Patent Number: 
Section: description

FIG. 1 shows a fuel assembly according to the invention. The fuel assembly comprises an upper handle 1, a lower end portion 2, and a plurality of fuel units 3 stacked one above the other. Each fuel unit comprises a plurality of fuel rods 4 arranged in parallel and in a definite space relationship to each other in a given lattice, and a top tie plate 5 and a bottom tie plate 6 for attachment of the fuel rods 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 is facing the bottom tie plate 6 in the next fuel unit in the stack, and such that the fuel rods in all the fuel elements are parallel to each other. A fuel rod 4 comprises fuel in the form of a stack of pellets 7 of uranium arranged in a cladding tube 19. FIG. 2 shows a section II-IIxe2x80x2 through the fuel assembly in FIG. 1. The fuel assembly is enclosed in a fuel channel 8 with a substantially square cross section. The fuel channel is provided with a hollow support member 9 of cruciform cross section which is secured to the four walls of the fuel channel. In the central channel 14 formed by the support member 9, moderator water flows. The fuel channel with the support member surrounds four vertical channel-formed parts 10, so-called sub-channels, with an at least substantially square cross section. The four sub-channels each contain a stack of fuel units. Each fuel unit comprises 24 fuel rods 4 arranged in a symmetrical 5xc3x975 lattice. The fuel assembly in FIG. 2 comprises 10xc3x9710 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. In certain fuel assemblies, a number of fuel rods have been replaced by one or more water channels. The introduction of a water channel changes the number of fuel rods but not the number of fuel rod positions. FIG. 2a shows another embodiment of a fuel assembly according to the invention. The figure 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 assembly. The channel 14a is surrounded by a tube 9a with a substantially square cross section. The fuel units are kept in position by being fitted onto the tube which surrounds the vertical channel. 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 9a and 9c through which water is conducted from below and upwards through the assembly. The water rods have a diameter which is somewhat larger than the diameter of the fuel rods and are designed with a substantially circular cross section. The fuel units are kept in position by being fitted onto the water rods. FIGS. 3a-3c show examples of what is meant by a substantially square cross section. The fuel assembly in FIG. 3a has a reduced corner portion 11. The fuel assembly in FIG. 3b has two reduced corner portions. The fuel assembly in FIG. 3c has four reduced corner portions. The reduction of a corner portion reduces the number of fuel rods in the fuel unit by one fuel rod compared with a fuel unit without a reduced corner portion. In a boiling water reactor, cooling water flows upwards through the fuel, whereby part of the water is transformed into steam. This results in a greater pressure drop in the upper part of the fuel assembly than in the lower part thereof. This difference gives rise to a force which tends to raise the fuel upwards. In conventional fuel assemblies, the fuel bundles are kept in position because of their weight. In a fuel assembly with short fuel units there is a risk of the upper fuel units being raised upwards by these forces. To prevent certain fuel units from being pressed upwards, a spring means 12 is arranged in the upper part of the fuel assembly. FIG. 4a shows a section IVA-IVAxe2x80x2 through the fuel assembly in FIG. 1. FIG. 4b shows in more detail the appearance of a spring means 12 in FIG. 1. The spring means 12 comprises a spiral spring 13 arranged in a slit in the support means 9 around the central channel 14. The spring 13 is provided with four radially extending arms 15, each of which presses down a stack of fuel units. This arrangement gives each stack of fuel units a freedom to grow in relation to the fuel channel independently of how the other stacks grow. Two types of growth occur in the fuel units, namely, thermal growth and irradiation growth. FIG. 5a shows another embodiment where the fuel units 3 are kept in position by being fitted onto a common supporting member. The common supporting member may, for example, be a tube 50a which conducts non-boiling water. The advantage of using one or more water-filled tubes as a common supporting element is that non-boiling water may be moved into the central parts of the fuel assembly and hence attain an improved moderation. The common supporting member may, for example, comprise a plurality of joined-together short fuel rods (50b) as shown in FIG. 5b. The advantage of allowing the supporting member to comprise a plurality of short fuel rods instead of one long fuel rod is that the previously mentioned risks in case of fuel damage are reduced. FIGS. 6a and 6b show another arrangement for keeping the fuel units in place. Two fuel units are connected to each other by four connecting springs 51 arranged between the top tie plate of the lower fuel unit and the bottom tie plate of the upper fuel unit. The connecting spring comprises an attachment loop 52, which may, for example, be of Inconel or some other nickel-base alloy. The connecting springs are easy to open such that the fuel units may change be rearranged or replaced during refuelling. The springs cannot be unintentionally opened when the bundle stands in the fuel channel or is being raised. FIG. 7 is a section VII-VIIxe2x80x2 through the fuel assembly in FIG. 1 and shows an example of a bottom tie plate 6. FIG. 8 shows the bottom tie plate in a section Dxe2x80x94D in FIG. 7. The bottom tie plate comprises an orthogonal latticework composed of tubular sleeves 16, 17 and surrounded by a frame 18. The function of the frame 18 is to guide the fuel units when charging the fuel, keep the fuel rods at a certain distance