Abstract: A screwing-unscrewing apparatus is provided, more especially for screwing-unscrewing the stud-bolts securing the lid of the reactor vessel of a nuclear reactor, comprising a frame, at least one traction assembly, at least one screwing-unscrewing assembly and at least one support assembly for a stud-bolt. The piston rod of the traction cylinder and the traction head of the traction assembly are hollow. The screwing-unscrewing assembly comprises a hollow rod which passes through the piston rod and through the traction head, is provided at its lower end with a male drive member and is connected kinematically at its upper end to a rotational drive mechanism, which is situated above said traction cylinder. Said support assembly comprises another rod, which is mounted for rotation, while being prevented from axial movement, inside said hollow rod and which is threaded at its lower end, and another rotational drive mechanism which is connected kinematically to the upper end of said other rod.

Abstract: A traction block is provided comprising an elongate frame with U shaped cross section having a flat web and two spaced apart lateral flanges perpendicular to the web, two clamping blocks of elongate shape which are mounted for movement between the lateral flanges of the frame so that a relative longitudinal movement between each movable clamping block and the adjacent flange of the frame causes a transverse movement of the respective movable clamping block, and a flat cover plate parallel to the web of the frame and detachably fixed thereto, said lateral flanges of the frame being provided with tenons which project from their end faces facing towards the cover plate and which are tightly fitted in corresponding cavities in the cover plate.