Patent Number: 046719261
Section: description

DESCRIPTION OF PREFERRED EMBODIMENT In the drawings, 1 designates a fuel channel with approximately square cross-section. The fuel channel 1 is made of a zirconium alloy, for example Zircaloy II.sup.R, and surrounds a bottom portion 23, a top tie plate 19 and a plurality of vertical fuel rods 20 and 24, which together with the above-mentioned components are included in a fuel assembly for a boiling reactor. A minority of the fuel rods are made as tie rods 24 having upper end taps 24' and lower end taps 24" and corresponding nuts 25' and 25", respectively. The fuel rods 21 have upper end taps 21' and lower end taps 21". The top tie plate 19, the bottom tie plate 23 and the fuel rods are interconnected with each other into a bundle which may be lifted by means a lifting force applied to a lifting handle 22 attached to the top tie plate. The fuel channel is attached to said bundle by means of a fixing device and can thus be lifted in the same lifting operation as the other components of the fuel assembly. The fixing device comprises a fixing member 2, which is cast in stainless steel and formed with an upper portion having a substantially L-shaped cross section. The fixing body is resting on the upper edge of the fuel channel, being fixed thereto by means of a plurality of snap connections. The top tie plate 19 has four upwardly-directed projections, which are located at corresponding corners of the fuel channel and which, similar to the rest of the top tie plate, are made of the same material as the fixing member 2. One of these projections is designated by the reference numeral 3, the remaining ones by the reference numeral 18. The fixing member is fixed to the projection 3 by means of a tensile force-transmitting bolt 4 with a threaded portion 5 which is screwed into a threaded vertical bore 6 of the projection 3. The threaded portion 5 has a larger diameter than the tensile force-loaded portion of the bolt 4 positioned above the portion 5. The bolt 4 has a bolt head 7 with a varying circular cross section. A spring 8 is suspended from the bolt head 7. A lower portion of the bolt head 7 is arranged -- with a play -- in a through-going, vertical bore 9 in the fixing member 2, said bore 9 also containing a circular sleeve 10 surrounding the bolt 4 and a spring washer 11 clamped between the bolt head 7 and the sleeve 10, said spring washer being made of the same material as the bolt 4. A metallic ring 12, arranged in an annular slot in the bore 9, prevents the bolt 4 from leaving the fixing member 2 when the fuel channel is dismantled. The bore 9 has an upper circular-cylindrical part, the diameter of which is somewhat larger than the diameter of a lower circular-cylindrical part, positioned immediately below said upper circular-cylindrical part. Thus, an annular, upwardly-directed contact surface positioned in a horizontal plane is formed. The sleeve 10 makes contact with said surface with a corresponding, downwardly-directed contact surface. At the upper and lower end, the sleeve 10 has a plurality of radial channels 13 and 14, respectively. At room temperature and when the bolt is tightened, the sleeve 10 transmits a compressive force to the projection 3 via a substantially triangular sheet body 15, made of the same material as the fuel channel and being welded thereto, the bolt 4 being loaded with an equally great tensile force. The bolt 4 is made of a steel alloy with a very high nickel content and has a coefficient of thermal expansion which is considerably smaller than the coefficient of thermal expansion of the material used for the fixing member 2 and the projection 3, usually at least 20% smaller. The sleeve 10 is made of the same material as the bolt 4, or alternatively of a material with a smaller coefficient of thermal expansion. The sleeve 10 has a vertical extension which constitutes at least 40%, preferably at least 60% of the total extension of that part of the bolt 4 which is loaded with a constant tensile force. Upon heating of the fuel assembly to operating temperature, consequently the play C is considerably diminished, however without being reduced to zero. If the sleeve 10 had been made of the same material as the fixing member 2, the difference in thermal expansion would have resulted in a dangerous increase of the tensile force acting on the bolt 4. Because the sleeve 10 has a coefficient of thermal expansion which is equal to or smaller than the coefficient of thermal expansion of the bolt material, instead an acceptable increase of the tensile force acting on the bolt 4 is obtained, or no increase at all. Between the bolt 4 and the sleeve 10 there is a hollow-cylindrical gap 16. Water is supplied to the gap 16 via the channels 14 and leaves the gap via the channels 13, which communicate with a plurality of horizontal outlet channels 17 extending through the fixing member 2. In this way a permanent renewal of the water present in the gap 16 is obtained. Since stationary water is generally considered to be a necessary condition for stress corrosion, the probability of stress corrosion occurring in the bolt 4 is thus reduced. The arrangement shown in FIGS. 4, 5 and 6 differs from that shown in FIG. 3 in that the bolt head 7' has a conventional shape, in that the sleeve 10' has the same vertical dimension as the bore in which it is arranged and in that the outlet channels 17 have been replaced by upwardly-directed openings. In FIGS. 7-12 the bolts 74, 84, 94, 104, 114, 124 and the sleeves 107, 108, 109, 110, 111 and 112 are made of the same material as the bolt 4 described above. The sleeves 107, 108, 111 and 112 are arranged with a press fit along a distance A, whereas the sleeve 109 is arranged with a press fit and the sleeve 110 with a play along the entire length of the sleeve. In some of the shown variants, the top tie plate has a projection 3' which differs from the projection 3 in that its uppermost portion is formed in a different way. In the arrangement shown in FIG. 13, the bolt 134 is made of the same material as the bolt 4 described above, whereas the coefficient of thermal expansion for the projection 3' extending from the top tie plate is the same as for the fixing member 2. If the coefficients of thermal expansion of parts 113, 134 and 2 are designated k.sub.113, k.sub.134 and k.sub.2, respectively, then k.sub.113 is considerably smaller than k.sub.134. More particularly, the difference k.sub.2 -k.sub.134 is considerably smaller than the difference k.sub.134 -k.sub.113.