JPH0785114B2 - Control rod - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a control rod, and more particularly to a control rod suitable for application to a boiling water reactor.

[Conventional technology]

A control rod used in a boiling water reactor has a cross-shaped cross section and has four blades. Each blade is configured by arranging a large number of neutron absorbing rods inside a U-shaped bent sheath. Each sheath is attached to a tie rod provided at the axis of the control rod. The neutron absorbing rod is not fixed in the space formed by the tie rod, the sheath, the upper support member attached to the upper end of the tie rod, and the upper surface of the velocity limiter attached to the upper end of the tie rod. They are arranged in a line. Between the upper and lower surfaces of the neutron absorbing rod and the upper support member and the upper surface of the velocity limiter, a shock absorber,
That is, a thin stainless pipe is inserted horizontally. Such a control rod is disclosed in JP-A-57-171293. This stainless steel pipe is provided to reduce the impact of the neutron absorbing rod on the upper support member and the velocity limiter when the control rod is driven during normal operation or scram.

[Problems to be solved by the invention]

JP-A-57-171293 discloses that by inserting a stainless steel pipe as a shock absorber between the neutron absorbing material and the structural materials at the upper and lower ends of the blade, the neutron absorbing rod and the structural material It reduces the gear gap that occurs in the. However, the neutron absorbing rod has a length of 4 m,
Moreover, the cladding tube and the end plug are joined by welding. Therefore, it is difficult to equalize the axial lengths of all the neutron absorbing rods arranged in one blade, and the lengths vary. That is, the length of the neutron absorbing rod has an average of 2 to 3 mm and a maximum variation of 5 to 6 mm.
Therefore, even if the stainless pipe is sized so as to absorb the thermal expansion of the neutron absorbing rod, it is difficult to reduce the gap between all the neutron absorbing rods in the blade and the stainless pipe to zero. . Even when the neutron absorbing rod is stretched due to heat rod tension during operation of the reactor, both end faces of a certain neutron absorbing rod are in contact with the upper and lower stainless pipes.The upper end face of the neutron absorbing rod is in contact with the stainless pipe. Without doing so, a maximum gap of 5 to 6 mm is formed between the upper end surface and the stainless pipe. All neutron absorber rods are in contact with the stainless pipe located at the lower end due to the action of gravity.

During the reactor scram, the control rods are rapidly inserted into the core in about 1 second and finally stopped rapidly. An extremely large acceleration is applied to each neutron absorbing rod during rapid insertion, and a gear tap is formed between the stainless pipe located at the upper end due to the rapid deceleration that leads to a rapid stop at the completion of insertion. The neutron absorbing rod violently collides with the stainless pipe. Therefore, the stainless steel pipe is crushed by one scrum operation (especially by the neutron absorbing rod having a large gear gap between the stainless steel pipe) and has a buffer function for the subsequent scrum operation. The structure material on the insertion end side of the control rod is likely to be impacted by the neutron absorbing rod. In particular, when the neutron absorbing rod is made of Hf, the stainless steel pipe is easily crushed by one scrum operation as described above.

It may be possible to attach a coil spring to each neutron absorbing rod instead of the stainless pipe and use it as a shock absorber, but the number of parts is large and the structure is complicated, and the control rod is complicated to manufacture.

The purpose of the present invention is to prevent the impact of the neutron absorbing rod on the structural material on the control rod insertion end side during scram over a long period until the control rod reaches the end of its life, and the neutron absorbing capacity of the core lower region at the time of scrum. The purpose is to supply control rods that can prevent deterioration.

[Means for solving problems]

In order to achieve the above-mentioned object, the present invention comprises a neutron absorbing rod in an absorption nuclear chain type neutron absorbing material, the neutron absorbing rod in the axial direction of the control rod insertion end side region and the control rod driving device coupling side region. It is divided into two regions, the thickness of the neutron absorbing rod in the latter region is made thinner than the thickness of the neutron absorbing rod in the former region, and the control rod insertion end side region of the neutron absorbing rod is a holding member attached to the sheath. By being engaged and supported, the axial movement of the blade is restrained and the blade is supported in a suspended state.

