Patent Number: 051184672
Section: summary

The invention relates to a fuel assembly for a boiling water reactor having mutually parallel fuel rods disposed in an elongated fuel assembly case, and inner walls extending longitudinally between the fuel rods. The invention also relates to a boiling water reactor having such a fuel assembly. 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 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 transmission with high boiling output and the most uniform possible cooling at the fuel rods, a flow with a pressure difference is maintained along the 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 easily buckle the case. 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. 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 baffles in the intermediate spaces between the channels and the case walls in order to effect a uniform flow and to create turbulence in vapor bubbles and liquid droplets, so that the fuel rods are acted upon uniformly by water. In particular, the flow 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 which extend at right angles to the flow direction are milled into the inner surfaces of the case walls and lead to an enlargement of the flow cross section and therefore counteract an elevated pressure resulting from the increase in volume during evaporation. The upper edge of a groove is constructed as a narrow surface at right angles to the flow direction so that a film of water creeping along the wall is detached there and steered into the interior of the case. Such a structure is known as a "flow tripper". It is accordingly an object of the invention to provide a fuel assembly for a boiling water reactor, which overcomes the hereinafore-mentioned disadvantages of the heretoforeknown devices of this general type and which increases the boiling capacity or output of the fuel assembly in a boiling water reactor. The invention assumes that according to a variant of the invention the flow of thermal neutrons is increased, above all in the upper region of the fuel assembly, by means of inner walls that laterally define a channel carrying non-boiling water with respect to the fuel rods. There, the fuel rods are largely surrounded by water vapor, which can only inadequately slow down the neutrons produced in the decomposition. In these channels, which do not contain any fuel rods, a suitable flow cross section and flow speed assure that the convective heat entering through the channel wall is dissipated without causing the formation of steam in the channels. The non-boiling water in these channels therefore improves the distribution of the thermal neutrons. However, this leads to an increased heating capacity or output in the upper part of the fuel assembly only if sufficient water for evaporation is supplied to the fuel rods at that location as well. Furthermore, the supply of liquid water to the fuel rods is made more difficult by the fact that a considerable portion of the water entering the base of the fuel assembly forms a film of liquid on the channel walls and therefore does not come into contact with the fuel rods. According to this variant, at least in the aforementioned upper part of the fuel assembly, grooves are therefore provided on the surfaces of the inner walls forming the water channels that are oriented toward the fuel assemblies. The grooves extend transversely to the rods, or in other words transversely to the flow direction. However, the neutron flow can also be increased by providing that the thickness of the case walls is reduced, and thus the total quantity of neutron absorbing material is decreased. In order to assure adequate stability of the case walls even with a low wall thickness, another variant of the invention accordingly provides inner walls that interconnect oppositely disposed case wall parts. However, a film of liquid forms on these reinforcing inner walls as well, and as a result the percentage of water that can come into contact with the fuel rods in liquid form is reduced in the upper part of the fuel assembly that carries steam. Suitable grooves in these reinforcing walls cause a liquid film creeping along them to detach or break away and become swirled together with the mixture of water and steam flowing past it in such a way that once again the boiling capacity or output of the fuel assembly is increased. 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 fuel assembly case, mutually parallel fuel rods having longitudinal axes and being disposed in the case, and longitudinally extending inner walls between the fuel rods having lateral surfaces facing or oriented toward the fuel rods, at least some of the lateral surfaces having grooves formed therein extending perpendicular to the longitudinal axes of the fuel rods. In accordance with another feature of the invention, the inner walls form at least one water channel for non-boiling water being laterally closed off from the fuel rods. In accordance with a further feature of the invention, the grooves extend continuously around the outside of the water channel. In accordance with an added feature of the invention, the lateral surfaces with the grooves are planar outer surfaces of the water channel. In accordance with an additional feature of the invention, the case has mutually opposite walls, a plurality of the inner walls interconnect at least two of the mutually opposite walls, and the inner walls each have two of the lateral surfaces in which the grooves are formed. In accordance with yet another feature of the invention, the inner walls have sides being parallel to the longitudinal axes of the fuel rods, and the grooves formed in one of the lateral surfaces each extend as far as the sides of the inner walls. In accordance with yet a further feature of the invention, the inner walls have sides being parallel to the longitudinal axes of the fuel rods, and the grooves formed in one of the inner walls end in a rounded portion at a distance from the sides of the inner walls. In accordance with yet an added feature of the invention, the case has a polygonal and preferably square cross section and oppositely disposed side walls, a plurality of the inner walls are parallel to the oppositely disposed side walls of the case, each of the inner walls interconnect two respective oppositely disposed side walls of the case, and the grooves are formed in all of the lateral surfaces facing toward the fuel rods. In accordance with yet an additional feature of the invention, the case has a regular polygonal and preferably square cross section and side walls, the inner walls include a first group of inner walls forming a water channel for non-boiling water extending at least approximately centrally in the case and being laterally closed off from the fuel rods, and a second group of inner walls connecting each of the side walls of the case to the water channel, and at least the second group of inner walls has the grooves formed in all of the surfaces facing toward the fuel rods. In accordance with again another feature of the invention, the grooves have narrow sides perpendicular to the longitudinal axes of the fuel rods. In accordance with a concomitant feature of the invention, the case has a lower end with a base having inlet openings for boiling water, an upper portion with a top part having outlet openings for boiling water, and a plurality of spacers being longitudinally spaced apart from one another and having ribs between the fuel rods extending transversely to the longitudinal axes of the fuel rods, the grooves being formed only in the upper portion and each of the grooves being upstream of a respective one of the spacers, as seen in flow direction of boiling water. With the objects of the invention in view, there is also provided a boiling water reactor, comprising a boiling water circuit having a pressure vessel and a steam turbine, and a plurality of mutually parallel fuel assemblies disposed in the pressure vessel, each of the fuel assemblies including an elongated fuel assembly case, mutually parallel fuel rods having longitudinal axes and being disposed in the case, and longitudinally extending inner walls having lateral surfaces facing toward the fuel rods, the lateral surfaces having grooves formed therein extending perpendicular to the longitudinal axes of the fuel rods, and the fuel rods being bathed by boiling water flowing around them in the longitudinal direction of the fuel rods. 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 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.