JP4768407B2 - Control rod drive mechanism - Google Patents

Description

  The present invention relates to a drive mechanism for a control rod in a nuclear reactor, particularly a boiling water reactor.

  A boiling water reactor is generally configured as shown in FIG. That is, reference numeral 1 in the figure denotes a reactor pressure vessel, and a shroud 2 is provided in the reactor pressure vessel 1, and a core 3 is disposed in the shroud 2. The core 3 is configured by arranging a large number of fuel assemblies 4 in a lattice pattern, and control rods having a cross-shaped cross section are inserted into the gaps between the fuel assemblies 4 from below to adjust the output of the core 3. Is configured to do.

  A control rod drive mechanism for inserting and withdrawing the control rod into and from the core is provided at the lower part of the reactor pressure vessel 1, and controls the position of the hollow piston to which the control rod is connected by driving with an electric motor. When one reactor is brought to an emergency stop (scram), the control rod is rapidly inserted by hydraulic drive.

  FIGS. 8A and 8B are diagrams showing the configuration of the control rod drive mechanism, in which a housing 5 is vertically penetrating from the bottom of the reactor pressure vessel 1, and a guide is provided in the housing 5. FIG. A tube 6 is accommodated, and a hollow hollow piston 7 having a drive piston portion 7a formed at the lower end is inserted into the guide tube 6 so as to be movable up and down. A control rod 8 disposed in the pressure vessel 1 is connected to the top end of the hollow piston 7. A ball screw 9 having a ball screw groove formed on the outer periphery is inserted in the center of the guide tube 6, and the lower end of the ball screw 9 is a rotary drive motor (see FIG. The ball screw 9 is rotationally driven in both forward and reverse directions by its rotational drive motor. The ball screw 9 is locked with respect to the guide tube 6 so as to be movable only in the vertical direction, and a ball nut 11 provided with a latch guide 10 on the upper side is screwed. Therefore, when the ball screw 9 is rotated by the rotational drive motor, the ball nut 11 is lifted together with the latch guide 10 and comes into contact with the lower end of the drive piston portion 7a of the hollow piston 7 to push up the hollow piston 7 and control rod 8 Is inserted into the core 3. When the ball screw 9 is rotated in the reverse direction, the ball nut 11 is lowered together with the latch guide 10, and the hollow piston 7 and the control rod 8 are lowered together with the ball nut 11 by gravity, so that the control rod 8 is moved into the core. It is pulled out from 3. Further, if the ball screw 9 is stopped, the control rod 8 will not be lowered and pulled out from the core, and the control rod 8 can be stopped at an arbitrary position.

  However, since the moving speed of the ball nut 11, that is, the driving speed of the control rod 8 cannot be increased with such a configuration, the control rod 8 cannot be rapidly inserted into the core 3 during scram. Therefore, at the time of scrum, high-pressure scram water is supplied into the guide tube 6 via the scram pipe 12, and the hollow piston 7 is pushed up by the pressure of the scram water, and only this hollow piston 7 is released from the latch guide 10 and the ball nut 11. The control rod 8 is configured so as to rise away and rapidly insert the control rod 8 into the core 3 (FIG. 8B).

  FIG. 9 is a diagram showing the relationship between the ball nut 11 and the hollow piston 7, and the drive piston portion 7 a provided at the lower end of the hollow piston 7 has an axis perpendicular to the axis of the hollow piston 7. A claw mechanism 13 is pivotally attached. The claw mechanism 13 has a pair of claw pieces 13b each having a hanging part 13a at the lower end, and each claw piece 13b is urged in the opening direction by a spring 13c.

  Therefore, at the end of the sudden rise of the control rod 8, in other words, at the end of the scrum, the hollow piston 7 and the ball nut 11 are disconnected, so that the claw piece 13b of the claw mechanism 13 is pushed and opened by the spring 13c. , Engaging with the opening 6a of the guide tube 6 to determine the stop position of the hollow piston 7 to prevent natural fall.