Further, the neutron absorbing rod is composed of an absorption nucleus chain type neutron absorbing material, and the neutron absorbing rod is axially divided into two regions of a control rod insertion end side region and a control rod driving device coupling side region, and in the latter region. The thickness of the neutron absorbing rod is made thinner than the thickness of the neutron absorbing rod in the former region, the control rod insertion end side region of the neutron absorbing rod is supported by being engaged by a holding member attached to the sheath, The neutron absorbing rod has means for restraining its axial movement at the engaging portion with the holding member.

[Action]

According to the present invention, since the neutron absorbing rod is supported while being restrained from moving in the axial direction, even when the control rod is urgently inserted into the core during scram over the life of the control rod, control of the neutron absorbing rod is performed. It is possible to prevent the end portion on the rod insertion end side from colliding with the upper holding portion which is the structural material on the control rod insertion end side.

Also, because the control rod insertion end side region, which is the large diameter part of the neutron absorbing rod, is supported, the load of the large diameter part does not act on the small diameter part, so the control rod was urgently inserted into the core during scram. Even in such a case, the neutron absorbing material filling region is less likely to be shortened due to the bending phenomenon of the small diameter portion, so that it is possible to prevent the neutron absorbing ability of the lower core region from being reduced.

〔Example〕

B ₄ C as a neutron absorber to prolong control rod life
It is considered to use hafnium instead of neutron absorber, and it is possible to replace all neutron absorbers with long-life neutron absorbers such as hafnium. However, the long-lived neutron absorber is heavier than B ₄ C which is generally used in the past, and when the control rod using only the long-lived neutron material is used as the neutron absorber, the weight of the control rod is It has been found that the number significantly increases, and the control rod operation by the control rod drive device may be hindered.

Therefore, when the Hf rod is used as a neutron absorbing rod, the inventors have attempted to reduce the weight of the control rod by thinning a part of the Hf rod arranged in the blade.

An embodiment of the present invention will be described with reference to FIGS. 1, 2, 4, and 5.

The control rod has the external shape shown in FIG. That is, the control rod 1 has a cross-shaped cross section and has four blades 2 extending in all directions from the axis. A velocity limiter 17 is provided below the blade 2. The handle 18 is attached to the upper end of the blade 12. Although not shown, the control rod is detachably connected to a control rod drive device installed in the reactor pressure vessel in the reactor pressure vessel. The lower end of the velocity limiter 17 is detachably connected to the control rod drive device. Blade 2 of control rod 1
The shape will be described with reference to FIGS. 1, 4, and 5. Each blade 2 is made of SUS placed at the axis of the control rod 1.
It is composed of a U-shaped sheath (made of SUS) 12 having both ends attached to a tie rod 11 made of neutron, and a neutron absorbing rod 3 provided in the sheath 12. The upper holding portion 6 is attached to the upper end of the tie rod 11 and the lower holding portion 7 is attached to the lower end of the tie rod 11 by welding. The upper end of the U-shaped sheath 12 surrounds the upper holding portion 6 and is attached to the upper holding portion 6 by spot welding. The lower end of the sheath 12 surrounds the lower holding portion 7 and is attached to the lower holding portion 7 by spot welding. Sheath 12
The control rod 1 is installed in the reactor, and the cooling water in the reactor flows into the sheath 12 through these holes when the control rod 1 is installed in the reactor. This cooling water cools the neutron absorbing rod 3. As shown in FIGS. 4 and 5, the neutron absorbing rod 3 is disposed inside the sheath 12 and is even retained between the upper and lower retaining portions 6 and 7 made of SUS attached to the tie rod 11. To be done. The neutron absorbing rod 3 is a hafnium metal round rod. The handle 18 is mounted on the upper holding portion 6. Velocity Limiter 17 is a
It is engaged with the lower end of 11. The lower holding portion 7 is fixed to the velocity limiter 17. In particular, the handle 18 may be used as the upper holding portion 6 without newly providing the upper holding portion 6. Further, the velocity limiter 17 may be used as the lower holding portion 7 without newly providing the lower holding portion 7.

