JPS63271194A - Control rod for nuclear reactor - Google Patents

Abstract

PURPOSE:To enable use of control rods for a long-life type nuclear reactor without changing the design of a control rod driving mechanism by decreasing the total amt. of the Hf to be inserted into sheaths so that the weight of the control rods for the nuclear reactor is reduced. CONSTITUTION:The thin-wall sheaths 15 made of a high-purity stainless steel having a U-shaped section are fixed to the respective projecting legs of a central tie rod 14 having a cruciform cross section and rod-shaped long-life type neutron absorbers consisting of Hf metallic rods 18 are inserted therein. The Hf average density of plural pieces of rod-shaped Hf 18 to be inserted from the terminal side of the control rods into an insertion space 25a in the range of 3/4 the overall length of a rod-shaped Hf insertion space 25 except the side end parts thereof is decreased to the density smaller than the Hf average density of the rod-shaped Hf 18 to be inserted into the insertion space except said insertion space by combining the central Hf rods 18a and hollow Hf rods 18b. The longer life and lighter weight of the control rods are thereby realized while an allowance for stopping the nuclear reactor is assured. The use of the driving mechanism for the control rods without changing the design thereof is thus enabled.

Description

[Detailed Description of the Invention] [Object of the Invention] (Industrial Application Field) The present invention relates to a control rod for a nuclear reactor that adjusts and controls the reactor power of a nuclear reactor, particularly for a long-life 11m water type nuclear reactor. related to control rods.

(Prior art) Nuclear reactor control rods have a central tie rod with an elongated U? It is constructed by loading a large number of neutron absorption rods into a plurality of wings formed by attaching a shaped sheath. The neutron absorption rod is 5
The cladding tube is filled with boron carbide (84C) powder, and partitions are placed at regular intervals within the cladding tube to prevent powder movement.

The 84C filled in the neutron absorption rod absorbs neutrons and gradually loses its neutron absorption ability. Meanwhile, boron-10 ("B) reacts with the neutrons to generate He gas and reduce the pressure inside the cladding tube. The life determined by the neutron absorption capacity is called the nuclear life, and the life determined by the gas pressure inside the cladding tube is called the mechanical life.

By the way, control rods that are moved in and out of the core of a nuclear reactor are not uniformly irradiated with neutrons; for example, the side edges and upper ends of each wing are exposed to strong neutron irradiation. For this reason, the neutron-absorbing rods near the side edges and top ends of each wing of the control rod absorb a large amount of neutrons, and therefore reach their nuclear lifetime earlier than the other neutron-absorbing rods. As a result, the control rod had to be disposed of as radioactive waste, even though the other neutron-absorbing rods still had sufficient nuclear lifespan.

In order to solve such problems, the present applicant has developed an AILJ control rod for nuclear reactors in which a long-life neutron absorber with a long nuclear life is placed near the side edge of the wing, which is exposed to strong neutron irradiation. This reactor control rod is published in Japanese Patent Application Publication No. 53-74697.
It is disclosed in the publication No.

However, the reactor control rod disclosed in JP-A-53-74697 has a lifespan only about twice that of a normal type fIIJIll rod, and is not necessarily satisfactory in terms of extending the lifespan of the control rod. There wasn't.

In order to address this problem of longer life, the present applicant has developed a control rod for nuclear reactors of a much superior long life type. This 1i11tll rod for nuclear reactor is published in Japanese Patent Publication No. 58-55887.
As disclosed in the publication, each wing of the Ill Ill rod is loaded with a solid neutron absorbing plate made of a long-life neutron absorbing material. This neutron absorption plate is designed to remove as much plate material as possible in areas where the axial distribution of reactor shutdown margin is small, and conversely to remove a large amount of plate material in areas where it becomes large. has been established.

(Problem to be solved by the invention) However, this control rod for a nuclear reactor uses an expensive hafnium (Hf) metal plate with a high specific gravity as a neutron absorbing material, so the IQ is very large and the control rod is expensive and has a large specific gravity. At the same time, the design of the control rod drive i side that handles this control rod needed to be changed in terms of its weight resistance, and the conventional lIJ rod drive mechanism could not be used as is.

On the other hand, there is still room for deletion of the hafnium metal plate, which is a long-life neutron absorbing material used in $11,611 rods for nuclear reactors, in terms of FF1M, and if it is possible to reduce the weight of nuclear reactor control rods by removing excess metal, the control rod Subsequent studies confirmed that the drive mechanism could be used without any design changes.

