Patent Number: 047643404
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENT Relief of thermally induced stresses in a nuclear fuel assembly may require the use of a stress relieving fastener. The invention described herein is a nuclear fuel assembly stress relieving fastener capable of relieving thermally induced stresses developed in a nuclear fuel assembly when the nuclear fuel assembly is disposed in a nuclear reactor and when the nuclear reactor is brought to its operating temperature. A more satisfactory solution to the problems described above in the Background of the Invention is to employ a stress relieving fastener for attaching a first nozzle to a stud; the fastener having a deformable portion for relieving thermally induced stresses developed in the stud by the differential thermal expansion of the stud and the first nozzle. Therefore, this device comprises a stress relieving fastener nut which may include a small, thin and deformable ring disposed on the seating surface of the nut whereby the deformable ring contacts the first nozzle. This ring is machined integrally with the nut such that there is no potential for additional loose parts in the fuel assembly. During the process of mounting the first nozzle on the channel, the nuts are torqued on the studs to provide a small preload between the first nozzle and the channel. This eliminates the need for a small gap between the nut and the first nozzle. A joint, which is herein defined by the nut, stud, and first nozzle tightens as the fuel assembly is heated when the reactor is brought to its operating temperature. As the temperature increases, the yield and ultimate strengths of the nut are substantially reduced. The tightening of the joint and the reduction in yield and ultimate strengths cause the ring on the seating surface of the nut to deform. The deformation of the ring limits the thermal stresses in the studs to desirable levels. Referring to FIG. 1, the nuclear fuel assembly is referred to generally as 20 and comprises a first material first nozzle 30, a second material channel 40 and a first material second nozzle 50. The first nozzle 30, which may be 304L stainless steel, may be an elongated square approximately 5.5 inches on each side and approximately 4 inches in length. Channel 40, which may be Zircaloy, is disposed such that first nozzle 30 is mounted thereon. The channel 40 may be an elongated square approximately 5.5 inches on each side and approximately 16.8 inches in length. Attached, which may be by a plurality of Inconel-800 screws, to the lower portion of channel 40 is second nozzle 50, which may be 304L stainless steel. The second nozzle 50 may be a substantially elongated square approximately 5.5 inches on each side and 5.8 inches in length and having a substantially tapered lower end portion. The channel 40 is disposed substantially in vertical alignment with first nozzle 30 and with second nozzle 50. Again referring to FIG. 1, a water cross 60 which may be of the type generally used in the art is disposed in fuel assembly 20 and extends from near first nozzle 30 to near second nozzle 50, which water cross 60 defines a plurality of elongated chambers 70 extending the length of water cross 60. Water cross 60 generally comprises eight sides substantially defining a cross-shaped cavity 80 which is disposed in a horizontal plane perpendicular to the vertical axis of fuel assembly 20. Disposed in cavity 80 is a substance, such as water, for moderating neutrons produced by the fission of nuclear material. Disposed in each chamber 70 and extending substantially the length thereof is a plurality of cylindrical fuel rods 85 having nuclear fuel material therein. Referring to FIGS. 1 and 2, disposed in first nozzle 30 is a bearing member 90, which may be integrally formed with first nozzle 30, horizontally extending a predetermined distance from an inside vertical surface of first nozzle 30 and having an upwardly facing bearing surface 100 disposed thereon. Extending through bearing member 90 and bearing surface 100 is a vertical continuous aperture 110 for receiving an attachment device generally referred to as 120, which attachment device is capable of attaching channel 40 to first nozzle 30. Attachment device 120, which may be Zircaloy, may comprise a substantially L-shaped stud 130 having a horizontal leg 140 integrally formed with a vertical leg 150 which is disposed perpendicularly to horizontal leg 140. Disposed about a predetermined upper portion of the external surface of vertical leg 150 is a plurality of helically aligned longitudinal threads 160. An end 170 of horizontal leg 140 is attached, which