JP2557365B2 - Control rod drive mechanism - Google Patents

Description

The present invention relates to a control rod drive mechanism suitable for a boiling water reactor (hereinafter referred to as BWR), and particularly to a motor for shim operation. The present invention relates to a control rod drive mechanism that inserts and pulls out a control rod from below the core of a nuclear reactor by driving.

(Prior Art) Generally, a BWR is configured as shown in FIG. 7, and a reactor core 2 is housed in a reactor pressure vessel 1 by being immersed in reactor water.

A plurality of fuel assemblies (not shown) are loaded in the core 2, and a plurality of control rods 3 each having a cross-shaped cross section are provided below the core 2 in a required gap between the fuel assemblies. Inserted or pulled out.

Each control rod 3 is a control rod drive mechanism (hereinafter referred to as CRD)
4 are driven respectively to insert into and extract from the core 2, and the output of the nuclear reactor is controlled by appropriately adjusting the degree of insertion.

Conventionally, this type of CRD 4 has a control rod drive mechanism housing (hereinafter referred to as a CRD housing) 5 vertically penetrating the bottom portion 1a of a reactor pressure vessel 1,
The CRD housing 5 is filled with reactor water from the reactor pressure vessel 1.

Then, as shown in FIG. 8, the hollow piston 6 is housed in the CRD housing 5 so as to be able to move up and down, and the control rod 3 and the hollow piston 6 are integrally connected by a connecting member provided at the top of the hollow piston 6. To be done.

CRD4 is a relatively slow insertion / extraction operation (shim operation) of control rod 3 that is performed to maintain and maintain the reactor power in a prescribed state during normal operation, and control rod 3 that is used to emergency stop the reactor in an emergency. It has the function of performing rapid insertion (scrum operation). Generally, the shim operation is performed by a motor drive, and the scrum operation is performed by a hydraulic drive.

That is, at the time of shim operation, the motor 7 is driven, and its rotational movement is transmitted to the hollow ball screw shaft 9 via the drive shaft 8, and the ball screw nut screwed to the ball screw shaft 9 so as to be axially movable. 10 and the hollow piston 6 placed on the ball screw nut 10 are moved up and down. This allows
The control rod 3 supported and connected to the hollow piston 6 is inserted into or pulled out from the core 2.

On the other hand, at the time of scram operation, high-pressure scrum water from a water pressure control unit (HCU) not shown passes through the insertion pipe 11 and further through the insertion flow path formed in the outer peripheral flange 12 to inject high-pressure water into the CRD housing 5. The high-pressure water pressurizes the hollow piston 6 and pushes it up, so that the control rod 3 is inserted into the core 2 at high speed.

When the control rod 3 is shimmed, the lower ends of the pair of left and right latch fingers 13a, 13b provided at the lower end of the hollow piston 6 as shown in FIG. 9 have a pair of left and right claw housings 14a, 14b.
It is fitted inside.

As shown in FIG. 10, the pair of latch fingers 13a and 13b are urged by springs 15a and 15b so as to be constantly expanded outward, and the tip portions thereof are as shown in FIG. It is closed by being sandwiched by a pair of claw storage portions 14a, 14b.

Therefore, since the lower end of the hollow piston 6 is simply placed on the ball screw nut 10 in the center hole 16a of the ring-shaped latch guide 16, the shim of the control rod 3 is operated by the motor 7. Ball screw shaft 9 driven by rotation
Rotates about an axis and the ball screw nut 10 and the latch guide 16 rise, the ball screw nut 10 pushes up the hollow piston 6, and the control rod 3 is inserted into the core 2.

On the other hand, when the ball screw nut 10 and the latch guide 16 descend, the hollow piston 6 descends by its own weight while the pair of latch fingers 13a and 13b are fitted in the pair of claw housings 14a and 14b, and is controlled. The rod 3 is pulled out from the core 2.

During the scram operation of the control rod 3, the hollow piston 6 pressurized with high-pressure water rapidly rises, while the ball screw nut 10 and the latch guide 16 remain stopped. The latch fingers 13a and 13b of the hollow piston 6 are pulled out and released from the pawl storage portions 14a and 14b of the latch guide 16 to release the pair of latch fingers 13a and 13b.
The tips of a and 13b expand outward.

For this reason, as shown in FIG. 10, the pair of latch fingers 13a, 13b that expand outward are engaged with the pair of notches 17a, 17b of the guide tube 17 to maintain the raised position of the hollow piston 6. It

Therefore, the insertion position of the control rod 3 connected to the hollow piston 6 into the reactor core 2 is held, and the reactor scram is held.