On the other hand, the latch guide 10 provided on the upper surface of the ball nut 11 has a claw piece operating portion 10a formed to protrude upward, and raises the ball nut 11 to its upper limit when the reactor is restarted. Then, the claw piece operating portion 10a of the latch guide 10 is engaged with the hanging portion 13a of each claw piece 13b, and each claw piece 13b is released from the opening 6a of the guide tube 6. Therefore, if the ball nut 11 is lowered via the ball screw 9, the hollow piston 7 and the control rod 8 are lowered on this and the control rod 8 can be pulled out.
Japanese Unexamined Patent Publication No. 63-166957 JP-A 63-204196

  However, in the control rod drive mechanism configured as described above, the pulling force when the control rod 8 and the hollow piston 7 are lowered is only their own weight, so that interference such as friction between the control rod 8 and the fuel assembly occurs. In such a case, the control rod 8 may not be pulled out. When the pulling operation is performed in this state, a separation event occurs in which the hollow piston 7 connected to the control rod 8 is disconnected from the ball nut 11. If this separation event occurs, it becomes impossible to pull out the control rod 8 coupled to the control rod drive mechanism, which hinders plant operation. Even when the control rod drive mechanism is removed for repair, the hollow piston 7 and the control rod 8 are in a restrained state in the furnace, so that the normal control rod drive mechanism cannot be removed. There is a possibility that a great deal of work such as partial cutting of the components of the worst control rod drive mechanism, opening of the reactor pressure vessel, removal of peripheral devices, etc., and restoration of the control rod drive mechanism will occur. There are problems such as.

  In view of such a point, the present invention is to obtain a control rod drive mechanism having a forced pulling mechanism that can transmit transmission power even during the pulling operation of the control rod drive mechanism without affecting the hydraulic drive performance. Objective.

The present invention is provided in a control rod drive mechanism housing that is inserted through the bottom wall of a reactor pressure vessel, and is rotated around an axis by a motor, and the ball screw is screwed into the ball screw and rotated. A ball nut that moves up and down in the axial direction, and a hollow piston that is placed on the ball nut and has a control rod connected to the top end, and is normally pushed up by the ball nut, and at the time of scrum is rapidly raised by scram high pressure water And a latch guide having a ball key which is provided on an upper portion of the ball nut and which can be engaged with and disengaged from a claw restraining groove formed on an outer periphery of a drive piston portion at a lower end portion of the hollow piston. The ball key is slidable up and down along the ball nut, and the ball key is It holds in engagement in tight constraint channel, and having a movable guide for holding the ball key during scram operation disengaged from the pawl restraining groove.

  Since the present invention is configured as described above, transmission power can be reliably transmitted to the hollow piston even during the pulling operation of the control rod drive mechanism, and the hollow piston to which the control rod is coupled is separated from the ball nut. Occurrence of a separation event can be prevented, or both can be easily connected even when the separation phenomenon occurs, and the control rod can be forcibly pulled out. In addition, it can be set to operate sufficiently in a friction test by water pressure driving at a pressure smaller than the water pressure acting at the time of scrum, and does not hinder the scrum function during plant operation.

  Hereinafter, embodiments of the present invention will be described with reference to FIGS. The same parts as those in FIGS. 8 and 9 are denoted by the same reference numerals, and detailed description thereof is omitted.

  FIGS. 1A and 1B are longitudinal sectional views showing a first embodiment of the present invention, in which a ball screw 9 rotated around an axis by a motor (not shown) is concentric in a guide tube 6. A ball nut 11 is screwed into the ball screw 9, and the ball nut 11 is moved up and down in the axial direction of the ball screw 9 by the rotation of the ball screw 9. A latch guide 10 is provided on the ball nut 11 so that the latch guide 10 and the ball nut 11 can move up and down integrally. The latch guide 10 is mounted with a large-diameter drive piston portion 7a formed at the lower end of a hollow piston (not shown) having a control rod connected to the top end.

  A ball key engagement groove 15 extending in the circumferential direction is formed on the outer periphery of the drive piston portion 7a. On the other hand, a ball key mounting piece 16 is projected on the upper portion of the latch guide 10, and the ball key mounting A ball key 17 detachably attached to a ball key engagement groove 15 formed in the drive piston portion 7a is mounted on the piece 16 so as to be able to advance and retract inward.

  A movable guide 20 is disposed outside the ball nut 11 and the latch guide 10 via guide rollers 18 and 19 so as to be slidable up and down along the ball nut 11 and the latch guide 10. The movable guide 20 is urged downward by a movable guide driving spring 21, and the drive piston portion is located on the inner surface when the movable guide 20 moves to the raised position against the movable guide driving spring 21. A recess 22 is formed for maintaining the ball key 17 detached from the claw restraining groove 15 formed in 7a in a retracted state.

