Patent Number: 040100707
Section: description

DETAILED DESCRIPTION OF THE INVENTION As indicated above, FIG. 1 shows only the lowermost portion of an absorber element 1 having its leading tip 2, which must directly penetrate the pebble bed, having a substantially hemispherical shape, although other known tip shapes could be used. As shown, behind the tip 2, the element 1 has the shape of a substantially helically coiled rod, forming a single thread as contrasted to a multi-threaded or double-helical shape which would result in a considerably larger angle of pebble-bed penetration and, therefore, would require a considerably greater force of penetration P. The rod is preferably made of a suitable metal and is hollow or tubular, and is filled with the absorber material. Assuming that the pebbles or fuel spheres of the pebble bed have a characteristic cross-sectional size or diameter of 60 mm, the outside diameter of the element's rod or tube should likewise be about 60 mm. The element has an inside diameter and an axially directed coil convolution separation distance which are both greater than the cross-sectional dimension or diameter of the rod or tube used, and of the pebbles or fuel spheres. The tip 2 is formed by the leading end of the rod or tube, possibly as a integral part of the balance. In FIG. 2, the reactor container 3 is shown, containing the pebble bed 4 made up of a large number of pebbles or fuel spheres of which only a few are indicated at 4a. Only one of the new absorber elements 1 is shown, but of course, a larger number would ordinarily be provided. These would be distributed over the circumference of the bed 4 which has a generally cylindrical contour excepting for the conical run-out 3a of the container 3. The element 1 is shown substantially completely inserted in the bed 4. The upper end of the element 5 is provided with a tubular shank 6 provided with a rotative drive 7 which may also provide for vertical movement to the extent required for the initial insertion of the element 1, after which rotation of the element 1 effects its complete penetration into the bed 4. As previously indicated, the active portion of the rod 1 is itself hollow or tubular as shown by the broken-away segment in FIG. 1, the absorber material 1a then being on the inside of the rod or tube 1. It can be seen that the tip 2 is directed to penetrate the bed 4 in a non-vertical direction, and that this direction varies angularly during penetration of the tip in the bed. Behind the tip 2 the element 1 has the shape of the substantially helically coiled rod or tube. The interior of the helical shape and the spaces between the helical convolutions are open and permit the passage of the fuel sphere or pebbles. The new absorber element in its form of a helical spiral coil with its hollow or empty spaces both within the convolutions and from one convolution to another, has additional advantages over the described easier penetration. From the viewpoint of nuclear physics, the absorber effect of an absorber element depends essentially on the size of its surface, so the new element, for the same absorber volume, has a considerably greater absorber effect than would a conventional solid cylindrical rod, of the same diameter as the outside diameter of the new helical element, because the ratio of the surface to the volume is greater for the helical form. Therefore, the number of absorber element required for the reactor, can be reduced substantially, which is important with respect to the space requirement and the costs for the absorber drives, such as the drive illustrated schematically at 7.