Patent Number: 048204790
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A typical fuel assembly 1 for a pressurized water nuclear reactor, partially illustrated in FIG. 1, includes a top nozzle 3, a bottom nozzle 5, and a plurality of guide tubes 7 extending between the top nozzle 3 and the bottom nozzle 5. The top nozzle 3 includes an upper hold down plate 9, a lower adapter plate 11, below, and spaced from, the upper hold down plate 9, and an enclosure 13 forming a sidewall and extending between the upper hold down plate 9 and the lower adapter plate 11. A fuel assembly 1 is positioned below the upper core plate 15 of the nuclear reactor so that the guide tubes 7 are aligned with the control rods (not shown) in the nuclear reactor core. In this way, the control rods can be inserted within, and withdrawn from, the reactor core through the guide tubes 7. The raised corners of the upper hold down plate 19 of the top nozzle 3 include second bores 19, which are aligned with first bores 21 in the upper core plate 15 using the guide pin assembly 23 of the invention to properly position the fuel assembly 1. Further, the corners of the enclosures 13 of the top nozzles 3 of the four adjacent fuel assemblies 1 form a recess 25 to receive a portion of the guide pin 27 of a guide pin assembly 23 extends below the second bore 19 so that it is disposed within the enclosure 13 of the top nozzle 3. Generally, two guide pin assemblies 23 are used, one each at opposite corners (17) of the upper hold down plate 9 of each top nozzle 3. The guide pin assembly 23 of the invention includes a guide pin 27, nut 29 and locking cup 31. The guide pin 27 has a generally circular cross-section, and includes a nose 33, a threaded portion 35, a shaft 37 and an end 39. The nose 33 of the guide pin 27 (FIG. 2) has engagement means 41 thereon which coact with the locking cup 31 to secure the locking cup 31 to the nose 33 once the guide pin 27 is ihserted within the first bores 21 in the upper core plate 15 and the nut 29 is tightened on the threaded portion 35. Preferably, the engagement means 41 is a plurality of elongated slots 43 spaced annularly about the nose 33. The nose 33 is pointed so as to facilitate the insertion of the guide pin 27 within the first bores 21 of the upper core plate 15. The threaded portion 35 of the guide pin 27, adjacent the nose 33, has a threaded outer surface 45. The threaded outer surface 45 coacts with the nut 29 to enable the nut 29 to be tightened against the threaded portion 35 of the guide pin 27. The shaft 37 of the guide pin 27, adjacent the threaded portion 35, has an upper section 47, a lower section 49, and a radial alignment section 51 between the upper section 47 and lower section 49. The radial alignment section 51 of the shaft 37 has a diameter that is approximately the diameter of the first bore 21 in the upper core plate 15 of the nuclear reactor fuel assembly 1 so that the guide pin 27 cannot move radially within the first bore 21. Preferably, the diameters of the upper section 47 and the lower section 49 of the shaft 37 are less than those of the radial alignment section 51 so that the guide pin 27 can be easily inserted within the first bores 21 of the upper core plate 15. The end 39 of the guide pin 27, adjacent the shaft 37, includes a top 53 having a diameter greater than that of the diameter of the first bore 21 in the upper core plate 15. The shoulder 55, resulting from the difference in diameters between the top 53 of the end 39 of the guide pin 27 and the lower section 49 of the shaft 37 of the guide pin 27, abuts the bottom surface 57 of the upper core plate 15 and serves to stabilize the guide pin 27 against vibration. The inner surface 59 of the top 61 of the nut 29 includes a first groove 63 to receive an annular flange 65 about the bottom 67 of the locking cup 31, and a second groove 69, disposed below the first groove 63. The inner surface 59 of the nut 29 further includes a tapped portion 71 that, when tightened against the threaded surface 45 of the threaded portion 35 of the guide pin 27, secures the guide pin 27 within the second bore 19 of the upper hold down plate 9 and the first bore 21 of the upper core plate 15. An annular flange 73 about the lower end 75 of the nut 29 is adapted to abut the upper surface 77 of the upper core plate 15 when the nut 29 is tightened on the theaded portion 35 of the guide pin 27. Thus, the annular flange 73 has a diameter greater than that of countersunk portion 79 in the upper end 81 of the first bore 21 of the upper core plate 15. The nut 29 may further include a plurality of indentations (not shown) spaced annularly about the outer surface 83 of the nut 29 by which a tightening tool can grip the nut 29 to tighten the nut 29 by about the guide pin 27. The locking cup 31 is securable, preferably permanently securable, about the nut 29. Thus, the annular flange 65 about the bottom 67 of the annular locking cup 31 is securable within the first groove 63 in the inner surface 59 of the top 61 of the nut 29, such as by a weld. The locking cup 31 coacts with the engagement means 41 on the nose 33 of the guide pin 27 when the nut 29 has been tightened onto the threaded surface 45 of the threaded portion 35 of the guide pin 27 so that the flange 73 of the pin 29 abuts the upper surface 77 of the upper core plate 15 of the nucler reactor. Thus, the locking cup 31 can be crimped about the slots 43 in the nose 33 of the guide pin 27. The guide pin assembly 23 of the invention is typically installed by first placing the end 39 of the guide pin 27 within a second bore 19 in the upper hold down plate 9 of the top nozzle 3 of the nuclear reactor. The nose 33 of the guide pin 27 is then inserted within the aligned first bore 21 in the upper core plate 15 from the bottom surface 57 of the upper core plate 15 until the shoulder 55 formed between the shaft 37 and the end 39 of the guide pin 27 contacts the bottom surface 57 of the upper core plate 15. The nose 33 and at least a portion of the threaded portion 35 of the guide pin 27 protrude above the upper surface 77 of the upper core plate 15. The nut 29 is then threaded onto the threaded surface 45 of the threaded portion 35 of the guide pin 27 and tightened to a torque of preferably between about 40 and about 45 foot pounds. The locking cup 31 is then crimped about the slots 43 in the nose 33 of the guide pin 27. The guide pin assembly 23 of the invention is remotely installable within the nuclear reactor. That is, the guide pin assembly 23 can be installed within the second bores 19 and first bores 21 of the upper hold down plate 9 of the top nozzle 3 and the upper core plate 15, respectively, in an operating nuclear reactor without the risk of exposing plant personnel to dangerous levels of radiation.