  Thus, as shown in FIG. 1A, the movable guide 20 is normally held at the lower position by the movable guide driving spring 21 so that the ball key 17 is moved to the ball key engaging groove by the movable guide 20. 15, the latch guide 10 and the drive piston portion 7 a are restrained by the ball key 17. Accordingly, the rotation of the ball screw 9 causes the ball nut 11, the latch guide 10 and the drive piston portion 7a to move up and down integrally, and the control rod can be inserted and pulled out. Drawing can be performed.

  On the other hand, when high-pressure water is supplied during scrum, the water pressure acts on the lower surface of the movable guide 20, and the movable guide 20 is raised against the movable guide driving spring 21 by the water pressure. Accordingly, as shown in FIG. 1B, the ball key 17 is retracted into the concave portion 22 of the movable guide 20 and can be detached from the ball key engaging groove 15 formed in the drive piston portion 7a, and the drive piston portion 7a. And the latch guide 10 are released, the drive piston portion 7a is separated from the latch guide 10, and the control rod is rapidly inserted into the core together with the hollow piston 7 by the high-pressure water.

  As described above, the control rod drive mechanism of the present embodiment can always perform forced insertion / extraction by electric drive at normal times, and can prevent the occurrence of a separation event. Further, since the restraint between the drive piston portion 7a and the latch guide 10 is released as described above at the time of hydraulic driving, that is, scram, there is no problem in the scram function.

  Further, the movable guide driving spring 21 is set so that the movable guide 20 is operated by the water pressure applied to the control rod drive mechanism function confirmation test smaller than the water pressure acting at the time of the scrum, so that the scram function is not impaired. Confirmation tests can also be performed.

  FIG. 2 is a diagram showing a second embodiment of the present invention, and a claw restraining groove 23 is formed on the outer surface side of the drive piston portion 7a. Further, the latch guide 10 has a claw portion 24a that can be engaged and disengaged with a claw restraining groove 23 formed in the drive piston portion 7a at one end and a water pressure receiving plate 24b at the other end. 24 is pivotally attached by a shaft 25. During normal electric drive, the claw portion 24a is urged by the claw operating spring 26 so as to engage with the claw restraining groove 23.

  2A, the claw portion 24a of the forcible extraction claw 24 is engaged with the claw restraining groove 23 formed in the drive piston portion 7a. Thus, the latch guide 10 and the drive piston portion 7a are restrained. Accordingly, the rotation of the ball screw 9 causes the ball nut 11, the latch guide 10 and the drive piston portion 7a to move up and down integrally, and the control rod can be inserted and pulled out. Drawing can be performed.

  On the other hand, when high-pressure water is supplied during scram, the water pressure acts on the water pressure receiving plate 24b, and the water pressure causes the forcible pull-out claw 24 to swing against the claw operating spring 26, as shown in FIG. As shown, the claw portion 24a is disengaged from the claw restraint groove 23 formed in the drive piston portion 7a, the restraint between the drive piston portion 7a and the latch guide 10 is released, and the drive piston portion 7a is separated from the latch guide 10. The control rod is rapidly inserted into the core together with the hollow piston 7 by the high-pressure water.

  After the hydraulic pressure drive, the drive piston portion 7a and the latch guide 10 are joined by electric insertion because the contact point between the claw restraining groove 23 of the drive piston portion 7a and the claw portion 24a of the forcible extraction claw 24 is the rotation fulcrum of the forced claw claw 24 Therefore, when the weight of the control rod and the hollow piston acts on the forcible extraction claw 24, the claw opens, and the latch guide 10 and the drive piston portion 7a can be easily restrained even after separation. Further, the claw actuating spring 26 can be a spring having a very small force because there is no problem as long as the claw actuating spring 26 has a force that does not cause the forced extraction claw 24 to be in the open state. For this reason, the scram function is not impaired by setting the claw actuating spring 26 so that the forcible extraction claw 24 is operated by the water pressure applied to the control rod drive mechanism function confirmation test smaller than the water pressure acting at the time of scram. The confirmation test can also be performed.

  3 (a) and 3 (b) are views showing a third embodiment of the present invention. A ball key engagement groove 28 is formed on the outer periphery of the drive piston portion 7a, while a latch guide is shown. A ball key mounting piece 16 projects from the upper portion of the ball key 10. The ball key mounting piece 16 has a ball key 17 that can be engaged with and disengaged from the ball key engaging groove 28 formed in the drive piston portion 7a. It is installed so that it can move forward and backward.

