Patent Number: 058621962
Section: summary

FIELD OF THE INVENTION The present invention relates to a fuel assembly and a spacer for a light-water nuclear reactor. More particularly, the reactor is a boiling water reactor or a pressurized-water reactor. The fuel assembly comprises a bundle of elongated fuel rods retained and fixed by a number of spacers arranged in spaced relationship to each other along the bundle. The spacers comprise a number of cells for mutually fixing the fuel rods. A coolant, for example water, is adapted to flow from below and up through the normally vertically arranged fuel assembly and, during a nuclear reaction, to cool the fuel rods arranged in the assembly. BACKGROUND OF THE INVENTION A fuel assembly in a boiling water nuclear reactor consists of an elongated tubular container, often with a rectangular or square cross section. The container is open at both ends forming a continuous flow passage. The coolant of the reactor is able to flow through the passage. The fuel assembly comprises a large number of equally elongated tubular fuel rods, arranged in parallel in a certain definite, normally symmetrical pattern. At the top, the fuel rods are retained by a top tie plate and at the bottom by a bottom tie plate. To allow coolant in the desired manner to flow past the fuel rods, it is important to keep them at a distance from each other and prevent them from bending or vibrating when the reactor is in operation. For this purpose, a plurality of spacers are used, distributed along the fuel assembly in the longitudinal direction. A fuel assembly for a pressurized-water nuclear reactor has, in principle, the same design as a fuel assembly for a boiling water nuclear reactor, apart from the fact that the fuel rods are not enclosed by any tubular container and the fact that their number is higher. Between the fuel rods secondary channels are formed, through which the coolant flows through the fuel assembly. An important factor when designing a spacer for a light-water reactor is to ensure that the spacer brings about a good cooling of the fuel rods by a suitable mixing of the coolant. In those cases where the cooling is not sufficient, so-called dryout may occur in a boiling water reactor. A so-called DNB (Departure from Nucleate Boiling) may occur in a pressurized-water reactor. In serious cases, dryout and DNB, respectively, give rise to penetration of the fuel rods. The spacers thus influence the flow of the coolant and hence the cooling of the fuel. It is known that, in a region immediately below the spacer where the coolant has still not passed through the spacer, a deterioration of the coolant film on the fuel rods occurs. On the other hand, in a region above the spacer, where the coolant has just passed through the spacer, a reinforcement of the water film instead occurs. The reinforcement of the coolant film is due to the turbulence which arises in the coolant when it passes through a spacer. The greatest risk of dryout exists in the upper part of the fuel immediately below the spacers. One way of increasing the cooling of fuel rods by means of the arrangement of guide vanes at the downstream edge of the spacer in the fuel assembly is described in PCT patent document WO 91/13442. Two or four guide vanes are arranged extending from the mentioned edge and curved in such a way that a swirl is formed in the upwardly flowing coolant. Further, the guide vanes are exposed and may become damaged in connection with the mounting of the fuel rods. Another method is described in Swedish patent document PCT patent document SE 9303583-0. The spacer described in this document comprises an orthogonal latticework formed from sleeves. In this spacer, guide vanes are formed as tabs in the sleeve material. The disadvantage of this solution is that the larger the tab chosen, the larger part of the supporting length of the fuel rod is lost. Known spacers are often designed completely or partially of Inconel which is a material with a good strength and good properties as far as manufacture of spacers is concerned. However, it is desirable to manufacture spacers of a material with less ability to absorb neutrons, that is, a low-absorbing material, for example a zirconium alloy. Spacers of zirconium alloy are an advantage from the point of view of reactivity economy. In addition, spacers of a low-absorbing material are less radioactive after irradiation. This is an advantage when handling these spacers after reactor operation. The object of the invention is to provide a spacer which gives good cooling, small flow resistance, and which is simple to manufacture and, in addition, is possible to construct of zirconium alloy. SUMMARY OF THE INVENTION The present invention relates to a fuel assembly and a spacer completely or partially made of zirconium alloy for a nuclear reactor. The spacer comprises an orthogonal latticework, composed of spacer cells. Each cell positions an elongated element, extending through the cell, at a plurality of spacer levels, for example a fuel rod or a control rod guide tube. Between the fuel assemblies, secondary channels are formed for the passage of coolant flow through the fuel assembly. In at least the majority of these secondary channels, at least in the majority of spacer levels, guide vane trees with a trunk and branches are arranged. The trunk consists, for example, of a rectangular plate with its longitudinal edge arranged parallel to the flow. Alternatively, the trunk includes a pipe or sheet-metal strip, shaped like a helical spring, arranged with its respective conceived center line parallel to the coolant flow. The branches project from the trunk at at least two axially separated levels along the trunk and conduct the coolant flow towards the fuel rods arranged around the guide vane tree. The coolant flowing in the fuel assembly, that is, through the primary channel, from below and upwards, first reaches the trunk of the tree which preferably has a minimum extension across the direction of flow. The trunk conducts the coolant to the branches, or guide vanes, of the tree. By the axial distribution of the guide vanes along the length of the trunk, the coolant flow is diverted in several steps, whereby the control of the coolant flow towards the fuel rods is ensured. The guide vane tree is intended to be arranged in a secondary channel and between the spacer cells which surround the secondary channel, or at the downstream edge of the spacer cells. The size and length of the guide vane tree may be varied depending on where in the fuel assembly it is intended to be arranged. The shape of the guide vanes may vary for optimization of the control of the coolant flow. The guide vanes may, for example, be smooth or arched. The need to stir and direct the coolant towards the fuel rods is greatest downstream of the fuel assembly where the steam content, and hence the risk of so-called dryout, is greatest. Each guide vane contributes to the pressure drop across the fuel assembly. Upstream of the fuel assembly, guide vanes may be completely omitted in order to reduce the total pressure drop. In a particularly advantageous embodiment, the guide vane tree has the same extent in the flow direction as the spacer cells and are arranged therebetween so that the guide vane tree does not project from the spacer. The advantage of this design is that the cells in the spacer reflect the coolant towards the guide vane tree such that a greater part thereof may be diverted thereby. Each spacer which is provided with a plurality of guide vane trees according to the present invention may, in addition, be provided with conventional structural parts such as a four-sided frame or skirt comprising the whole bundle. The advantage of the present invention is that a spacer, which in the majority of the secondary channels is provided with guide vane trees, provides good cooling of the fuel rods arranged around the guide vane tree, whereby the output power from the fuel assembly may be increased. Further, the guide vane tree constitutes a small flow resistance which is important, not least from the point of view of two-phase pressure drop. The two-phase pressure drop is the pressure drop that usually arises in the upper part of the fuel assembly where the steam mixture contains the two phases water and steam. Another advantage of the present invention is that the design of the guide vane tree permits a simple manufacturing process. Still another advantage of the present invention is that the guide vane tree may be constructed of zirconium alloy. Using zirconium alloy instead of Inconel in the spacers entails, among other things, a reduction of the radiation to which reactor personnel are exposed, or the so-called personnel dose. The reason for the influence on the personnel dose is that, upon irradiation of nickel which is part of Inconel, radioactive cobalt isotopes are formed in the process water. Smaller amounts of nickel thus lead to a lower cobalt content, which causes the personnel dose to be reduced. The design of the guide vane tree according to the present invention permits considerable options for guiding the flow in an optimum manner and achieving good cooling. Some of these embodiments will be described below under the description of the preferred embodiments.