Patent Number: 047553510
Section: description

DETAILED DESCRIPTION OF EMBODIMENT The apparatus shown in the drawings is part of a nuclear reactor including a nuclear fuel assembly 11. The assembly 11 includes a skeleton formed of an upper nozzle 13, a lower nozzle 15 and a plurality of spaced screens 17 formed of straps interlaced in the manner of an egg crate to define aligned pocket 19. The nozzles 13 and 15 and the screens 17 are formed into a rigid body by thimble tubes 20 which extend around their peripheries and are joined to the screens. Fuel rods 21 are held in aligned pockets by springs (not shown) mounted on the part of the strap (not shown) bounding each pocket and dimples (not shown) in the part of each opposite strap bounding the pocket (see Andrews No. Re. 30047). The fuel assembly 11 is interposed between an upper core-support plate 23 and a lower core-support plate 25. These core plates 23 and 25 are connected to the core barrel (not shown) of the reactor. The upper nozzle 13 is in the shape of a plate of generally rectangular transverse cross-sections having a bottom which has holes 29 for transmitting the coolant and also holes (not shown) through which control rods 31, suspended from a spider 33, penetrate. The upper nozzle 13 also has, diagonally disposed, upwardly extending feet 35 and 37. The feet 35 are provided with load pads 39. The feet 37 are provided with axial holes. The upper core plate 23 has alignment pins 41 which extend generally axially with the holes. The pins 41 are of sufficient length to accommodate the full anticipated upward and downward movement of the fuel assembly 11. The plate which constitutes the upper nozzle is thin and the feet 35 and 37 are short compared to the corresponding components of the prior-art nozzle (because the springs are eliminated). For this reason the fuel rods can be extended and are longer than the fuel rods in prior-art assemblies. The lower nozzle 15 (FIG. 2) has a top 43 of generally rectangular transverse cross section having holes 45 for transmitting the coolant. Diagonally opposite feet 47 and 49 extend downwardly from the top 43. Each foot 47 is a snubber including a cup-shaped cylinder 51 (FIGS. 3, 4) within which an inverted cup-shaped piston 53 is slidable. A sealing ring (O-ring) 55 is embedded in the outer sliding surface of the piston 53. A helical compression spring 57 is connected to the piston 53 and cylinder 51 in such manner that it exerts downward pressure on the cylinder 51. Each leg 49 has a cylindrical hole 59 (FIG. 5) into which an alignment pin 62 projecting from the lower core-support plate 17 extends generally coaxially. A retaining spring 63 is interposed between the inner surface of the hole 59 and the pin 61. The spring assures that while the pin 61 is slidable relative to the hole, the pin engages the hole snugly so that vibrations of the fuel assembly 11 from which the leg 49 depends, under transverse forces of the coolant, are suppressed. A like retaining spring (not shown) is interposed between each pin 41 of the upper core support and the hole in leg 37 of the upper nozzle 13. In the use of the fuel assembly 11 in a reactor, the fuel assembly is in the state shown in FIG. 1 in the quiescent conditioning of the reactor with the coolant pumps (not shown) not in operation. In this state the piston 53 is advanced into the cylinder 51 by the weight of the assembly. Since the pressure of the coolant is high, coolant has penetrated into the cylinder under the piston notwithstanding the sealing ring 55. The upper nozzle 13 in this state of the reactor is spaced from the upper core plate but the pins are inserted in the holes in leg 37 and the pins 61 in the holes 59. When the coolant pumps are turned on, the coolant flows upwardly through the lower core plate 25, the lower nozzle 15, the interior of the assembly 11, the upper nozzle 13 and the upper core plate 23. The fuel assembly 11 is raised by the force of the coolant and the loads pads 39 engage the lower surface of the core plate 23 under pressure. The reactive force on the spring 57 causes the cylinders 51 to move downwardly so that the snubbers 47 assume the state shown in FIG. 2. The spring 57 causes the cylinder 51 to engage the upper surface of the bottom core plate 25. The spring 57 is dimensioned so that the pressure exerted by the cylinder on the core plate 25 is the required pressure. The retaining springs 63 suppress vibration of the fuel assembly. While a preferred embodiment of this invention has 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.