Patent Number: 052079759
Section: summary

FIELD OF THE INVENTION AND RELATED ART STATEMENT The present invention relates to a hydraulic control rod driving system of a reactor, and more particularly to a configuration of a piston portion formed at an upper end of a driving shaft. Start-up, shut-down and power change of a reactor are performed by putting a control rod in and out a reactor core. Further, the control rod is driven by means of a control rod driving system fitted to a reactor vessel cover. There have been heretofore various types of control rod driving systems, but one of them is a hydraulic driving system. FIG. 5 shows a structure of a conventional hydraulic control rod driving system schematically. The control rod driving system 1 includes a cylinder 2 installed vertically on a reactor vessel cover (not shown) and a driving shaft 3 which extends into the cylinder 2 penetrating through the reactor vessel cover coupled with a spider (not shown) of a control rod cluster, and moves the driving shaft 3 vertically so as to put the control rod cluster in and out a reactor core by controlling fluid pressure above and under a piston portion 4 formed at an upper end of the driving shaft 3. The inside of the cylinder under the piston portion 4 forms a high pressure side A since it communicates with the inside of the reactor vessel, and the inside of the cylinder above the piston portion 4 forms a low pressure side B. Pipes 7 and 8 provided with switching valves 5 and 6 extend from the high pressure side A and the low pressure side B of the cylinder 2, respectively. These pipes 7 and 8 are connected to a pipe 9 which is opened to atmospheric pressure side. The pipe 9 is provided with a switching valve 10. When the switching valve 5 of the tube 7 is closed, and the switching valves 6 and 10 are opened in such a structure, the low pressure side B is opened to the atmospheric pressure side, a force acting on the piston portion 4 by the difference between fluid pressure, above and below, becomes larger than the dead weight of the driving shaft 3, and the driving shaft 3 moves upward. Further, when the switching valve 5 is opened gradually in a state that the switching valve 10 is closed and the switching valve 6 is opened, the differential pressure between the high pressure side A and the low pressure side B becomes smaller, and the driving shaft 3 moves downward by the dead weight thereof. In FIG. 5, a reference numeral 11 denotes a bearing provided at the lower part of the cylinder 2, which supports the driving shaft 3 radially. Further, radial clearance is formed between the piston portion 4 and the cylinder 2 so that both do not come in contact with each other. If the stroke of vertical movement of the driving shaft 3 becomes longer, however, there is such a possibility that the piston portion 4 at the upper end of the driving shaft 3 runs out from the central axis of the cylinder 2 and comes in contact with the internal surface of the cylinder 2. Thus, generally, piston packings 12 have been heretofore fitted to the piston portion 4 of the driving shaft 3 so as to support the driving shaft 3 radially at two locations, upper and lower, as shown in FIG. 6 in addition to the bearing 11. FIG. 7 shows a hydraulic control rod driving system which is used practically in a reactor in a more concrete manner. In this control rod driving system 1, the cylinder 2 is fitted as one body in a housing 14 which is fixedly attached to a reactor vessel cover 13. Further, metallic piston rings 17 are fitted to the piston portion 4 at the upper end of the driving shaft 3 coupled with a spider 16 of a control rod cluster 15, thereby to support the driving shaft 3 radially and also to generate a driving force. In the structure shown in FIG. 7, the driving shaft 3 is supported radially by means of the piston rings 17 only, and no bearing is provided at the lower part of the cylinder 2. As described previously, in the control rod driving system shown in FIG. 5, there is a fear that the piston portion moves vertically in a state that the piston portion is in contact with the internal surface of the cylinder in case the stroke of vertical movement of the driving shaft is long, thus damaging the internal surface of the cylinder. When piston packings 12 are added to the piston portion 4 as shown in FIG. 6, the piston portion 4 does not come in contact directly with the cylinder 2, but smooth movement of the driving shaft is impeded sometimes since piston packings are always in contact with the internal surface of the cylinder. Further, since the piston packings slide on the internal surface of the cylinder, there is the possibility that the internal surface of the cylinder is damaged in this case, too. Furthermore, in such a type that uses piston rings as shown in FIG. 7, contact between the piston ring and the cylinder is made between metals, and metallic adhesion is produced thereby to prevent smooth movement of the driving shaft where there is a foreign matter or a flaw on the internal surface of the cylinder. OBJECT AND SUMMARY OF THE INVENTION It is an object of the present invention to provide a hydraulic control rod driving system which makes a piston portion of a driving shaft movable vertically without coming in contact with the cylinder so as to solve above-described conventional technical problems. In order to achieve above-described object, in a hydraulic control rod driving system according to the present invention, radial clearance is formed between the piston portion and the cylinder, one or two or more step portions 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 the upper part are provided on the piston portion along an axial direction thereof, circumferential grooves for partitioning off each of the step portions are provided, and the ratio of the length of each step portion to the maximum outside diameter of the piston portion is set at 0.5 to 1.0. In the above-described structure, the fluid flows from the high pressure side under the piston portion to the upper low pressure side through radial clearance between the piston portion and the cylinder. The operation at respective step portions at this time is as follows. That is, pressure distribution in an axial direction between the piston portion and the cylinder is determined by the ratio of the size h.sub.i of an inlet portion (high pressure side) to the size h.sub.o of an outlet portion (low pressure side) of the radial clearance. In case h.sub.i =h.sub.o, the pressure drops at a constant rate from the high pressure side to the low pressure side. Whereas, when h.sub.i /h.sub.o becomes larger as in the present invention, the rate of pressure drop between the lower part of the piston portion and the cylinder becomes smaller. Accordingly, when the central axis of the piston portion runs out of the central axis of the cylinder, h.sub.i /h.sub.o on the runout side becomes larger than that on the opposite side and the pressure on the runout side becomes larger than the pressure on the opposite runout side. As a result, a force which is going to return the runout acts on each step portion of the piston portion, and the piston portion is always held in a concentric state with respect to the cylinder. Further, a plurality of step portions each composed of an upper part having a large diameter and a lower part having a small diameter are provided, and circumferential grooves are also provided as partitions among respective steps. Therefore, it is possible to set the ratio of the length of each step portion to the maximum outside diameter of the piston portion at 0.5 to 1.0. At this ratio, the greater part of the flow along the piston portion shows the axial direction and almost no flow is produced in the circumferential direction, which will be described in detail later. Thus, the maximum restoring force is applied to the piston portion.