Patent Number: 039792587
Section: summary

The invention relates to nuclear reactors of the kind which are charged with spherical reaction elements and in which control rods are arranged to be thrust directly into the charge. In the operation of nuclear reactors which are charged with a bulk charge consisting of spherical reaction elements the problem arises that control rods in the reflector are not in themselves sufficient to keep the reactor, for example a power reactor, operating below the critical temperature. It is therefore necessary to provide extra control rods which are thrust either directly or indirectly into the bulk charge. Using the indirect method the rods are for example guided in bores in graphite noses which themselves penetrate into the charge. Using the direct method, on the other hand, each control rod is thrust directly into the charge of spherical reaction elements, for example by means of a piston working in a pneumatic cylinder. During the penetration of the rod into the charge a rounded nose on the rod thrusts the spherical elements sideways out of the way. During deeper penetration of the rod the spherical reaction elements can become stressed mechanically so greatly that they are damaged. A further disadvantage is that repeated penetration and withdrawal of the rods results in an undesired compacting of the charge. Furthermore the control rods themselves may be stressed sideways to an undesired degree. With the aim of reducing the mechanical stresses which are applied to the reaction elements and to the control rods when the rods are thrust into the charge of the elements, according to the invention, in a reactor of the kind described, each control rod which is arranged to be thrust directly into the charge has at least one screw thread on its external surface. The effect obtained is that during penetration of the rods into the charge each rod rotates, drive being applied tangentially to the rod as a result of the charge acting on the thread as the rod is advanced. The rotational driving effect is increased by the fact that the reaction elements, which are mainly graphite, have a high coefficient of friction because they are immersed in an ambient atmosphere of pure helium. The rotation of the control rods enable them to penetrate the charge more easily, the reaction elements being pushed aside by the threads. The elements and rods are stressed less than in conventional reactors of this type, and the charge also remains more loosely packed. During the penetration of the rods into the charge the greatest mechanical stress is applied to the elements which are most directly in line with the axes of the rods, the worst case occurring when a rod axis passes through the centre of a sphere. An advantageous construction in accordance with the invention comprises each control rod having a thread on its leading section having a greater pitch than a thread on the remaining length of the rod. With this arrangement, the leading section of each rod penetrates the charge relatively easily, the greater pitch thread causing only slight rotation, but as the rod penetrates deeper and resistance becomes greater, the trailing thread of lesser pitch causes the rotation to increase and thus facilitate penetration. The leading section which is thus rotated more than its own thread would otherwise cause tends to push the deeper elements sideways and upwards which tends to loosen the path for the rod. Preferably the axial length of the thread of greater pitch is limited to the ratio of 360.degree. to the number of threads. This limitation ensures that the part of the rod which has a thread of lesser pitch is considerably longer than the part where the thread has a greater pitch. This ensures that the rod rotates sufficiently rapidly. A further advantage provided by the construction in accordance with the invention is that, due to the rotation, the control rods penetrate straighter into the charge and are therefore subjected to less bending stress. It should be observed that this effect is obtained without it being necessary to provide any external drive for rotating the rods and consequently there are no extra sealing and lubricating problems. As an alternative to having a helical thread on the leading section of each control rod, this section may be provided with a number of ribs extending axially along the section, the screw thread or threads being on the remaining length of the rod. This arrangement makes it even easier to drive the rod initially into the upper layers of the charge, without fracturing or excessively abrading the reaction elements and facilitates control of the rod movement. When the rod initially penetrates the charge it does not yet rotate, the ribs near the nose of the rod allowing the nose to push the upper elements aside with the least possible friction. When the rod penetrates deeper into the charge the thread or threads on the longer upper part of the rod automatically brings the rod into rotation, the ribs on the leading end of the rod loosening the spheres in the deeper and more densely packed region of the charge. The ribs may be formed integrally with the rod or may be welded on. A similar effect may be obtained if the leading section of each control rod has an oval or polygonal cross section rather than having distinct ribs.