Patent Number: 056174578
Section: summary

CROSS-REFERENCE TO RELATED APPLICATION This application is a Continuation of International Application PCT/DE94/00256, filed Mar. 4, 1994. BACKGROUND OF THE INVENTION Field of the Invention The invention relates to a pressurized-water reactor having a multiplicity of fuel assemblies being mutually adjacently disposed in the interior of a pressure vessel on a core support at the bottom of the pressure vessel, each of the fuel assemblies containing a bundle of fuel rods disposed around control-rod guide tubes and being supported at apertures in a grid plate by a top carrying a top plate covering the bundle, a plenum being formed above the grid plate inside the pressure vessel, attachments protruding into the plenum at the upper surface of the grid plate, the plenum having a lateral outlet, and the pressure vessel having a device for deflecting a coolant flow from an inlet into the pressure vessel through the core support, for distributing it over the individual fuel assemblies and for guiding it along the fuel rods, through passage openings in the top plates of the fuel assemblies and through apertures in the grid plate into the plenum. Fuel assemblies of pressurized-water reactors contain a bundle of fuel rods which are disposed around control-rod guide tubes, with the bundle of fuel rods being covered by a top plate in the top of the fuel assembly. Inside a pressure vessel, a multiplicity of such fuel assemblies are disposed adjacently on a core support at the bottom of the pressure vessel and are supported with their top parts on the apertures of a grid plate. Formed above the grid plate inside the pressure vessel is a plenum into which attachments on the upper surface of the grid plate protrude. The plenum contains a lateral outlet, such as one or more outlet nozzles for a coolant flow. The coolant flow is guided by appropriate devices from an inlet in the pressure vessel to the core support at the bottom of the pressure vessel, is distributed there over the individual fuel assemblies, then flows along the fuel rods and emerges through passage openings in the top plates of the fuel assemblies in order to enter the plenum through the apertures in the grid plate. In this case, the core support at the bottom of the pressure vessel can already carry throttle plates or inserts in order to distribute the coolant flow uniformly over the entire cross section of the pressure vessel. The interstices between the fuel rods and the guide tubes of a fuel assembly are connected to one another and to the interstices between adjacent fuel assemblies so that coolant flows directed transversely to the fuel assemblies can occur in the pressure vessel. This may be desirable in order to achieve thorough mixing between hotter and cooler regions of the coolant, for which purpose spacers with appropriate deflection devices may be provided on different axial planes of the fuel assembly. Such spacers are required in any case in order to fix the lateral spacing of the fuel rods. However, apart from such spacers supporting the fuel rods, their own grid structures may also be provided additionally to hold further mixing devices on the fuel assemblies. The attachments which protrude from the upper surface of the grid plate into the dome of the pressure vessel (that is to say the plenum for the coolant heated on the fuel rods) are necessary, for example, to support the grid plate mechanically and to receive the control rods which can be introduced into the control-rod guide tubes. The partial flows of the coolant, into which the coolant is divided at the lower core support and which emerge after flowing through the individual fuel assemblies through the individual apertures in the grid plate, therefore have to overcome an individual flow resistance on their path to the outlet, which flow resistance is determined by the length of the respective flow path and the obstacles disposed in that path. A pressure thus occurs in the coolant when passing through the passage openings in the plates, which pressure is distributed inhomogeneously over the cross section of the pressure vessel. When passing through the top plates, the coolant thus suffers damming-up which, due to the geometrical configuration of the attachments on the top plate and of the lateral outlet, may be different for each fuel assembly and already leads to pressure differences and resultant transverse flows in the axial zone of the pressure vessel in which the fuel rods are seated. The pressure differences are already one of the causes of bending of the fuel rods and fuel assemblies. Additionally, the transverse flows cause the fuel rods and the structural elements of the fuel assemblies to vibrate and to be subjected to mechanical loading. In total, other physical loads on the fuel assemblies are thus intensified, in such a way that damage may occur on the fuel assemblies. In order to avoid horizontal pressure differences and transverse flows, the coolant can be guided vertically through the plenum in the dome of the pressure vessel and conducted away through corresponding vertical outlet nozzles which take into account the geometry of the attachments on the grid plate. However, that leads to a complicated construction or an impermissible structural height of the pressure vessel. SUMMARY OF THE INVENTION It is accordingly an object of the invention to provide a pressurized-water reactor with individually adapted pressure distribution in the coolant, which overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type and which avoids an inhomogeneous flow speed inside a reactor pressure vessel in such reactors with undesirable transverse flows in an axial region in