The present invention has been made in consideration of the above-mentioned circumstances, and by reducing the total amount of hafnium inserted into the sheath and reducing the weight of nuclear reactor control rods, 1ilIIj rod drive
The purpose is to provide a long-life nuclear reactor control rod that can be used without changing the design of the mechanism.

[Structure of the invention]

(Means for Solving the Problems) In the nuclear reactor control rod according to the present invention, the tip structural member and the terminal structural member are connected by a central tie rod, and a sheath is fixed in the axial direction to each protrusion of the central tie rod. In the control rod for a nuclear reactor, in which a long-life neutron absorber is inserted into the sheath, the long-life neutron absorber is composed of a plurality of rod-shaped hafnium rods, and a rod-shaped hafnium insertion space is inserted from the control rod end side. The average hafnium density of the plurality of bar-shaped hafnium rods inserted into the insertion space ranging from 3/4 to 2/3 of the total length, excluding the side ends thereof, is:
This is lower than the average hafnium density of rod-shaped hafnium inserted into an insertion space other than the insertion space.

(Function) In the control rod for a nuclear reactor according to the present invention, a plurality of rod-shaped hafnium rods are inserted as long-life neutron absorbing bodies in the sheath, so that a long life of the control rod can be achieved.

In addition, the hafnium average of multiple rod-shaped hafnium rods inserted into the insertion space within the range of about 3/4 to 2/3 of the total length of the rod-shaped hafnium insertion space from the end side of the control rod, excluding the side ends thereof. Since the density is reduced b from the average hafnium density of the rod-shaped hafnium inserted into an insertion space other than the insertion space, the total amount of hafnium in the f, II Ill gold rod is reduced. Therefore, the light a of the υ control rod is
can be achieved.

(Example) Hereinafter, an example of a control ill rod for a nuclear reactor according to the present invention will be described with reference to the accompanying drawings.

FIG. 1 is an overall perspective view showing an embodiment of a υJIB rod for a nuclear reactor according to the present invention, and this control rod 10 for a nuclear reactor has a tip structure member 12 provided with a handle 11 and an end structure member 13 integrally. It has a central tie rod 14 having a cruciform cross section. A high purity stainless steel sheath 15 with a deep U-shaped cross section is fixed to each protruding leg of the central tie rod 14 to form a wing 16. A rod-shaped long-life neutron absorber 18 made of a hafnium (Hf) metal rod is inserted into the sheath 15 .

In the control rod 10 for a long-life nuclear reactor, the tip structural member 12 is subjected to extremely high neutron irradiation, and there is a possibility that this neutron irradiation may cause brittleness. problem is alleviated. In addition, the tip structure material 12, the end structure material 13, this end structure material 1
The speed limiter 22 attached to 3 is made as thin as possible and lightweight. Further, in the nuclear reactor control rod 10 of this embodiment, a gap 23 is formed in the lower part of the tip structure member 12, and this gap 23 is configured as an auxiliary handle part. The auxiliary handle part 23 is a part that does not require neutron absorbing material in terms of neutron absorption performance, and the void 2 of the auxiliary handle part
3, the reactor control rod 10 is further reduced in weight.

On the other hand, it has been experimentally confirmed that the fast neutron irradiation G on the upper part of the auxiliary handle is about 1/3 to 1/5 or less than the fast neutron irradiation G on the upper part of the handle. From this, the embrittlement of the auxiliary handle part 23 due to neutron irradiation is about 1/3 to 115 or less than the embrittlement of the handle part. It has a handle backup function. The auxiliary handle 23 is not necessarily required and may not be provided depending on the construction of the reactor control rod and the conditions of use.

In addition, the sheath 15 fixed to the central tie rod 14 is
It is made as thin as possible without losing its strength, and the weight of the 1 blJm rod 10 for a nuclear reactor is reduced.

FIG. 2 is a tlI7i plane view showing the nuclear reactor control rod of the above embodiment. The left half of this sectional view shows the insertion space 25 for the rod-shaped hafnium 18 in the sheath 15, and the right half shows the insertion space 25 for the rod-shaped hafnium 18 in the sheath 15, and the hafnium average density of the rod-shaped hafnium 18 to be inserted. They are conceptually divided and shown.

In this embodiment, in the insertion space 25a that covers about 3/4 of the total length of the rod-shaped hafnium insertion space 25 from the control rod end side,
The average hafnium density of the plurality of rod-shaped hafnium 18 inserted into the insertion space excluding the side ends is lower than the hafnium average density of the rod-shaped hafnium 18 inserted into the insertion space other than the insertion space. Therefore, the distribution of the reactivity effect (neutron absorption characteristics) in the control rod axis direction is as shown in FIG. That is, the rod-shaped hafnium insertion space 2
If the total axial length of the control rod of No. 5 is L, then the reactivity effect in the range of 3/4L from the end of the control rod is much lower than the reactivity effect in the range of 3/4L.

