Patent Number: 047524405
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT The control rod shown in FIGS. 1 and 2 is substantially manufactured from stainless steel and consists of an absorber section 12 supported by a vertically arranged coupling bar 11. Ths absorber section consists of four elongated absorber plates 13-16 arranged in cruciform configration, the centre line of the cross coinciding with that of the coupling bar. The absorber plates are provided at the centre of the control rod with cutouts 17 and connection pieces 27 and are welded to each other at the connection pieces and at a supporting filling piece having square cross-section which is arranged adjacent to the connection pieces. The plates which have a thickness of 8 mm are provided with a large number of bored channels 18 (18a-c) having a diameter of 6 mm and a depth of 100 mm. The distance between the envelope surfaces of two adjacent holes is 2 mm. The uppermost channels 18a, which are most exposed to radiation, are filled with rods of hafnium metal with the same length as the channels. Hafnium does not swell upon irradiation. The channels 18b arranged therebelow are filled with powdered boron carbide 20 except for the part positioned furthest from the centre which is filled with a circular-cylindrical body 21 of hafnium metal. At least 50% of the boron carbide consists of grains having a grain size exceeding 0.13 mm. The body 21 has a diameter which is 0.06 mm smaller than the diameter of the channel and a length of 15 mm. The channels 18c arranged at the bottom, where the least irradiation occurs, are filled in their entirety with powdered boron carbide 20 of the above-mentioned kind. That edge of each absorber plate which faces away from the centre line of the rod is provided with a slot 22, in which here is arranged a bar 23. At least in the upper part the bar is suitably made of a metallic neutron-absorber material, e.g., hafnium metal. It may also in its entirely, and at least in the lower part, be of, for example, stainless steel. The application of the bar in the slot and the sealing of the slot are described in greater detail in connection with the explanation of FIGS. 3-5. For guiding the control rod in the relatively narrow gaps between the fuel boxes of the reactor, it is provided at the top with guide pads 24 of Inconel. In addition, it is provided with a lifting handle 25 for handling the rod during installation and replacement. At the lower part the rod is arranged with a coupling head 26 by means of which the rod can be connected to a rod drive mechanism of the reactor. FIG. 3 shows a number of the 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. As is illustrated more clearly in FIGS. 4 and 5, the absorber plate has an edge portion 30 comprising a gas-tight edge 31, a longitudinal space 32 arranged inside the gas-tight edge, and a longitudinal bar 23 arranged in the slot 22 and having the same width as the slot and being provided with a longitudinal indentation 33 which serves as a limiting wall for the space 32. The bar does not entirely cover the orifices 34 of the channels which communicate with the slot as its width is smaller than the diameter of the channels, which diameter, as previously mentioned, amounts to 6 mm in the exemplified case. In this way a gap 35 is formed between the bar and the side walls of the slot, where the channels are located, but not where the side walls of the slot make contact with the bar in the portions 38 (FIG. 3) where no channels are bored and the side walls of the slot are therefore thicker. The channels 18b, as well as the channels 18a and 18c, are in open communication via the gaps 35 with the space 32, so that gas formed upon irradiation of the powdered absorber material is able to flow between the different channels in the plate and a pressure equalization be achieved. The gap 36 between the body 21 of hafnium metal and the inner wall of the channel 18b allows a gas flow but prevents or counteracts radically a simultaneous transportation of absorber material from the channel. In this way, the transportation of absorber material from an undamaged channel to a channel exhibiting a crack, and from there to the surroundings of the control rod, is prevented or counteracted. The gap 36 (the distance between the inner wall of the channel 18b and the surface of the body 21) is thus narrower than the gap 35 (the distance between the inner wall of the edge 31 and the surface of the bar 23 at the side of the space 32) and furthermore longer. The gap 36 is also narrower than the cross-section of the space 32, that is, narrower 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 a longitudinally extending edge on a plane-parallel plate. Thereafter, the slot 22 is cut out. After filling the channels with absorber material, the bar 23 is applied in the slot and the side walls of the slot, which are straight at the beginning, are pressed against the bar and welded together at the end surfaces while forming a gas-tight edge 31. The weld joint is designated 37. The slot is also sealed with weld joints at its ends at the top and bottom of the absorber plate. The invention has been described in greater detail in connection with the use of boron carbide as powdered absorber material and hafnium as metallic absorber material. The invention is also applicable to the use of other known powdered absorber materials such as europium in the form of oxide or another compound and other known metallic absorber materials such as a silver-indium-cadmium alloy (80% Ag, 15% In, 5% Cd) and stainless boron steel.