Further, as shown in FIG. 11, a plurality of rollers 18a, 18b, 18c, 18d that rotate by contacting with the inner peripheral surface of the guide tube 17 are arranged on the outer peripheral portion of the latch guide 16 at a predetermined pitch in the circumferential direction. The rollers 18a to 18d guide the up and down movement of the latch guide 16 and the hollow piston 6.

Of these rollers 18a to 18d, the adjacent required roller
18a to 18d Between the circumferential directions of the guide rails, which are arcuate plates 19
And the convex curved surface of the guide rail 19 is fixed to the inner peripheral surface of the guide tube 17. Therefore, the vertical movement of the adjacent rollers 18a and 18d is guided by the guide rail 19, while the circumferential rotation of the latch guide 16 is restricted by the both side surfaces of the guide rail 19.

By thus restricting the rotation of the latch guide 16, the rotation of the ball screw nut 10 fixing the latch guide 16 is restricted, and the rotational movement of the ball screw shaft 9 is controlled by the ball screw nut 10. Converted to a lifting movement.

In FIG. 8, reference numeral 20 denotes a shaft sealing portion, which seals the drive shaft 8 which is axially separably connected to the output shaft of the motor 7. A boss 22 having a key groove formed on the inner peripheral surface is coaxially fixed to the ball screw shaft 9 at the lower end of the back seat 21 fixed to the lower end of the ball screw shaft 9.

A spline shaft portion, which is an upper end portion of a drive shaft 8 which is detachably coupled to the output shaft of the motor 7 and which is a spline shaft, is fitted into the boss 22. For this reason, the drive shaft 8 can be removed from both the output shaft of the motor 7 and the boss 22.

Further, the upper end portion of the division seat 23 having a seat portion which is watertightly adhered to the lower surface of the back seat 21 is screwed into the lower end portion of the outer peripheral flange 12, and the back seat 21 is seated on the seat portion of the division seat 23. As a result, it is possible to prevent the reactor water from leaking in the CRD housing 5 below this portion. The upper inner peripheral surface of the shaft sealing portion 20 is watertightly adhered to the outer peripheral surface of the divided portion sheet 23.

(Problems to be Solved by the Invention) However, when the control rod 3 is pulled out by such a conventional control rod drive mechanism, the motor 7 is rotationally driven in a predetermined direction to rotate the ball screw shaft 9 around its axis. The ball screw nut 10 is lowered, and the hollow piston 6 connecting the control rod 3 is caused to follow the ball screw nut 10 by its own weight.

Therefore, although the weight of the control rod 3 is large and sufficient for pulling out, it is considered that the control rod 3 cannot be pulled out by its own weight due to friction between the control rod 3 and the fuel assembly. However, even in such a case, it is necessary to take measures to forcibly pull out the control rod 3 from the core 2.

Therefore, it is an object of the present invention to provide a control rod drive mechanism capable of forcibly pulling the control rod from the core if the control rod cannot be pulled out from the core by its own weight.

[Structure of Invention]

(Means for Solving Problems) According to the present invention, a latch guide is provided with a latching portion that is latched by a latch finger of a hollow piston connecting a control rod,
If the control rod cannot be pulled out of the core for some reason, hook the latching part of the latch guide to the latch finger, connect the control rod and ball screw nut through the latch guide and hollow piston, and The control rod is forcibly pulled out from the core by retracting the nut, and the first invention of the present case is configured as follows.

A ball screw nut that is screwed around a shaft that is rotated by a motor and that moves up and down in the axial direction, and a latch finger that is connected to a control rod and that is mounted on the ball screw nut are used to expand the latch fingers. A hollow piston that can be opened freely, a latch guide that is fixed to the ball screw nut, and a latching portion that is locked to the latch finger, and an axial direction that regulates the rotation of the ball screw nut. And a guide for guiding the movement of the control rod drive mechanism housing which is fixed to the reactor pressure vessel by penetrating the bottom of the reactor pressure vessel and stores the reactor water.

In addition, the second invention of this case, when the first invention is carried out,
It is an invention of an apparatus used exclusively for preventing reactor water from leaking from the control rod drive mechanism housing, and is configured as follows.

