FUEL ASSEMBLY FOR A BOILING WATER REACTOR WITH TAPERING FUEL RODS

Abstract: A fuel assembly for a boiling water reactor has at least one fuel rod which has a lower column (6) of fuel pellets with a relatively larger diameter and an upper column of fuel pellets (7) with a relatively smaller diameter, and a cladding tube (5) with a lower coarser part (5a) which surrounds the lower column of fuel pellets, an upper narrower part (5b) which surrounds the upper column of fuel pellets, and a transition portion (13) between the lower and upper parts. A spacing member (12), which is movable in the longitudinal direction of the tube, is arranged between the lower and upper columns. The upper end of the spacing member is arranged above or in the transition portion and the lower end of the spacing member is arranged below and at a distance from the transition portion. A spring (11) arranged in the upper end of the fuel rod exerts a compressive force on the upper column and, via the spacing member, on the lower column.

Description: Fuel assembly for a boilinσ water reactor with taoerinσ fuel rods TECHNICAL FIELD The present invention relates to a fuel assembly for a boiling water reactor comprising at least one fuel rod which comprises - a lower column of fuel pellets with a relatively larger diameter, and - an upper column of fuel pellets with a relatively smaller diameter, and - a cladding tube comprising a lower coarser part which surrounds the lower column of fuel pellets, an upper narrower part which surrounds the upper column of fuel pellets, and a transition portion between the lower and upper parts. BACKGROUND ART The advantage of a fuel assembly in which at least certain fuel rods are tapering, that is, they have a larger rod diameter in their lower part than in their upper part, compared with conventional fuel rods, that is, fuel rods which have the same rod diameter along the whole of their axial extent, is that the fuel-to-moderator ratio in the axial direction becomes more uniform. The high proportion of steam and hence the low volume of water in the upper part of the core are compensated by the smaller quantity of uranium in the upper part of the fuel rods. An improved fuel-to-moderator ratio leads to an increased reactivity during operation, which gives improved fuel economy, and to an improved shutdown margin, which results in increased safety. An additional advantage achieved with tapering fuel rods is a reduced pressure drop in the upper two-phase region of the fuel assembly, which improves the ratio between the pressure drops in the lower single-phase region of the fuel assembly and the upper two-phase region of the fuel assembly. This improved pressure ratio means that the fuel is given improved stability properties during operation. During operation, the size of the fuel pellets varies. At the beginning of the operating cycle, the density of the pellets increases, which means that they shrink m size. When the pellets are subjected to neutron irradiation, they grow m size because they absorb neutrons, which leads to the forma¬ tion of gaseous fission products which are accumulated m the pellets. The size of the pellets also depends on the tempera¬ ture thereof. Also the cladding tubes grow when they are subjected to neutron irradiation and their size varies with the temperature. When designing a fuel rods comprising two columns of pellets with different diameters, t is necessary to give these a possibility of growing and shrinking in relation to the cladding tube, both axially and radially. If the pellets in the lower column grow in relation to the cladding tube and if there is not sufficient space, the top pellet in the column will be pressed against the upper part of the cladding tube and give rise to stresses in the cladding tube If, instead, the lower column of pellets shrinks in relation to the cladding tube, pellets from the upper column may fall down into the lower part of the cladding tube and be positioned crosswise. If the smaller pellets fall down m the lower coarser part, the distance from these pellets to the wall of the cladding tube becomes so large that the heat transfer from the pellets is deteriorated. Patent application EP-0 514 121 Al discloses a fuel assembly for a boiling water reactor which comprises the characteristic features described m the introduction The upper and lower columns of pellets are pressed against each other, or against a reducer provided between the columns of pellets, by an upoer and a lower spring. The upper spring is arranged m a space lr the upper part of the cladding tube and takes up movements in the upper column of pellets. The lower spring is arranged in a space in the lower part of the cladding tube and takes up movements in the lower column