As shown in FIG. 1, the neutron absorbing rod 3 is composed of a large diameter portion 4 and a small diameter portion 5. The large diameter portion 4 and the small diameter portion 5 are hafnium round bars. The large-diameter portion 4 and the small-diameter portion 5 have different diameters. The large-diameter portion 4 has a diameter of 4.8 mmφ and the small-diameter portion 5 has a diameter of 3.4 mmφ.
The large diameter portion 4 is arranged above the blade 2 and the small diameter portion 5 is arranged below the blade 2. The neutron absorbing rod 3 is formed by joining the lower end of the large-diameter portion 4 to the upper end of the small-diameter portion 5 with the axes of the large-diameter portion 4 and the small-diameter portion 5 aligned. 21 neutron absorbing rods 3 are loaded in one blade 2. The large-diameter portion 4 and the small-diameter portion 5 are neutron absorbing rods 3 (neutron absorbing material filled region).
It occupies one half of the axial length of. Therefore, in the blade 2 of the present embodiment, when the upper half and the lower half are compared, the amount of absorption nucleus chain type neutron absorber is smaller in the lower part. Here, the absorption nuclear chain type neutron absorbing material means a neutron absorbing material in which a nuclide that absorbs neutrons further appears in a nuclide generated by a neutron absorbing reaction or a decaying nuclide thereof and the neutron absorbing effect is slow to decay. Specific examples of the absorption nuclear chain type neutron absorber include Hf, Eu ₂ O ₃ , T
a, Dy ₂ O ₃ and Ag—In—Cd.

There are four blades 2 on one control rod. Blade 2
The large diameter part 4 of the neutron absorbing rod 3 is arranged in the upper region of the neutron because the neutron importance distribution is large, and the small diameter part 5 of the neutron absorbing rod is arranged in the lower region of the blade 2 because the neutron importance distribution is small. . In this embodiment, the boundary between the upper region and the lower region is arranged at a position half the axial length of the neutron absorber filling region from the lower end of the neutron absorber filling region of the control rod. However, when the filling amount of the absorption nuclear chain type absorber in the blade 2 is changed to two regions of the upper region and the lower region, the boundary between these regions is from the lower end of the neutron absorber filling region to the axial direction of this filling region. It may be located within the range of 3/8 to 5/8 of the total length. FIG. 3 shows the neutron importance distribution (relative value) along the axial position during reactor shutdown. In FIG. 3, the solid line shows the distribution, and the broken line shows the average value of the values in the upper and lower two regions.
From this figure, it is clear that the boundary between the upper and lower two regions should be within the range of 3/8 to 5/8 of the total length.

That is, the neutron importance distribution in the axial direction of the core of the boiling water reactor shutdown diagram is as shown in FIG. The neutron importance distribution at the time of this reactor shutdown is
It has a peak in the upper part of the core.

A control rod having an upper region with a large amount of hafnium and a lower region with a small amount of hafnium as shown in JP-A-59-192992, specifically, a large diameter portion in the upper region as shown in FIG. When a control rod equipped with a hafnium rod having a small diameter portion in the lower region is completely inserted into the core and the reactor is stopped, one core rod with the maximum value is not inserted into the core. The effective multiplication factor of changes with respect to the control rod where the position of the boundary between the upper area and the lower area is different,
It is shown in FIG. The characteristics shown in FIG. 11 were obtained in a nuclear reactor that produces a neutron importance distribution as shown in FIG. 3 during a reactor shutdown. When the effective multiplication factor Keff is 1, the reactor enters a critical state.

The horizontal axis of FIG. 11 shows a large amount of neutron absorbing material in the transverse section perpendicular to the axis of the neutron absorbing material-filled area in which the neutron absorbing material is arranged. The position of the boundary with the lower region, that is, the position of the boundary from the lower end of the neutron absorber filling region is shown. Zero is the lower end of the neutron absorber filled region and 8/8 is the upper end of the neutron absorber filled region. The vertical axis of FIG. 11 represents the effective multiplication factor.

Even if one control rod is not inserted as described above, the effective multiplication factor Keff should be set so that the reactor will not become critical due to external disturbances.
It is desirable to set it to 0.99 or less. When the boundary between the upper region and the lower region is located more than 5/8 of the axial total length of the neutron absorber filling region from the lower end of the neutron absorber filling region, the neutron importance of the neutron importance is shown in the upper region shown in FIG. Despite the presence of the peak, the amount of hafnium in the upper region decreases and the effective multiplication factor sharply increases. this is,
This means that the reactor shutdown margin is sharply reduced, and the stability of the reactor at the time of reactor shutdown will be impaired. Further, the boundary between the upper region and the lower region is 3/8 of the axial total length of the neutron absorber filling region from the lower end of the neutron absorber filling region.
When placed below, the effective multiplication factor changes due to the relationship with the neutron importance distribution shown in Fig. 3 when the boundary is placed at 3/8 of the axial total length. do not do. This means that the reactor shutdown margin does not increase even if the boundary between the upper region and the lower region is arranged too downward, and this only increases the weight of the control rod.
The increased weight of the control rod makes it difficult to operate the control rod with the control rod drive. In particular, the absorption nuclear chain type neutron absorber such as hafnium is significantly heavier than B ₄ C, and reducing the weight of the control rod is also a major issue.