A ball screw nut that is screwed around a shaft that is rotated by a motor and that moves up and down in the axial direction, and a latch finger that is connected to a control rod and that is mounted on the ball screw nut are used to expand the latch fingers. A hollow piston that can be opened freely, a latch guide that is fixed to the ball screw nut, and a latching portion that is locked to the latch finger, and an axial direction that regulates the rotation of the ball screw nut. And a guide for guiding the movement of the control rod drive mechanism housing which is fixed to the bottom of the reactor pressure vessel and contains the reactor water. In the control rod drive mechanism housing, a closed casing is installed which is detachably and watertightly installed in place of the mounting position of the section to accommodate the reactor water. Seated on the back seat that seals the reactor water and supports and detaches the split seat that is screwed to the outer peripheral flange, and is detachably connected to the boss of the back seat. And a ball screw handling device for rotating the ball screw shaft at a latching position where the latching portion of the latch guide is engaged with and latched on the latch finger. Each of the operating ends of the device and the divided sheet handling device is watertightly extended to the outside of the closed casing.

(Function) If the control rod cannot be pulled out of the core for some reason, engage the latch finger of the hollow piston with which the control rod is connected by engaging the latch guide latching part, and then lower the latch guide downward. And pull out the control rod forcibly from the core.

However, it is necessary to safely remove the shaft sealing part of the control rod drive mechanism housing for this work.
The motor is rotationally driven to rotate the ball screw shaft so that the back sheet at the lower end of the ball screw shaft is brought into close contact with the division sheet, and once the reactor water is prevented from leaking, the shaft sealing portion is removed.

Next, after installing the sealed casing in a watertight position at the position where the shaft sealing part is removed, operate the operating end of the split sheet handling device from the outside of the sealed casing, remove the split sheet from the outer peripheral flange, and remove the control rod. The sealed casing receives the reactor water in the drive mechanism housing.

Thereafter, the operating end of the ball screw handling device is operated from the outside of the closed casing, and the ball screw shaft is rotated to a position where the latching portion of the latch guide is latched by the latch finger of the hollow piston.

Further, the ball screw handling device pushes the ball screw shaft upward to hold the ball screw nut in a state in which its rotation is restricted.

Next, the motor is rotationally driven to rotate the ball screw shaft, raise the ball screw nut and the latch guide, and latch the latching portion of the latch guide to the latch finger.

As a result, the control rod is connected to the ball screw nut via the hollow piston and the latch guide.

Therefore, next, the control rod can be forcibly pulled out by driving the motor in the reverse direction to lower the ball screw nut.

(Embodiment) An embodiment of the present invention will be described below with reference to FIGS. 1 to 5. In FIGS. 1 to 5, the same parts as those in FIGS. 7 to 11 are designated by the same reference numerals.

FIG. 3 is a longitudinal sectional view of an essential part of an embodiment of the first invention of the present case. In FIG. 3, a hollow ball screw shaft 9 is coaxially housed in a cylindrical guide tube 17.

A ball screw nut 10 is screwed in the middle of the ball screw shaft 9 so as to be movable in the axial direction, and a ring-shaped latch guide 16 is coaxially fixed on the ball screw nut 10.

As shown in FIG. 1, the latch guide 16 is provided with, for example, four rollers 18a to 18d arranged at an appropriate pitch in the circumferential direction, and each roller 18a to 18d has an outer peripheral surface on the inner peripheral surface of the guide tube 17. It is inscribed and rotates in the ascending / descending direction to guide the ascending and descending of the latch guide 16.

Further, an arc plate-shaped guide rail 19 which is a guide is fixed to the inner peripheral surface of the guide tube 17 partitioned by adjacent required rollers, for example, 18a and 18d, over substantially the entire length of the guide tube 17, and the guide rail Both side surfaces of both rollers 18a and 18d are in contact with the left and right side surfaces of 19, respectively.

Therefore, the guide rail 19 guides the vertical movement of both rollers 18a, 18d in the axial direction, while the guide tube 17
The rotation of both rollers 18a and 18d in the circumferential direction is restricted.

This allows the latch guide 16 and ball screw nut
The rotation of the ball screw shaft 9 is restricted, and the rotational movement of the ball screw shaft 9 is converted into the vertical movement of the ball screw nut 10.

The latch guide 16 has a pair of diametrically opposed claw housings 14a, 14b provided upright on the outer periphery thereof, and a pair of left and right latch fingers 13a, 13b are housed in the claw housings 14a, 14b. The lower end of the hollow piston 6 is placed on the latch guide 16.

