Patent Number: 052079759
Section: description

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be described in detail hereinafter with reference to the drawings. FIG. 1 shows a piston portion 21 of a control rod driving system 20 formed in accordance with the present invention. The basic construction of the control rod driving system 20 is the same as those shown in FIG. 5 and FIG. 6 which have been described previously, and includes a cylinder fitted vertically to a reactor vessel cover (not shown) and a driving shaft 3 which extends into the cylinder 2 while penetrating through the reactor vessel cover. The piston portion 21 is formed at an upper end of the driving shaft 3, and the driving shaft 3 is moved vertically by regulating the fluid pressure above and under the piston portion 21, thereby to put a control rod cluster (not shown) coupled with the lower end of the driving shaft 3 in and out a reactor core. According to the present invention, a plurality of steps 23 each consisting of a cylindrical upper part 22a and a cylindrical lower part 22b are provided axially on the piston portion 21 at the upper end of the driving shaft 3. In each step portion 23, the lower part 22b is arranged adjacent to the lower side to the upper part 22a, and the outside diameter of the lower part 22b is smaller than the outside diameter of the upper part 22a. Further, in each step portion 23, radial clearances 24 and 25 are provided between the inner wall of the cylinder 2 and the upper part 22a and the lower part 22b of the piston portion 21. Furthermore, circumferential grooves 26 are provided for partitioning the step portion 23 in each step. In such a construction, the fluid flows from the high pressure side A under the piston portion 21 to the low pressure side B on the upper side through radial clearances 24 and 25 in respective step portions 23 between the piston portion 21 and the cylinder 2 and circumferential grooves 26 partitioning respective step portions 23 while a reactor is in operation. The operation of respective step portions 23 at this time will be described hereinafter with reference to FIG. 2 showing a model in which only one step portion 23 is provided in the piston portion 21. When the piston portion 21 runs out of the central axis of the cylinder 2 in one direction (left direction in the figure) as shown in FIG. 2 in a state that the fluid is flowing, the pressure distribution in the radial clearances 24 and 25 changes. Now, when it is assumed that the eccentric side is named C and the opposite eccentric side is named D, the size h.sub.ci of the radial clearance 25 on the high pressure side of the eccentric side C and the size h.sub.co of the radial clearance 24 on the low pressure side thereof become smaller than the size h.sub.di of the radial clearance on the high pressure side and the size h.sub.do of the radial clearance on the low pressure side of the opposite eccentric side D, respectively. Therefore, the relationship of the ratio of clearance on the high pressure side to the low pressure side becomes as follows. EQU h.sub.ci /h.sub.co &gt;h.sub.di /h.sub.do Since the bigger the ratio is, the smaller the ratio of pressure drop in the radial clearance 25 on the high pressure side becomes smaller as described previously, radial pressure distribution on the eccentric side C and the opposite eccentric side D appears as shown in FIG. 3. In FIG. 3, a point E corresponds to a portion E where the outside diameter of the piston portion 21 changes, and the rate of pressure drop changes at the point E. As it is realized from FIG. 3, the sum of radial fluid pressure on the eccentric side C becomes bigger than the sum of the fluid pressure on the opposite eccentric side D, and a force is applied to the piston portion 21 in the direction that the eccentricity with respect to the cylinder 2 becomes smaller by pressure difference between them, thus maintaining the piston portion 21 always in a concentric state. Thus, a non-contact state is maintained between the piston portion 21 and the cylinder 2. The operation and effects are equivalent also in the case that one or two or more step portions 23 are provided as the construction of the present invention. On the other hand, it is required to reduce the flow quantity in the radial direction flowing in the radial clearance between the piston portion 21 and the cylinder 2 in order to stabilize vertical actuation of the driving shaft 3 of the hydraulic control rod driving system 20. For such a purpose, it is thinkable to make the length l of the cylindrical upper portion 22a and the cylindrical lower portion 22b in FIG. 1 of each step portion 23 longer so as to increase the resistance of the passage in the axial direction. When the resistance of the passage in the axial direction is increased, however, the flow quantity flowing through radial clearances 24 and 25 between the piston portion 21 and the cylinder 2 is reduced. When the magnitudes of passage resistances both in the axial direction and the circumferential direction are compared with each other, it is found that the passage resistance in the axial direction is larger than the passage resistance in the circumferential direction. Therefore, the fluid flows in the circumferential direction. Thus, with reference to FIG. 3, the difference between the pressure distribution in the radial clearance on the eccentric side C and the opposite eccentric side D becomes small. Accordingly, the force (restoring force) for maintaining the piston portion 21 in a concentric state becomes smaller, and non-contact state between the piston portion 21 and the cylinder 2 is apprehended. Further, the restoring force of the piston portion 23 per one step depends on the configuration of the piston. As the configuration parameters, there are a ratio of l to d in FIG. 1, a ratio of the clearance 24 to the clearance 25 in FIG. 1, and a ratio of the axial length 22a to 22b in FIG. 1. Further, a value showing an optimum value of the restoring force exists with respect to each parameter. In particular, the parameter which exerts a big influence upon the restoring force is l/d. When the clearance ratio, the length ratio, the cylinder diameter and the cylinder differential pressure which are parameters other than l/d are made constant, the restoring force becomes small in the region where l/d is small as it is seen from the fact that BA in FIG. 3 becomes shorter since the axial length becomes shorter and the area of the location showing the restoring force in FIG. 3 becomes smaller. Further, in the region where l/d is big, the pressure distribution on the eccentric side and the opposite eccentric side is equalized (the side C and the side D in FIG. 3 approaching to each other) because of the fact that the axial length gets longer but a circumferential flow is produced. With this, the restoring force becomes smaller due to the fact that the area of the location showing the restoring force in FIG. 3 becomes smaller. With the foregoing, it is comprehended that l/d has a point where the restoring force reaches the maximum. FIG. 4 shows the result of computing that value. As it is seen from FIG. 4, l/d where the restoring force reaches the maximum is at 0.5 to 1.0. According to the present invention, a plurality of step portions 23 are formed. Accordingly, it is possible to set l/d of each step portion 23 at 0.5 to 1.0, to control the flow flowing through the clearance between the piston portion 21 and the cylinder 2 in the circumferential direction, and to maintain the maximum restoring force. Moreover, since the total length of the piston portion 21 can also be made long, it is possible to reduce the flow quantity flowing through the clearance in the axial direction so as to aim at stabilization of vertical movement of the piston portion 21. As described above, according to the present invention, it is possible to produce hydrostatic bearing effects on the piston portion by forming the configuration of the piston portion into a step-shaped configuration composed of an upper part having a large diameter and a lower part having a small diameter. Furthermore, by forming the step-shaped configuration into a plurality of steps and providing circumferential grooves which partition the step portions, 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 0.1 and to maximize the restoring force of the piston portion. Thus, the piston portion is always maintained in a concentric state with respect to the cylinder, thus preventing contact between both of them. Therefore, there is no fear that the piston portion comes in contact with the internal surface of the cylinder and damages it, and vertical movement of the driving shaft is also performed smoothly. Thus, it is possible to aim at improvement of actuation characteristics of the control rod driving system. In addition, since it is possible to reduce the flow quantity flowing in the axial direction through the radial clearance between the piston portion and the cylinder, it is possible to stabilize all the more vertical movement of the driving shaft.