Patent Number: 050892150
Section: summary

The invention relates to a method and assembly for securing a cylindrical centering pin, which is used in particular for centering nuclear fuel assemblies. In order to secure a centering pin on a plate of a reactor core support structure or framework, such as a grid plate, it is known to use a centering pin having one end that has a collar and is provided with a thread. The centering pin with a thread is passed through the bore of the plate and is screwed firmly against the shoulder by a nut on the back of the plate. In that device, two components must be kept on hand for each centering pin, and when they are installed they must each be joined together from a different side of the plate. Accordingly, the prior art method and assembly requires two components and is quite time consuming to install. It is accordingly an object of the invention to provide a method and assembly for securing a centering pin, which overcome the hereinafore-mentioned disadvantages of the heretofore-known methods and devices of this general type and with which a one-piece centering device can be anchored in an existing bore more quickly than before. With the foregoing and other objects in view there is provided, in accordance with the invention, a method for securing a cylindrical centering pin, especially for centering fuel assemblies, in a bore formed in a plate, which comprises: a) introducing an end of a centering pin to be secured to a plate into a bore formed in the plate until at least part of a coaxial, radially deformable wall portion of the end to be secured protrudes beyond the plate; and b) generating pressure in a closed hollow space surrounded by the wall portion with a pressure fluid until the wall portion radially expands and anchors the centering pin. With the objects of the invention in view, there is also provided a method for securing a cylindrical centering pin, especially for centering fuel assemblies, in a bore formed in a plate, which comprises: a) introducing an end of a centering pin to be secured to a plate into a bore formed in the plate until at least part of the end to be secured having a periphery and an interior with radial slits formed therein protrudes beyond the plate; and b) bringing a mandrel-like tool partially into the slits in the interior of the end of the centering pin and radially expanding the end of the centering pin with the tool for anchoring the centering pin. In accordance with another mode of the invention, there is provided a method which comprises introducing the mandrel-like tool into an axial bore formed in the centering pin from a free end of the centering pin for expanding the centering pin. In this way, only one side of the plate need be accessible. This makes the positioning of the centering pin particularly simple. A common feature of both methods according to the invention is that the assembly time can be considerably reduced, because the centering pin is anchored as a component in a simple manner. This gains particular significance if there is a need to replace the centering pin in a core support structure or framework, because the radiation exposure to operating personnel can be markedly reduced as compared with methods used previously. With the method using a centering pin having a closed hollow space that can be acted upon by pressure in the end to be secured, it is advantageous if, in accordance with a further mode of the invention, there is provided a method which comprises generating the pressure by igniting an explosive charge with an igniter connected to the charge through electrical lines. This method permits particularly fast anchoring of the centering pin. In accordance with an added mode of the invention, there is provided a method which comprises generating the pressure by connecting the hollow space through a line to a pressure fluid reservoir. In this way, very precise metering of the pressure can be attained. With the objects of the invention in view, there is furthermore provided an assembly for securing a cylindrical centering pin in a bore formed in a plate, comprising a cylindrical centering pin having an end to be secured in a bore in a plate, the end having a closed hollow space formed therein defining a coaxial, radially deformable wall portion surrounding the space, and means associated with the hollow space for inflating the end, expanding the wall portion and anchoring the centering pin in the bore. An advantage of this embodiment is that the centering pin is formed in one piece and can be secured to a plate that is accessible from only one side. In accordance with another feature of the invention, there is provided an outer surface, electrodes disposed on the outer surface, and ignition lines extended through the centering pin to the electrodes, the inflating means including an explosive charge connected to the ignition lines. The anchoring is then accomplished by merely connecting and switching on an igniter. In accordance with a further feature of the invention, there is provided an adaptor, and a pressure fluid reservoir, especially a hydraulic reservoir, the inflating means including a pressure fluid line having a controllable valve and being connected from the hollow space through the adaptor to the fluid reservoir. This embodiment permits controllable anchoring of the centering pin. In accordance with an added feature of the invention, there is provided a connecting web extending through the hollow space in the axial direction. With such a configuration, the centering pin preferentially expands in the radial direction, which leads to particularly good anchoring. In accordance with an additional feature of the invention, the hollow space is an axial blind bore with a bottom being formed in the end to be secured, and the connecting web is a head bolt disposed in the axial blind bore, the head bolt having a head covering the blind bore and being tightly joined to the wall portion, and the head bolt having an end retained in a force-locking manner on the end to be secured at the bottom of the blind bore. This provides a particularly stable version of the centering pin. With the objects of the invention in view, there is additionally provided an assembly for securing a cylindrical centering pin in a bore formed in a plate, comprising a cylindrical centering pin having an end to be secured in a bore in a plate, the end having a periphery and an interior with radial slits formed therein extending in longitudinal direction of the centering pin, the slits formed in the periphery being offset from the slits formed in the interior, and a mandrel-like tool partly inserted into the slits formed in the interior for expanding the end of the centering pin. This one-piece apparatus is particularly simple to manufacture. In accordance with again another feature of the invention, the end to be secured has a cylindrical opening formed therein, and the cylindrical opening and the slits formed in the interior together form a star shape with the cylindrical opening at the center or point of the star. This embodiment also enables particularly uniform expansion of the centering pin. In accordance with again a further feature of the invention, the centering pin has an axial bore formed therein with a restriction in the vicinity of the slits. Such a centering pin can be anchored from the free end thereof by means of the mandrel-like tool, which is introduced into the bore. In accordance with again an added feature of the invention, the restriction is formed by the cylindrical opening. In accordance with again an additional feature of the invention, the end to be secured has a radial groove formed thereon forming a shoulder on the plate in the expanded condition of the centering pin. This enables the centering pin to engage the plate from behind in hook-like fashion. In accordance with yet another feature of the invention, the end to be secured is beveled at least at one side of the groove. Such an embodiment offers highly secure anchoring. This security can be increased if, in accordance with yet a further feature of the invention, the end to be secured has at least one sawtooth-like side of the groove. Such a centering pin can then be used even if the bore or plate has rough areas. The centering pin digs in behind the bore making the anchoring very reliable. In accordance with a concomitant feature of the invention, the mandrel-like tool has a rod remaining in the centering pin as a dowel. 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 method and assembly for securing a centering pin, 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.