from the fuel channel 8, and to scrape off water from the walls of the fuel channel, especially in the upper part of the channel. The frame is provided with guiding vanes l9a, the function of which is two-fold, namely, to facilitate the introduction of the fuel unit into the cladding tube, and to increase the mixing of cooling flow. The sleeves are of two different types, namely, fixing sleeves 16 in which the fuel rods 4 are fixed, and supporting sleeves 17 which support and fix the fixing sleeves. The fixing sleeves have the same or almost the same diameter as the cladding tube of the fuel rods. The fixing sleeves are arranged in a symmetrical 5xc3x975 lattice which corresponds to the lattice of the fuel rods, and the supporting sleeves are arranged between the fixing sleeves to support these. The supporting sleeves 17 may be provided with mixing vanes 19b to increase the mixing of the coolant flow. The mixing should primarily be performed in the upper part of the fuel assembly, where the risk of dryout is greatest. The fuel assembly preferably comprises two types of fuel units, of which one type has bottom tie plates with mixing vanes and the other type has bottom tie plates with no mixing vanes. The fuel units whose bottom tie plates have mixing vanes are arranged in the upper part of the fuel assembly and those without mixing vanes are arranged in the lower part of the fuel assembly. The top tie plate 5 may be designed as the bottom tie plate described above. The frame of the top tie plate is suitably provided with a marking for identification of the respective fuel unit. The top tie plate shall also be capable of being gripped by a lifting tool. FIG. 8 shows how the fuel rods 4 are attached to the top tie plate 5 and to the bottom tie plate 6. In the lower part of the fuel rod 4, a bottom plug 20 is arranged, the free end of which is inserted into the fixing sleeve 16 in the bottom tie plate 6. In the uppermost part of the fuel rod, a top plug 21 is arranged, the free end of which is inserted into a fixing sleeve 22 in the top tie plate 5. During the burnup of the nuclear fuel, fission gases contained in the fuel rod are released. To prevent the pressure on the cladding from becoming too great, an expansion space for the fission gases is needed. The bottom plug 20 is provided with a cavity 23 to receive fissile gases, and that part of the bottom plug which faces the uranium pellets has an opening between the cavity and the remainder of the fuel rod. In the upper part of the fuel rod, the stack of uranium pellets ends somewhat below the top plug 21 which is provided with a cylindrical recess 25, the opening of which faces the uranium pellets. The space between the top plug and the uranium pellets and the space in the top plug may be utilized for expansion of the fissile gases. The uranium-free parts of the fuel rods give a reduced neutron absorption, which leads to an increase of the effect in the uranium pellets nearest the top and bottom plugs. To reduce the effect and to further increase the space for the fission gases, uranium pellets with holes may preferably be used nearest the top and bottom plugs. It may also be suitable to give these pellets a lower enrichment. It is important that the emission of fission gases be kept at a low level, such that the required fission space becomes as small as possible. This is achieved by a low linear rod load (kW/m) which is made possible by a large number of fuel rods (96 in the embodiment) in the highly loaded cross section of the assembly. An additionally larger number of rods may also be advantageous. The number of fuel rod positions should at least be 80, preferably more than 90, for the fission gas emission to be sufficiently low to be taken care of in the short fuel units. The fission gas emission may be further reduced by additions to the fuel pellets. A fuel unit comprises a small number of, for example two, retaining fuel rods which are fixed to the top tie plate and the bottom tie plate. The retaining fuel rods retain the fuel unit such that the other fuel rods are kept in position. FIG. 9a shows how a retaining fuel rod 4a may be fixed to the fixing sleeve 16 of the bottom tie plate with a cleaving rivet 26. FIG. 9b shows a section IXB-IXBxe2x80x2 in FIG. 9a. FIG. 10 shows how a retaining fuel rod 4b may be fixed to the fixing sleeve 16 with a screw joint 27. In a conventional fuel assembly, with full-length fuel rods, which are retained by a plurality of spacers along their axial length, abrasion damage normally arises on a level with the spacers because debris adhering thereto remain and wear holes in the cladding. Because no spacers are needed in a fuel assembly according to the invention, the risk of abrasion damage to the fuel is reduced. However, a risk of abrasion damage remains in the region below the top tie plate. FIGS. 11a-11c show different alternatives for reducing the risk of cladding damage caused by abrasion in this region. Because the rods need not be drawn through spacers during mounting, a larger outer diameter may be allowed in this region, for example by a wear-resistant coating. FIG. 11a shows a top plug 33 which has an upper solid part 34, for connection to the fixing sleeve 22 in the top tie plate, and a lower solid 35. The lower part 35 is longer than the upper part 34. The lower part is arranged in the region with the greatest risk of abrasion damage. FIG. 11b shows a top plug 28 which has an upper solid part 29 and a lower hollow part 30. The lower part 30 is longer than the upper part 29 and is provided with a coating 31 which protects against abrasion damage, for example zirconium oxide or aluminium oxide. FIG. 11c shows a fuel rod, the cladding tube 19 of which in its upper part, where the risk of abrasion damage is greatest, is provided with a coating 32 protecting against abrasion damage. For several different reasons, it is desirable to reduce the amount of uranium in the upper part of the