Patent GB-1052777-A (GB)
Abstract: 1,052,777. Fuel elements. UNITED KING- DOM ATOMIC ENERGY AUTHORITY. Nov. 24, 1965 [Dec. 10, 1964], No. 50344/64. Heading G6C. A fuel element comprises a cluster of parallel fuel rods supported at their lower ends by support means which also serve for independently supporting a surrounding sleeve at its lower end, there being provided a member serving for lateral spacing of the fuel rods at their upper ends and optionally one or more of lateral spacing members intermediate the upper lateral spacing member and the support means, the lateral spacing member(s) being secured to a central longitudinally extending member which is itself secured to the support means, the fuel rods being slidable relative to the lateral spacing member(s) and the latter being slidable relative to the sleeve. In the embodiment described (Fig. 1, not shown), the central member 23 is tubular and provides accommodation for a lifting member by means of which the fuel element or a stack of such fuel elements can be charged into or discharged from a fuel channel of a reactor. An annular support member 24 has a radial flange 26 which supports an outer graphite sleeve 31, and an upper peripheral portion 25 which supports an inner graphite sleeve 32 spaced from the outer sleeve 31 and to which is secured a grid supporting the lower ends of the fuel rods 1. The support member 24 also has a lower peripheral portion which engages within the upper end of a lower adjacent fuel element when the fuel elements are stacked within a fuel channel and serves to laterally locate the adjacent fuel element. The spaces between the sleeves 31, 32 provide static gas gaps which assist in thermally insulating the sleeve 31 from the heat produced by the fuel rods. The fuel rods 1 are individual and of nearly the full length of the fuel element, each consisting of a stack of cylindrical fissile ceramic (e.g. UO2) pellets 5 housed within a metallic sheath 3 provided with external roughening in the form of either helical or circumferential integral ribs 4 or helically wrapped and secured wire. The pellets 5 have recesses 6 at their ends which co-operate to form a succession of voids along each fuel rod. An end cap 8 of dished form is welded to the upper end of each sheath 3, and an end cap 9 having an extended central spigot 10 to the lower end of each sheath 3. A non- fissile heat insulating disc 11 (e.g. Al2O3) is interposed between each of the end caps 8, 9 and the adjacent pellets 5. The spigots 10 which are accommodated in openings 14 of the support grid are annularly recessed at 20<1> and these recesses are engaged by deformable clips 50 having wings which can be inwardly deformed by squeezing with a suitable tool. The support grid comprises a fuel rod supporting portion (Fig. 2, not shown) consisting of four concentric rings 13 formed from bent double thickness strip material and shaped so as to provide the openings 14 which are spaced equidistantly around the rings 13 and a load bearing portion consisting of six radial supports each consisting of two spaced limbs 17 of strip material. The rings 13 are engaged in shallow slots in the limbs 17 and retained by inverted and elongated U-shaped tie members 19 spot welded to the limbs 17. Two pairs of opposed support limbs 17 have at their inner ends arcuate portions 20 to which is welded or brazed a short tube 21 having an internal shoulder 21 upon which the end of an elongated central tube 23 is supported, the tube 23 being welded to the tube 21. The remaining pairs of limbs 17 are secured by welding or brazing to one or other of the limbs 17 which have the arcuate portions 20 and adjacent pairs are also secured to one another. The outer ends of the limbs 17 are bent and welded or brazed to the upper peripheral portion 25 of the support member 24. Each lateral spacing member 36 is secured by welding or brazing arcuate portions 39 of the member 36 in a peripheral recess 38 of a sleeve 37 welded to the tube 23 and consists (Fig. 3, not shown) of a plurality of tubular members 40 secured to one another by web members 42. Each tubular member 40 has at least three equally spaced inward projections 41 for contacting a fuel rod 1, and may either be of constant diameter or of varying cross-section along its length so that it is of minimum cross-section at its middle and maximum cross-section at both its ends. The inner sleeve 32 and the support grid have portions which are engaged with bayonet slots in the lower end portion of the outer sleeve 31 so as to locate the inner and outer sleeves and the support grid relative to one another and to restrain them against relative axial movement.