Patent Number: 062755579
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows a fuel assembly of a boiling water type comprising an upper handle 1, a lower end portion 2 and a plurality of fuel units 3 stacked one above the other. Each fuel unit 3 comprises a plurality of fuel rods 4 arranged in parallel and in spaced relationship to each other in a given lattice. Further, each fuel unit 3 comprises a top tie plate 5 and a bottom tie plate 6 for attachment of the fuel rods 4 in their respective positions in the lattice. The fuel units 3 are stacked on top of each other in the longitudinal direction of the fuel assembly and they are stacked in such a way that the top tie plate 5 in one fuel unit 3 is facing the bottom tie plate 6 in the next fuel unit 3 in the stack and such that the fuel rods 4 in all the fuel units 3 are parallel to one another. A fuel rod 4 contains fuel in the form of a stack of fuel pellets 7b of uranium arranged in a cladding tube 7a. The cladding tube 7a is suitably made of a zirconium-base alloy or an alloy which, in addition to zirconium, comprises niobium, iron, tin and chromium. A coolant is adapted to flow from below and up through the fuel assembly. FIG. 2 shows that the fuel assembly is enclosed in a fuel channel 8 with a substantially square cross section. The fuel channel 8 is provided with a hollow support member 9 of cruciform cross section, which is secured to the four walls of the fuel channel 8. In the central channel 14 formed of the support member 9, moderator water flows. The fuel channel with support members surround four vertical channel-formed parts 10, so-called sub-channels, with an at least substantially square cross section. The four sub-channels each comprises a stack of fuel units 3. Each fuel unit 3 comprises 24 fuel rods 4 arranged in a symmetrical 5.times.5 lattice. The fuel assembly in FIG. 2 comprises 10.times.10 fuel rod positions. By a fuel rod position is meant a position in the lattice. All the fuel rod positions in the lattice need not be occupied by fuel rods 4. In certain fuel assemblies, a number of fuel rods 4 are replaced by one or a plurality of water channels. The introduction of a water channel changes the number of fuel rods 4 but not the number of fuel rod positions. FIG. 2a shows an alternative embodiment of a fuel assembly according to the invention. FIG. 2a shows a horizontal section through the fuel assembly which is provided with an internally arranged vertical channel 14a through which water is conducted in a vertical direction from below and upwards through the fuel assembly. The channel 14a is surrounded by a tube 9a with a substantially square cross section. The fuel units 3 are kept in position by being fitted onto the tube which surrounds the vertical channel 14a. FIG. 2b shows an additional embodiment of a fuel assembly according to the invention. The figure shows a horizontal section through the fuel assembly which is provided with two centrally arranged vertical water rods 14b through which water is conducted from below and upwards through the fuel assembly. The water rods 14b have a diameter which is somewhat larger than the diameter of the fuel rods 4 and are formed with a substantially circular cross section. The fuel units 3 are kept in position by being fitted onto the water rods 14b. FIG. 3 shows a pressurized-water fuel assembly of square cross section. In the same way as the fuel assembly in FIG. 1, it comprises a plurality of fuel units 3 stacked on top of each other. Each fuel unit 3 comprises a plurality of fuel rods 4 arranged in parallel and in spaced relationship to each other in a given lattice. Each fuel unit 3 further comprises a top tie plate 5 and a bottom tie plate 6 for attachment of the fuel rods 4 in their respective positions in the lattice. The fuel units 3 are stacked on top of each other in the longitudinal direction of the fuel assembly and they are stacked in such a way that the top tie plate 5 in one fuel unit 3 is facing the bottom tie plate 6 in the next fuel unit 3 in the stack, and such that the fuel rods 4 in all the fuel elements 3 are parallel to each other. A fuel rod 4 contains fissionable material in the form of a stack of fuel pellets 7b of uranium arranged in a cladding tube 7a. A coolant is adapted to flow from below and upwards through the fuel assembly. A number of so-called control rod guide tubes 4b are arranged extending through the whole fuel assembly. The control rod guide tubes 4b are intended to receive finger-shaped control rods (not shown) which are inserted into and withdrawn from, respectively, the guide tubes 4b for the purpose of controlling the power of the nuclear reactor. The guide tubes extend between a top part 15 and a bottom part 16. The top part 15 is arranged above the uppermost fuel unit 3 in the fuel assembly and the bottom part 16 is arranged below the lowermost fuel unit 3 in the fuel assembly. The fuel units 3 are kept in position by being fitted onto the control rod guide tubes 4b. FIG. 4 shows a fuel rod 4 for a fuel assembly according to FIG. 1 or FIG. 3. The fuel rod 4 comprises, as mentioned above, a cladding tube 7a and a stack of fuel pellets 7b arranged in the cladding tube. At the top, the cladding tube 7a is sealed with a top plug 17 and at the bottom with a bottom plug 18. The fuel rod 4 is formed with an inner cavity, an axial gap 19, in which fission gases may accumulate. The axial gap 19 is also intended to permit thermal expansion of the column of fuel pellets 7b. A spacer 20 made of a zirconium-base alloy is arranged in the column of fuel pellets 7b to achieve the axial gap 19 at the desired level in the fuel rod. The axial gap 19 is arranged such that at least one fuel pellet 7b is arranged between the axial gap and either the top plug 17 or the bottom plug 18 of the fuel rod 4. The spacer 20 is formed as a sleeve with V-shaped slits 21 arranged in the respective ends. The outer parts