Patent Number: 056174578
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the figures of the drawing in detail and first, particularly, to FIG. 1 thereof, there is seen a multiplicity of fuel assemblies 2 which are disposed inside a reactor pressure vessel 1. The fuel assemblies are supported by tops or top fittings 3 of the fuel assemblies at apertures or meshes 4 in a grid plate 5. Feet or bottom fittings 6 of the fuel assemblies rest adjacently on a lower core support 7 with a flow distribution plate 7' at a bottom 8 of the pressure vessel 1. A plenum 10 which is below a lid 9 of the pressure vessel is separated by the grid plate 5 from a zone of the pressure vessel containing the fuel assemblies. Support columns 11 protrude from the grid plate, through the plenum 10 up to a top plate 12 above an upper core support 13, to hold the grid plate 5 and the tops of the fuel assemblies. For reasons of clarity, a control-rod guideway 14 is shown above only one fuel assembly. The guideway holds the control rods which can be introduced into control-rod guide tubes of individual fuel assemblies to control the nuclear reaction. The lid therefore bears passages 15 which are assigned to drive elements in each case for a plurality of control rods. Elements 11 and 14 are plenum attachments. The fuel-assembly zone is surrounded laterally by a core shroud 17 inside a core barrel 18, so that the coolant flowing in through an inlet 16 is guided to a flow skirt 19 at the bottom of the pressure vessel. Elements 17, 18, 19 provide a device for deflecting a coolant flow. In each case the coolant then enters the foot of a fuel assembly from below through the lower core support 7, and flows essentially vertically along the fuel rods to passage openings in the top plate which covers the corresponding fuel assembly at the top of the fuel assembly. The coolant then enters the plenum 10 from the respective fuel assembly through the top 3 of the fuel assembly and enters the aperture 4 of the grid plate 5. In the plenum the coolant is deflected to one or more lateral outlet nozzles or an outlet 20. FIG. 2 shows the horizontal components of the coolant flow which occur within a quadrant of the core shroud. In this case, the fuel assemblies are disposed in rows 22 having an alignment which is shown in FIG. 2. In this case, gaps 23 are formed between the individual rows of fuel assemblies, with the coolant being able to pass through the gaps 23 from one row of fuel assemblies to another in a largely unobstructed manner. These transverse flows are produced by pressure differences below the grid plate. The pressure differences occur due to damming-up which the coolant suffers because of obstructions to flow which it encounters in the plenum after it passes through the grid plate and on the path to the outlet. These pressure differences and flows cause bending and horizontal vibrations, above all in the upper region of the fuel assemblies, which can lead to damage to the fuel rods and fuel assemblies that are bent in any case in the course of reactor operation due to the high temperatures, temperature loading and radiation-related changes. In order to be certain that a spacing, which is sufficient for the coolant flow, is always ensured between individual fuel rods of a fuel assembly despite this bending, the fuel rods are guided through apertures of spacers 24 shown in FIG. 1 and mechanically supported at different axial positions. It is already known (from European Patent Application 0 246 962 A1, corresponding to U. S. Pat. No. 4,804,516) to select the axial spacing of the spacer grids to be narrower in the upper region of the fuel assembly. However, ruptures of welds or other mechanical damage may occur even on the spacers. FIG. 2 shows some positions 26 at which turbulences and particular mechanical loads occur, for example due to great changes in the flow and the flow direction. Whereas a controlled transverse flow is desirable per se to increase the thorough mixing and cooling effect and can be produced intentionally by guide surfaces on the spacer webs, such inhomogeneous flow conditions as are shown in FIG. 2 are also undesirable with respect to a uniform cooling of the reactor core. Whereas in boiling-water reactors the coolant flow, which is already distributed relatively uniformly on a lower grid plate or the lower core support 7 over the different regions of the core cross section, is guided individually in the individual fuel assemblies (the feet of the fuel assemblies contain funnel-shaped, laterally closed transition pieces and the bundle of fuel rods is surrounded laterally by a fuel-assembly water canal), the feet of customary