which the fuel rods are seated, through the use of an individual adaptation of flow conditions in a region of the tops of the fuel assemblies and of a grid plate supporting the tops. This object is achieved by an individual throttling of the coolant flow when it passes through the individual tops of the fuel assemblies or, at the latest, when it passes through the grid plate. With the foregoing and other objects in view there is provided, in accordance with the invention, a pressurized-water reactor, comprising a pressure vessel having a bottom, an inlet, a lateral outlet, and an interior; a core support disposed at the bottom of the pressure vessel; a grid plate having apertures formed therein and having an upper surface, the grid plate defining a plenum above the grid plate in the interior of the pressure vessel, the plenum leading to the lateral outlet; plenum attachments protruding into the plenum at the upper surface of the grid plate; a multiplicity of mutually adjacent fuel assemblies disposed in the interior of the pressure vessel on the core support, each of the fuel assemblies containing a bundle of fuel rods and a top carrying a top plate covering the bundle and having passage openings formed therein, and each of the fuel assemblies being disposed around control-rod guide tubes and being supported at the apertures in the grid plate by the top; a device disposed in the pressure vessel for deflecting a coolant flow from the inlet, into the pressure vessel and through the core support, for distributing the coolant flow over the individual fuel assemblies and for guiding the coolant flow along the fuel rods, through the passage openings in the top plates of the fuel assemblies, through the apertures in the grid plate and into the plenum; and throttle plates each being attached in the top of a respective one of a plurality of the fuel assemblies, the throttle plates having throttle openings formed therein for an individual adaptation of pressure in the coolant flowing through the top of the respective fuel assembly. These features provide such an individual throttling when the flow passes through the top of the fuel assembly. In accordance with another feature of the invention, the throttle openings in the throttle plate have a smaller cross section than the passage openings in the top plate, the throttle openings have a cross-sectional area, and the greatest part of the cross-sectional area of the throttle openings is disposed above the passage openings in the top plate. In accordance with a further feature of the invention, the throttle plate is attached releasably in the top of the fuel assembly. In accordance with an added feature of the invention, the throttle plate and the top plate are bolted together to an upper end of the control-rod guide tubes. In accordance with an additional feature of the invention, there are provided common holding-down devices holding the top plate and the throttle plate in the top of the fuel assembly. In accordance with yet another feature of the invention, each of the throttle plates is disposed above a respective one of the top plates. With the objects of the invention in view, there is also provided a pressurized-water reactor, comprising a pressure vessel having a bottom, an inlet, a lateral outlet, and an interior; a core support disposed at the bottom of the pressure vessel; a grid plate having apertures formed therein and having an upper surface, the grid plate defining a plenum above the grid plate in the interior of the pressure vessel, the plenum leading to the lateral outlet; plenum attachments protruding into the plenum at the upper surface of the grid plate; a multiplicity of mutually adjacent fuel assemblies disposed in the interior of the pressure vessel on the core support, each of the fuel assemblies containing a bundle of fuel rods and a top and being supported by the top at the apertures in the grid plate; a device disposed in the pressure vessel for deflecting a coolant flow from the inlet into the pressure vessel, for distributing the coolant flow through the core support over the individual fuel assemblies and for guiding the coolant flow along the fuel rods, through the tops of the fuel assemblies and the apertures in the grid plate and into the plenum; and throttle elements associated with a plurality of the apertures in the grid plate, the throttle elements each having at least one passage opening formed therein for an individual adaptation of pressure in the coolant emerging from the tops of the fuel assemblies supported at the apertures in the grid plate. These features provide a corresponding individual throttling when the flow passes through the apertures in the grid plate. In accordance with a concomitant feature of the invention, the throttle inserts are inserted in the apertures in the grid plate. Correspondingly, a throttle plate may thus be attached in the top of a plurality of fuel assemblies (in particular in each case above the top plate). The throttle plate contains one or more throttle openings for the individual adaptation of the pressure in the coolant which flows through the top of the respective fuel assembly. However, a plurality of grid apertures, on which the respective fuel assemblies are supported, may also have throttle elements in each case with one or more passage openings which bring about the individual adaptation of the pressure in the coolant that flows through the top of the fuel assemblies. 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 pressurized-water reactor with individually adapted pressure distribution in the coolant, 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. The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.