Patent Number: 042784980
Section: description

Referring now to the drawing and first, particularly, to FIG. 1 thereof, there is shown a reactor pressure vessel identified as a whole by 1 and having a convex cover 2 having a spherical part or cover calotte 2a and a cover flange 2b. Control rod drives identified as a whole by 3 are connected to the vessel cover 2 in the region of the spherical part or cover calotte 2a thereof by means of control rod drive stub tubes, which are not visible in FIG. 1 since they are covered by the support grid that is yet to be described herein and are indicated only by corresponding raster points. The control rod drives 3 can be seen better in FIG. 5, where only two such control rod drives 3 are shown in the interest of greater clarity in the presentation thereof, each thereof being formed of a control rod drive stub tube 3.1 pressure-tightly extending through the cover 2, and a pressure tube 3.2, which is shown in broken lines and is flanged pressure-tightly to the flanges 3.1a of the stub tubes 3.1 in an otherwise non-illustrated manner. In the interior of the pressure tube 3.2, control drive shafts 3.5 diagrammatically indicated in phantom i.e. by respective dot-dash lines, are mounted so as to be axially shiftable and lockable, and are provided with transverse slots and teeth at the outer periphery thereof. Locking and lifting pawls of an electromagnetic ratchet step lifter 3.3 can be brought into engagement with the peripheral teeth of the control drive shafts 3.5, the pawls being controlled by armatures which are movable, axially limited, against stops within the pressure tube, the actuating coils of the armatures, namely, a lifting coil 3.31, a gripper coil 3.32 and a holding coil 3.33 being slid onto the pressure tube 3.2 at the outside thereof and being mounted within a tubular coil housing 3.4 which surrounds the pressure tube 3.2 coaxially. A respective pressure tube 3.2 and a respective stub tube 3.1 form the drive housing for the control rod drive shafts 3.5. The specific construction of the ratchet step lifter is not essential to the invention of the instant application and is moreover well known, for example, from the German Pat. No. 1,439,948. The control drive shafts are connected to the control rods 3.6, likewise shown as dash-dot lines, by couplings which are diagrammatically indicated at 3.7. In addition, the core internals of the pressure vessel 1, and especially the fuel assemblies, which have the absorber channels into which the control rods 3.6 with absorber fingers can be inserted to a greater or lesser extent for the purpose of controlling the reactivity, are not illustrated in the interest of clarity. In FIG. 5, the lower part of the substantially cylindrical hollow pressure vessel 1 is identified as 1.1; also shown is one of the coolant nozzles 1.2 of the vessel that are distributed over the periphery thereof in a plane 1' normal to the axis; additionally shown is one of a number of support lugs 1.3 thereof which are distributed over the periphery of the vessel 1 and by means of which the vessel 1 rests on support brackets 1.6 that are mounted on an annular box girder or beam 1.5 anchored in a concrete structure 1.4 and is secured, as well, against lifting and turning. Also provided is a hollow prestressed-concrete cylinder 1.7 of the biological shield which, with an insulating layer 1.8 fastened to the inner periphery thereof, surrounds the pressure vessel 1 with an annular gap therebetween. Cover studs 1.9 with nuts secure the pressure vessel cover 2a pressure-tightly against the lower vessel part 1.1 and are anchored in the latter. Due to the curvature of the spherical cover 2a, which is outwardly convex in the illustrated embodiments, considered together with the fact that the flanges 3.1a of the stub tubes 3.1 are disposed in a horizontal connecting plane and the pressure tubes 3.2 of all of control rod drives 3 terminate at a level represented by the broken line 4, the control rod drive stub tubes 3.1 are consequently of different lengths i.e. the total length of the control rod drives 3 with stub tubes 3.1 disposed in the outer zones of the spherical cover part or calotte 2a is greater than that of the control rod drives 3 located in the farther inwardly disposed zones, and the latter control rod drives 3 are, in turn, greater in length than the control rod drives 3 with stub tubes 3.1 that are disposed in the central region of the spherical cover part or calotte 2a. The control rod drives 3, therefore, have different resonance frequencies, depending upon the respective length of the control rod stub tubes 3.1 associated therewith. According to FIGS. 1 and 2 and 5, the upper