As described above, in the control rod using the neutron absorber composed of the absorption nuclear chain type neutron absorber such as hafnium, the boundary between the upper region and the lower region is the neutron absorber from the lower end of the neutron absorber filling region. 3/8 to 5/8 of the total axial length of the filling area
Must be placed in the range (range Ra in Fig. 11). By arranging the boundary between the upper region and the lower region in the above range, the control rod using the neutron absorber composed of absorption nuclear chain type neutron absorber corresponds to the neutron importance distribution during reactor shutdown As a result, an effective neutron absorption capacity can be obtained while the reactor is shut down, and the weight can be operated by the control rod drive device.

The control rod shown in FIG. 4 of JP-A-59-192992 has a neutron absorbing rod composed of a solid portion of hafnium and a hollow portion of hafnium. The boundary between the solid part and the hollow part is located at the position 3/4 of the axial total length of the neutron absorbing rod from the lower end of the neutron absorbing rod. Such a control rod has a small neutron absorption capability when the reactor is shut down.

In this embodiment, since the neutron absorbing rod 3 including the large diameter portion 4 and the small diameter portion 5 is arranged in the blade 2 as described above,
The weight of the control rod 1 is almost the same as that of the control rod composed of the neutron absorbing rod filled with B ₄ C. Therefore, the control rod 1 of this embodiment can be easily operated by the control rod drive device for operating the conventional control rod. That is, good operability of the control rod 1 can be obtained. In addition, the blade 2 of the control rod 1
Has the same width as a conventional control rod. For this reason, it is not necessary to make changes such as enlarging the fuel rod fittings installed on the control rod guide tubes and the core support plate provided below the core in the reactor pressure vessel.

The neutron absorbing rod 3 uses Hf as a neutron absorbing material. Hf, which is an absorption nuclear chain type neutron absorber, does not generate gas even when absorbing neutrons. Therefore, the mechanical life of the neutron absorbing rod 3 of the control rod 1 is increased. Moreover, since the neutron absorbing rod 3 uses Hf which is an absorption nucleus chain type neutron absorbing material, new neutron absorbing material is generated by absorbing neutrons,
Nuclear life is also increased. Here, the diameter of the large diameter portion 4 of the neutron absorbing rod 3 is almost equal to the diameter of the neutron absorbing rod filled with B ₄ C provided in the conventional control rod used in the boiling water reactor. The axial length of the neutron absorbing rod 3 is substantially equal to the active fuel length of the fuel assembly, like the neutron absorbing rod of the conventional control rod. The active fuel length of a fuel assembly is the axial length of the region filled with fuel pellets.

With such a structure, the weight of the control rod is suppressed to the same level as that of the conventional control rod, and the reactivity control capability of the control rod becomes approximately the same as that of the conventional control rod. In the lower area, it has tripled.

Next, a structure for holding the neutron absorbing rod 3 will be described. The large diameter portion 4 of the neutron absorbing rod 3 is supported by the holding member 13 fixed to the sheath 12 by spot welding. Therefore, the neutron absorbing rod 3 is supported in the sheath 12 in a suspended state (suspended state) without the both ends contacting the upper holding part 5 and the lower holding part 6. The support structure of the neutron absorbing rod 3 will be described in detail with reference to FIGS. 6 and 7.

