Patent Number: 06310931&
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1a shows a boiling water fuel assembly 1 which comprises a long tubular container, of substantially rectangular cross section, referred to as fuel channel 2. The fuel channel 2 is open at both ends so as to form a continuous flow passage through which the coolant of the reactor flows. The fuel assembly 1 comprises a large number of equally long tubular fuel rods 3, arranged in parallel in a bundle, in which pellets 4 of a nuclear fuel are arranged. The fuel rods 3, are arranged spaced from each other in four orthogonal sub-bundles by means of a cruciform support means 8 (see also FIG. 1b). The respective sub-bundle of fuel rods 3 is retained at the top by a top tie plate 5 and at the bottom by a bottom tie plate 6. The fuel rods 3 in the respective sub-bundle are kept spaced apart from each other by means of spacers 7 and are prevented from bending or vibrating when the reactor is in operation. The bottom tie plate 6 comprises a plurality of through-holes 6a for passage of coolant. Further, the bottom tie plate 6 comprises a plurality of plug holes 6b for guiding the elongated elements 3, 3a with associated end plugs 3b (see FIG. 4). In FIG. 1a, the rod which is arranged nearest the centre line is made as a part-length rod. The part-length rod extends from the bottom tie plate 6 and upwards through the fuel assembly 1 without reaching the top tie plate 5. FIGS. 2a, 2b show a fuel assembly 1 of pressurized-water type which comprises a number of elongated tubular fuel rods 3 and control rod guide tubes 3a, arranged in parallel. The fuel rods 3 accommodate pellets 4 of a nuclear fuel. The control rod guide tubes 3a are retained at the top by a top nozzle 5 and at the bottom by a bottom nozzle 6. The fuel rods 3 are kept spaced apart from each other by means of spacers 7. The holes in FIG. 2b provided with centre lines are plug holes 6b for receiving elongated elements. The other holes 6a are intended for passage of coolant flow. FIG. 3a shows in a view from the side an end plug 3b connected to an elongated element 3, 3a. The end plug 3b is provided with a first locking member in the form of a recess 9a in the form of a slot with a substantially spherical shape which extends around the periphery of the end plug. The lower part of the end plug 3b, designated 3c', may be provided with a bevelled edge to facilitate guiding of the end plug into the plug 6b in the bottom tie plate 6. FIG. 3b shows an alternative embodiment of a first locking member 9b arranged in the end plug 3b. Here, the first locking member is made as a recess in the form of a cavity with a substantially spherical shape. FIG. 4 shows part of a bottom tie plate 6 which comprises two adjacently located plug holes 6b and a locking hole 6c extending between these plug holes 6b and opening out thereinto. Further, a second locking member 10 arranged in the locking hole 6c is shown. In the embodiment chosen, the second locking member 10 made as a substantially circular-cylindrical body 10a which is slidable in the axial direction of the locking hole 6c. Each end of the substantially circular-cylindrical body 10a is made with a radius and has a spherical shape 10b. FIG. 5 shows how the locking plug 10 with its spherical end 10b cooperates with the recess 9a of an end plug 3b according to FIG. 3a. During mounting, the end plug 3b" which is to be locked in the plug hole 6b", that is in the plug hole 6b' shown on the righthand side in FIG. 5, is first inserted, whereby the locking plug 10 is moved laterally thorough the locking hole 6c and partly into the plug hole 6b' which is shown on the lefthand side in FIG. 5. Then, an adjacently located end plug 3b' is inserted into the plug hole 6b'. The end plug 3b' pushes the locking plug 10 in the opposite direction through the locking hole 6c such that it is arranged, with its spherical end 10b, in the spherical recess 9a in the end plug 3b". Thus, one of the end plug 3b" is locked to the bottom tie plate 6 whereas the other end plug 3b' is arranged guided in the bottom tie plate 6. When dismantling elongated elements 3, 3a which are arranged guided with their respective end plugs 3b' in the bottom tie plate, these end plugs are lifted from the plug holes 6b'. To dismantle elongated elements which are arranged with their respective end plugs 3b" locked to the bottom tie plate 6, a certain lifting force in the axial direction is required to achieve a displacement of the locking plug 10 in the lateral direction in the locking hole 6c such that the end plug 3b" with the first locking member 9a is separated from the locking plug 10. By arranging the first locking member as an opening 9b in the end plug 3b, the end plug 3b may be locked so as to prevent rotation in the bottom tie plate 6. Prevention of rotation in the bottom tie plate 6 is desirable, especially when it is a question of locking of spacer holder rods. To prevent the locking plug 10 from leaving the locking hole 6c, when the elongated elements 3, 3a are not mounted in the bottom tie plate 6, a locking pin 11 may be arranged from one of the flat surfaces of the bottom tie plate, in FIGS. 4 and 5 shown from the upper flat surface and at an angle to the locking plug 10. The locking pin 11 may be fixed to the bottom tie plate 6, for example by welding, and is arranged to extend into the locking hole 6c such that the possible displacement of the locking plug 10 between two adjacently located plug holes 6b is limited to a distance which may correspond to the length of a part 10c of the circular-cylindrical body 10a of the locking plug which is provided with a smaller diameter than the rest of the body 10a. The locking plug 10 is suitably placed protected inside the bottom tie plate and is advantageously made of stainless steel, which is good from the point of view of corrosion. Alternatively, the locking plug 10 is made of a nickel-base alloy, for example Inconel. By placing the locking plug 10 inside the bottom tie plate 6, the entire arrangement is protected against the coolant flowing through the fuel assembly 1, thus preventing vibration and abrasion associated therewith. The first and second locking members 9a, 9b, 10 need not, of course, be made with spherical surfaces. The surface may have an arbitrary double-curved shape, a plane shape, or a bevelled shape. The essential point is that the concave shape of the first locking member 9a, 9b may cooperate with the shape of the second locking member 10 for locking the end plug 3b" to the bottom tie plate 6. The locking plug 10 is thus provided at each end with surfaces which are suitable for cooperation with the first locking member 9a, 9b in that end plug 3b which it is to lock and to facilitate dismantling thereof from the bottom tie plate 6. FIG. 6 shows a locking plug 10 which at each end is provided with bevelled edges designated 10d. The end plug 3b" which is to be locked is provided with a waist with corresponding bevelled edges designated 3c" to cooperate with the locking plug 10. FIG. 7 shows a locking plug 10 which at each end is provided with plane end surfaces designated 10e. The end plug 3b" which is to be locked is provided with a waist with corresponding plane surfaces to cooperate with the locking plug 10. To facilitate mounting and dismantling of the elongated elements 3, 3a, the end plug 3b" is further provided with a bevelled edge 3d. FIGS. 5, 6 and 7 show the end plug 3b" with a bevelled edge designated 3e for cooperation with a bevelled edge 6d formed in the bottom tie plate in connection with the mounting of the elongated elements 3, 3a in the bottom tie plate 6. FIG. 8 shows a locking plug 10, the diameter of which substantially corresponds to the diameter of the locking hole 6c. Further, it is shown that a hole for insertion of the locking plug 10 into the bottom tie plate is plugged up (see reference numeral 12). The hole may, for example, be welded up after the locking plug 10 has been inserted into the locking hole 6c. In this way, particles in the coolant are prevented from entering the locking hole 6c, thus causing possible seizing between the locking plug 10 and the locking hole 6c. Plugging of the hole also ensures that the locking plug 10 does not leave the bottom tie plate 6.