Patent Number: 047524405
Section: summary

BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a control rod for nuclear reactors which comprises a number of elongated absorber plates which are each provided with a plurality of channels, these channels extending substantially perpendicularly to the longitudinal direction of the absorber plate, and containing powdered boron carbide or other powdered absorber material which gives off gas and swells upon irradiation, these channels being hermetically separated from the surroundings of the control rod by an edge portion which is arranged at an edge running in the longitudinal direction of the absorber plate and which comprises a gas-tight edge, a longitudinally-extending space being provided inside the edge and in open connection with and permitting a gas flow between the different channels in the absorber plate. The Prior Art A control rod of this kind is known from U.S. Pat. No. 3,448,008. According to this U.S. patent, the edge portion comprises an outwardly sealed, longitudinally extending slot in the absorber plate in which a longitudinal bar is arranged to cover in part the orifices of the channels at the bottom of the slot. When boron carbide is subjected to irradiation, helium gas is formed. Since absorber plates in a control rod are not subjected to uniform irradiation, the amount of gas developed will be different in different channels with absorber material. In the known control rod described above, an equalization of the gas pressure arising is achieved in the different channels by the fact that, as mentioned above, the channels are sealed with an edge portion which permits a gas flow between the different channels. Another property of boron carbide is that it swells upon irradiation. The present invention is based on the realization that the life of a control rod can be considerably extended if measures are taken to counteract the consequences of the swelling. An extension of the life is of the utmost importance. It involves not only reduced costs of new control rods but also reduced costs for taking care of and disposal of spent control rods. SUMMARY OF THE INVENTION The swelling of the abosrber material in a channel may cause stress corrosion in the construction material in the channel surrounding the absorber material, that is, in the material of which the control rod is manufactured. The risk of cracks in the construction material caused by stress corrosion increases with increasing irradiation and swelling of the absorber material. If a crack occurs at a channel, a transportation of boron carbide from the channel takes place so that the channel is depleted of boron carbide. In addition, a transportation of absorber material from other nearby, undamaged channels occurs, especially in the region located nearest the edge portion, because the edge portion does not efficiently prevent the transportation of liquid from the defective channel to an undamaged channel and the transportation of liquid together with absorber material from an undamaged channel to the defective channel and from there to the surroundings. According to the present invention, the above-mentioned transportation of powdered absorber material is prevented or radically counteracted by arranging, between the powdered absorber material and the edge portion in a plurality of channels, a body consisting of hafnium or other metallic absorber material, this body forming between it and the inner wall of the channel a gap which permits a gas flow but which is smaller than at least the main part of the grains in the powdered absorber material. The channels and this body preferably have a circular cross-section and the body preferably a circular-cylindrical shape. A transportation of absorber material can be very efficiently counteracted according to the present invention by the fact that a gap with an accurately predetermined width can be achieved without difficulty between the channel and the body of metallic absorber material. This result can be achieved without the outer dimensions of the control rod having to be changed or its neutron-absorbing capacity having to be significantly reduced. Preventing such transportation only by the use of the edge portion would require additional, very time-consuming operations in connection with the pressing and welding operations which are part of its manufacture, and would furthermore yield an unreliable result owing to the nature of these work operations. In the powdered absorber material, preferably at least 50% of the grains have a size exceeding 0.13 mm. The gap between the inner wall of the channel and the body of metallic absorber material (the distance between the inner wall of the channel and the surface of the body in a direction perpendicularly to the longitudinal direction of the channel) preferably amounts to 0.01-0.13 mm. If the channel and the body have circular cross-section, the body has a diameter which is preferably 0.01-0.13 mm smaller than the diameter of the channel. The length of the body preferably amounts to 5-25 mm.