Flexible control rod for a nuclear reactor
TECHNICAL FIELD 
     The present invention relates to a flexible control rod for a nuclear reactor which is built up of a number of absorber wings that are supported by a control rod shaft. 
     In certain types of nuclear reactors in which the power is controlled by means of control rods inserted into the reactor core, it is of great importance to make the control rod gaps between the fuel assemblies in the core as narrow as possible. The width of these gaps is determined by the thickness of the absorber wings and by the fact that there must be a certain play between the absorber wings and the limiting surfaces of the gap. This play is necessary to prevent the control rods from becoming stuck in the gap, e.g., as a result of deformation of the control rods and/or the fuel channels, or for some other reason. If the control rod is very stiff, it may be necessary to dimension the gaps so that their width becomes considerably greater than the thickness of the absorber wings, which in turn results in a deterioration of the fuel economy of the reactor. 
     DISCUSSION OF PRIOR ART 
     It is thus important to reduce the noted play to a minimum. One known way of achieving this is to make the control rod more flexible, permitting it to pass through a narrower gap without getting stuck, even in the face of deformations of the gap which will arise in use of the reactor. However, the prior art embodiments of flexible control rods have normally been of a construction which has made them too weak, which is clearly not desirable. 
     BRIEF STATEMENT OF INVENTION 
     The control rod according to the present invention is composed in a known manner of oblong first and second wing portions of absorber material, which wings are arranged in a cruciform configuration. By means of slots extending substantially perpendicularly to the imaginary crossing line of the wings, each wing portion is divided into a plurality of wing elements. Characteristic of the invention is that the first and second wing portions of one wing are arranged in a first plane with the slots in these wing portions located pairwise in line with each other and the first and second wing portions of the other wing are arranged in a second plane and also with the respective slots therein aligned pairwise, and that the slots in the first plane are parallel-displaced in the elongate direction of the control rod relative to the slots in the second plane, and that portions of the wing elements adjoining the crossing line of the wings are provided with recesses. 
     In the known embodiment, transversely positioned slots in one coplanar pair of absorber wing portions are disposed at the same axial level as the corresponding slots in the other coplanar pair of wing portions, and the control rod at these levels is more or less severed and therefore becomes too flexible. In a control rod according to the present invention, two slots at most will be located at any given axial level, thereby obtaining a suitable balance between flexibility and stiffness in the control rod. Control of the flexibility of the control rod is achieved by providing the wing elements--in the portions adjoining the crossing line of the wings--with recesses. By making the recesses longer or shorter in a direction along the crossing line, the flexibility in the control rod can be increased or decreased. 
     To obtain an evenly distributed flexibility along the control rod, the slots are suitably displaced by approximately half the length of a wing element, whereby the distance between levels at which two adjacent slots, lying in different planes, traverse the crossing line becomes substantially constant along the entire length of the control rod. 
     Stiffening of the control rod can be achieved by reducing the width of the slots by means of projections arranged in the slots. If the stiffening becomes excessive, it can easily be reduced by reducing the height of such slot-located projections. 
    
    
     DESCRIPTION OF DRAWINGS 
     Two embodiments of control rods in accordance with the invention will now be described, by way of example, with reference to the accompanying schematic drawings, in which: 
     FIG. 1 shows part of a first embodiment of control rod according to the invention, partially sectioned and viewed from the front, 
     FIG. 2 shows the control rod of FIG. 1 viewed from the side, sectioned on the line II--II, 
     FIG. 3 shows part of a second embodiment of control rod provided with projections in the slots, and 
     FIG. 4 shows a perspective view of the part of the first embodiment of control rod shown in FIGS. 1 and 2. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     In FIGS. 1 and 2, the numeral 1 designates part of a flexible control rod, the lower portion of which is provided with a control rod shaft (not shown) for connection to a drive device (also not shown). The illustrated part of the control rod consists of two wings A and B of absorber material arranged in a cruciform configuration and joined together along an imaginary crossing line (shown as the chain line 6). Each wing is composed of two wing portions 2,4 and 3,5, respectively. By means of slots 7, which extend substantially perpendicularly to the crossing line 6, each wing portion is divided into wing elements 8a, 8b,  8c and 8d. 
     The wing elements 8a and 8b are arranged in the same plane and at the same axial level in the control rod, so that the slots 7 between the elements will be in alignment. Similarly, the wing elements 8c and 8d are positioned in the same plane and at the same axial level but are parallel-displaced in the axial direction relative to the elements 8a and 8b. The control rod is given an increased flexibility by providing the elements 8a to 8d, in their portions adjoining the crossing line 6 of the wings, with recesses 9. Further, these recesses 9 are each disposed so as to bridge the slots 7 in the wing plane perpendicular thereto. Between the recesses 9, the wing elements 8 are fixed to each other in any convenient way adjacent to the imaginary crossing line 6. With the construction described, deformation zones are provided at spaced-apart intervals along the control rod, these zones being centered on each aligned pair of slots 7, each zone being of a length approximately corresponding to the length of the bridging recess 9. 
     To obtain an evenly distributed flexibility along the length of the control rod, the slots 7 in the wing portions 2 and 4 are displaced by approximately one half of the length of an element in relation to the slots 7 in the wing portions 3 and 5. 
     Finally, FIG. 3 shows how the flexibility of a control rod in accordance with the invention can be reduced by providing the wing elements with projections 10 locally arranged in the slots 7 so as to provide the slots with two portions 7a and 7b, portion 7a being in communication with a recess 9 and having a thinner width in the elongated direction of the control rod as compared to the second portion 7b. Upon sufficient bending of the control rod, contact between adjacent projections 10 occurs, resulting in a tension load in the appropriate bending wing element(s) 8. These projections 10 thus increase the flexural resistance of the control rod within a wide range.