The latch fingers 13a, 13b are biased by springs 15a, 15b (see FIG. 10) so that the lower ends thereof are opened, but they are stored in the claw storage portions 14a, 14b and are closed. Since it is sandwiched, the lower end of the hollow piston 6 passes through the center hole 16a of the latch guide 16 and is merely placed on the ball screw nut 10.

Therefore, the hollow piston 6 and the control rod 3 connected to this hollow piston 6 are pushed up by the elevation of the ball screw nut 10 and ascended, and the hollow piston 6 follows and descends by its own weight due to the fall of the ball screw nut 10.

Then, as shown in FIG. 1, on the latch guide 16,
A pair of hooks 30a, 30b are provided upright to face each other in a direction substantially orthogonal to the facing direction of the pair of claw storage portions 14a, 14b.

As shown in FIG. 2, these hooks 30a, 30b have their upper ends bent inward at a substantially right angle, and as shown in FIG. 6, they are hooked by a pair of latch fingers 13a, 13b. It is formed on the stop.

Further, a taper is formed on the inner side of the upper end of each hook 30a, 30b so as to sequentially expand upward, and the hooks 30a, 30b are raised from below the pair of latch fingers 13a, 13b to abut against each other. The expanding lower ends of the latch fingers 13a and 13b are guided by the taper of the hooks 30a and 30b, introduced into the hooks 30a and 30b, and then engaged with each other.

Therefore, the latch guide 16 and ball screw nut
Rotate 10 by a required angle, for example 90 °, and hook each 30a, 30b
The hooks 30a, 30b on the latch fingers 13a, 13b by moving the latch fingers 13a, 13b to the latching positions of the latch fingers 13a, 13b.
Can be locked.

However, the rotation of the latch guide 16 requires the required rollers 18a, 18
Since it is regulated by the guide rail 19 interposed between d, the latch guide 16 cannot be rotated 90 ° in this state.

Therefore, it is necessary to retract the latch guide 16 below the lower end of the guide rail 19 and rotate the latch guide 16 by, for example, 90 ° at the position released from the regulation of the guide rail 19.

However, in order to retract the latch guide 16 to below the lower end of the guide rail 19, there is not enough space in the lower space of the CRD housing 5, and the bottom must be opened.
Moreover, if this is simply released, the reactor water in the CRD housing 5 will leak to the outside, which is inconvenient from the viewpoint of reducing the radiation exposure of the worker.

Therefore, in such a case, in order to prevent the leakage of the reactor water in the CRD housing 5 and proceed with the work, the device of the second invention of the present case is required, and one embodiment thereof is as shown in FIG. It is configured.

That is, as shown in FIG. 5, the closed casing 40, which has a cylindrical shape with a bottom and is transparently formed, has the shaft sealing portion 20 shown in FIG. It is designed to be replaced and to be detachably fixed by a bolt 40a.

As shown in FIG. 5, the axial length of the closed casing 40 has such a length that the boss 22 can be accommodated even when the rollers 18a to 18d of the latch guide 16 are set rearward below the lower end of the guide rail 19. Split seat handling device at the bottom of the closed casing 40
41 and a ball screw handling device 42 are mounted.

The split portion sheet handling device 41 has a main shaft portion 41a that vertically penetrates the bottom portion of the closed casing 40 in a watertight and rotatably manner, and at the upper end housed in the closed casing 40, at least two pairs extending in the direction perpendicular to the axis. The arms 41b and 41c are fixed.

The division sheet 23 is provided on each upper surface of the arms 41b and 41c.
Pins 41d and 41e that are fitted in the recesses that are recessed in the bottom of the are respectively planted.

A handle 41f extending in a direction perpendicular to the axis is fixed to the lower end of the main shaft portion 41a of the split portion sheet handling device 41.
By rotating the 1f, the arms 41b and 41c are rotated and the pins 41d and 41e are fitted in the recesses.
Is rotated in an appropriate direction to remove or attach the division sheet 23 from the outer peripheral flange 12.

The main shaft portion 41a of the divided portion sheet handling device 41 is coaxially rotatable by the main shaft portion 42a of the ball screw handling device 42 and penetrates in a watertight manner, and the upper end portion of the main shaft portion 42a has a boss 22a.
It is formed on a spline shaft 42b that is detachably fitted inside, and a lower end portion thereof is formed on a handle 42c.
Ball screw shaft 9 and latch guide 16
Is rotated in an appropriate direction.

 Next, the operation of each of the above embodiments will be described.