ends 3' of the control rod drives 3 and the pressure tubes 3.2, respectively, are articulatingly connected to each other by grid bars 5 and 6 of a support grid identified as a whole by G. In the illustrated embodiment, the support grid G is formed of two subgrids G5 and G6 (FIG. 2) which are disposed at a spaced distance al from each other and, respectively, in a lower grid plane el and an upper grid plane e2. The upper subgrid G6 of the plane e2 is disposed in the form of a square screen, the grid bars 6 thereof being indicated in FIG. 1 by solid black lines. The other subgrid G5 of the lower grid plane el is formed by diagonally extending rods 5 which are indicated in FIG. 1 by double lines. The grid joints are identified as a whole in FIGS. 1 and 2 by the reference numeral 7. As is apparent, the joints of the grid bars 5 of the lower subgrid G5 lie approximately in the projection of the corners of the upper subgrid G6 onto the lower subgrid G5. Both subgrids G5 and G6 are disposed in respective horizontal planes and are therefore perpendicular to the vertical pressure tubes 3.1 of the control rod drives 3. Due to the square screen or raster construction of the upper subgrid G6, grid bars 6a extend in X-direction and grid bars 6b in Y-direction. The square grid fields or mesh of the upper subgrid G6 are identified by the reference character 6. In FIG. 1, the coordinate cross or intersection of the two principal directions x and y for the subgrid 6G is shown together with the coordinate cross intersection of the likewise mutually perpendicular principal directions n and m of the subgrid G5, the two coordinate crosses or intersections being rotated relative to each other through and angle .alpha.=45.degree.. The grid bars of the subgrid G5 extending in the direction m are identified by the reference character 5a and those extending in the direction n by 5b. It is apparent therefrom that the grid bars of the subgrid G5 are disposed in zig-zag fashion i.e. alternatingly in the principal direction m or in the principal direction n, as viewed in the projection of the raster or screen squares 6' onto the subgrid G5. The grid configuration shown has been found to be particularly advantageous for a control rod field with a square screen or raster. Depending upon the intensity of the earthquake that is anticipated, more than two subgrids disposed on top of each other could also be used. As shown in FIG. 2 in conjunction with FIGS. 3 and 4b, the ends of the grid bars identified as a whole by the reference character g are spherical and, for this purpose, have ball heads 8 (FIG. 3) threadedly secured thereon. The joints 7 constructed as crossheads are fastened to the upper ends 3' of the pressure tubes (see FIG. 2), the grid bars g extending through slots 9 formed in the crosshead plates 10 and being movably supported with the ball heads 8 thereof within cylindrical bores 11 serving as ball joint sockets. FIG. 2 shows that the respective grid joints are formed by two crosshead plates 10, 10 disposed on top of each other and, respectively, provided with ballhead receiving bores 11 and through-slots 9 (FIGS. 4a and 4b) for the grid bars g as well as with a central through-bore 12 for a fastening screw or bolt 13. The bores 11 and 12 and slots 9 formed in the crosshead plates 10, 10 are closed off at the bottom of the joint 7 by a bottom plate 14 and at the top thereof by a cover 15, as viewed in FIG. 2. The pressure tube end 3' is of solid construction and is provided with a central tapped bore 16 for the fastening screw 13 as well as with a flat or planar mounting surface 17. In this manner, a simple and strong means for fastening the grid joint 7 can be attained by providing that the fastening screw 13 disposed in the central bore 12 passes through the crosshead plates 10, 10 together with the bottom plate 14 and the cover 15 and are tightened or clamped against the planar mounting surface 17, a conventional device 18 for preventing unscrewing being advantageously provided at the fastening screw 13, the device 18, in the illustrated embodiment of FIG. 2 being formed of a washer with a bent-up lip engaging one of the lateral flat surfaces of the hexagonal head of the screw 13. In FIG. 3, the grid bars g are shown divided, and the two grid bar halves g' and g" thereof connected together by means of a turnbuckle 19. The latter is formed of a bushing with an internal thread which is brought into threaded engagement with the threaded shank 20 of the one grid bar half g", for example, by means of a right-hand thread 19a and, accordingly, with the threaded shank 20 of the other grid bar half g' by means of a left-hand thread 19b. At the outer periphery of the