Patent Number: 047524405
Section: claims

1. In a control rod for nuclear reactors which comprises a number of elongated absorber plates, each absorber plate including a plurality of channels which extend substantially perpendicularly to the longitudinal direction of the absorber plate, some of said channels containing a powdered neutron-asborber material composed of grains and which gives off gas and swells upon irradiation, said channels which contain a powdered neutron-absorber material which gives off gas and swells upon irradiation having a circular cross section, each absorber plate including an edge portion which extends in the longitudinal direction of the absorber plate and comprises a gas-tight edge and a longitudinal space located inside said gas-tight edge, said longitudinal space being in open communication with and permitting a gas flow between said channels in the absorber plate, the improvement wherein a separate elongated body of metallic neutron-absorber material is located between the powdered neutron-absorber material and the edge portion in a plurality of said channels which contain powdered neutron-absorber material which gives off gas and swells upon irradiation, each said elongated body having a circular cross section and a predetermined diameter which is less than the diameter of the channel in which it is positioned, thereby providing a predetermined gap between said elongated body and the inner wall of the channel in which it is positioned, said predetermined gap being smaller than the size of at least 50% of the grains of said powdered neutron-absorber material, thus allowing gas to flow therethrough but counteracting simultaneous movement of the grains of said powdered neutron-absorber material therethrough. 2. A control rod according to claim 1, wherein each said separate body of metallic neutron-absorber material has a circular-cylindrical shape. 3. A control rod according to claim 1, wherein each said edge portion comprises an outwardly sealed slot which contains said longitudinal space. 4. A control rod according to claim 3, wherein a longitudinal bar is located in said slot between the longitudinal space and the channels, said longitudinal bar covering a portion of the cross sections of the orifices of all of said channels. 5. A control rod according to claim 4, wherein said bar consists of a metallic neutron-absorber material along at least a part of its length. 6. A control rod according to claim 1, wherein said edge portion of each absorber plate constitutes an edge facing outwardly, as seen from the centre of the control rod. 7. A control rod according to claim 1, wherein said powdered neutron-absorber material consists of boron carbide. 8. A control rod according to claim 1, wherein said metallic neutron-absorber material consists of hafnium.