4 and 5 (A) and (B) show the rod-shaped hafnium 1 inserted into the rod-shaped hafnium insertion space 25 shown in FIG.
FIG. 8 is a diagram showing an example of No. 8;

Among the rod-shaped hafnium 18 shown in FIG. 4, the cross section of the rod-shaped hafnium 18 in the 3/4 L mustard range is as shown in FIG. 5(A).

The rod-shaped hafnium rod 18 is constructed by combining a solid hafnium rod 18a and a hollow hafnium rod 18b. Each hafnium bar 18a, 18b has substantially the same width, that is, an outer diameter, and the outer diameter is formed to be substantially the same as the wing thickness direction dimension of the insertion space 25b formed by being surrounded by the sheath 15.

In other words, within the sheath 15 each hafnium rod 18a,
18b are arranged densely at equal pitches, and the hafnium rods 18a
, 18b within the sheath 15 is restricted. The hollow hafnium rods 18b may have the same wall thickness, or may have different wall thicknesses.

Further, a rod-shaped member (hereinafter referred to as a stiffener) 26 made of metal or the like is provided in the center of the insertion space 25b, and the opposing surfaces of the sheath 15 are fixed to each other via the stiffener 26. The reason why the opposing surfaces of the sheath 15 are fixed to each other in this way is that the sheath 15 is thinned to almost its strength limit, so that the U-shaped sheath 15 opens up and the hafnium rods 18a, 18b are arranged in a similar manner. This is to maintain strength so that the control rods will not be disturbed when withdrawing from the core due to turbulence and thickening of the blade portions.

As shown in FIG. 4, the rod-shaped hafnium 18 inserted into the insertion space 25b has a shortened insertion tip at the inner end of the wing corresponding to the auxiliary handle portion 23. This part has almost no effect in terms of reactivity, and due to the presence of water as a moderator in the auxiliary handle part 23, the combustion of the nuclear fuel adjacent to this part progresses, so when the control rod is withdrawn, The current customer (hereinafter referred to as blade history effect) where the output of the fuel portion abnormally increases is alleviated.

Further, the rod-shaped hafnium 18 of the auxiliary handle portion 23 is
Is it possible to reduce the weight of control rods because of the savings? It helps in the development of

Moreover, at the insertion tip of the rod-shaped hafnium 18, the rod-shaped hafnium 18 is alternately shortened. The reason why the lengths of the control rods are alternately shortened is that the average hafnium angle of 7 degrees in this part is reduced and the neutron absorption capacity is weakened, which increases the neutron flux in this part and causes a sudden increase in the length when the control rod is withdrawn. This is because the increase in fuel output is alleviated (follower effect or gray nose effect). Since the average hafnium density in that part is reduced, the lff1 quantity of the control rod can be quantified.

A solid hafnium rod 18a is inserted into the wing inner end and the wing outer end of the insertion space 25b to secure the hafnium base of these parts.

Since these Is components have a particularly high neutron irradiation dose, more hafnium material is arranged to prevent a decrease in neutron absorption ability. In addition, since the range from 3/4L to L is a region where it is necessary to increase the reactivity, a large amount of hafnium material is placed in this range to effectively increase the reactivity.

On the other hand, among the rod-shaped hafnium 18 shown in FIG.
The cross section of the rod-shaped hafnium 18 within 3/4L from the end of the rod is as shown in FIG. 5(8).

The rod-shaped hafnium rod 18 is constructed by combining a solid hafnium rod 18a and a hollow hafnium rod 18b, similar to the case shown in FIG. 5(A). And this insertion space 25
The average hafnium density of the plurality of hafnium rods 18 inserted into the insertion space excluding the side ends of a is reduced.

That is, solid hafnium rods 18a are disposed at the outer end of the wing and the inner end of the wing of the insertion space 25a, and hollow hafnium rods are provided in the insertion space other than the outer and inner ends of the insertion space 25a. 18b is provided. That is, since the hafnium in the center of the hollow hafnium rod 18b is removed, the average density of hafnium is lower than that of the solid hafnium rod 18a.

Further, a stiffener 26 is provided in the insertion space 25a as well as in the insertion space 25b, and the opposing surfaces of the sheath 15 are fixed to each other.

The hollow hafnium rod 18b, as shown in FIG.
When the solid hafnium rod 18a is disposed on the side of the insertion space 25b, it is joined to the solid hafnium rod 18a. This joining is carried out, for example, by shaving the joining end of the solid hafnium rod 18a and fitting it into the hollow portion of the hollow hafnium rod 18b. By joining them in this way, work efficiency during assembly of the control rods will be improved.

