Abstract: In a nuclear reactor including a core, a plurality of control rods, a support supporting the control rods and movable for displacing the control rods in their longitudinal direction between a first end position in which the control rods are fully inserted into the core and a second end position in which the control rods are retracted from the core, and guide elements contacting discrete regions of the outer surface of each control rod at least when the control rods are in the vicinity of the second end position, the control rods being longitudinally movable relative to the guide elements to thereby cause the outer surface of the control rods to experience wear as a result of sliding contact with the guide elements, there is provided a displacement device operatively coupled to the control rods for periodically rotating the control rods in order to change the locations on the outer surfaces of the control rods at which the control rods are contacted by the guide elements.