The holding member 13 is composed of a pair of holding portions 13A and 13B. The holders 13A and 13B have semicircular notches 15 as many as the number of neutron absorbing rods 3 in one blade 2. The holding portions 13A and 13B are arranged above the blades 2 and in the sheath 12 such that the notches 15 face each other in the width direction of the blades 2 (direction perpendicular to the axis of the control rod 1). . The holding portions 13A and 13B are made of SUS like the sheath 12, and are attached to the sheath 12 by spot welding. 15 is the spot weld. In a part of the large diameter part 4 of the neutron absorbing rod 3,
A small diameter portion 4A having a diameter slightly smaller than that of the large diameter portion 4 is formed. This small-diameter portion 4A is a holding portion 13A that is arranged facing each other.
And 13B are arranged in the cutout portion 14. Therefore, each neutron absorbing rod 3 is supported by the holding member 13 at the large diameter portion 4 and is suspended. Between the upper end of the neutron absorbing rod 3 and the upper holding part 6 and between the lower end of the neutron absorbing rod 3 (the end on the control rod drive device connection side) and the lower holding part 7, due to thermal expansion in the reactor. A gap 17 for absorbing the axial extension of the neutron absorbing rod 3 is formed.

Although it is conceivable that the small diameter portion 5 is supported by the holding member 13 and the neutron absorbing rod 3 is suspended in the sheath 12, it is preferable to support the large diameter portion 5 by the holding member 13 in terms of strength.

In this embodiment, since the neutron absorbing rod 3 is engaged with the holding member 13 fixed to the sheath 12 as described above, even when the control rod 1 is rapidly inserted into the core during scram of the reactor, It is possible to completely prevent the end portion (upper end portion in FIG. 1) of the neutron absorbing rod 3 on the control rod insertion end side from colliding with the upper holding portion 6. That is, the small-diameter portion 4A provided in the middle of the large-diameter portion 4 and the notch 14 of the holding member 13 are engaged with each other so that the upper end of the large-diameter portion 4 below the small-diameter portion 4A is the lower surface of the holding portions 13A and 13B. Since they are in contact with each other, it is possible to completely prevent the neutron absorbing rod 3 from moving upward due to sudden deceleration immediately before the completion of the scrum, and to prevent a collision between the end portion of the neutron absorbing rod 3 on the insertion end side and the upper holding portion 6. it can. Since the engagement state between the small diameter portion 4A and the lower support portion 7 is maintained for the life of the control rod 1, the collision between the neutron absorbing rod 3 and the upper support portion 6 causes the control rod 1 to reach the life. Cannot happen over a long period of time.

The engagement between the neutron absorbing rod 3 and the holding member 13 as described above is
The problems peculiar to the control rod 1 including the neutron absorbing rod 3 having the large diameter portion 4 in the upper portion and the thin system portion 5 in the lower portion can be solved. When the neutron absorbing rod 3 including the large diameter portion 4 and the small diameter portion 5 is installed in the blade 2, the large diameter portions 4 of the adjacent neutron absorbing rods 3 are in contact with each other as shown in FIG. However, a gap 16 as shown in FIG. 1 is formed between the small diameter portions 5 of the adjacent neutron absorbing rods 3. Therefore, if the lower end of the small diameter portion 5 of the neutron absorbing rod 3 is not engaged with the lower support portion 7, the neutron absorbing rod 3 will move upward during the rapid deceleration of the control rod during the reactor scram and the upper portion While colliding with the holding part 6, the small diameter part 5 of the neutron absorbing rod 3 is curved as shown in FIG. 8 due to the inertial force at the time of the collision, and the lower end of the small diameter part 5 is lifted upward by a dimension H. To be Therefore, when the neutron absorbing rod 3 and the upper holding portion 6 collide with each other, the neutron absorbing material filling region is temporarily moved upward and has a dimension H.
Therefore, the neutron absorption capacity of the lower part of the core during scram is reduced. In this embodiment, as shown in FIG. 6 and FIG. 7, the neutron absorbing rod 3 and the holding member 13 are engaged with each other and the load of the large diameter portion 4 does not act on the small diameter portion 5. In the rapid insertion operation of the control rod, the bending phenomenon of the small-diameter portion 5 shown in FIG. 8 is unlikely to occur, and the neutron absorbing material filling region is also unlikely to be shortened. Therefore, in this example, the neutron absorption in the lower part of the core region is not hindered during scram.

The gap in the width direction of the blade 2 of the small diameter portion 5 due to the formation of the gap 16 between the adjacent small diameter portions 5 can be prevented by inserting a spacer (not shown) into the gap 16. . Further, the movement of the small-diameter portion 5 in the direction perpendicular to the side surface of the blade 2 can be prevented by the protrusion 8 provided on the sheath 12. The lowest protruding portion 8 is preferably provided as close to the lower holding portion 7 as possible.

