Patent Number: 047088427
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. 1 of the drawings, there is depicted threin a portion of the core and the reactor lower internals. A fuel assembly generally designated by the numeral 10 includes a plurality of fuel rods 11, an upper outlet flow nozzle 12, and a lower inlet flow nozzle 13. Fuel rods 11 are parallel arranged and may be held in spaced relationship to each other by Inconel grids (not shown) as is well known in the art. A plurality of moderator control tubes 14 are interspersed among fuel rods 11. The Inconel grids (not shown) also support and are attached to the moderator control tubes 14. Upper outlet flow nozzle 12 is fixedly attached to the upper end 15 of the moderator control tubes 14 as by welding. A manifold 16, within upper outlet nozzle 12, flow connects the upper ends 15 of moderator control tubes 14. Similarly, the lower inlet flow nozzle 13 is seal welded to the lower end 17 of the moderator flow tubes 14. A lower manifold 18 flow connects the lower end 17 of all but one of the moderator flow tubes 14. Fuel assembly 10, as described, is of a type used with a light water, pressurized nuclear reactor which utilizes the spectral shift concept by varying the amount of deuterium oxide or heavy water within the fuel assembly, thereby varying the amount of light water moderator within the core. The deuterium oxide enters the fuel assembly via the channels 19 in the lower core support plate 20, through inlet seal connector 21, to lower nozzle manifold 18. The deuterium oxide is then distributed to the moderator control tubes 14 which, upon rising to the upper nozzle manifold 16, displaces the light water therein. The deuterium oxide then flows down the return flow moderator tube 22 to outlet seal connector 23 to the outlet channel 24 in the lower core support plate 20. A more complete description of fuel assembly 10, lower core support plate 20 and the flow path of the deuterium oxide may be found in copending U.S. patent application Ser. No. 626,847, filed Feb. 7, 1984, by R. K. Gjertsen, et al., entitled "Fuel Assembly" and assigned to Westinghouse Electric Corporation. It is to be noted, however, that once the desired amount of deuterium oxide is achieved within the moderator control tubes 14 and 22, the flow of deuterium oxide is reduced to the extent necessary to provide for any makeup heavy water required as a result of internal leakage and more importantly to provide a controlled continuous flow to remove radiation energy and to maintain the temperature of the heavy water below the boiling point. Hence, each seal connector 21 and 23 experiences the substantially same internal pressure. Moreover, since each seal connector 21 and 23 are located at the same axial core station, they each experience the same external pressure. Therefore, the inlet seal connector 21 is exposed to virtually the same environment as the outlet seal connector 23, and, by adjusting the internal pressure of the deuterium oxide to that of the coolant moderator at the inlet to the fuel assembly 10, there is substantially no pressure differential between the internal portion and the external portion of the seal connectors 21 and 23. Because of the substantially similar operating environment of the seal connectors 21 and 23, each seal connector may be made precisely the same and can be interchangeable. Details of seal connector 21 or 23 are shown in FIG. 2. Notwithstanding the near zero pressure differential internal and external of the seal connector, they are made to be leak free in the unlikely event of the existence of a negative or a positive pressure differential. A positive pressure differential coupled with a leaking seal connector could result in an inadvertent increase in moderation by an unplanned reintroduction of moderator within the fuel assembly 10. Obviously, such a condition is undesirable. On the other hand, a negative pressure differential coupled with a leaking seal connector could result in an unplanned decrease in moderator which is also undesirable. Seal connector 21 or 23 in general comprises a static upper portion 30 which is connected to the inlet end of fuel assembly 10, a movable lower portion 31 which is inserted within an insert 32, the lower core support plate 20, a bellows 33 connecting the upper 30 and lower 31 portions and a skirt 34 connected to upper portion 30 and which encircles bellows 33. Flow channels 35 and 36 are provided within upper portion 30 and lower portion 31, respectively, for purposes of introducing deuterium oxide or any other fluid (liquid or gas) which is less effective in slowing down neutrons than light water into the fuel assembly 10 or reintroducing the light water reactor coolant into said fuel assembly 10. Since the bottom or the moderator