  A movable guide 29 is disposed on the outer surface of the latch guide 10 so as to be vertically slidable along the latch guide 10. The movable guide 29 is urged upward by a movable guide driving spring 30. When the movable guide 29 moves to the lowered position against the movable guide driving spring 30, the drive piston portion is provided on the inner surface. A recess 29a is formed for maintaining the ball key 17 released from the ball key engagement groove 28 formed in 7a in a retracted state. The latch guide 10 is provided with a movable guide stopper 32 urged by a spring 31 so as to protrude in the direction of the guide tube 6, and in a normal state, a protrusion 32 a provided on the movable guide stopper 32. Engages with an opening 29 b formed in the movable guide 29, and holds the movable guide 29 in the lowered position against the movable guide driving spring 30.

  On the other hand, on the inner surface of the guide tube 6, a stopper operating projection 33 is provided so as to project against the movable guide stopper 32 at a position corresponding to the latch guide 10 when the control rod is fully inserted.

  Therefore, in a normal state, the protrusion 32 a provided on the movable guide stopper 32 engages with the opening 29 b formed in the movable guide 29, and the movable guide 29 is opposed to the movable guide driving spring 30. Since the ball key 17 is held in the lowered position, the ball key 17 is maintained in the retracted state in which it is detached from the ball key engaging groove 28 formed in the drive piston portion 7a, and the latch guide 10 and the drive piston portion 7a are restrained from each other. It is in a state that has not been done. Therefore, the pulling-out operation is performed by a self-weight dropping method that can be hydraulically driven as in the conventional control rod driving mechanism.

  However, when a separation event occurs in which the drive piston portion 7a and the latch guide 10 are separated as described above during the operation of the nuclear reactor, and it is necessary to remove the control rod to check the control rod drive mechanism. It becomes impossible to pull out the control rod to the full pulling position.

  Therefore, when the ball screw 9 is driven by electric drive to raise the ball nut 11 and move to a position where the movable guide stopper 32 provided in the latch guide 10 contacts the stopper operating protrusion 33, the stopper operating protrusion 33, the movable guide stopper 32 is pressed against the spring 31 in the direction of the latch guide 10, the protrusion 32a of the movable guide stopper 32 is released from the opening 29b formed in the movable guide 29, and the movable guide 29 is moved to the movable guide stopper. 32 is released. Therefore, as shown in FIG. 3B, the movable guide 29 is raised to the position of the stopper pin 34, and the ball guide 17 is engaged with the ball key engaging groove 28 of the drive piston portion 7a by the movable guide 29, and is latched. The guide 10 and the drive piston portion 7a are restrained by the ball key 17. Therefore, the control rod can be pulled out by electric drive.

  In other words, since this embodiment is an operation-type forced restraint release type when necessary as described above, there is no change from the conventional control rod drive mechanism drive method, and the operation does not change. .

  4 (a) and 4 (b) are diagrams showing a fourth embodiment, and a tapered claw restraining groove 35 is formed on the outer periphery of the drive piston portion 7a at the lower end portion of the hollow piston. An L-shaped forcible pull-out pawl 36 that can be engaged with and disengaged from the tapered pawl restraining groove 35 is pivotally attached to the latch guide 10 provided on the upper portion of the ball nut 11 by a horizontal shaft 37. The tip end claw portion 36a of the forced pulling claw 36 is urged by a claw operating spring 38 in the direction of the ball screw 9.

  In the normal state, the forced pulling pawl 36 pivotally attached to the latch guide 10 is engaged with the tapered pawl restraining groove 35 formed in the drive piston portion 7a, so that the latch guide 10 and the drive piston portion 7a are engaged. Is always restrained and can be forcibly inserted and withdrawn by a pulling operation by electric drive.

  On the other hand, when high pressure water is supplied at the time of scram and an upward driving force is applied to the drive piston portion 7a by the high pressure water, the claw restraining groove 35 is tapered. Is automatically disengaged from the claw restraining groove 35, and the restraint by the forcibly withdrawing claw 36 is released, so that the hydraulic drive operation is not hindered.

  Further, when the latch guide 10 is connected to the drive piston portion 7a raised by the water pressure drive, if the latch guide 10 is moved to a position where it abuts on the drive piston portion 7a by electric drive, the forced pulling claw 36 is moved. Automatically engages with the claw restraining groove 35 and can be forcibly pulled out by a pulling operation by electric drive.

