moreover, the intermediate section has a tapering, conical outer diameter to maintain constant the stresses in the split-leaf base section. A washer is disposed around the first pin portion between the locking nut and the control rod guide tube flange and includes a concave spherical upper surface which is shiftable during mounting of the locking nut to compensate for nonperpendicular alignment between the support pin and the control rod guide tube flange. The support pin system preferably has a support pin fabricated from a nickel based alloy, most preferably strain-hardened 316 stainless steel which is more resistant to stress corrosion cracking than Inconel-750.sup.1 which also may be used. Such cold worked 316 stainless steel has an excellent operating history in reactor internals applications. The locking nut and spherical washer portions of the support pin system also preferably are formed of cold worked 316 stainless steel. The preferred material for the crimp cap and locking cap are 304 stainless steel. FNT .sup.1 Inconel is a U.S. registered trademark owned by the International Nickel Corporation. The invention can be better understood by referring to the Figures and particularly FIGS. 1 and 2 which illustrate elevational side views, partly in cross-section, of a novel, improved support pin system 8 constructed in accordance with the present invention. The cooperative parts of support pin system 8 are shown as being used in a nuclear reactor power plant for securing control rod guide tubes 10 to upper core plate 12. The support pin system 8 may be installed initially or may be retrofitted when an existing conventional guide tube support pin system requires repair or replacement due to failure under, for example, stress corrosion cracking conditions. The vertically positioned control rod guide tubes are secured to upper core plate 12 through annular guide tube flanges 14, one flange 14 being formed around the lower periphery of each guide tube. In order to interconnect guide tube flange 14 to upper core plate 12, a plurality of vertically disposed support pins 16 are used, two such pins 16 being shown substantially in full view in FIG. 1. The support pin orientation here is vertical, although any orientation would be within the scope of the invention. Pins 16 are disposed in a circumferential array within the annular guide tube flange 14, usually in pairs. An upper or first pin portion 18 is disposed within guide tube flange 14 and a lower or second pin portion 20 is disposed within upper core plate 12. In order to accommodate the disposition of the support pins 16, the upper surface of upper core plate 12 is provided, at locations corresponding to each support pin 16, with a bore 22 within which a lower, split-leaf base section 24 of the support pin is adapted to be frictionally inserted and retained in a biased engagement. Although bore 22 is shown as a through bore, in some applications it may be a blind bore, whereby the split leaves are retained therewithin should they shear. Guide tube flange 14 is provided, at locations corresponding to each support pin 16, with a through bore 26 for accommodating an intermediate shank portion 28 of support pin 16. Shank portion 28 and base section 24 of support pin 16 are integrally connected by an annular shoulder 30. A counter-bore 32 is defined within the lower surface of guide tube flange 14 proximate to upper core plate 12 to be coaxially or concentrically disposed with respect to guide tube flange through bore 26. In this manner, support pin annular shoulder 30 is appropriately accommodated and seated within the lower surface of guide tube flange 14. First pin portion 18 of support pin 16 has an upper end section 34 and an externally threaded section 36. Upper end section 34 of the support pin 16 projects vertically upwardly and axially outwardly from the guide tube flange through bore 26, along longitudinal axis 38 and is adapted to be threadedly mated with an annular, axially elongated, internally threaded, securing or locking nut 40. Internally threaded section 42 of nut 40 threadedly engages the externally threaded section 36 of support pin 16. Nut 40 fixedly retains guide tube flange 14 in its mounted mode upon each support pin 16, and therefore fixedly secures or fastens flange 14 of nuclear reactor control rod guide tube 10 on upper core plate 12. Extending downwardly from annular shoulder 30 of first pin portion 18 is a solid body section 44 of second pin portion 20. Solid body portion 44 has a constant outer diameter 46 which is accommodated by core plate bore 22 by a close clearance fit. The split-leaf base section 24 extends from solid body section 44 and has a split intermediate section 48 which extends from solid body section 44 and terminates in a split end section 50. Split end section 50 biasingly engages at least a portion of the wall of core plate bore 22, whereby support pin 16 is fixedly secured within upper core plate 12 by a frictional fit. Split intermediate section 48 includes two leaves 52 which are separated by longitudinal gap 54. As shown in the figures, gap 54 extends into solid body