Patent Number: 
Section: description

FIG. 1 shows a horizontal section through four fuel assemblies 1 with an associated control rod 2 placed therebetween. The section is laid through the upper part of the control rod 2. Each fuel assembly 1 comprises 10xc3x9710 fuel rods 3. The fuel rods 3 are enclosed in a fuel channel 4 with a square cross section. Each fuel rod 3 comprises a number of fuel pellets of uranium dioxide stacked on top of each other and enclosed in a cladding tube. The spaces between the fuel rods 3 in the fuel assembly 1 is traversed by water which constitutes both coolant and moderator. Between the fuel assemblies 1, gaps 5a and 5b are formed which are traversed by water. The control rod 2 is inserted into the gaps 5a and 5b and has four absorber blades 6, 7, 8, 9 which form an orthogonal cross. The cruciform centre of the control rod is designated 10. FIG. 2a shows a vertical section through a control rod 2 according to the invention. The control rod 2 is substantially made of stainless steel and comprises an absorber part 11, which is supported by a vertical coupling bar 12. The control rod 2 comprises an upper part 15 and a lower part 16. The length of the upper part constitutes at most one-third of the total length of the absorber part 11. Two of the four absorber blades 6 and 8 are visible in FIG. 2a. Each one of the absorber blades 6, 8 is provided with a large number of bored channels 13, 14 extending perpendicularly to the longitudinal direction of the control rod 2, from the outer edge of the absorber blade 6, 8 and towards the cruciform centre 10. Each channel 13, 14 is filled with an absorber material, for example natural or enriched boron carbide and/or metallic hafnium, gadolinium or erbium, and is hermetically sealed outwards by means of, for example, welding. In the following, these channels will be referred to as absorber channels. The absorber channels 13 which are arranged in the upper part 15 of the absorber blades are considerably shorter than the absorber channels 14 which are arranged in the lower part 16 of the absorber blades. The absorber channels 13 are arranged in an outer part, disposed radially outside an inner part, of the upper part 15 of the absorber blade. The absorber channels 13 each have a length which is at least 30% shorter than the length of one of the absorber channels 14 in the lower part 16. The absorber channels 13, 14 in the upper part 15 and in the lower part 16 of the absorber blades, respectively, are preferably circularly cylindrical in shape and all have the same diameters d. To obtain a maximum service life, the diameters d of the absorber channels 13, 14 should be the largest possible taking into consideration the thickness of the material of the absorber blades. The service life of the absorber material depends on the quantity of absorber material per unit of surface, described under the background art above. On the other hand, a halving of the length of the absorber channel with a retained diameter d only has a marginal effect on the service life. In this way, the service life of the control rod is retained in spite of the fact that the percentage of absorber material is reduced in its upper part. That part of the absorber material which is located inside the absorber channels 13, 14 is provided with recesses 17, 18 in the form of through-holes. In the upper part 15 of the control rod, the recesses 17 are considerably wider in the radial direction than the recesses 18 in the lower part 16 of the control rod. All the recesses have the same extent in the axial direction. It is their respective radial extent that varies. The recesses 17 in the upper part 15 are equally large. When the control rod 2 is inserted into the core, the recesses 17, 18 are filled with moderator, whereby the burnupxe2x80x94due to the good moderationxe2x80x94increases further for the fuel pellets which are to be exposed the next time the control rod 2 is to be extracted somewhat. For guiding the control rod 2 in the relatively narrow gaps 5a, 5b between the fuel assemblies 1, it is provided on top with guide pads 19 in the form of bulges extending outwards from the blade. In addition, the control rod 2 is provided with a handle 20 for handling the control rod during installation and replacement. The only thing that distinguishes the control rod 2 in FIG. 2b from that in FIG. 2a is that in the lower part 16, in the vicinity of the coupling bar 12, a recess 21 is arranged which is considerably wider in the radial direction than the other recesses 18 in the lower part 16 and the absorber channels 14 which have been made correspondingly shorter. One advantage of this embodiment is that a certain increased burnup of the fuel rods 3 nearest the cruciform centre 10 of the control rod 2 is achieved as long as the control rod 2 is fully inserted. Another advantage of this embodiment is that the need of reactivity control in the area around the recess 21 decreases since the core normally has a blanket ion, that is, a region with uranium with low enrichment, at the bottom. An additional advantage with this embodiment is that the saved weight of the control rod 2 may be used for placing more absorber material in a part of the control rod 2 where it is better utilized. The latter is of more importance when a heavy absorber material, such as hafnium, is used. FIG. 2c shows an embodiment of the invention where the recesses 17a, 17b, 17c in the upper part 15 of the control rod successively increase in width and the absorber channels 13a, 13b, 13c successively decrease in length the closer to the top of the control rod that they are arranged. The advantage of the absorber content successively decreasing towards the top of the control rod is that this gives a gradual and more ideal increase of the load on the fuel pellets when the control rod is extracted. The disadvantage of this embodiment is that it is more complicated to manufacture in comparison with the embodiments mentioned above. FIG. 2d shows a control rod 2 where each one of the absorber blades 6, 8 is provided with a large number of channels 22, 23, 24 extending in the longitudinal direction of the control rod from the lower part and up towards the upper part of the control rod 2. The cooling channels 22, 23, 24 are filled with an absorber material. A number of channels 22 are arranged in the outer part of the respective absorber blades 6, 8 and extend along the active length of the control rod. One or more channels 23, 24 are arranged nearest the cruciform centre 10 and are shorter than the channels 22 in the outer part of the respective absorber blades. In the upper part 15, recesses 17a, 17b, 17c are arranged which are wider than the recesses 18 which are arranged in the lower part 16 of the control rod. The channels 23, 24 nearest the cruciform centre 10 terminate below the wider recesses 17a, 17b, 17c and possibly above the attachment of the coupling bar 12. The absorber channels 22, 23 and 24 are preferably circularly cylindrical in shape and all have the same diameter d. The control rod 2 according to the invention is suitable to use as a complement to conventional control rods. The control rod according to the invention is especially suitable to use as a controlling rod, for control of reactor power and for burnup compensation (cf. the description under the background art). During a considerable part of the operating cycle, the controlling rods are inserted into the core. To attain the above-mentioned advantages of the invention compared with a conventional control rod, that is, reduced risk of fuel failure on the cladding tube, better utilization of fuel and longer service life of the control rod, it may be sufficient with a small number of control rods according to the invention, for example 4, 8, 12 or 16, depending on the size of the core and the manner of utilizing the control rods. The shutdown margin of a reactor, that is, the inherent negative reactivity of the reactor which causes it to be safely shut down, is deteriorated by control rods which are inserted into the core during operation. Since only a small number of the control rods in the core are of a type according to the invention, the deterioration of the shutdown margin becomes small.