Patent GB-1466887-A (GB)
Title: Fuel assembly for a nuclear reactor
IPC: G21C3/334
Filed: 1974-06-24
Abstract: 1466887 Fuel elements COMBUSTION ENG Inc 24 June 1974 [25 June 1973] 27974/74 Heading G6C A nuclear fuel assembly has an easily disassembled threaded connection which is formed between upper and/or lower alignment posts and vertically extending control rod guide tubes and which clamps a fuel assembly end plate therebetween. In the embodiment described, the fuel assembly comprises four guide tubes 12 removably attached to upper and lower end fittings 14, 16 to provide the structural framework for the fuel assembly. A plurality of rectangular spacer grids 20 are welded at spaced locations along the guide tubes 12 and serve to support a plurality of elongated fuel rods 22. The upper end fitting 14 (Fig. 2) comprises an end plate 26 and four upstanding alignment posts 28 adapted to slidably engage in corresponding openings in the reactor core alignment plate (not shown). Openings 32 in alignment with the four guide tubes 12 each extend through a raised boss 34 formed in the end plate 26 and include a small diameter bore 32 and a larger diameter bore 36, between which is a shoulder 38. The alignment posts 28 each include a spindle 40 of reduced diameter projecting downwardly and sized to pass through the small bore 32. The upper end of each of the guide tubes 12 forms a close fit with the larger bore 36 and is provided with internal threads 50 which mate with external threads 42 provided on the spindle 40 so that when the members are assembled the enlarged upper ends 48 of the guide tubes 12 bear against the shoulders 38, and shoulders 44 of the alignment posts 28 engage the bosses 34 to form a rigid mechanical connection. As an added safety feature to prevent rotation of the various parts during operation, the spindles 40 are provided with axial extensions 52 which are deformable into engagement with indentations 54 formed within the enlarged upper ends 48 of the guide tubes 12. The lower end fitting 16 (Fig. 3) comprises an end plate 56 and four alignment pins 58 adapted to slidably engage in corresponding openings in the lower core support plate (not shown). Openings 60 within the end plate 56 are in alignment with the four guide tubes 12 and each includes a large diameter bore 62 and a smaller diameter bore 64. Each alignment pin 58 includes an externally threaded portion 68 sized to be received within the smaller bore 64 while the main body of each pin 58 fits within the large bore 62. Each guide tube 12 is provided with an inwardly extending annular flange 72 at its lower end upon which rests cylindrical nut 74 having internal threads to engage those on the pin 58. The nut 74 is restrained from rotating and to some extent axially moving by a pin 76 extending through a U-shaped slot (78) (Fig. 6, not shown) in the nut into oppositely disposed openings in the sides of the guide tube 12 and being secured by welds 80. The threaded portion 68 of the pin 58 is threadably engaged with the nut 74 and tightened to the desired torque level, after which a deformable locking collar 81 is slipped over the pin 58, welded to the bottom of the end plate 56 as at 82, and crimped to engage the several longitudinal slots 84 provided in the pin 58. Dissimilar metals having different coefficients of linear expansion are used for the various components of both the upper and lower end fittings 14, 16. For example, the guide tubes 12 may be made from Zircaloy, the end fitting plates 26, 56 from stainless steel, and the alignment posts 28/pins 58 from a Ni-Cr-Fe alloy. The relative dimensions of the components are related to the relative coefficients of linear expansion, in a manner specified, such that the end fittings will maintain their structural integrity when subjected to thermal cycling.