Abstract: Apparatus for tightening and releasing one of the clamping nuts which act to maintain a pre-established pressure-tight sealing of a flanged reactor pressure vessel, the bolts for which clamping nuts are held and pretensioned in the flange of the vessel. The clamping nut has teeth and is turned on its bolt, via a reduction gearing cooperating with the teeth, by a three phase motor whose rotor has the smallest number of poles for a small moment of inertia and a high speed of rotation. A connecting lead for the motor has a current sensor disposed in one phase thereof, and a triac contactor responsive to an output signal from the current sensor and arranged to open-circuit the connection lead when the sensed current exceeds a predetermined threshold at a limit position of the nut. The apparatus also includes an override means which acts to prevent the motor being automatically switched off in response to start-up current during a preselected period from switch-on of the motor.