  FIG. 5 is a view showing a modification of the embodiment shown in FIG. 4. A tapered claw restraining groove 38 is formed on the inner periphery of the drive piston portion 7a at the lower end portion of the hollow piston. The latch guide 10 provided on the upper portion of the guide shaft is pivotally attached by a horizontal shaft 40 to a forcible pull-out pawl 39 that can be engaged with and disengaged from the tapered pawl restraining groove 38. The forcibly withdrawing claw 39 has a front claw portion 39a urged outward by a claw operating spring 41.

  In the normal state, the forcible pulling pawl 39 pivotally attached to the latch guide 10 is engaged with the tapered pawl restraining groove 38 formed in the drive piston portion 7a, so that the latch guide 10 and the drive piston portion 7a are engaged. Is always restrained and can be forcibly inserted and withdrawn by a pulling operation by electric drive.

  On the other hand, when high pressure water is supplied at the time of scram and an upward driving force is applied to the drive piston portion 7a by the high pressure water, the claw restraining groove 38 is tapered. Is automatically disengaged from the claw restraining groove 38, the restraint by the forcibly withdrawing claw 39 is released, and there is no hindrance to the hydraulic drive operation, and the same action as in the embodiment shown in FIG. 4 is performed.

  By the way, the claw actuating springs 38 and 41 are shape memory alloy springs, and by adjusting the pressing load to be less than the load generated during the friction test, the forcibly withdrawing claws 36 and 39 are automatically released by water pressure driving and water pressure driving. Does not interfere with operation. When forced pulling is impossible due to this pressing load, hot water is passed through the control rod drive mechanism or the claw actuating springs 38 and 41 made of the shape memory alloy spring are forcibly raised in temperature, and the shape memory alloy spring is The pushing load of the claw actuating springs 38 and 41 consisting of can be increased, and the forced pulling force can be increased.

  FIGS. 6A and 6B are views showing a fifth embodiment of the present invention, in which the separation detection mechanism is used to restrain the drive piston portion 7a and the latch guide 10. FIG. is there.

  That is, a claw restraining groove 42 is formed on the outer periphery of the drive piston portion 7a at the lower end of the hollow piston, and the latch guide 10 provided on the upper portion of the ball nut 11 is engaged with and disengaged from the claw restraining groove 42. A possible L-shaped forced extraction claw 43 is pivoted by a horizontal shaft 44. Outside the ball nut 11 and the latch guide 10, a guide lot 45 slidable in the vertical direction along the ball nut 11 and the latch guide 10 is provided. The guide lot 45 is urged downward by a spring 46, and a long hole 47 extending in an oblique direction is formed in the upper portion of the guide lot 45, and a pin 48 projecting from the forcibly withdrawing claw 43 is formed. Is engaged with the elongated hole 47. Therefore, the guide lot 45 is normally in the lower position by the spring 46, and the forcibly withdrawing claw 43 is held away from the claw restraining groove 42 (FIG. 6A).

  By the way, an internal gear 50 is fixed to the lower end of the ball screw 9, and a gear 53 provided at the upper end of the drive shaft 52 of the electric motor 51 is meshed with the internal gear 50. In addition, the internal gear 50 is slidable in the vertical direction with respect to the gear 50. The drive shaft 52 is slidably mounted with a slide plate 54 that contacts the lower end of the internal gear 50, and the slide plate 54 and a support plate 55 fixed to the drive shaft 52 are provided. A spring 56 is interposed therebetween.

  A forced pulling ball nut 57 is screwed onto the ball screw 9 at a position below the ball nut 11, and a reset spring 58 is interposed between the forced pulling ball nut 57 and the ball nut 11. The forced pull-out ball nut 57 is urged downward with respect to the ball nut 11. The lower end of the guide lot 45 is connected to the ball nut 57 for forced extraction.

  Since the forced extraction ball nut 57 is interlocked with the ball nut 11 at a regular interval in normal times, the forced extraction claw 43 is restrained by the claw of the drive piston portion 7a as shown in FIG. The forced withdrawal pawl 43 is not engaged with the groove 42 and does not restrain the drive piston portion 7a, and performs the same operation as that of the conventional control rod drive mechanism.