In this embodiment, the thickness of the absorption nuclear chain type neutron absorber material region in the direction perpendicular to the outer surface of the blade 2 is different in the axial direction of the blade 2. That is, the thickness of the absorption chain neutron absorber in the direction perpendicular to the side surface of the blade 2 is thick in the upper region of the blade 2 where the large diameter portion 4 of the neutron absorbing rod 3 having a large diameter is present, and the neutron absorbing material having a small diameter is absorbed. It is thinned in the lower region of the blade 2 where the small diameter portion 5 of the rod 3 exists.

The sheath 12 has protrusions 8 protruding inward on both side surfaces. These protrusions 8 are formed by projecting a part of the structural material of the sheath 12 inward. As shown in FIG. 1 and FIG. 2, the protrusion 8 is a region where the thickness of the absorption nuclear chain type neutron absorber in the direction perpendicular to the side surface of the blade 2 is thin, that is, the small-diameter portion 5 of the neutron absorbing rod 3. It is provided in the lower region of the arranged control rod 1. Neutron absorption rod 3
The small-diameter portion 5 is sandwiched by two opposing protruding portions 8 provided on both sides of the sheath 12. Such a pair of protrusions 8 are provided in the lower portion of the blade 2 at several positions in the longitudinal direction thereof.

Since the small-diameter portion 5 of the neutron absorbing rod 3 is held by the protruding portion 8 protruding on the inner surface of the side surface of the sheath 12, the small-diameter portion 5 of the neutron absorbing rod 3 is located inside the sheath 12, that is, on the side surface of the sheath 12. It is possible to prevent the blade 2 from moving in a direction perpendicular to the side surface. Therefore, while the control rod 1 is being inserted into the reactor, the small diameter portion 5 of the neutron absorbing rod 3 moves in the sheath 12 in the thickness direction of the blade 2 due to an earthquake, flow vibration of cooling water, or the like. However, there is no possibility of affecting the reactivity control. Further, the provision of the protrusions 8 capable of suppressing such fluctuations in reactivity leads to an increase in bending rigidity of the sheath 12, and the thickness of the sheath 12 can be reduced. This can also reduce the weight of the control rod 1.

Further, in this embodiment, the holding portions 13A and 13B are provided inside the sheath 12.
Since the is attached, the outer bulge of the sheath 12 can be prevented. In particular, the bulging of the sheath 12 to the outside can be prevented more reliably by connecting the holding portions 13A and 13B with screws or welding.

Holding portion 13 in which the radius of the notch portion 14 matches the radius of the small diameter portion 5.
By disposing A and 13B at the lower part of the blade 2, that is, at the position of the small diameter portion 5 of the neutron absorbing rod 3 and attaching it to the sheath 12, the width direction of the blade 2 of the small diameter portion 5 and the side surface of the blade 2 The movement in the direction perpendicular to can be restricted.

Another embodiment in which the neutron absorbing rod 3 is held in a suspended state in the blade 2 will be described below with reference to FIGS. 9 and 10. FIG. 9 shows a portion corresponding to FIG. 7 described above.

The embodiment shown in FIG. 9 is the holding member 13 shown in the above-mentioned embodiment.
Is replaced with a holding member 13C. Holding member 13C is 1
The holding member 13C has a cutout portion 14A extending in the direction perpendicular to the side surface of the blade 2 from the center of the holding member 13C. The number of notches 14 is equal to the number of neutron absorbing rods 3. Small diameter part of the total neutron absorption rod 3 arranged in the sheath 12
4A is inserted into each notch 14A. The holding member 13C is
Both side surfaces are fixed to the sheath 12 at the spot welds 15.

The control rod of this embodiment can also obtain the same effects as those of the above-described embodiments. Further, the holding member 13C is made of one member and is easier to manufacture than the above-mentioned holding member 13.

In the embodiment shown in FIG. 10, the end portion of the neutron absorbing rod 3 on the control rod insertion end side is suspended, that is, hung, from the upper holding portion 6 (or the handle 18) located on the control rod insertion end side. It is a thing. The control rod insertion end is one end of the control rod that is first inserted into the core, and is located at a position opposite to the other end of the control rod, which is one end on the control rod drive device connecting side. A groove 20 having an inverted L-shaped cross section is continuously provided in the upper holding portion 6A in the width direction of the blade 2, and the groove 20 is provided at the upper end portion of the neutron absorbing rod 3A (upper end portion of the large diameter portion 4). L-shaped engaging part 21
Is inserted to engage the upper holding portion 6A and the neutron absorbing rod 3A. In the present embodiment, the small diameter portion 4A is not provided in the large diameter portion 4 as in the above embodiments. However, the neutron absorbing rod 3A is made of Hf, and like the neutron absorbing rod 3, the large diameter portion 4
And a small-diameter portion 5.

