Patent Number: 046831169
Section: description

DETAILED DESCRIPTION OF EMBODIMENTS The apparatus shown in the drawings includes a reactor 11 (FIG. 1) having a pressure vessel including a generally cylindrical body 13 having a semi-spherical base and a dome-shaped head 15. The body 13 and the head 15 have flanges 17 and 19 which are engaged and sealed pressure tight by bolts 21. The body 13 has inlet nozzles 23 and outlet nozzles 25 for the coolant. In the lower part of the body 13 there is a core 27. The core 27 is encircled by a core barrel 29 having a flange 31 by which the core barrel 29 is suspended from a ledge 33 on the flange 17 of the body 13. The core 27 has a plurality of fuel assemblies 35. These fuel assemblies are conventional. Typically, such assemblies are shown in U.S. Pat. No. 4,522,780 granted June 11, 1985 to Shallenberger et al. for Removal and Replacement of Nuclear Reactor Fuel Assemblies, assigned to Westinghouse Electric Corporation. Each fuel assembly 35 includes a plurality of fuel rods 37 interposed between a top nozzle 39 and a bottom nozzle 41. Each fuel assembly 35 also has a plurality of thimbles 43. The thimbles are secured to the top nozzle 39 and the bottom nozzle 41 and bind the fuel assembly into an integrated unit. The manner in which the thimbles 43 is secured to the top and bottom nozzles is conventional. Typical structure is shown in Shallenberger et al. (supra). Each fuel assembly 35 has grids (not shown, but see Andrews supra) for holding the fuel rods 37 together. The fuel assemblies 35 are mounted between upper and lower core plates 45 and 47 which are supported by core barrel 29. Control rods (not shown) are moveable into and out of the thimbles 41 of each fuel assembly 35. The control rods associated with each assembly 35 are suspended in a cluster from a spider 49. The control rods are moveable in guides 51 above the upper core plate 45. The assembly of guides 51 is referred to as the upper internals of the reactor. The guides are supported in a generally cup-shaped member 53 having a flange 55 by which it is suspended above the barrel flange 31. The member 53 is also supported by columns 57 which extend between the upper core plate 45 and the member. The control rod clusters are each movable upwardly or downwardly by drive rods 59, which are operated by a mechanism (not shown) above the head 15. The core 27 also includes fuel assemblies 61 (FIGS. 1, 3) with which control rods are not associated. Each of the assemblies 61 includes a top nozzle 63, a bottom nozzle 65, fuel rods 67 interposed between the nozzles 63 and 65 and structural members or tie rods 69 (FIG. 4) secured to the top and bottom nozzles. Typically, each structural member may be threaded onto a screw 77 (FIG. 5) extending from the lower nozzle 65 and secured by a nut 70 to the top nozzle as shown in FIG. 3. The tie rods 69 bind the assembly 61 into an integrated unit. There is also a central tube 71 for instrument secured to the top and bottom nozzles. Such a central tube (not shown) is also included in each of the conventional fuel assemblies 35. The fuel rods 67 and structural members 69 are held together by grids 73 as disclosed in Andrews (supra). The grids 73 are prevented from being displaced longitudinally of the assembly 61 by the force of the sed in Andrews (supra). The grids 73 are prevented from being displaced longitudinally of the assembly 61 by the force of the sed in Andrews (supra). The grids 73 are prevented from being displaced longitudinally of the assembly 61 by the force of the coolant by bulges 75 (FIG. 8) in the structural members on both sides of each grid 73. The coolant flows at a high velocity, typically 50 ft./sec., and is under high pressure, typically 2000 lb./sq. inch. Typically, the structural members 69 have a substantially greater thickness than the thimbles 43 of the conventional fuel assemblies 35 and there are substbers 69 have a substantially greater thickness than the thimbles 43 of the conventional fuel assemblies 35 and there are substantially fewer structural members 69 in each assembly 61 than thimbles in each assembly 35. The remaining locations in each assembly 61 which correspond to those occupied by thimbles in each conventional fuel assembly 35 are occupied by fuel rods in the fixed assembly 61. For example, there are typically 24 thimbles in an assembly 35; in an assembly 61 there are only 8 structural members and 16 additional fuel rods in the remaining locations. The lower nozzle 65 is generally similar to the lower nozzle of the conventional fuel assemblies 35 except that it has fewer mechanisms, specifically screws 77 (FIG. 5) for securing the structural members at their lower ends. Thimbles 43 may be secured as shown in Shallenberger (supra). The top nozzle 63 is of smaller vertical height (or length) than the top nozzle 39 of a conventional fuel assembly 35 because it need not include a groove for receiving the spider 49 (FIG. 1) of the control rod cluster during scram. The structural member 69 (FIG. 4) includes a tube or shell or cladding 81 sealed by end plugs 83 and 85 at the top and bottom. The shell 81 has an extension 86 (FIG. 8) at the top to receive the end plug 83. The end plug 83 at the top has a threaded tip 87 to be engaged by the nut 70 (FIG. 3). The end plug 85 at the bottom has a tapped central hole 89 into which the screw 77 (FIG. 5) from the lower nozzle 65 is threaded. Burnable neutron-absorber pellets 91 are stacked in the tubes 81 of at least certain of the structural members. The others may be empty. Above the stack of pellets 91 there is a space in which there is a spring 93. The spring 93 is compressed between the plug 83 and a cylindrical member 95 which engages the upper pellet 91 of the stack thus maintaining the pellet stack rigid. The burnable neutron absorber burns out to a small residual absorption capability during the early part of a fuel cycle. In fabricating the fuel assembly 61, a skeleton 101 (FIGS. 5, 6) is first formed. The skeleton includes the tubes 81 of the structural members 69. Each tube is empty and is closed by the plug 85 at the bottom and is open at the top. The open tubes 81 are secured to the bottom nozzle 65. The skeleton also includes the instrument tube 71 which is also secured to bottom nozzle 65. The grids 73 are mounted spaced along the tubes 81. Each tube 81 passes through an array of coaxial cells 103 of the grids 73. Each cell 103 is of rectangular transverse cross-section but is lined by an annular sleeve 105 (FIGS. 7, 8) which extends a short distance above and below its associated cell. Each tube 81 engages the coaxial sleeves 105 along which it extends and is a sliding fit in these sleeves. Each tube 81 and the sleeve 105 through which it passes are bulged out to produce bulges 75 above and below each grid 73 by a bulge tool. Once the skeleton 101 is assembled, the burnable neutron-absorber pellets 91 are inserted in each tube 81. The spring 93 and the block 95 for transmitting the pressure of the spring to the stack is then inserted. The tubes 81 are then evacuated and back-filled with an inert gas, and plug 83 is welded to the top of each tube. The fuel rods 67 are then inserted in coaxial cells 103 in the skeleton 101 and the top nozzle 63 is secured to the structural members 69. The assembly 61 is now complete and may be inserted in the appropriate locations in the core 17. FIG. 2 shows the distribution of assemblies 35 and 61 in the core 17. The assemblies 35 are disposed in the cells 107 which are lettered. The assemblies 109 are disposed in the cells which are not lettered. While preferred embodiments of this invention have been disclosed herein, many modifications thereof are feasible. This invention is not to be restricted except insofar as is necessitated by the spirit of the prior art.