Patent Number: 049960210
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT; The numeral 10 in FIG. 1 generally designates a fuel assembly for a pressurized water nuclear reactor. The fuel assembly 10 includes an upper end fitting 12 and a lower end fitting 14 connected by a plurality of guide tubes or thimbles 16 for receipt of control elements in known manner. Cell-defining spacer grids 18, 20 and 22 have fuel or poison rod support features in the form of arches 24 and opposing springs 26, in known manner, and are secured at spaced intervals to the guide tubes 16 with their cells in register. The fuel rods or poison rods 30 are dotted-in in FIG. 2. The preferred embodiment of the invention is shown in FIGS. 3 and 4 in which the novel attachment of the zircaloy guide tube 16 to the stainless steel lower end fitting 14 is illustrated. The main feature of the attachment is a fastener 40 in the form of a stainless steel bolt having a head 42 configured for receipt of a hex key or wrench for torquing. A peripheral slot 44 can be used for staking to lock the bolt 40 against rotation. The shank of bolt 40 includes threads 46 through which a longitudinal slot or groove 48 is cut. The groove 48 creates a plurality of points 50 which tend to dig into mating threads in a zircaloy plug 52 having a threaded central passageway of threads mating with threads 46 to increase the frictional grip therebetween. The slot or groove 48 of the stainless steel fastener 40 further provides "give" to avoid cracking of either the plug 52 or guide tube 16 due to thermal dimensional changes during temperature recycling in the reactor. The end 54 of the fastener 40 opposite the headed end 42 and lower end fitting 14 into which it is countersunk in bore 56, has a transverse opening 58 in fluid communication with groove 48. This enhances coolant flow out of the guide tube 16 and flushes it, as well as giving it a "dashpot" or retarding function upon the insertion of a control rod. Between the zircaloy plug 52 and zircaloy guide tube 16 is the bottom of a stainless steel tube receiving cup 60. The cup 60 extends upwardly a distance sufficient to permit the lower grid 22, which is of stainless steel, to be attached to it. The aperture or bore 62 or cup 60 permits passage of the stainless steel fastener 40 therethrough to accomplish the fastening function. Because it is of stainless steel, as is the lower end fitting and the fastener itself, the thermal expansion characteristics of these three elements are substantially the same, thus permitting a firm and secure attachment between them even during temperature cycling. The end zircaloy guide tube 16 and zircaloy plug 52 are "captured" even though their expansion characteristics are different from the stainless steel elements. They are held firmly because of the increased friction and "give" afforded to the threaded connection by means of the groove 48 and sharp points 50 it forms with the threads 46 and their engagement with the corresponding threads in the plug 52.