Abstract: A pressurized water nuclear reactor having a safety vessel containing a steam generator from which a live steam line extends, a shut-off and safety device includes valve means having a displaceable valve member connected in the live steam line within the safety vessel, and control piston means connected to the valve member for controlling opening and closing of the valve means, the control piston means comprising a control cylinder and a control member displaceable in the control cylinder and having a pair of opposite sides having working surfaces respectively subjectible to differences of pressure applied thereto in respective portions of the control cylinder located adjacent thereto for displacing the control member in respective directions so as to displace the valve member in corresponding directions to open and close the valve means, the portions of the control cylinder located adjacent the sides of the control member having means for effecting a pressure difference between the respective cylinder portio