of the tongues 22 formed between the slits are bent in towards the center of the spacer 20 at an angle of the order of magnitude of 100.degree.. The spacer 20 is adapted to make contact, by its upper end, with a lower end of a fuel pellet 7b and, by its lower end, to make contact with an upper end of a fuel pellet 7b. This design of the spacer 20 permits the spacer to be deformed in the axial direction when the fuel pellets 7b because of thermal expansion grow in the axial direction. When the spacer 20 is deformed, it will make contact with the inner surface of the cladding tube 7a. This means that the pellets column across such a spacer 20, because of its friction against the cladding tube, also when the pellets 7b shrink due to densification, is retained in its position. In this way, axial gaps 19 are prevented from forming between the top plug 17 and the fuel pellet 7b arranged at the top of the column. The spacer 20 may, of course, be formed in many different ways. It may, for example, be provided with an edge, folded towards the center, without slits 21. Alternatively, it may be formed as a spiral spring. It may also be suitable to arrange different types of spacers 20 in different parts of the fuel rod 4, for example non-deformable spacers 20 in certain axial gaps 15a. In FIG. 4, it is indicated that the pellet 7b arranged at the top and bottom of the fuel rod 4, as well as the pellets 7b arranged adjacent the spacer 20, are made with through-holes 23. With this embodiment, the maximum temperature in the fuel pellets 7b may be reduced in the region where power peaks due to good moderation arise. At the same time, the amount of released fission gas may be reduced and space for accumulation of released fission gases be created in the pellets 7b. Further, the fuel pellets 7b are provided with cupped upper and lower end surfaces (see reference numeral 24). Because of the thermal expansion, the fuel pellets 7b grow more in the central, warmer parts than in the outer, colder parts. The cup shape 24 thus permits thermal expansion to a certain extent before the axial gap 19 is utilized for this purpose. Because of the hollowed 23 and cup-shaped 24 pellets 7b, a smaller axial gap 19 is sufficient for the thermal expansion and for accumulation of the released fission gases. Alternatively, fuel pellets 7b with lower enrichment may be used adjacent the spacers 20. This has, in principle, the same effect as hollowed pellets when it comes to limiting power peaks, however, not with regard to reducing the power at the center of the fissionable material or accumulating fission gases. FIG. 5a shows two fuel rods 4 arranged adjacent to each other, each with an axial gap 19. The axial gaps 19 in the two adjacently arranged fuel rods 4 are arranged at axially separate levels. Arranging axial gaps 19 at axially separate levels in adjacently located fuel rods 4 results in an equalization of the power along the fuel rod 4 and a reduced risk of high power peaks as a result of too good moderation in these regions which lack fissionable material 7b. In an alternative embodiment, an axial gap 19 is arranged at random in the fuel rod 4 during the manufacture. It is then suitable to determine in advance a region within which the location of the axial gap 19 may be varied. The random location of the axial gap 19 may, for example, be achieved with the aid of a conventional random number generator. FIG. 5b shows an alternative embodiment of the fuel rod 4 according to FIG. 5a. The axial gap 19 is here divided into two smaller axial gaps 19a in each fuel rod 4. The axial gaps 19a are arranged at different axial levels in the respective fuel rods 4. The fuel rods 4 in FIG. 5a and FIG. 5b are designed preferably identical, but when putting these together into a bundle for a fuel assembly, every other fuel rod 4 is placed upside down. FIG. 6a shows another alternative embodiment of the fuel rod 4. In this fuel rod 4, the axial gap 19 is divided into four smaller axial gaps 19b arranged at axially separate levels. In this case, fuel pellets 7b without through-holes 23 may be used. In FIG. 6b, a spacer 20 is shown which is adapted to such a short axial gap 19b. A fuel assembly which has a length of the order of size of 400 millimeters is provided with a gap which is 20-30 millimeters, alternatively two gaps which are each of the order of size of 10 millimeters, etc. FIGS. 7a and 7b show an alternative embodiment of the spacer 20. This spacer 20 is formed by punching a sheet and forming the sheet into a sleeve and folding the tongues 22 inwards towards the center of the sleeve. The dash-lined slit 25 shown in the figure indicates where the ends of the sheet meet. By forming the slit 25 with a hook 25a, the spacer 20 may be given a stable design in the axial direction. This spacer 20 is simple to manufacture since it is punched out in a piece of sheet, whereafter it is formed into a sleeve. FIG. 8 shows an embodiment of a fuel rod 4 with an upper and a lower end pellet 7c with a smaller diameter than that of the other fuel pellets 7b. Because of this arrangement, the power peaks at the axial gap between fissionable material 7b which is formed between two fuel units stacked on top of each other may be reduced (see reference numeral 19c in FIGS. 1 and 3, respectively). To prevent gaps from arising between the fuel pellet 7c and the top plug 17 and the bottom plug 18, respectively, the material surrounding the end pellet 7c is made with a correspondingly smaller inner diameter. In FIG. 8, the top plug 17 and the bottom plug 18, respectively, are provided with a larger thickness of material in relation to the cladding tube 7a. The material around the end pellet 7c may, of course, be provided with a correspondingly smaller inner diameter in some other manner than with the aid of the top plug 17 and the bottom plug 18, respectively; for example, the cladding tube 7a itself may be designed in this way.