Patent Number: 042808758
Section: description

Referring now to the figures of the drawing and first, particularly, to FIGS. 1 and 1a thereof, there is shown an upper core structure 1 of a reactor pressure vessel 2, held by a ring flange 1a against a support flange 4a placed on the pressure vessel 2 at the inner circumference thereof, and more specifically, a shoulder 2a of the core barrel 4, with the interposition of pre-tensioning cup springs 3 constructed in the form of a column or a stack. Hold-down plates 5 (see also FIGS. 2 to 5) are fastened for holding down the cup springs 3, as a first structural part or member, to the ring flange 1a of the upper core structure 1 by means of screws 6, which are secured against rotation. The ring flange 1a and core structure 1 form a second structural part or member. The ultimate clamping of the core structure 4 is accomplished, as shown in FIGS. 1, 1a and 2, by the flange 7a of the pressure vessel cover or head 7, which extends over the hold-down plates 5 and through which clamping studs 8 projecting beyond the hold-down plates 5 of the cup springs 3 (see FIG. 4) can be pushed inward (downward in the drawing) for supplying the rest tension for the cup springs 3. In other words, according to FIG. 4, the cup spring column 3 is already pre-tensioned when the hold-down plate 5 is screwed down and secured against unscrewing and has thereby transmitted the pre-tension to the spring column 3 through the clamping stud 8. Starting from the pre-tensioned condition shown in FIG. 4, the clamping stud 8 is pushed inward additionally by the spring travel distance f for the ultimate clamping; this condition can be seen in FIG. 2. This ensures a particularly vibration-proof fastening of the upper core structue 1. The thermal expansion of the upper core structure 1 and the flange 4a of the core barrel 4 (which consist of austenitic material) relative to the pressure vessel 2 and its head 7 (of ferritic material with austenitic cladding on the inside) is assured. During the fuel assembly change mentioned above, the cup springs 3 can now be exposed, with the reactor pressure vessel open, and examined for proper functioning. It is important in this regard to unscrew the hold-down plates 5 by remote control from the top, and as quickly as possible. On the other hand, the screws 6 must be secure against rotation since loosening of the seating of the hold-down plates 5 must not happen under any circumstances. These requirements are met by the device against unscrewing according to the invention in a particularly advantageous manner. Before further explanation hereinafter, one of the reactor coolant nozzles 9 of the pressure vessel should be pointed out. The pressure vessel in question is for a pressurized-water nuclear reactor power plant with an electric power output of, say, 900 MW. The head screws of the core structure 1 are not shown in FIG. 1 for the sake of simplification, only the upper grid 1b with supports 1c can be seen. A so-called poller or trunnion 10 i.e. an eye- or hook-shaped element for hanging the core structure to the lifting traverse of a building crane and one of the head screw bore holes 11 in the head flange 7a are shown. As shown especially in FIGS. 5 and 6, a circular groove or slot 12 is cut into the wall 5a of the receiving hole 5b of the hold-down plate 5 (first structural part or member). This slot 12 is formed in such a manner that the head 6a of the screws 6, shown in FIG. 7, if disposed within this receiving hole 5b with the screw 6 in the tightened condition, is surrounded by a plastically deformable ring collar 5c with an annular gap r. This can be seen particularly clearly in FIGS. 9 and 10. The screw head 6a is provided with arcuate or citriform recesses 13, similar to the shape of the small end of a lemon, at two diametrically opposite points of its outer circumference. If the screws 6 are in the tightened condition (see particularly FIGS. 3, 4 and 9, 10), the wall portions 5c1 of the ring collar 5c, which are situated in the vicinity of the circumferential angle of the recesses 13, can now be pushed into the recesses 13. FIGS. 3 and 4 show an overall view of the screw 6 in tightened and rotation-secured condition. It can be seen that each screw 6 has an inner hexagon 6b, extends through the hold-down plate 5 as the first structural part or member, engages behind the latter and clamps it against the ring flange 1a of the upper core structure 1 as the second structural part. In the ring flange, corresponding blind tapped holes 14, which are provided with internal threads are formed. The receiving holes 5b of the holddown plate 5 surround the screw head 6a over its entire axial length. The enlarged view of FIG. 10 shows an inner hexagon 6b. However, this element may generally take the form of most any inner polygon, i.e., in principle, a square, an octogon or the like could be used. The important thing being that corresponding cap screw wrenches, for tightening and unscrewing the screws 6 preferably operated by remote control, can be inserted into this polygonal blind hole in the screw head 6a. The wall thickness d (see FIGS. 9 and 10) of the ring collar 5c and the shape of the recesses 13 are chosen so that the loosening torque of the