Patent Number: 048225584
Section: description

DESCRIPTIN OF THE PREFERRED EMBODIMENTS Turning to FIG. 1, a pressurized water reactor (not shown in detail) has a hemispherical bottom end wall 1 and supports a horizontal lower core plate 2, on the top face 3 of which there are supported fuel assemblies, of which one is shown and generally designated at 4. The fuel assembly 4 comprises a plurality of fuel rods 5 supported at their lower end by a bottom nozzle generally designated at 6. The nozzle 6 comprises a horizontal platform 7 having a bottom face or underside 8, as well as spaced legs 9 which extend downwardly from the platform 7 and stand on the top face 3 of the lower core plate 2, thus supporting upright the entire fuel assembly 4. Through axially aligned vertical passages 10, 11 and 12 provided, respectively, in the reactor wall 1, the lower core plate 2 and the platform 7 there passes a thimble 13 which is a slender stainless tube reaching to the top part of the fuel assembly 4 at its upper end and projectin out of the reactor wall 1 in a downward direction to be associated with measuring instrumentation, not shown. The thimble 13 serves the purpose of guiding therein a probe (also not shown) which travels vertically along the length of the fuel assembly 4 to sense the neutron flux. The thimble 13 is axially moved out of the fuel assembly during refueling and is subsequently reinserted into the fresh fuel assembly. In the zone between the reactor wall 1 and the lower core plate 2 the thimble 13 is surrounded by a permanently installed sleeve construction 14 whose securing elements and thimble guide components are generally designated at 14a. A thimble guide 15 constituted by an elongated sleeve-like component is inserted at the upper end of the passage 11 of the lower core plate 2 and extends vertically upwardly, in alignment with the passage 12 provided in the nozzle platform 7. The thimble guide 15 is rigidly held--for example, by a tightened threaded connection--in the core plate 2. The thimble guide 15 serves as a sliding guide and for positioning and stabilizing the thimble 13. Above the nozzle platform 7 the thimble 13 is positioned within an instrumentation tube 16 extending upwardly from the platform 7 and forming a permanent part of the fuel assembly 4. On the thimble guide 15 there is inserted an only symbolically shwon thimble guide extender generally designated at 17 for the particular purpose of providing a shield about the thimble 13 in the zone situated between the top of the thimble guide 15 and the underface 8 of the platform 7. The thimble guide extender 17 constitutes a first preferred embodiment of the invention, now to be described in detail with reference to FIG. 2 in which the extender 17 is shown in its installed state. The extender 17 has a longitudinal axis A and comprises a lower fitting 18 having a flange 19 and an upper fitting generally designated at 20, being vertically spaced from and generally in alignment with the lower fitting 18. The upper fitting 20 has a hollow cap 21 having an inner radial terminal shoulder 21a and provided with a passage 22 adapted to closely surround the thimble 13 (not shown in FIG. 2) and an integral or welded sleeve 23 having a radially inwardly extending flange 24 of downwardly flaring configuration at 25 and a narrow, annular contact area 25a. A cylindrical bellows 26 of circumferentially closed configuration is sealingly welded at its top to a radial lower face of the sleeve 23 and is, at its bottom, sealingly welded to a radial top face of the flange 19 of the lower fitting 18. The bellows 26 is resiliently deformable in several directions and may be made, for example, of No. 316 stainless steel. Within the bellows 26, coaxially therewith, there is arranged a retainer ring generally designated at 27 having an upper terminal radial face 31a. Approximately the lower two-thirds of the length of the retainer ring 27 is slit to form a plurality of circumferentially arranged, radially resilient legs 28 each having an inwardly extending bottom projection 29. The retainer ring 27 further has at its bottom, approximately at the axial location of the leg projections 29, radially outwardly extending ring segments 30. At the top of the retainer ring 27 there is formed a radially outwardly extending flange 31. The retainer ring 27 is, at its lower part, in a stabilizing engagment with the inner face of the lower fitting 18 by means of the ring segments 30. At its upper portion, the retainer ring 27 is in circumferential engagement with the surrounding sleeve 23 of the upper fitting 20. The circumferential contact between the sleeve 23 and the retainer ring 27 is along the narrow, annular contact area 25a. Such small area contact permits a rocking motion of the upper fitting 20 relative