Patent Number: 
Section: description

FIGS. 1 and 2 show a preferred embodiment of a control rod according to the invention. The control rod comprises a top piece 1 and a bottom piece 2 for fixing an absorber part 11 extending therebetween. The top piece comprises a handle 1a and distance buttons 1b, which ensure that the control rod, when being inserted into the reactor core, is kept at a definite distance from the fuel. The absorber part comprises a central part 4 and four absorber blades 3 extending from the central part. The central part and the absorber blades together form a cruciform channel 10. The absorber part comprises four plates 5 of thin hafnium sheet with a substantially L-shaped cross section in the longitudinal direction of the control rod, that is, a plate which is bent through 90xc2x0 such that each side of the bending line is equally long. The plates are arranged facing each other so as to form a right-angled cross with four absorber blades 3. For joining the walls of the channel together and keeping them at a distance from each other in the absorber blades, each one of the opposite walls is provided with indentations 7 intermittently arranged in the longitudinal direction. Each indentation in one wall is directed against and welded to a corresponding indentation belonging to the other wall. In this way, the walls of the channel are joined together intermittently along the length of the whole channel. FIG. 3 shows in detail the appearance of the joined indentations in a section Bxe2x80x94B in FIG. 2. The control rod is provided with two rows of indentations in each absorber blade. The plates are not joined together along the outer edge of the absorber blades. An opening 6 has been left between the plates, which gives a good exchange of water between the inside and outside of the control rod. The opening 6 extends along the length of the whole absorber part. The water in the control rod causes a slowing down of the fast neutrons such that they can be more easily absorbed by the hafnium sheet. During the slowing down, heat is released. Neutron absorption in the hafnium sheets and the radiation absorption also contribute to a significant heat generation in the control rod. For the control rod to function satisfactorily, it must have thorough cooling. The water used as moderator also serves as coolant in the control rod. The openings 6 in the outer ends of the absorber blades provide a flow of cooling water and, in addition, eliminate the risk of pressure increase in the control rod. Steam formation in the control rod would also be a safety hazard as it would reduce the effectiveness of the control rod. The cruciform channel formed from the joined L-shaped plates is open at both top and bottom such that the cooling water which flows upwards through the core can pass through the control rod. The top piece and the bottom piece are fixed to the absorber part by rivets. Openings 11a are arranged between the absorber part and the top piece. Between the absorber part and the bottom part, openings 11b are arranged. The task of these openings is to improve the flow of water through the absorber part. The openings shall be of such a size to create a thorough circulation of the cooling water. In an embodiment of a control rod according to the invention shown in FIG. 4, the control rod only has one row of indentations in each absorber blade, and instead the outer ends of the absorber blades are joined together. The joining of the plates in the outer ends of the absorber blades can be made by means of, for example, spot welding or rivets. An all-welded edge is unsuitable since stresses are thus built into the control rod. In further embodiments the outer end of the absorber blade may be partially open and partially joined together. Instead of using indentations as spacers, an inwardly facing fold may be arranged on each wall in the longitudinal direction thereof. Two opposite side walls are joined by means of, for example, spot welding along the folds. The folds divide the large cruciform channel into five smaller channels, which consist of a central cruciform channel and four channels located in the absorber blades. FIGS. 5a-5f show different possible attachments between two hafnium sheets. FIG. 5a shows a spot weld and FIG. 5b shows a fusion weld, for example a TIG weld. Welding has the advantage that it does not add any small parts to the construction which may subsequently loosen and cause problems. A disadvantage with welding is that it requires expensive and complicated welding equipment. FIG. 5c shows a conventional rivet joint, and FIG. 5d shows a rivet joint with a safety device which prevents the whole rivet from loosening if the rivet breaks. FIG. 5e shows a two-part rivet joint which is welded or threaded and weld-locked. FIG. 5f shows an upset rivet joint where the plates are pressed together in such a way as to lock against each other. Rivets may be of a material other than hafnium, for example titanium, zirconium, stainless steel or Inconel (nickel-base alloy). The advantage of a rivet joint is that it is simple and inexpensive to manufacture. The disadvantage is that it comprises a large number of small rivets which may possibly loosen after some time in operation. FIG. 6 shows a further embodiment of the invention, which comprises a tube of hafnium on which are arranged four absorber blades 3 of hafnium sheet. The hafnium tube constitutes a centrally located channel 10. The channel has a substantially circular cross section and is at least partially filled with a moderator. Each absorber blade constitutes a channel which is at least partially filled with a moderator. The neutron irradiation of the control rod does not take place uniformly across the whole surface; for example, an outer portion of the absorber blades is exposed to a higher irradiation than other portions. The upper part of the control rod, that is, that part which is nearest the top piece, is also exposed to a higher irradiation than the lower part, that is, that part which is nearest the bottom piece. Those parts of the neutron absorber which are exposed to a higher irradiation are burnt out faster than the remainder of the control rod. To obtain an optimum hafnium use, the thickness of the sheet should vary, that is, thicker sheets in those parts which are burnt out fastest. However, the strength of the control rod must also be taken into consideration. In an alternative embodiment of the invention, a bar of neutron-absorbing material, for example hafnium, is arranged at the extreme end of each absorber blade in order to increase the amount of neutron-absorbing material at particularly exposed points. This bar may also serve as a base for attachment of the bottom