may be by welding, to an inside surface of channel 40 at a predetermined location above fuel rods 85 such that leg 150 and threads 160 extend through aperture 110. Referring to FIGS. 2 and 3, engaged on stud 130 is a stress relieving fastener comprising a first embodiment of a nut 180 having a screw threaded bore 190 formed therethrough, which bore 190 matingly engages threads 160. Formed in a top-most surface 179 of nut 180 is a substantially rectangular first slot 181 having predetermined height and width and extending from one marginal edge of top-most surface 179 to the other marginal edge thereof. Slot 181 is capable of providing a means for engaging nut 180 on stud 130. Disposed on nut 180 is a deformable ridge 200 contacting bearing surface 100 when nut 180 is engaged on stud 130. The ridge 200, which is shown in FIGS. 3 and 7, is capable of deforming for relieving thermally induced stresses developed in stud 130. The ridge 200, which may be substantially recessed from the marginal edge of nut 180 downwardly extends a predetermined distance from the bottom surface of nut 180 and extends substantially circumferentially around the bottom surface of nut 180. Referring to FIGS. 4-6, engaged on stud 130 is a stress relieving fastener comprising a second embodiment of a nut 210 having a screw threaded bore 220 therethrough, which bore 220 matingly engages threads 160. Formed in a top-most surface 209 of nut 210 is a substantially rectangular second slot 211 have predetermined height and width and extending from one marginal edge of top-most surface 209 to the other marginal edge thereof. Slot 211 is capable of providing a means for engaging nut 210 on stud 130. Disposed on nut 210 is a deformable portion which may comprise a circumferential, deformable ridge 230 contacting bearing surface 100 when nut 210 is engaged on stud 130. As shown in FIGS. 5, 6 and 8, ridge 230, which may be disposed substantially flush with the marginal edge of nut 210 downwardly extends a predetermined distance from the bottom surface of nut 210 and extends substantially circumferentially around the bottom surface of nut 210. Referring to FIGS. 5, 6 and 8, the deformable portion of nut 210 may further comprise a circumferential, deformable first groove 240 having a predetermined height and formed in the lower portion of nut 210. First groove 240 extends substantially circumferentially around the external surface of nut 210 and horizontally extends from the marginal edge of nut 210 to substantially near the central longitudinal axis of nut 210. Again referring to FIGS. 5, 6 and 8, the deformable portion may further comprise a second groove 250 having predetermined height and width and formed in the bottom portion of nut 210 and horizontally extending from substantially near the marginal edge of nut 210 to substantially near the central longitudinal axis of nut 210. When reactor heatup occurs, thermal expansion occurs in stud 130 and first nozzle 30 causing ridge 230, first groove 240 and second groove 250 to deform such that the thermally induced stresses in stud 130 are relieved. During reactor heatup, stainless steel first nozzle 30 expands at a different rate than Zircaloy stud 130 because the thermal expansion rate for stainless steel is approximately three times that of Zircaloy. When the first embodiment of the nut is utilized, as the temperature of fuel assembly 20 increases, the joint defined by nut 180, stud 130 and first nozzle 30 tightens due to the differential thermal expansion rate between stud 130 and first nozzle 30. Thermal stresses develop in stud 130 because the expansion of first nozzle 30 is restrained by nut 180 and stud 130 when nut 180 threadedly engages stud 130 and when ring 200 contacts bearing surface 100. As the temperature increases, the yield and ultimate strengths of nut 180 decrease. Therefore, due to the joint tightening and due to the decrease in the yield and ultimate strengths of nut 180, ring 200 deforms thereby reducing the thermal stresses developed in stud 130. Similarly, when the second embodiment of the nut is utilized, as the temperature of fuel assembly 20 increases, the joint defined by nut 210, stud 130 and first nozzle 30 tightens, thereby deforming ridge 230, first groove 240 and second groove 250 such that thermally indluced stresses in stud 130 are relieved. Therefore, the invention described herein provides a nuclear fuel assembly stress relieving fastener for relieving thermally induced stresses developed in a nuclear fuel assembly when the fuel assembly is disposed in a nuclear reactor and when the reactor is brought to its operating temperature.