section 44 to increase the flexibility of split intermediate section 48. In an alternative embodiment, gap 54 may terminate without extending into solid body section 44. Split intermediate section 48 has an outer diameter 56 which is less than outer diameter 46 of solid body section 44 so that split intermediate section 48 does not engage the wall of bore 22. Thus, split-leaf base section 24 frictionally engages bore 22 at split end section 50, but not along split intermediate section 48. As a further improvement over the support pin of the '448 patent, split intermediate section 48 has a varying outer diameter 48 with a preferably uniformly tapered cross section such that outer diameter 48 is a maximum and is substantially equal to outer diameter 46 of solid body section 44 adjacent solid body section 44. Outer diameter 52 decreases along longitudinal axis 38 of support pin 16 in a direction toward split end section 50 of split-leaf base section 24 to a minimum diameter adjacent split end section 50. The varying of outer diameter 52 creates a split intermediate section 48 having a substantially fructroconical shape. Moreover, the tapered leaf design of split intermediate section 48 adds to the flexibility of the support pin and accommodates insertion displacements due to misalignment of the support pin and hole size interferences. The tapered shape of split intermediate section 48 is designed to develop sufficient preload to prevent fatigue of the leaves 52. The stresses in the leaves are limited by the interference preload and radial clearance between the second pin portion 20 of support pin 16 and the upper core plate bore 22. The present structure minimizes the bending stress on intermediate shank portion 28 by restricting bending of support pin 16 along its length by the close clearance fit provided between solid body section 44 and the walls of core plate bore 22. Prior art support pins were susceptible to stress cracking corrosion in the crotch portion where the split intermediate section joins the solid body section. Such prior art support pins had solid body sections and split intermediate sections having the same outside diameter so that applied loads reacted in bending as well as in shear and machining tolerances could be more relaxed to accommodate misalignments since the prior art configurations did not provide a close clearance fit. The present support pin 16 accommodates misalignments by increasing the length of second pin portion 20. The present support pin 16 also is preferably provided with a solid body section 44 having an outer diameter 46 which is greater than outer diameter 56 of the intermediate shank portion 28 of the upper or first pin portion 18, for additional strength when loads are reacted in shear therethrough. Furthermore, by forming split intermediate section 48 of a varying, tapering outer diameter 56 which is never greater than outer diameter 46 of solid body section 44 (and only equals outer diameter 46 at the common connecting plane between solid body section 44 and split intermediate section 48), the stresses in split-leaf base section 24 which reacts to loads in pure shear are reduced. The tapered design holds these stresses at a fairly constant level which is below the yield point of support pin 16. This reduces the chances of failure of the support pin during use. Preferably, guide tube flange 14 is approximately 2.38 cm thick and upper core plate 22 is approximately 3.81 cm thick, and support pin 16 is advantageously about 12.21 cm in length. Bore 22 provided in upper core plate 12 is a through bore and accommodates second pin portion 20 which has a length of 3.81 cm. Furthermore, the outer diameter of intermediate shank portion 28 is approximately 1.45 cm, the outer diameter of annular shoulder 30 is 2.78 cm, the outer diameter of solid body section 44 is 2.08 cm, the total outer diameter of split intermediate section 48 is less than 2.08 cm, and the total outer diameter of the split end section 50 in its free state is 2.13 cm. The diameter of core plate bore 22 is 2.10 cm such that a close clearance fit exists between core plate bore 22 and solid body section 44, but a much larger clearance exists between split intermediate section 48 and core plate bore 22. The locking system for support pin system 8 includes locking nut 40 having an internally threaded section 42 and a crimpable cylindrical section 58 extending from and integrally connected to internally threaded section 42. Alternatively, and as shown in the figures, the crimpable cylindrical section may be separate from locking nut 40 and may be formed on a separate crimp cap 60. Internally threaded section 42 threadedly engages externally threaded section 36 of first pin portion 18 of support pin 16. Threaded sections 42, 36 cooperate to retain guide tube flange 14 between locking nut 40 and annular shoulder 30. Guide tube flange 14 should be retained between locking nut 40 and solid body section 44 of second pin portion 20 of the support pin 16. Once locking nut 40 has been threadedly engaged upon threaded section 36 of support pin 16 and appropriately torqued to a