In the unlikely event that the control rod 3 inserted in the core 2 shown in FIG. 7 cannot be pulled out for some reason, first the reactor operation is stopped, and then the latch guide 16 shown in FIG. It is necessary to engage the hooks 30a, 30b with the latch fingers 13a, 13b of the hollow piston 6 and hook them.

For this purpose, it is necessary to release the engagement between the rollers 18a and 18d of the latch guide 16 and the guide rail 19 and rotate the latch guide 16 and the ball screw nut 10 by 90 °, for example.

However, such an operation needs to be performed without leaking the reactor water to the outside of the CRD housing 5 as described above, and for that purpose, it is necessary to use the split portion seat handling device 41, the ball screw handling device 42, and the like. .

Therefore, after stopping the reactor operation first, the motor 7
To rotate the ball screw shaft 9 around its axis to lower the ball screw nut 10 to the lowermost end of the ball screw shaft 9, and to divide the lower surface of the back seat 21 into the dividing parts as shown in FIG. The seat 23 is seated in a watertight manner to prevent leakage of reactor water.

In this state, the motor 7 is removed from the bottom of the CRD housing 5, and then the drive shaft 8 connected to the output shaft of the motor 7
Is pulled out from the boss 22, and the shaft sealing portion 20 shown in FIG. 8 is removed from the outer peripheral flange 12.

Next, as shown in FIG. 5, a transparent hermetic casing 40 is watertightly fixed to a position where the shaft sealing portion 20 of the outer peripheral flange 12 is removed, and the operator sees the inside from the outside of the hermetic casing 40 while dividing the divided portion. The pins 41d and 41e of the arms 41b and 41c of the seat handling device 41 are fitted into the recesses at the bottom of the split seat 23, and the handle 41f is rotated to remove the split seat 23 from the outer peripheral flange 12 to form a closed casing. Move to the bottom of 40. Reactor water leaking from the detached position of the division sheet 23 is received by the closed casing 40, and leakage to the outside is prevented.

Then, the ball screw handling device 42 is operated to fit the spline shaft portion 42b of the main shaft portion 42a into the boss 21, and the handle 42c is rotated in a required direction to operate the latch guide 16 as shown in FIG. The rollers 18a-18d are lowered below the lower end of the guide rail 19, and the ball screw nut 10 is once lowered to a position where the engagement between the rollers is released.

Next, the handle 42c is rotated by, for example, 90 ° in a required direction to rotate the hooks 30a, 30b of the latch guide 16 to the engaging position where the latch fingers 13a, 13b of the hollow piston 6 are engaged with each other. Push the handle 42c upwards,
The latch guide 16 is raised to a position where the guide rail 19 is interposed between the required two rollers 18c and 18d, and the rotation of the latch guide 16 is held by the guide rail 19 to be held.

At this time, the lower end of the hollow piston 6 connected to the control rod 3 which is not pulled out from the core 2 is suspended above the inside of the guide tube 17, and the pair of latch fingers
13a and 13b are expanded downward.

After that, the dividing sheet 23 is attached to the outer peripheral flange 12 again by an appropriate operation of the dividing sheet handling device 41, and is brought into close contact with the back sheet 21 to prevent leakage of the reactor water in the CRD housing 5. Remove the sealed casing 40.

Next, the drive shaft 8, the motor 7 and the shaft sealing portion 20 are attached to the respective attachment positions.

Then, the motor 7 is rotationally driven to raise the ball screw nut 10 and the latch guide 16, and the pair of latch fingers 13a and 13b are opened downward to extend downward, and the pair of hooks of the latch guide 16 are provided. The lower ends of the latch fingers 13a, 13b are introduced into the hooks 30a, 30b by contacting the taper of the upper ends of the latch fingers 13a, 30b to guide the introducing direction, and as shown in FIG. On each hook 30a, 3
Engage 0b and lock it.

Next, rotate the motor 7 in the reverse direction to rotate the ball screw nut 10
Is lowered, the hollow piston 6 having the latch fingers 13a, 13b hooked on the hooks 30a, 30b of the latch guide 16 and the control rod 3 connected to the hollow piston 6 are gradually lowered, and the control rod 3 is It is forcibly pulled out of the core 2.

Therefore, according to the present embodiment, even if the control rod 3 cannot be pulled out from the core 2 by any chance, the control rod 3 can be forcibly pulled out from the core 2.

Moreover, the forced withdrawal of the control rod 3 can be performed safely and reliably.