of pellets. During the burnup of the nuclear fuel, fission gases, which are contained within the fuel rod, are released. To prevent the pressure on the cladding from becoming too large, an expansion space is needed for the fission gases. In conven¬ tional fuel rods there is a fission gas space m the upper part of the fuel rod. The fuel rod described m the above- mentioned EP application has two fission gas spaces, one in the upper and one in the lower part of the rod. Current boiling water reactors are designed for fuel rods with a certain height of the uranium column and with the fission gas space in the upper part thereof. Therefore, there are physical limitations to the height of the uranium column in the fuel rod. Such limitations are, for example, the range of the control rods and the location of the core grid. If a fission gas space is located in the lower part of the fuel rod, the uranium column must decrease in size corresponding to the size of the fission gas space. From the point of view of fuel economy, therefore, it is more advantageous to locate the fission gas space in the upper part of the fuel rod. A disadvantage with the fuel rod described above is that the uranium column loses m height because the fuel rod has a fission gas space in the lower part of the fuel rod, which leads to deteriorated fuel economy. OBJECT OF THE INVENTION The invention aims to provide a fuel assembly of the kind described m the introductory part of the description, --nich m a simple way absorbs the above-mentioned pellets movements without any limitations as to the height of the uranium column and with retained fuel economy. What characterizes a fuel assembly according to the invention will become clear from the appended claims. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 schematically shows a fuel assembly according to the invention. Figure 2 shows a fuel rod according to a first embodiment of the invention. Figures 3 shows part of a fuel rod according to a second embodiment of the invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Figure 1 schematically shows a fuel assembly according to the invention. The fuel assembly comprises an upper handle 1, an inlet part 2, a plurality of fuel rods 3 and a number of axially distributed spacers 4, which retain the spaced relationship of the fuel rods. At the top the fuel rods are retained by a top tie plate (not shown) and at the bottom by a bottom t e plate (not shown) . The shown fuel rods have two different diameters, a smaller diameter in the upper part of the fuel rod and a larger diameter in the lower part of the fuel rod. Figure 2 shows a fuel rod according to a first embodiment of the invention. The fuel rod comprises a cladding tube 5 which consists of a lower part 5a with a larger diameter and an upper part 5b with a smaller diameter, and a transition portion 13 between the lower and upper parts of the cladding tuoe The upper and lower parts are joined together by means of a weld joint. The lower part of the cladding tube surrounds a lower column of fuel pellets 6 with a larger diameter The upper part of the cladding tube surrounds an upper column of fuel pellets 7 with a smaller diameter. The lower part of the cladding tube is sealed by means of a bottom plug 8, and the upper part is sealed by means of a top plug 9. Between the upper and lower pellets columns, a spacing member 12 is arranged. The spacing member comprises a third column with fuel pellets 14 with a diameter which is smaller than the diameter of the pellets 7 in the upper pellets column and a tube 15 which surrounds the third column of fuel pellets 14. The tube 15 has a diameter corresponding to the diameter of the upper pellets column and is made of a material with a low neutron absorption, for example Zr or Ti or various alloys thereof. The spacing member 12 is arranged at the transition portion 13 between the lower and upper parts of the cladding tube in such a way that it extends both into the upper narrower part 5b and into the lower coarser part 5a of the cladding tube. The spacing member 12 is ovably arranged in the longitudinal direction. Between the upper pellets column and the top plug there is a fission gas space 10. The fission gas space houses a helical spring 11, the task of which is to take up movements in the pellets columns as well as to press the upper pellets column against the spacing member, which in turn presses the lower pellets column against the bottom plug. The spacing member permits the lower pellets column to grow in relation to the cladding tube without any risk that the uppermost pellet in the column is pressed against the upper part of the cladding tube and gives risk to stresses in the cladding tube, and permits the lower pellets column to shrink