screws 6 is not, or for the most part not, greater than their tightening torque, and the ring collars 13 can be used again for further rotation-securing deformations after the wall portions 5c1 pushed into the recesses 13 have been formed back to their original shape. An advantageous value for the wall thickness d for so called M10 socket head or allen cap screws is, for instance, 0.6 mm. This applies when an austenitic, ductile steel is used for the holddown plates, such as is required for a core structure holddown system. The arcuate or lemon-shaped of the recesses 13 shown is particularly advantageous for pushing-in the wall portions 5c1 during the plastic deformation as well as for pushing them out for unscrewing the screws 6. It has been found that wall portions 5c1 which have been pushed into the recesses 13 could be deformed plastically again to assume the rotation-prevention function. For repeated usability of the holddown plates 5 and their ring collars 5c, however, it is particularly advantageous if wall portions of the ring collars 5c which has not yet been deformed in a previous deformation process can be used for the further deformations. To illustrate this, three different sets of wall region pairs 5c1, 5c2 and 5c3 which are rotated relative to each other by 120.degree. are illustrated in FIG. 5 by different shadings. In order to use the above-mentioned three different pairs of wall portions sucessively for each rotation-securing operation, the heads 6a of the screws 6 can be provided with several, selectable pairs of recesses which are disposed at different points of the circumference. For instance, the pair of recesses 13', 13' shown in FIG. 10 could be provided. This pair is shifted 90.degree. relative to the pair of recesses 13, 13. Alternatively, three pairs of recesses according to the wall portion construction of FIG. 5 can be provided. However, it is also possible to use groups of screws with only one pair of recesses 13, 13 as can be seen from FIGS. 3 and 7. In this case, however, the connecting lines v1, v2, v3 of the recess 13, 13 are rotated from group to group by a predetermined circumferential angle, for instance, 120.degree. or 60.degree., relative to a screw reference point. An advantageous value for the maximum depth of the arcuate or lemon-shaped recesses 13 is two to four ring collar wall thicknesses. This depth is designated t in FIG. 7b. As is illustrated in FIGS. 7c and 9, the arcuate or lemon-shaped recesses 13 cover only a part a1 of the axial length a of the screw head 6, so that an unweakened stop flange 6c of the screw head 6a is left (see also FIG. 7c). The thread of the screws 6 is designated 6d. Referring now to the previously partly explained FIGS. 9 and 10, as well as to FIGS. 8a and 8b, the tool 15 for the plastic deformation of the ring collar 5c1 will now be explained. This tool 15 comprises a substantially cylindrical deforming punch 15a (FIGS. 8a and 8b) which can be inserted into the inner polygonal opening 6b of the respective screw 6 in the direction of the screw axis, in centering relationship. For this purpose a centering pin 16 which protrudes centrally beyond its bottom surface 15b and is spring-loaded axially movably and in a restoring manner is provided. The deforming punch 15a further has two diametrially opposed deforming plugs 17a and 17b with a deformation profile 17 corresponding to the impressions 5c1 to be made. To this end, the deforming edges of the deformation profile 17 of the deforming plugs 17a, 17b are provided with rounded deforming edges 17.1, 17.2 which extend at an angle downward and outward as is clearly shown, especially in FIG. 9. The deforming plugs 17a, 17b themselves each are round pins having inclined surfaces 17.1, with which they engage the upper edge of the not yet deformed ring collar 5c and then curve the ring collars 5 c inward with their rounded edges 17.2 upon further lowering of the deforming punch, so that a shape with the impressions 5c1 shown in FIGS. 9 and 10 is obtained. The deforming plugs 17a, 17b are inserted into corresponding axial holes 18 of the tool 15 and are pinned there by means of dowel pins 19. The centering pin 16 has an elongated hole 2 formed therein and is provided with a crosswise pin 20 guided in the elongated hole 21 so as to secure the centering pin 16 against dropping out of the punch 15a. Between the end 22a of the blind guide hole 22 (see FIG. 8a) and the inner end 16a of the centering pin 16, a compression coil spring 23 is inserted. The deforming plugs 17a, 17b protrude from the punch 15a by a distance a2 such that when the bottom surface 15b of the punch 15a is placed on the structural part 5, the precise desired deformation length of the wall regions 5c1 is obtained. In the deformation process, the punch 15a with the pin 16 is axially guided at the walls of the inner polygon 6b. As illustrated in FIG. 10, a slight lateral offset e can be permitted without interfering with the protection against unscrewing. The punch 15a can be fastened at the lower end to a suitable holder, not shown. The offset e shown is exaggerated and can result from the fact that the inner hexagon 6b is slightly wider than the outside dimension of the plug 6 for the purpose of easier insertion.