Patent Number: 051436914
Section: summary

The invention relates to a fuel assembly for a boiling water reactor, having mutually parallel fuel rods containing nuclear fuel and being disposed in an elongated case between a base part such as a lower tie plate, having inlet openings discharging into the interior of the case for flowing coolant and a head or cap part such as an upper tie plate, having outlet openings for the flowing coolant leading out of the interior of the case, the inner surfaces of the case having a profile with an alignment extending rectilinearly as seen in the flow direction and with flow trippers extending transversely to the alignment. When constructing a fuel assembly, various and sometimes mutually contradictory conditions must be considered, in order to optimize the production and flow of thermal neutrons, to increase steam production at the active fuel rods, to assure adequate mechanical stability and corrosion resistance, and to preclude other factors, such as corrosion or thermal changes. The fuel rods containing the nuclear fuel, such as uranium dioxide or uranium-plutonium mixed oxides, are held by mechanical fixtures that simultaneously provide guidance for the flow of water and steam and form the so-called skeleton of the fuel assemblies as well. The fuel rods are disposed longitudinally parallel to one another inside a fuel assembly case which extends longitudinally between a base part and a cap part, such as a lower and upper tie plate, and through which water flows. The water enters through openings in the base part, flows along the fuel rods as partial evaporation occurs, evaporates to a large extent on the hot surfaces of the rods, and emerges through outlet openings of the cap part in the form of a mixture of water and steam. In order to attain high heat transfer with high critical power (boiling output) and the most favorable cooling at the fuel he longitudinal axis of the case. To this end, suitable feed devices are provided in a closed water cycle or circuit extending from the outlet openings through a steam turbine to the inlet openings of the case. Non-boiling water for slowing down the neutrons and a controllable absorber for controlling the neutron balance are located outside the case. In order to keep the neutron absorption low within the active zone of the reactor, a special material, as a rule a zirconium alloy, is used at that location for the skeletal parts and in particular for the fuel assembly case. However, that material has little mechanical strength. The pressure difference between the interior and exterior of the case can therefore buckle the case a little. In order to improve the neutron flow, some of the positions provided for the fuel rods in the interior of the case are often occupied by so-called "water rods", that is channels which are in the form of fuel rods, but which carry non-boiling water. It has also already been proposed in German Petty Patent G 88 02 565.9 which deals with a case having a square cross section, to use a water channel having a cross section which is in the form of a regular polygon, in particular a square. Accordingly, the water channels are laterally closed off from the fuel rods by inner walls extending in the interior of the case, parallel to the fuel rods. It may be advantageous in such a case to place flow tabs or baffles in the intermediate spaces between the channels and the case walls in order to effect a favorable distribution of vapor and liquid within the coolant flow, so that the fuel rods are acted upon uniformly by water. In particular, the flow tabs or baffles may be attached to grid-like structures, which serve as spacers for the fuel rods. In order to increase mechanical strength, Published European Application No. 89 119 176.9 proposes providing inner walls in the case that interconnect oppositely disposed case walls. In such a device it is advantageous if the inner walls are constructed as a unit along with the spacers, fuel rods and suitable fasteners for the fuel rods in the cap part and the base part. Such a unit is displaceable relative to the case wall so that the case can be removed from the fuel assembly for inspection. In order to optimize the various requirements of a fuel assembly, U.S. Pat. No. 4,749,543 proposes a profile for the outer surfaces of the case that forms a reinforcing case wall at the corners of the square case and above all forms wall reinforcements in the lower part of the case that extend longitudinally and/or transversely. Due to the reinforcements, a relatively small wall thickness in the regions located between the reinforcements is then sufficient so that less neutron-absorbing material is needed overall for the case. Additionally, grooves extending at right angles to the flow direction are milled into the inner surfaces of the case walls. On one hand, as flow trippers have the task of detaching or separating a liquid flow creeping along the inner surfaces of the case and incorporating it in the form of liquid droplets into the flow of the boiling water. In the upper third of the fuel assembly, where the boiling water flow increasingly includes vapor because of the evaporation of water on the hot fuel rods, this feature is intended to supply a higher proportion of liquid water to the fuel rods. On the other hand, by increasing the flow cross section, the grooves are intended to counteract a pressure produced by the increase in volume upon evaporation, while in order to maintain the mechanical strength in the region where the wall thickness is reduced by the grooves, the outer surfaces of the case walls have the aforementioned profile, which provides reinforcement regions of correspondingly increased wall thickness. It is accordingly an object of the invention to provide a fuel assembly with flow trippers for a boiling water reactor, which overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type and which has improved flow trippers. These flow trippers are intended to detach or separate the liquid film which carries unused coolant at the case walls, and to direct it at the surfaces of the fuel rods. This provides better cooling and more favorable heat transfer as well as increased critical power at the fuel rods, but at same time, the mechanical stability, and the neutron flow distribution should not be disturbed, if at all possible. With the foregoing and other objects in view there is provided, in accordance with the invention, a fuel assembly for a boiling water reactor, comprising an elongated case having an interior and inner surfaces, a base part usually a lower tie plate having inlet openings discharging into the interior of the case for coolant flowing in a given flow direction, a head or cap usually an upper tie plate part having outlet openings leading out of the interior of the case for the coolant flowing in the given flow direction, mutually parallel fuel rods containing nuclear fuel and being disposed in the case between the base and head parts, the inner surfaces of the case having a profile with a straight or rectilinear alignment as seen in the given flow direction and flow trippers extending transversely to the alignment in the form of bulges protruding inward into the given flow direction and slightly reducing the flow cross section for the coolant, each of the bulges having a sloped surface facing toward the coolant flow with an impact surface substantially perpendicular to the given flow direction. In accordance with another feature of the invention, the case has wall parts with a substantially constant wall thickness, the bulges are formed by bending the case wall parts, and the impact surfaces are formed by removing material from the sloped surfaces. In accordance with a further feature of the invention, the case has wall parts with a substantially constant wall thickness, and the bulges are formed of additional wall material disposed on the case wall parts. In accordance with an added feature of the invention, the additional wall material is welded on. In accordance with an additional feature of the invention, the additional wall material is welding material, and the impact surfaces are formed by mechanical removal of part of the welding material. In accordance with yet another feature of the invention, the additional wall material is in the form of reinforcing metal sheets welded to the wall parts, and the reinforcing sheets have a profile forming the bulges and the impact surfaces In accordance with yet a further feature of the invention, the additional wall material is in the form of ribs being attached to the inner surface of the case and perpendicular to the alignment. In accordance with yet an added feature of the invention, the case has upper and lower portions, and the bulges are disposed only in the upper portion. In accordance with yet an additional feature of the invention, there are provided spacers disposed in the case, the spacers having ribs between the fuel rods extending perpendicular to the longitudinal axis of the case, and each of the bulges being disposed upstream of a respective one of the spacers as seen in the given flow direction. In accordance with a concomitant feature of the invention, there are provided inner walls being attached to the case in the interior of the case, the inner walls having the inner surfaces with the impact surfaces disposed substantially perpendicular to the given flow direction. Other features which are considered as characteristic for the invention are set forth in the appended claims. Although the invention is illustrated and described herein as embodied in a fuel assembly with a flow tripper for a boiling water reactor, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.