in relation to the cladding tube without any risk of pellets from the upper column falling down into the lower part of the cladding tube. Figures 3 shows a second embodiment of the invention, wherein the spacing member comprises a helical spring 17 and a number of fuel pellets 18 surrounded by the helical spring. The helical spring is arranged in the lower part of the cladding tube 5a with its upper end at the transition portion 13. The pellets 18 which are surrounded by the spring have the same diameter as the pellets 7 in the upper column. When the lower pellets column grows, the helical spring 17 is compressed and the pellets 18 are pushed upwards and into the upper part of the cladding tube. When the lower pellets column shrinks, pellets 7 from the upper pellets column are pressed down into the lower part of the cladding tube and are surrounded by the helical spring 17. The helical spring 17 has two important functions, namely, to prevent the narrower pellets which are pressed down in the coarser part of the cladding tube from being obliquely positioned, and to improve the heat transfer from these pellets to the cladding tube. The helical spring 17 in this example consists of spring wire with a circular cross section. The helical spring 17 may advantageously consist of a spring wire with a rectangular cross section to further improve the heat transfer. In spite of these different measures to maintain a good heat transfer between the pellets and the cladding tube, it may, however, be locally deteriorated. It is part of known technique to fill the fuel rod with He to a certain pressure, at the end of the manufacture, in order to maintain a good heat transfer between the pellet and the cladding. The invention may advantageously be supplemented with this technique. In the above embodiments, fuel rods with one diameter transi- tion have been shown. The invention may, of course, also be applied to fuel rods with several diameter transitions. In the above embodiments, the upper and lower parts are joined by a weld joint. Another way of joining them is to provide them with a tap and a thread and screw them together. It is also possible to manufacture the whole cladding tube in one single piece, for example by rolling it into the final design during manufacture. The transition between the lower and upper parts of the cladding tube may be distinctive as in this example, or successively tapering. The lower column of pellets need not, of course, rest direct on the bottom plug, as shown in these examples, but there may, for example, be a washer between the bottom plug and the lowermost pellet.

Claims: CLAIMS 1. A fuel assembly for a boiling water reactor comprising at least one fuel rod which comprises - a lower column of fuel pellets (6) with a relatively larger diameter, and - an upper column of fuel pellets (7) with a relatively smaller diameter, and - a cladding tube (5) comprising a lower coarser part (5a) which surrounds the lower column of fuel pellets, an upper narrower part (5b) which surrounds the upper column of fuel pellets, and a transition portion (13) between the lower and upper parts, characterized in that - a spacing member (12), which is movable in the longitudinal direction of the tube, is arranged between the lower and upper columns, - the upper end of the spacing member is arranged above or in the transition portion (13) , - the lower end of the spacing member is arranged below and at a distance from the transition portion (13) , and - a spring (11) is arranged at the upper end of the fuel rod and provides a compressive force on the upper column and via the spacing member on the lower column. 2. A fuel assembly according to claim 1, characterized in that the upper end of the spacing member is arranged above and at a distance from the transition portion, and that the spacing member comprises a tube (15) with a diameter corre- spondmg to the diameter of the upper pellets column and a number of fuel pellets (14) surrounded by the tube. Ξ. A fuel assembly according to claim 1, characterized m that the spacing member comprises a helical spring (17) and a number of fuel pellets (18) surrounded by the helical spring. 4. A fuel assembly according to claim 3, characterized in that the fuel pellets (18) surrounded by the helical spring have the same diameter as the fuel pellets in the upper column. 5. A fuel assembly according to claim 3, characterized in that the helical spring (17) has a substantially rectangular cross section.

Inventors: ALBERTSSON MARGRIT, GROENLUND MAGNUS, ALBERTSSON, MARGRIT, GROENLUND, MAGNUS
Applicants: ALBERTSSON MARGRIT, ASEA ATOM AB, GROENLUND MAGNUS, ABB ATOM AB, ALBERTSSON, MARGRIT, GROENLUND, MAGNUS
Application year: 1996
Main classification: G21C3
IPC codes: G21C3