predetermined load limit or value, it is desirable to insure that the pin and nut assembly remains intact in its assembled state to insure that guide tube flange 14, and therefore nuclear reactor control rod guide tube 10, remains positionally fixed with respect to nuclear reactor upper core plate 12. As seen in FIG. 2, upper end section 34 of support pin 16 above threaded section 36 is provided with a plurality of recesses 62, which are shown in as longitudinally or axially extending recesses 62 equiangularly spaced around support pin 16 at 90.degree. intervals and which serve as crimp receiving sections or grooves. For some modifications, the recesses need not be longitudinal, although longitudinal recesses are preferred. Upper end section 34 of support pin 16 passes through the coaxially aligned crimpable cylindrical section 58 of crimp cap 60, and once the crimp cap, locking nut, and support pin assembly is fully threadedly engaged and the predetermined torque load value or limit has been attained, crimps (not shown) are made by pressing upon and deforming portions of crimpable cylindrical section 58 into crimp receiving sections or longitudinal recesses 62 of support pin 16 as explained in the '448 patent. In practice, diametrically opposed crimps are formed by simultaneously pressing from opposite directions to operatively engage two diametrically opposed longitudinal recesses 62 formed upon upper end section 34 of support pin 16. This crimping operation must be performed at the location sites of the support pins 16, and must be performed by suitable, remotely controlled tools (not shown), whereby such crimping operations may be performed in an irradiated underwater environment without exposing maintenance personnel to the irradiated environment when support pin system 8 is retrofitted in an operating nuclear reactor. Spacial restrictions frequently do not permit use of an hexagonal nut since a hexagonal torque wrench cannot be positioned around the nut structure in an annular 360.degree. mode to impart the necessary torque to the securing nut. Accordingly, locking nut 40 is a spline nut and is provided with vertically oriented splines (not shown) disposed within the external surface in a circumferential array and alternatingly associated with adjacent spline grooves. In this manner, a suitable splined torque tool (not shown), can axially engage and apply rotational torque to the locked nut splines 66. Due to the spacial constraints and restrictions of the use environment, the splined torquing tool need not engage locking nut 40 in a complete 360.degree. annular relationship as is required with a conventional external hexagonal torque wrench. The external splined torque wrench may engage the locking nut splines over a circumferentially extending arcuate area of less than 180.degree., and the splined torque wrench stroke may be 36.degree.. The locking system for support pin system 8 also includes improvements over the support pin of the '448 patent. Disposed around intermediate shank portion 28 of first pin portion 18 below externally threaded section 36, between locking nut 40 and the upper surface of guide tube flange 14, are lock cup 64 and spherical washer 66. Spherical washer 66 includes a concave spherical upper surface 68 which serves as a stress reduction mechanism during mounting of locking nut 40 on first pin portion 18 during installation of support pin system 8 to guide tube flange 14 and upper core plate 12. The lower surface of locking nut 40 is formed with a complementarily-shaped convex spherical surface 70 which mates with spherical washer 66. Concave spherical upper surface 68 is shiftable during mounting of locking nut 40 to accommodate and compensate for a nonperpendicular alignment between either support pin 16 or locking nut 40 and guide tube flange 14. This virtually eliminates bending in intermediate shank portion 28 of support pin 16 during installation and eliminates any requirement of performing costly machining operations to machine a flat clamping surface during support pin replacement in operating plants. In operation, where guide tube flange bore 26 is not perfectly perpendicularly formed in guide tube flange 14, intermediate shank portion 28 of support pin 16 is tilted at a nonperpendicular angle relative the top of the flange. Upon installation, locking nut 40 would first contact an edge of the flange and try to flatten out to rest flat against the flange. This would bend the intermediate shank portion of the support pin. However, use of spherical washer 66 permits locking nut 40 to align in a nonperpendicular orientation relative guide tube flange 14 without imparting bending forces to intermediate shank portion 28, because locking nut 40 may rotate relative to spherical washer 66 so that the relative spherical portions matingly engage each other. Lock cup 64 serves to hold spherical washer 66 and locking nut 40 together as a unit to prevent separation during shipping. Lock cup 64 serves a similar purpose during and after installation and the lock cup is rotatable relative spherical washer 66 due to the clearance therebetween.