fuel assembly in a fuel assembly intended for a boiling water reactor. One reason is that the high percentage of steam in the upper part of the fuel assembly leads to deteriorated neutron moderation, which results in the fuel not being burnt up as quickly in the upper part of the fuel assembly as in the lower part thereof. Another reason is that a reduction of the quantity of uranium in the upper part of the fuel assembly gives an improved shut-down margin. A consequence of the reduction of the quantity of uranium is that the free flow area increases, which leads to a reduction of the pressure drop in the upper part of the fuel assembly, and, therefore, the risk of thermohydraulic instability in the fuel assembly decreases. In a fuel assembly according to the invention, the quantity of uranium in different parts of the assembly may be varied in a simple manner. A fuel assembly may comprise fuel units with different numbers of fuel rods, different lattice configurations, and different fuel rod diameters. FIG. 12 shows a fuel assembly 39 which comprises fuel units (40, 41) of two different types, of which the first type contains fuel rods with a first diameter and the other type contains fuel rods with a second diameter. The first type of fuel units 40 is arranged in the lower part of the fuel assembly, and the second type of fuel units 41 is arranged in the upper part of the fuel assembly. The fuel rods in the lower fuel units 40 have a diameter which is larger than that of the fuel rods in the upper fuel units 41. FIG. 13 shows a fuel assembly 42 where the number of fuel rods in the fuel units 43 in the lower part of the fuel assembly is larger than the number of fuel rods in the fuel units 44 in the upper part of the fuel assembly. The number of axial zones with different rod diameters or different number of rods may, of course, be greater than the two shown in the examples. It is also possible to have different rod diameters within the fuel units to attain optimum properties in the cross section. A fuel assembly according to the invention may comprise fuel units with different height. FIG. 14a shows a fuel assembly which comprises eight equally long fuel units 60. FIG. 14b shows a fuel assembly which comprises eight fuel units with two different heights. The uppermost four fuel units 61 are shorter than the lowermost four fuel units 62. Since the top tie and bottom tie plates give rise to turbulence of the cooling water, it is advantageous, from the point of view of dryout, to have more top tie and bottom tie plates in the upper part of the fuel assembly than in the lower part thereof, which is achieved in this embodiment. FIG. 14c shows ten equally high fuel units 63. FIG. 14d shows five equally long fuel units 64, each one comprising a spacer 65 which keeps the fuel rods spaced-apart from each other and prevents them from bending or vibrating when the reactor is in operation. FIG. 15a shows an example of a fuel assembly where the fuel units 69 in the upper part and the fuel units 68 in the lower part of the fuel assembly have different lattices. FIG. 15b shows a cross section through the fuel unit 69, and FIG. 15c shows a cross section through the fuel unit 68. The number of fuel rods is larger in the fuel units in the lower part of the fuel assembly than in the fuel units in the upper part thereof. It is also possible to omit rods in occasional lattice positions, preferably in the uppermost fuel units. A fuel assembly according to the invention may also be optimized by the enrichment of uranium in the fuel rods varying between the different fuel units. Fuel units in the upper part of the fuel assembly may, for example, have a lower enrichment than fuel units in the lower part of the fuel assembly. The occurrence of burnable absorbers, for example gadolinium, may also vary between the fuel units. Recently, the development has gone towards fuel assemblies with narrower fuel rods which are more in number. However, there is a limit to how narrow the rods may be if they are to have a length of about four meters. If the rod is too narrow, mechanical difficulties arise which may become very difficult to solve. A solution to these problems is to manufacture short fuel units. FIG. 16 shows a cross section of a fuel assembly with 12xc3x9712 fuel rod positions. The rod diameter is about 8 mm. With a plurality of rods, the linear load and hence the fission gas emission are reduced. The need of space for fission gas in the short rods is thus reduced, which facilitates such a design. A fuel assembly with short fuel units has several advantages compared with a traditional fuel assembly with full-length fuel rods. One considerable advantage is the flexibility provided in designing the fuel assembly. This means that the fuel assembly in a simple manner may be optimized both in the axial direction and in the radial direction, for example with respect to lattices and fuel distribution. In connection with refuelling, certain fuel units may be replaced and certain may be allowed to remain in the fuel assembly. The fuel units which are allowed to remain in the fuel assembly may be given a new position. In this way, the service life of the fuel assembly increases. If the height of the fuel units is sufficiently low, no spacers are needed, which is an advantage since the spacers increase the risk of abrasion damage. It is also easy to design the upper end of the rods with special abrasion protection in the sensitive region below the top tie plates. The consequence of abrasion damage or other cladding damage is reduced as the length of the fuel rods is reduced, since the quantity of uranium and fission products which may leak out is smaller. The risk of secondary damage is also reduced for short rods. To achieve the above-mentioned advantages, the number of fuel units on top of each other may be at least three, preferably even more. To avoid using spacers, the number of fuel units should be more than six.