Patent Number: 047117567
Section: claims

1. In a nuclear reactor including a core, a plurality of cylindrical control rods, a single support means supporting said plurality of control rods from their upper ends in spaced apart positions and movable for displacing said control rods in their longitudinal direction between a first end position in which said control rods are fully inserted into said core and a second end position in which said control rods are retracted from said core, and guide means contacting discrete regions of the outer surface of each said control rod at least when said control rods are in the vicinity of the second end position, said control rods being supported by said support means for longitudinal movement without rotation into and out of said core relative to the guide means to thereby cause the outer surface of said control rods to experience wear as a result of sliding contact with the guide means, said support means being so arranged with respect to said core and said guide means that it is incapable of rotation relative to said guide means, the improvement comprising a plurality of displacement means each of said plurality of displacement means being operatively coupled to a respective one of said control rods for periodically rotating said control rod in a single angular direction through an angle selected to change the locations on the outer surfaces of said control rods at which said control rods are contacted by said guide means during subsequent longitudinal movement of said control rods. 2. A reactor as defined in claim 1 wherein said displacement means are operative for effecting such periodic rotation of said control rods each time said control rods are brought to the second end position. 3. A reactor as defined in claim 2 wherein said control rods are associated with said support means to enable said control rods to undergo a limited movement relative to said support means, in the longitudinal direction of said control rods, when said control rods are brought to the second end position, and said displacement means effect rotation of said control rods in response to such limited movement. 4. A reactor as defined in claim 3 wherein said displacement means comprise a camming mechanism coupling each said control rod to said support means for effecting the periodic rotation of each said control rod, whereby the periodic rotation is relative to said support means. 5. A reactor as defined in claim 4 wherein said displacement means periodically rotate each said control rod through an angle of the order of 45.degree.. 6. A reactor as defined in claim 3 wherein, for each said control rod, said displacement means comprise a first member structurally separate from said control rod and mounted in said support means to undergo rotation and vertical movement relative to said support means when said control rod is brought to the second end position, and a drive member carried by said control rod and arranged to undergo rotation, in response to rotation of said first member, in a first direction during movement of the control rod to the second end position. 7. A reactor as defined in claim 6 wherein, for each said control rod, said displacement means further comprise a compression spring interposed between said control rod and said support means for urging said control rod upwardly relative to said support means while permitting movement of said rod relative to said support means in the longitudinal direction of said rod when said rod is in the vicinity of said second end position. 8. A reactor as defined in claim 7 wherein said spring is operatively associated with said drive member for blocking rotation of said drive member in the direction opposite to said first direction. 9. A reactor as defined in claim 1 wherein said displacement means are operative for rotating said control rods while said reactor remains in operation.