inlet end of fuel assembly 10 is inaccessible during the initial placement of fuel assemblies within the core, during fuel shuffling operations or during core refueling, a fixed or permanent mechanical connection between fuel assembly 10 and the lower core support plate 20 is not practical. Hence, seal connector 21 or 23 may be integral with or permanently affixed only at one end ot either the lower nozzle 13 or the core support plate 20. In the embodiment illustrated in FIG. 2, the static upper portion 30 of seal connector 21 or 23 is integral with flow nozzle 13 while the movable lower portion 31 comprises a slip fit which is spring loaded within and against core support plate 20 by a combination of fuel assembly hold down spring forces, the weight of the fuel assembly and a bellows spring force to be described hereinafter. The sealed fit between lower portion 31 and core support plate 20 is effectuated upon placement of fuel assembly 10 within the reactor core and is disconnected upon removal of fuel assembly 10. It is to be noted that an integral connection at the lower core support plate 20 with a slip fit at the flow nozzle 13 would be equally satisfactory and such alternative is intended to be included within the scope of the invention described herein. Still referring to FIG. 2, upper static portion 30 may be connected by a rolled bulge joint 37 within a boss or cylindrical extension 38 which is an integral part of lower nozzle 13. A plurality of rolled bulge joints 37 are provided so that a substantially leak free joint obtains between upper portion 30 and boss 38. In this type of joint, the thin cylindrical portion 39 of static portion 30 is rolled into grooves 40 in the internal surface of boss 38. Another equally satifactory and adequate mechanical connection may comprise a "Swage-lok" or other similar type of tube to housing connection. Still another type of seal joint may comprise threading upper portion 30 into boss 38 and seal welding around the periphery thereof provided the materials used are capable of being welded. One method to effectuate the seal between the movable portion 31 of seal connector 21 or 23 and the lower core support plate 20 comprises a ball and cone seal 50 in combination with a plurality of "piston ring" seals 51. In this regard, insert 32 is inserted within an opening 53 in core support plate 20 and is welded 54 thereto. Weld 54 serves as a mechanical and a sealing joint, with the latter being necessitated because of the need for opening 53 to be flow connected to either flow channel 19 or 24 in core support plate 20, which in turn provides flow communication between the flow channels 35 and 36 in seal connector 21 or 23. Ball and cone seal 50 employs principles which are well known in the art. A truncated conical surface 56 is provided at the upper end of insert 32. Surface 56 may be directly machined in insert 32 or may comprise a conical insert 57 made of a material substantially harder than the material from which lower portion 31 of seal connector 21 or 23 is made which is welded to lower portions 31 and then machined to final dimensions. While being slightly more complicated, the conical insert 57 is preferable in that it negates the probability of damage to the insert 32 integral with the lower core support plate 20. Movable portion 31 of the seal connector 21 or 23 includes a machined truncated spherical surface 58 which sealingly mates with conical surface 56. A plunger 59 extends from movable portion 31 and has a plurality of piston ring seals 51 fitted to grooves 60 machined in the periphery thereof. Ring seals 51 are of any design which is well known in the art and serve as a backup seal to limit any leakage into or out from the moderator control tubes 14 in the event that the main ball and cone seal 50 is damaged or otherwise fails to operate properly. A bellows 33 sealingly connects lower portion 31 to upper portion 30 of seal connector 21 or 23. Bellows 33 is made of metal and is welded at 52 and 55 to a cylindrical extension 61 from upper portion 30 and a cylindrical extension 62 from lower portion 31. Clearance space 63 is provided between upper 30 and lower 31 portions of the seal connector. Clearance space 63 allows for relative axial motion between the upper 30 and lower portions 31 which motion allows for a compressive load to be applied to fuel assembly 10 when installed in the reactor core and assures positive sealing of spherical surface 58 within conical surface 56. As illustrated, bellows 33 comprises a spring which transmits its compressive force, the compressive spring force applied to fuel assembly 10 and the weight of the fuel assembly to the lower portion 31 of seal connector 21 or 23. Metallic bellows 33 and clearance space 63 also allows for proper sealing notwithstanding