threaded bushing 19, a region having a polygonal cross section 21, for example, a hexagonal section, is provided for engagement by wrenches. In the embodiment according to FIG. 6, a pressure vessel 1* of a boiling-water nuclear reactor is shown with a spherical bottom part of calotte 1a*, through which control rod drives 3* with respective pressure tubes 3.2* extend in a pressuretight manner and project downwardly and out of the pressure vessel 1* with an axial length 1.sub.a which is substantially the same for all of the control rod drives 3*. Tubular control rod drive shafts 3.5* are supported within the pressure tubes 3.2* so as to be movable lengthwise yet secured against rotation. They can be moved in axial direction, for example, hydraulically or, in combination, electrically and hydraulically by means of drive units 3.3* connected to the pressure tubes 3.2*, absorber rods 3.6* connected to the upper ends thereof being insertable to a lesser or greater extent into intermediate spaces (absorber channels) located between the fuel assemblies 3.0 for thereby controlling the reactivity in the manner explained hereinbefore in connection with the first embodiment of the invention shown in FIG. 5. The pressure tubes 3.2* include the feedthrough stub tubes like those at 3.1 in FIG. 5, but not shown separately in FIG. 6 and, together with the drive units 3.3*, form thimble-like tubular drive housings which are pressure-tight against the outside. Details of the construction of the drive units 3.3* are of no special significance for the invention of the instant application, and can be obtained, for example, from the German Pat. No. 1,169,596. It is important that also in this second embodiment of FIG. 6, earth-quake caused transversal vibrations of the control rod drives 3* having lengths 1.sub.a extending downwardly from the outwardly curving or convex sperical bottom part or calotte 1a* which would basically behave like spring rods clamped at one end thereof, are effectively prevented or reduced to harmless values, by providing that the drive housings be flexibly or articulatingly connected to each other in the vicinity of the lower or free ends thereof by grid bars g* of the support grid G* in such a manner that the free axial lengths 1.sub.a1 * of the drive housings, as measured from the outside of the spherical bottom part of calotte 1a* to the grid joint G6* are different as viewed over the cross section of the support grid G*. For this purpose, an upper subgrid G6* is provided which is disposed in an upper grid plane e2* and may be constructed like the hereinabove-described grid G6 of FIG. 2. The lower subgrid G5, however, is not disposed in a flat plane but in a curved surface e1* which corresponds substantially to the contour of the spherical cover part or calotte 1a*, because all of the drive housings have the same overall length 1.sub.a. Further length sections 1.sub.a2 * of the drive housings are thus provided between the two subgrids G6* and G5*, the respective lengths of which are likewise different as viewed over the cross section of the control rod drive field. Within the length sections 1.sub.a1 * as well as the length sections 1.sub.a2 *, drive housings of different length are coupled flexibly or articulatingly to each other through the subgrids G6* and G5* and also in a vibration-attenuating or vibration-cancelling manner because of the different resonance frequencies of the coupled length sections 1.sub.a1 *, 1.sub.a2 *. The subgrid G5* can be constructed in plan view like the hereinaforedescribed subgrid G5 of FIG. 2; in addition, however, the bars g* of the subgrid G5* extend inclined to the horizontal. For the subgrid G6*, the crossheads, which are not shown in FIG. 6, are hollow-cylindrical, the pressure tubes 3.2* passing through them, the crossheads being fastened at the outer periphery of the pressure tubes 3.2*. On the other hand, the crossheads for the subgrid G5* can, in principle, be constructed as described heretofore in connection with FIG. 2. Internal main reactor coolant pumps 22 are further shown in FIG. 6. The invention of the instant application is also applicable to control rod drives which are disposed in the spherical bottom part of calotte of a pressure vessel but have free ends thereof which extend, in a manner deviating from that of FIG. 6 and in accordance with FIG. 5, up to a common horizontal end plane, so that the lengths 1.sub.a of the drive housings extending out of the pressure vessel are inherently different. In such a case, the support grid construction of the first embodiment of FIG. 5 can again be used, but then, however, at the underside of the pressure vessel rather than at the top thereof.