Patent Number: 048881503
Section: description

The control rod shown in FIGS. 1 and 2 is mainly made of stainless steel and consists of an absorber part 12 supported by a vertically arranged coupling bar 11. The absorber part 12 consists of four absorber plates 13-16, which form a rectilinear cross, the centre line of which coincides with that of the coupling bar 11. The absorber plates are provided at the centre with recesses 17 and projections 27 and are welded to each other at the projections 27 and at a supporting block 28 with square cross-section arranged adjacent the projections 27. The plates, which have a thickness of 8 mm, are provided with a large number of bored channels 18 (18a-c) with a diameter of 6 mm and a depth of 100 mm. In order to make FIG. 1 more clear, the channels therein are drawn without the distances which in reality exist between them, as will also be clear from the following. The uppermost channels 18a, which are most subjected to radiation, are filled with rods of hafnium metal of the same length as the channels. Hafnium does not swell upon irradiation. The channels 18b arranged below channels 18a are filled with powdered boron carbide 20 with the exception of the outermost part which is filled with a circular-cylindrical body 21 of hafnium metal. The body 21 has a diameter which is somewhat smaller than the diameter of the channel. The channels 18c arranged at the bottom, where the irradiation is lowest, are completely filled with powdered boron carbide 20 of the kind mentioned above. The distance between the envelope surfaces on two adjacent channels of types 18a and 18c is at all places 2 mm. For each channel 18b, the distance to the adjacent channel on one of its sides is 0.4 mm (the distance designated a in FIG. 3), and the distance to the adjacent channel on the other side is 3.6 mm (the distance designated b in FIG. 3). The edge of each absorber plate facing away from the centre line of the rod is provided with a slot 22, in which a bar 23 is arranged. At least in its upper part the bar 23 is suitably of hafnium metal. It may also in its entirety, and at least in the lower part, be of, for example, stainless steel. The application of the bar 23 in the slot 22 and the sealing of the slot 22 are described in greater detail below with reference to an explanation of FIGS. 3-5. For guiding of the control rod in the relatively narrow gaps between the fuel boxes of the reactor, the upper part of the control rod is provided with guide pads 24 of Inconel. In addition, it is provided with a lifting handle 25 for handling the rod during insertion and replacement. At its lower part, the rod is provided with a coupling head 26 over which the rod can be connected to a drive mechanism (control device). FIG. 3 shows a number of channels 18b in the absorber plate 15 as well as the slot 22, which in the exemplified case has a width of 5.5 mm in its entire length. From the information given above concerning the thickness of the absorber plate 15, the diameter of the channels 18, and the distances a and b between different channels of type 18b, it is clear that for each channel 18b the distance (a) from its envelope surface to the envelope of an adjacent channel is smaller than the distance (c) from its envelope surface to the surface of the absorber blade. As will be more clear from FIGS. 4 and 5, the absorber plate 15 has an edge portion 30, which comprises a gas-tight edge 31, a longitudinal space 32 located inside that edge, and a longitudinal bar 23 located in the slot 22 and having the same width as the slot 22 and being provided with a longitudinal indentation 33 which constitutes a limiting wall for the space 32. The bar 23 does not completely cover the orifices 34 of the channels in the bottom of the slot 22 as its width is smaller than the diameter of the channels, which, as mentioned above, amounts to 6 mm in the exemplified case. This leads to the creation of a gap 35 between the bar 23 and the side walls of the slot 22, where the channels are located, by the side walls of the slot 22 making contact with the bar 23 in the portions 38 (FIG. 3) where no channels are bored and, consequently, the side walls of the slot 22 are thicker. The channels 18b, as well as the channels 18a and 18c, are in open communication with the space 32 via the gaps 35, so that gas formed upon irradiation of the powdered absorber material may flow between the different channels in the plate 15 and pressure equilization be achieved. The gap 36 between the body 21 of hafnium metal and the inner wall of the channel 18b allows gas flow but prevents or radically counteracts a simultaneous transport of absorber material from the channel. In this way, a transport of absorber material from an undamaged channel to a channel exhibiting a crack, and through there to the surroundings of the control rod, is prevented or counteracted. Thus, the gap 36 (the distance between the inner wall of the channel 18b and the surface of body 21) is thinner than the gap 35 (the distance between the inner wall of channel 31 and the surface of bar 23 at the side of space 32) and, in addition, longer. The gap 36 is also thinner than the cross-section of the space 32, i.e. thinner than transport paths for the gas which are located in the edge portion. In the manufacture of an absorber plate, the channels 18 are bored from an edge on a plane-parallel plate. Thereafter, the slot 22 is milled out. After filling the channels with absorber material, the bar 23 is applied in the slot 22 and the side walls of the slot 22, which are straight from the beginning, are pressed against the bar 23 and welded together at the end surfaces while forming a gas-tight edge 31. The welded joint is designated 37. Also at its ends at the top and bottom of the absorber plate, the slot 22 is sealed by welded joints. The invention has been described in detail in connection with the use of boron carbide as swelling absorber material. The invention is also applicable to the use of other known absorber materials such as europium in the form of oxide or another compound. It is, of course, also applicable to control rods where no bodies 21 of hafnium are arranged in the outermost portions of certain absorber plates.