any slight misalignment perpendicular to the axial centerline of the seal connector 21 or 23. Skirt 34 provides protection for bellows 33 and serves as an additional stop to limit the compressive travel of the movable lower portion 31 relative to the upper portion 30. Skirt 34 comprises a cylindrical tube 64 welded at 65 to an enlarged cylindrical portion 66 of the static upper portion 30. Skirt 34 extends down from said weld 65 covering the bellows 33 and terminates at end 67 which is spaced from insert 32 by a predetermined amount 68. Corresponding overlapping flanges or tabs 69 and 70 on movable portion 31 and skirt 34, respectively, limit the extension of space 63 when seal connector 21 or 23 is not fitted between nozzle 13 and core support plate 20. As described, seal connector 21 or 23 comprises a mechanically connected extension of a fuel assembly 10. Insert 32 is fixedly connected to core support plate 20. In the event seal connector 21 or 23 is damaged, it may be removed from fuel assembly 10 and replaced with a new seal connector. During such replacement, fuel assembly 10 is of course not assembled within a reactor core. When assembling a fuel assembly 10 equipped with seal connectors 21 and 23, normal procedures and precautions are utilized. Guide pins (not shown) as are commonly known in the art are used to guide the installation of fuel assembly 10 in its attachment to the core support plate 20. In this manner, the fuel assembly is properly aligned prior to any fit up between the seal connectors 21 and 23 and insert 32 so as to assure that the seal connectors 21 and 23 are not damaged during the installation procedure. Plunger 59 is further provided with a tapered entrance end 71 in the unlikely event of any misalignment of the fuel assembly 10 relative to core support plate 20. Taper 71 even further provides for minor misalignment, if any, of plunger 59 relative to opening 72 within insert 52. The transverse motion permitted by bellows 33, as previously described, then assures full sealing along the length of plunger 59. Contact between spherical surface 58 and conical surface 56 is effectuated after plunger 59 is substantially fully fitted within opening 72. Since bellows 33 exerts an extending force between upper 30 and lower 31 portions of the seal connectors 21 and 23, where contact is first made between the spherical 58 and conical 56 sealing surfaces, space 63 is greater than that shown in FIG. 2 by an amount approximately equal to the space 73 between flanges or tabs 69 and 70. The spring force on the exit end of fuel assembly 10 (not shown) as is commonly used in pressurized water nuclear reactors plus the weight of fuel assembly 10 serves to compress bellows 33 which then loads the spherical 58 and conical 56 surfaces. Upon completion of installation and the initiation of reactor operation, flow channels inthe core support plate 20 are in flow communication with the aligned flow channels 35 and 36 in seal connectors 21 and 23 which permit flow into and out of the moderator control tubes 14 and 15. Another embodiment of a seal connector 80 is shown in FIG. 3. In general, this embodiment differs from that of FIG. 2 by the provision of one or more ring seals 81 between the static portion 82 and the movable portion 83 of the seal connector 80. Ring seals 81 provide a backup seal in the unlikely event of failure of bellows 84. Ring seals 81 are fitted to grooves within end 85 of static portion 82; end 85 fits within an extension 86 of movable portion 83 which has an opening 87 therein and overlaps end 85. Further illustrated in FIG. 3 is an alternate method of attaching the static portion 82 to the lower nozzle 13. A flow connector 88 is inserted within an opening 89 in the lower nozzle 13. Opening 89 is in flow communication with either the lower nozzle manifold 18 and moderator control tube 22 (FIG. 1) depending upon whether seal connector 80 comprises an inlet connector or an outlet connector, rspectively. Flow connector 88 is welded 90 to lower nozzle 13. An extension 91 of the flow connector 88 fits within the upper end 92 of the static portion 83 of seal connector 80. As shown, static portion 83 is integrally seal connected to flow connector 88 by a plurality of rolled joints 93. As previously discussed, such connection may also comprise a "Swage-lok" type of connection. While the invention has been described, disclosed, illustrated and shown in certain terms or certain embodiments or modifications which it has assumed in practice, the scope of the invention is not intended to be nor should it be deemed to be limited thereby and such other modifications or embodiments as may be suggested by the teachings herein are particularly reserved especially as they fall within the breadth and scope of the claims here appended.