Also in this embodiment, since the neutron absorbing rod 3A is suspended in the sheath 12 (suspended state), the same effect as that of the embodiment of FIG. 1 can be obtained.

The support structure of the neutron absorbing rod of each of the above-mentioned embodiments is also a neutron absorbing rod composed of an absorption nucleus chain type neutron absorbing material, and also for supporting a neutron absorbing rod having a uniform cross-sectional area in the axial total length. Applicable.

〔The invention's effect〕

According to the present invention, it is possible to prevent the impact of the neutron absorbing rod on the structural member on the control rod insertion end side during scram for a long period until the control rod reaches the end of its life. Further, it is possible to prevent a decrease in the neutron absorption capacity in the lower core region during scram.

[Brief description of drawings]

FIG. 1 is a partial sectional view of a control rod which is a preferred embodiment of the present invention, FIG. 2 is an external view of the control rod of FIG. 1, and FIG. 3 is a characteristic diagram showing an axial distribution of neutron importance. 4 is a sectional view taken along line IV-IV in FIG. 2, FIG. 5 is a sectional view taken along line VV in FIG. 2, FIG. 6 is a sectional view taken along line VI-VI in FIG. 1, and FIG. 7 is shown in FIG.
VII-VII sectional view, FIG. 8 is an explanatory view showing a bending phenomenon which occurs in the small diameter portion of the neutron absorbing rod due to the control rod operation at the time of reactor scram, and FIGS. 9 and 10 are other diagrams of the present invention. FIG. 11 is a structural diagram showing a supported state of the neutron absorbing rod in the example, and FIG. 11 is a characteristic diagram showing a change in effective multiplication factor with a change in the position of the boundary between the upper region and the lower region in the neutron absorbing rod. 1 ... control rod, 2 ... blade, 3,3A ... neutron absorbing rod, 4 ... large diameter part, 4A, 5 ... small diameter part, 6 ... upper holding part, 7 ... lower holding part, 12 …… Sheath, 13,13C …… Holding member.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Zenichiro Endo 3-1-1, Saiwaicho, Hitachi-shi, Ibaraki Hitachi Ltd. Hitachi factory (56) Bibliography Sho 60-8895 (JP, U)

Claims (3)

[Claims] 1. A first support portion located on the control rod insertion end side,
A control rod having a blade comprising a second support portion located on the control rod drive device connection side and a sheath having both ends attached to the first and second support portions and surrounding the neutron absorption rod, wherein the neutron absorption The rod is composed of an absorption nuclear chain type neutron absorber, and the neutron absorbing rod is axially divided into two regions, a control rod insertion end side region and a control rod driving device coupling side region, and the neutron absorption in the latter region. The thickness of the rod is made thinner than the thickness of the neutron absorbing rod in the former region, and the control rod insertion end side region of the neutron absorbing rod is engaged and supported by a holding member attached to the sheath. The control rod is restrained from moving in the axial direction thereof and is supported in a suspended state in the blade. 2. The axial length of the control rod insertion end side region of the neutron absorbing rod is 3/8 to 5/8 of the total length of the neutron absorbing rod.
The control rod according to claim 1. 3. A first support portion located on the control rod insertion end side,
A second support portion located on the control rod drive device connection side, a plurality of neutron absorbing rods arranged between the first support portion and the second support portion, and both end portions of the first and second support portions. In the control rod consisting of a sheath surrounding the periphery of the neutron absorbing rod is attached, the neutron absorbing rod is composed of absorption nuclear chain type neutron absorbing material, the control rod insertion end side region in the axial direction of the neutron absorbing rod And a control rod driving device connecting side region, the thickness of the neutron absorbing rod in the latter region is made thinner than the thickness of the neutron absorbing rod in the former region, and the control rod of the neutron absorbing rod is The insertion end side region is engaged and supported by a holding member attached to the sheath,
A control rod, characterized in that it has means for restraining axial movement of the neutron absorbing rod at an engaging portion with the holding member.