〔The invention's effect〕

As described above, according to the first aspect of the present invention, since the latch guide is provided with the latching portion that is latched to the latch finger of the hollow piston that connects the control rods, the latching portion is locked to this latch finger. The control rod can be forcibly pulled out from the core by lowering the latch guide.

Further, the second invention of the present application allows the operation required when the latch finger is engaged with the latch portion in the first invention to be performed while preventing the core from leaking from the control rod drive mechanism housing. Therefore, the first invention can be carried out safely and reliably.

[Brief description of drawings]

1 is a lateral cross-sectional view of an essential part of an embodiment of a control rod drive mechanism according to the first aspect of the present invention, FIG. 2 is a cross-sectional view taken along the line BB of FIG. 1, and FIG. In the embodiment shown in the drawing, a longitudinal sectional view of a main part showing a state in which a latch finger is fitted in a claw accommodating portion of a latch guide, and FIG. 4 shows a state in which a shaft sealing portion is removed in the embodiment shown in FIG. Partial longitudinal cross-sectional view, fifth
FIG. 6 is a longitudinal sectional view of an essential part of an embodiment of a control rod drive mechanism according to the second aspect of the present invention, and FIG. 6 shows the latch finger of the hollow piston in the embodiment shown in FIG. FIG. 7 is a vertical cross-sectional view of a main part showing a state where hooks are engaged and locked, FIG. 7 is a schematic vertical cross-sectional view showing a configuration around a core of a general BWR, and FIG. 8 is an entire conventional control rod drive mechanism. FIG. 8 is a vertical cross-sectional view showing a partially cutaway structure, FIG. 9 and FIG.
FIG. 11 is a partially enlarged vertical sectional view for explaining the operation of the conventional example shown in the figure, and FIG. 11 is a partial transverse sectional view of the conventional example shown in FIG. 3 ... Control rod, 4 ... Control rod drive mechanism, 5 ... CRD housing, 6 ... Hollow piston, 7 ... Motor, 8 ... Drive shaft, 9 ... Ball screw shaft, 10 ... Ball screw nut ,
13a, 13b …… Latch fingers, 14a, 14b …… Claw housing, 16
...... Latch guide, 17 …… Guide tube, 18a-18d…
… Roller, 19 …… Guide rail, 20 …… Outer peripheral flange, 21 …… Back seat, 22 …… Boss, 23 …… Divided seat, 30a, 30b …… Hook (hooking part), 40 …… Sealed Casing, 41 …… Separating part sheet handling device, 42 …… Ball screw handling device.

Claims (2)

(57) [Claims] 1. A ball screw nut that is screwed onto a ball screw shaft that is rotated around an axis by a motor and that moves up and down in the axial direction, and an end portion that is connected to a control rod and is mounted on the ball screw nut. A hollow piston that has a latch finger that can be expanded and is fixed to the ball screw nut, and a latch guide that has a latching part that is latched to the latch finger and the rotation of the ball screw nut. A control rod drive mechanism, wherein a guide for regulating and guiding the axial movement is built into a control rod drive mechanism housing that is fixed to the bottom of a reactor pressure vessel and contains reactor water. 2. A ball screw nut which is screwed onto a ball screw shaft which is rotated around a shaft by a motor and which moves up and down in the axial direction, and an end portion which is connected to a control rod and is mounted on the ball screw nut. A hollow piston that has a latch finger that can be expanded and is fixed to the ball screw nut, and a latch guide that has a latching part that is latched to the latch finger and the rotation of the ball screw nut. A guide for guiding the axial movement while regulating is built in the control rod drive mechanism housing that is fixedly penetrated through the bottom of the reactor pressure vessel and contains the reactor water. The ball screw shaft is placed inside a hermetically sealed casing that is detachably and watertightly installed in place of the mounting position of the shaft sealing portion of the outer peripheral flange to accommodate the reactor water in the control rod drive mechanism housing. A split sheet handling device that is seated on a backsheet that is fixed to one end to seal the reactor water and that supports and detaches the split sheet that is screwed to the outer peripheral flange, and is attached to and detached from the boss of the backsheet. A ball screw handling device for rotating the ball screw shaft to a latching position where the latching portion of the latch guide is engaged with the latch finger and is latched is provided with an operating shaft that is freely coupled. A control rod drive mechanism characterized in that each operation end portion of the ball screw handling device and the divided seat handling device is extended in a watertight manner to the outside of the closed casing.