Patent Number: 052079759
Section: claims

1. A hydraulic control rod driving system provided with a cylinder installed on a reactor vessel cover and a driving shaft which is coupled with a control rod cluster, extends into said cylinder while penetrating through said reactor vessel cover and has a piston portion at an upper end thereof, in which said driving shaft is moved vertically by controlling fluid pressure above and under said piston portion, characterized in that: radial clearance is formed between said piston portion and said cylinder;  step portions each composed of a cylindrical upper part having a large diameter and a cylindrical lower part having a small diameter arranged adjacent to the lower side of said upper part are provided on said piston portion along an axial direction thereof;  circumferential grooves for partitioning said respective step portions are provided; and  a ratio of the length of each step portion to the maximum outside diameter of said piston portion is set at 0.5 to 1.0. 2. A hydraulic control rod driving system according to claim (1), wherein at least two or more step portions are provided.