pressurized-water fuel assemblies are formed only of a laterally open frame, and the fuel assemblies are not surrounded laterally by a canal. The invention therefore provides for uniform pressure conditions to be enforced in the fuel assemblies of a pres- surized-water reactor by corresponding throttling of the coolant flow when it enters the plenum 10, thus obviating the cause of the above-mentioned transverse flows below the grid plate. FIG. 3 shows a cross section through a top region of a fuel assembly. Upper closure caps 30 of individual fuel rods 31, in which a compensation space is provided through the use of compression springs, for gaseous fission products produced by nuclear operations, are held in the apertures or meshes of a spacer 24. The spacers themselves are supported by control-rod guide tubes 32 which are attached to a top plate 34 by a screwed-on nut 33. A similar attachment is also provided between the guide tubes and the feet 6 of the fuel assemblies, so that the top plate which is supported by a frame 35 of the top of the fuel assembly, the guide tubes with the spacers and the foot part, form a supporting skeleton for the fuel rods. As is also shown in FIG. 5, the frame 35 of the top of the fuel assembly with the associated top plate 34 is supported on the grid plate 5 through the use of compression springs 36. FIG. 4 shows that the top plate 34 has passage openings 40 which are advantageously disposed in such a way that they lie above interstices which are produced between the individual fuel rods 31. In the exemplary embodiments of FIGS. 3 and 5, a throttle plate 41 is releasably attached at least in the top of a plurality of fuel assemblies. The throttle plate can rest, in particular, on the top plate, with the throttle plate 41 being bolted to the top plate 34 through the nut 33 of some control-rod guide tubes 32 in FIG. 3. FIG. 5 shows that the top plate 34 and the throttle plate 41 are advantageously held through common holding-down devices, for example the springs 36, in the top of the fuel assembly. According to FIG. 4, the throttle plate contains throttle openings 43 which advantageously have, in total, a smaller cross-sectional area than the passage openings 40 in the top plate 34 and lie with the largest part of their cross-sectional area above the passage openings 40. The configuration of the individual throttle plates and the dimensioning of their passage openings are adapted individually to the position of the respective fuel assembly on the grid plate in such a way that, when the coolant passes through the tops of all of the fuel assemblies, for example, a uniform damming-up is produced everywhere, that is to say no horizontal changes in pressure occur. In this case, however, provision may also be made by appropriate construction of the throttle plates to maintain a particular uniform pattern of weak transverse flows to increase thorough mixing of the coolant. FIG. 6 shows that the underside of the grid plate 5 can carry fuel assembly alignment pins 60 which engage in corresponding bores in the top parts 3 of the fuel assemblies in order to position the tops of the fuel assemblies at the respective apertures in the grid plate 5. The upper surface of the grid plate 5 carries the control-rod guideways 14, the support columns 11 and holding structures 11' for the tops of the fuel assemblies. Webs of the grid plate 5 form individual apertures, with throttle elements 61 being inserted in at least a plurality of the apertures. During an exchange of used fuel assemblies, the throttle elements can be inserted together with new fuel assemblies in order to adapt the pressure in the coolant, which emerges from the top of the new fuel assemblies supported on the grid apertures, to the pressure which prevailed in the old fuel assembly. If, in the case of such a fuel-assembly exchange, fuel assemblies which have not yet been burnt out completely are transferred to a different site in the reactor core, the throttle elements which are advantageously constructed as inserts for the grid apertures, can again be placed at the old site. The throttle plates or throttle elements according to the invention thus influence the damming-up occurring in the coolant below the grid plate in such a way that virtually no horizontal pressure differences occur below the grid plate or at least these pressure differences lead to a desirable distribution of the coolant when it enters the plenum. Additionally, they can contribute to the stabilizing of the desired flow, as a result of which even the thermodynamic and hydrodynamic conditions in the reactor core can be calculated and controlled in a simpler manner.