Patent Number: 048881503
Section: summary

The present invention relates to a control rod for nuclear reactors, which comprises a number of absorber plates which are connected to each other along a centre line of the rod and which are each provided with a plurality of bored channels, which extend at least substantially perpendicularly to the centre line of the rod, contain boron carbide or other absorber material which swells upon irradiation and are sealed off from communication with the environment of the control rod. A control rod of the above-mentioned kind is known from U.S. Pat. No. 3,448,008 (Hellman). Each absorber blade in this control rod is provided with an edge portion, parallel to the centre line of the rod, in which is arranged an elongated space which is in open communication with and allows a gas flow between the different channels. At the edge facing away from the centre line of the control rod, the edge portion comprises an outwardly closed slot in the absorber plate, in which a longitudinal bar is arranged to make contact with the orifices of the channels at the bottom of the slot. When boron carbide is subjected to neutron irradiation, helium gas is formed. Since absorber plates in a control rod are not subjected to uniform irradiation, the developed amount of gas is different in different channels with absorber material. In the above-described known control rod, an equalization of the gas pressure arising in the different channels is achieved since--as mentioned--the channels are sealed by means of an edge portion which allows a gas flow between the different channels. The neutron irradiation of the boron carbide also results in the boron carbide swelling. This swelling of the absorber material in a channel may cause stress corrosion in the construction material around the channel which surrounds the absorber material, i.e. in the material from which the control rod is manufactured. The risk of cracks in the construction material caused by stress corrosion increases with increased irradiation and with increased swelling of the absorber material. If a crack arises in a channel, a transport of boron carbide, which is in powdered form in the known case, takes place from the space of the channel, so that the channel is depleted of boron carbide. In addition, a transport of absorber material occurs in the described control rod from other adjacently located undamaged channels, especially in the region positioned nearest to the edge portion, because the edge portion does not effectively prevent the transport of liquid from the defective channel to an undamaged channel and the transport of liquid together with absorber material from an undamaged channel to the defective one and from there to the environment. These processes, caused by the swelling of the absorber material, are described in U.S. patent apliction Ser. No. 071,590 (Ahlinder et al). To counteract the described processes, a body of hafnium or other metallic absorber material is arranged outside the boron carbide or other powdered absorber material in the outwardly-facing part of a channel. This body forms between it and the inner wall of the channel a gap which allows the passage of gas but prevents the passage of powdered absorber material from the channel. The present invention also relates to measures for counteracting harmful processes, which are associated with the swelling of an absorber material in a bored channel and with stress corrosion, arising in connection therewith, in the construction material around the channel. The invention is based on the realization that the harmful processes mentioned above can be counteracted by utilizing the property of the absorber plate that its inner parts are not subjected to the influence of a corrosive environment in comparison with what is the case with its superficially located parts. The measures taken according to the invention comprise arranging each channel, at least within that region of an absorber plate where the described harmful processes occur, with absorber material at a smaller distance to an adjacent channel than to the surface of the absorber plate. In this way, the swelling of the boron carbide is taken up by construction material which is not in contact with the corrosive medium, which minimizes the risk of the formation of cracks in the construction material around the channel. It is also possible to arrange a channel at such a small distance to an adjacent channel, i.e. make the partition between them so thin, that it is thereby ensured that the swelling of the absorber material gives rise to a strain and accomodation of the swelling of the boron carbide in this partition before any significant strain arises in other parts of the construction material around the channel. The distance between the channel and an adjacent channel should then be smaller than half of the distance between the channel and the surface of the absorber plate. More particularly, when characterizes the control rod according to the present invention is that, within at least one region of an absorber plate with channels, each channel is arranged at a smalller distance to an adjacent channel than to the surface of the absorber plate. According to an advantageous embodiment of the invention, each channel, within the stated region of the absorber plate, is arranged at a smaller distance to the adjacent channel on one of its sides than to the adjacent channel on its other side, i.e. the channels are arranged side-by-side in pairs. With this embodiment the following advantages are obtained: the swelling of the absorber material is taken up in the favourable manner described above, the quantity of absorber material can be adapted to an optimum value, and the absorber blades can be given the necessary mechanical stiffness. However, it is possible, per se, to arranged more than two of the channels side-by-side, for example three and three or four and four. The construction material in the absorber plates preferably consists of stainless steel, for example SIS 2352 (corresponding to Werkstoffnummer 1.4306 and DIN 17440) or SIS 2353 (corresponding to Werkstoffnummer 1.4435 and DIN 17443), but may also consist of other materials, such as INCONEL 600 or any other nickel-based alloy.