  On the other hand, when a so-called separation phenomenon occurs in which the load of the hollow piston 7 and the control rod 8 does not act on the latch guide 10 during the electric drive pulling operation, the latch guide 10 is not suddenly separated from the drive piston portion 7a. When the load acting on the ball screw 9 via the latch guide 10 is reduced, the internal gear 50 and the ball screw 9 are rotated upward by the spring 56 via the sliding plate 54 while rotating. Accordingly, the forcibly withdrawing ball nut 57 is moved upward by the movement of the ball screw 9 and the guide lot 45 connected thereto is moved upward to engage the forcibly withdrawing claw 43 by the engagement between the long hole 46 and the pin 54. The forced pulling claw 43 is engaged with the claw restraining groove 42 of the drive piston portion 7a (FIG. 6B). The drive piston portion 7a is restrained by the latch guide 10 by the engagement of the forcibly withdrawing claw 43 and the claw restraining groove 42 of the drive piston portion 7a, so that the electric forcible withdrawal of the hollow piston 7 and the control rod 8 becomes possible.

  When the separation is released by performing the forcible extraction in this way, the forcibly extracting ball nut 57 is lowered to a predetermined position by the reset spring 58, and returns to the normal operation state of FIG.

  As described above, this embodiment operates so that the forcible extraction can be performed only when the separation event occurs, and since it does not operate when there is no rotational power of the ball screw 9, the operation at the time of hydraulic drive is performed. Will not interfere.

(A), (b) is a figure which shows 1st Embodiment of the control-rod drive mechanism of this invention. (A), (b) is a figure which shows 2nd Embodiment of the control-rod drive mechanism of this invention. (A), (b) is a figure which shows 3rd Embodiment of the control-rod drive mechanism of this invention. (A), (b) is a figure which shows 4th Embodiment of the control-rod drive mechanism of this invention. The figure which shows the modification of 4th Embodiment of the control-rod drive mechanism of this invention. (A), (b) is a figure which shows 5th Embodiment of the control-rod drive mechanism of this invention. Sectional drawing which shows schematic structure of a boiling water reactor. (A), (b) is explanatory drawing of the conventional control rod drive mechanism. The figure which shows the relationship between the ball nut 11 and the hollow piston 7. FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Reactor pressure vessel 5 Housing 6 Guide tube 7 Hollow piston 7a Drive piston part 8 Control rod 9 Ball screw 10 Latch guide 11 Ball nut 13 Claw mechanism 15 Ball key engagement groove 16 Ball key attachment piece 17 Ball key 20 Movable guide 21 Movable guide drive spring 22 Recess 23 Claw restraint groove 24 Forced extraction claw 24b Hydraulic pressure receiving plate 26 Claw actuation spring 28 Ball key engagement groove 29 Movable guide 30 Movable guide drive spring 31 Spring 32 Movable guide stopper 33 Stopper actuating projection 34 Stopper Pins 35 and 38 Claw restraining grooves 36 and 39 Forced extraction claws 37 and 40 Horizontal shafts 38 and 41 Claw operating spring 42 Claw restraining groove 43 Forced withdrawal claw 45 Guide lot 47 Long hole 48 Pin 50 Internal gear 51 Electric motor 52 Drive shaft 53 Gear 54 Sliding plate 55 Support plate 57 Forced extraction ball Nut 58 Reset spring

Claims (3)

  A ball screw that is provided in a control rod drive mechanism housing that is inserted through the bottom wall of the reactor pressure vessel, is rotated about the axis by a motor, and is engaged with the ball screw in the axial direction by rotation of the ball screw. A control having a ball nut that moves up and down, and a hollow piston that is placed on the ball nut and has a control rod connected to the top end thereof, and is normally pushed up by the ball nut and rapidly raised by high-pressure water for scram during scram. In the rod drive mechanism, a latch guide having a ball key provided at an upper portion of the ball nut and detachable from a ball key engagement groove formed on an outer periphery of a drive piston portion at a lower end portion of the hollow piston, It is arranged so that it can slide up and down along the ball nut. It holds in engagement with the ball key engaging groove, the control rod drive mechanism; and a movable guide for holding the ball key during scram operation disengaged from said ball key engaging groove.   The movable guide is biased downward with respect to the ball nut by a movable guide driving spring, and is moved upward against the movable guide driving spring by water pressure during scram operation. The control rod drive mechanism according to claim 1.   3. The movable guide is formed with a recess for maintaining the ball key in a retracted state when the ball key is released from the ball key engaging groove. Control rod drive mechanism.

Images