piece and the top piece. FIG. 7 shows a section through a control rod according to the invention with a bar 8 inserted between two adjacent plates 5 which together, with the bar, form a control rod blade. It is desirable to be able to vary the absorption capacity, that is, the amount of hafnium, both axially and radially in the absorber part. FIG. 8 shows a control rod according to the invention where the amount of hafnium varies both axially and radially in the absorber part. In the regions marked 20, the hafnium thickness is greater than in the other parts of the absorber part. One way of achieving the desired variation in the absorption capacity is to vary the sheet thickness in the absorber part. FIG. 9a shows in a section Cxe2x80x94C in FIG. 8 how the sheet thickness varies radially in an absorber blade. The disadvantages of this method are the higher manufacturing costs. FIG. 9b shows in a section Cxe2x80x94C in FIG. 8 another method of varying the absorption capacity, which means that a hafnium sheet 21 is suspended loosely in the control rod blade. The hafnium sheet may be varied in length, width and thickness and may be provided with different holes and recesses. The attachment of the hafnium sheet may be made in connection with the attachment of the absorber blade to the top piece. To prevent the hafnium sheet from possibly becoming detached from the top piece, it may be secured with the aid of the intermittently arranged indentations which keep the control rod blade together. These indentations are not shown in FIGS. 9a and 9b.  One further way of varying the absorption capacity axially and radially in the control rod, which is shown in FIGS. 10a-10d, is to bend back one or both of the edges of the hafnium plates inwards toward the central part of the absorber part. Since the absorber part is of hafnium and the top piece and the bottom piece are of stainless steel, they cannot be welded to each other, which would be the case if they were of the same material. In the following, a few proposals for possible attachments will be given. In the event of reactor scram (fast insertion of the control rods), the stress is great on the attachment between the absorber part and the bottom piece. The attachment between the absorber part and the top piece, on the other hand, is exposed to lower stress and can therefore be made in combination with the distance buttons 1b (see FIG. 11a). This gives the distance buttons a twofold function, namely, as spacers between the control rod and the fuel, and as a large rivet for attachment between the absorber part and the top piece. As an alternative to using the distance button as attachment between the absorber part and the top piece, a rivet joint can be used (see FIG. 11b). The attachment with rivets 22 can be made in a plurality of different ways. FIGS. 12a-12c show different alternatives. FIG. 12a shows how the stainless part 23 surrounds one end of a tray 24 of hafnium and the other end of the tray is inserted between the hafnium sheets 25 in the absorber blades. The hafnium tray is welded to the hafnium sheets in the absorber blades. The stainless part and the hafnium tray are welded together by means of one or more rivets 22. In FIG. 12b, the lower parts of the hafnium sheets 25 in the absorber blades are compressed to make contact with each other and surrounded by the stainless part 26. The stainless part 26 and the hafnium sheets 25 are joined together by means of one or more rivets 22. In FIG. 12c, one end of the stainless part is formed as a lug 27. The lug is inserted between the hafnium sheets 25 in the absorber blades and is fixed by one or more rivets 22. FIG. 13 shows a rivet joint between the absorber part and the bottom piece. The attachment can be made using any of the alternatives shown in FIGS. 12a-12c. The advantage of a rivet joint is that it has a low weight and is simple to mount. A disadvantage of a rivet joint is that it contains many small parts which may possibly loosen during operation. An alternative to rivet joints is a locking-pin joint, which is shown in FIGS. 14a and 14b. The absorber part terminates in a lug 28 of hafnium which is locked in the bottom part by means of a pin 29. FIG. 14b shows in a section Exe2x80x94E how the hafnium lug is inserted into and welded between the hafnium sheets in the absorber part. A disadvantage of a locking-pin joint is, among other things, that the control rod becomes heavy because of the considerable weight of the hafnium lug. A large quantity of hafnium in the lower part of the control rod results in an inefficient use of hafnium, since the major burnup takes place in the upper part of the control rod. The absorber material in a control rod according to the invention may, for example, consist of pure hafnium or of some hafnium alloy, for example a hafnium-zirconium alloy. Another common absorber alloy is an alloy of cadmium, indium and silver, usually with a composition such that the nuclear properties of hafnium are imitated. The moderator consists of water or a solid moderator, for example zirconium hydride. During the manufacture of a hafnium sheet, it is given a directed texture. Upon neutron irradiation of the sheet, an irradiation growth takes place, substantially in the direction of the texture. A control rod made of hafnium sheet, as the one described above, thus runs the risk of being bent when subjected to neutron irradiation. One way of preventing this bending is to xcex2-quench the hafnium sheet in a manner analogous to that of a zirconium alloy. Such a method is described, for example, in Swedish patent specification 7502865-4. The method comprises heating the hafnium sheet up to such a high temperature that its texture is transformed from xcex1-phase to xcex2-phase, which has a random texture. To maintain the crystal structure in the hafnium sheet, it must be rapidly cooled again. In a random texture, the growth upon irradiation takes place equally in all directions and hence bending of the control rod is prevented. For hafnium the transformation temperature to xcex2-phase lies at about 1800xc2x0 C. The transformation temperature for a zirconium alloy lies at about 900xc2x0 C. By alloying hafnium with some suitable metal, for example Fe, Ni, Cr, Nb, the transformation temperature may be reduced to a transformation temperature which lies near the transformation temperature of the zirconium alloy. When choosing a suitable alloy, it is important to ensure that the corrosion properties and hydrogen pick-up properties of hafnium are retained and preferably improved. After the xcex2-quenching it is suitable to heat-treat the hafnium sheet to relieve stresses in the sheet caused by the xcex2-quenching, so-called stress-relieve annealing.