to the retainer ring 27 about an axis transverse to the longitudinal extender axis A for purposes to become apparent as the specification progresses. A cooperation between a lower radial face of the flange 31 of the retainer ring 27 and an upper radial face of the flange 24 of the sleeve 23 forming part of the upper fitting 20 limits an outwardly directed axial motion of the upper fitting 20 on the retainer ring 27 as urged by the bellows 26. The cylindrical bellows 26 is spacedly and fully surrounded by a cylindrical shield 32 which is secured at its bottom part to the lower fitting 18 and which has an open top through which projects the sleeve 23 of the upper fitting 20. The thimble guide extender 17 is installed on the thimble guide 15 previously secured to the lower core plate 2 from above, in the absence of the fuel assembly 4. For this purpose, the extender 17 is guided downwardly such that the tapered top part of the thimble guide 15 is introduced into the opening defined by the bottom part of the retainer ring 27 and is slid down thereon until the projections 29 of the respective legs 28 snap into a bottom circumferential groove 33 of the thimble guide 15. In this manner the retainer ring 27 is axially immobilized on the thimble guide 15 which closely fits into the retainer ring 27. In the absence of the fuel assembly 4 the biased bellows 26 ruges the sleeve 23 of the upper fitting 20 into abutting engagement with the flange 31 of the retainer ring 27 and, at the same time, the bellows 26 ruges the lower fitting 18 downwardly such that its smooth annular radial bottom face 18a surrounding the thimble guide 15 is in an annular contact with the top face 3 of the core plate 2. The fuel assembly 4 is lowered onto the core plate 2 such that the passage 12 of the nozzle platform 7 is in alignment with the passage 22 of the upper fitting 20 of the extender 17. The length of the extender 17 is designed such that before the legs 9 of the fuel assembly nozzle 6 contact the upper face 3 of the core plate 2, the upper annular radial surface area 34 of the cap 21 engages the bottom face 8 of the nozzle platform 7 and, as the fuel assembly 4 continues to move downwardly, the bellows 26 is compressed by virture of the downwardly moving upper fitting 20 causing an increasing, axially oriented resilient force of the bellows 26 to be exerted downwardly on the lower fitting 18 and upwardly on the upper fitting 20. Since the lower fitting 18 is free to slide axially along the ring segments 30 on the lower part of the retainer ring 27, no spring forces are taken up by the retainer ring 27; these forces are applied axially downwardly on the lower fitting 18 further pressing it into engagement with the upper face 3 of the core plate 2. The extender 17 is ready for use as the fuel assembly nozzle 7 assumes its supported position on the core plate 2 as shown in FIG. 1. The upper fitting 20, particularly by virtue of the only narrow annular contact area 25a between the sleeve 23 and the outer face of the retainer ring 27, is capable of executing a slight rocking motion about an axis transverse to the longitudinal extender axis A, readily permitted by the resilient bellows 26. By virtue of such rocking motion any deviation from a strict parallelism between the underside 8 of the nozzle platform 7 and the top face 3 of the core plate 3 and/or between the upper radial annular face 34 of the upper fitting 20 and the bottom annular face 18a of the lower fitting 18 are compensated for and thus face-to-face, sealing engagements between the upper fitting 20 and the platform 7 as well as between the lower fitting 18 and the core plate 2 are ensured. By virtue of the bellows construction and the above-described sealing engagement of the extender 17 with the bottom nozzle 7 and the core plate 2, the inside of the extender 17 is sealed from the surrounding environment, that is, the region between the core plate 2 and the platform 7 of the lower nozzle 6, without the need of accurately fitting, relatively sliding extender parts. Further, by virtue of its sealing contact with the underside 8 of the nozzle platform 7, the cap 21 of the upper fitting 20 seals the passage 12 in the nozzle platform from the surrounding environment and, with its upwardly tapering outer conical face allows the coolant water into platform passages 35 situated adjacent the thimble passage 12. Thus, the thimble extender 17 sufficient shields the thimble 15 from the coolant flow turbulences in the zone between the upper end of the thimble guide 15 and the underface 8 of the nozzle platform 7 and also in the passage 12 and the adjoining instrumentation tube 16. In sealing this area, the extender 17 defines a continuous flow path through the sleeve construction (instrumentation column) 14, the passage 11 and the instrumentation tube 16. By virtue of an orifice (not shown) at the top of the instrumentation tube 16 a reduced flow is obtained through this flow path, resulting, in turn, in smaller forces thus reducing the vibration-causing effect thereof. The cylindrical shield 32 surrounding the bellows 26 protects the bellows from flow turbulences that may cause vibrations thereof and further may be used --by virtue of the cooperation of its upper edge 36 with the underface 37 of teh sleeve 23--to limit the downward axial sliding motion of the upper fitting 20, thus preventing an excessive compression of the bellows 26 particularly during installation of the extender 17 on the thimble guide 15. Such a limiting effect may also be obtained by the cooperation beetween the radial face 21a of the cap 21 and the radial face 31a of the retainer ring 27. In FIG. 3 there is illustrated a thimble guide extender 40 constituting a second preferred embodiment of the invention and shown--similarly to the FIG. 2 embodiment--in its installed state, surrounding thimble guide 15 which is shown in section. The extender 40 comprises a lower fitting generaly designated at 41 having a radially outwardly oriented flange 42 at its upper portion and a cylindrical skirt 43 extending downwardly from the flange 42 and having, at its bottom part, a plurality of circumferentially distributed, radially inwardly projecting resilient locating fingers 44. An upper fitting generally designated at 45 has a cap 46 and a plurality of downwardly extending circumferentially distributed radially resilient fingers 47. The lower and upper fittinsg 41 and 45 are interconnected by a cylindrical, circumferentially closed resilient bellows 48 welded at the top to the lower part of the cap 46 and, at the bottom to the flange 42 of the lower fitting 41. Installation of the thimble guide extender 40 onto the thimble guide 15 is effected in a manner similar to that described in connection with the first embodiment. Thus, the upper conical terminus of the thimble guide 15 is, as the extender 40 is lowered, introduced into the bottom opening defined by the skirt 43 of the lower fitting 412 and the extender 40 is pushed down until the locating fingers 44 snap into the circumferential groove 33 of the thimble guide 15. The inner wall 42a of the flange 42 and the inner wall 43a of the lower part of the skirt 43 engage the respective outer surface portions of the thimble guide 15 with a close, sealing fit and by virtue of the locating fingers 44 engaging into the groove 33, the lower fitting 41 is axially immobilized on the lower part of the thimble guide 15. The sealing engagement with the bottom face 8 of the nozzle platform 7 and the upper annular face of the cap 46 is effected by the resilient force of the bellows 48. In this embodiment, however, the spring force of the bellows 48 is taken up at the bottom by the locating fingers 44 in the groove 33 of the thimble guide 15 and also, if required, by contact between the base of the skirt 43 with the top face 3 of the core plate 2. The lower seal is therefore effected by virtue of the close fit, between the lower fitting 41 and the thimble guide 15, rather than between the top face 3 of the core plate 2 and the lower edge of the skirt 43. The resilient finger 47 of the upper fitting 45 guidingly surrounding the thimble guide 15 permit a rocking motion of the upper fitting 45 about an axis transverse to the longitudinal axis A with the same result and advantages as described in connection with the first embodiment. Also similarly to the first embodiment, the resilient bellows 48 provides a fluidtight closure for the inside of the extender 40 without the need of small tolerances or closely interfitting, operationally sliding extender components. For limiting the extent of compression of the bellows 48, particularly during installation, that is, for limigint the relative motion between the lower fitting 41 and the upper fitting 45, the lower ends 47a of the fingers 47 are in alignment with an upper radial face 49 of the flange 42 of the lower fitting 41 whereby these components are brought into an abutting relationship upon a predetermined motion path which compresses the bellows 48. While in the two described preferred embodiments the sleeve-like sealing unit serves as an extender of and cooperates with a thimble guide mounted on a core plate, it is to be understood that the sleeve-like bellows-equipped sealing unit of the invention may find application in an environment which does not include a thimble guide. It will be understood that the above description of the present invention is susceptible to various modifications, changes and adaptations, and the same are intended to be comprehended within the meaning and range of equivalents of the appended claims.