Furthermore, if the joint parts are fixed and integrated by spot welding or the like, positional displacement will be less likely to occur during use in a nuclear reactor, and stability will be improved.

Further, the hollow hafnium rod 18b is provided with a large number of water passage holes through which water as a moderator passes.

In this embodiment, since the control rod insertion end side of the hollow hafnium rod 18b is shortened, the hollow hafnium rod 1
The dynamic pressure of the water flowing out inside the hollow hafnium rod 18b changes, causing an action (Bernoulli effect) to suck out the water inside the hollow hafnium rod 18b.

Next, the action of the aIII111 rod for nuclear reactors will be explained.

In a boiling water reactor, the axial fission nuclide concentration distribution curve A of a reactor core selected to have a certain degree of combustion is typically expressed as shown in FIG.

At the bottom of the reactor core, the progress of combustion is delayed during combustion, so
The fission nuclide concentration value is increasing.

Further, when the axial length of the reactor core is [,], from the center portion (2/41) to the upper end, a hardening phenomenon of the neutron spectrum occurs due to generated air bubbles (voids).

As a result, the plutonium production reaction (neutron capture reaction) is promoted, and the thermal neutron flux decreases due to the generated voids, and this decrease causes a combustion delay, resulting in the fission nuclide concentration distribution as shown in Figure 6. expressed.

When the nuclear fission nuclide concentration shown in FIG. 6 exists in the reactor core, the neutron multiplication factor at the time of reactor shutdown becomes the axial distribution curve B shown in FIG. 7.

The larger the value of the neutron multiplication factor, the smaller the reactor shutdown margin and the shallower the subcriticality. Figure 7 shows that the neutron multiplication factor decreases at the lower and upper ends of the reactor core. This is a phenomenon caused by neutron leakage.

FIG. 8 is an axial neutron irradiation dose distribution curve C of the nuclear reactor control rod 10 when the nuclear reactor control rod 10 according to the present invention is used. From this distribution curve C, it can be seen that the reactor control rod 10 is located at the upper end pole (limited area (usually 30 cm from the tip!
There is a region where the neutral dried fish o4ffi rapidly increases at around R). The other portions continuously and smoothly decrease toward the lower end of the reactor control rod 10.

In the above embodiment, the structure is such that a satisfactory control effect can be obtained for the neutron multiplication factor characteristics shown in FIG. 7 and the neutron irradiation amount characteristics shown in FIG. That is, the rod-shaped hafnium 18 inserted into the rod-shaped hafnium insertion space 25b in the range from 3/4L to L of the rod-shaped hafnium insertion space 25 has a high proportion of solid hafnium 18a, that is, a high average density of hafnium, as shown in FIG. As shown in Figure 2, the reactivity effect is enhanced. Therefore, from 3/4L to L
The measures are taken against the fact that the neutron multiplication factor rises in the range of (that is, the margin for reactor shutdown becomes smaller) and that the neutron irradiation dose increases and the margin for reactor shutdown tends to decrease.

Moreover, the rod-shaped hafnium 18 inserted into the rod-shaped hafnium insertion space 25a within a range of 3/4L from the control rod end side is
The proportion of hollow hafnium rods 18b is high, that is, the average density of hafnium is low, and the reactivity effect is reduced, as shown in FIG. This is because, as shown in Figures 7 and 8, the neutron irradiation amount and neutron multiplication rate are relatively small in the range of 3/4L from the end of the control tube insertion.
In response to this, hafnium m is reduced as much as possible.

By the way, the range in which the neutron multiplication factor increases as shown in FIG. 7 is not constant, but has been shifting downward due to recent measures to improve the distribution of reactor shutdown margins in the field of reactor fuel design.

Therefore, the range in which the average hafnium density is reduced needs to be determined flexibly depending on the state of the range in which the neutron multiplication factor increases. Therefore, if the range where the neutron multiplication factor increases shifts downward, the range for reducing the average hafnium density should be changed from 3/4L to 2/3L from the end of the control shaft to increase the reactor shutdown margin. It is necessary to choose within the limits that can be secured.

Figure 9 shows a typical neutral dried fish DJ (i1 distribution curve) in the width direction of each wing 16 of the reactor control rod 10. higher, inside the wing (center tie rod 14)
side) is slightly higher. In this embodiment, the solid hafnium rods 18a are disposed at both ends of the wing 16, and the number of hollow hafnium rods 18b in the other portions is increased by 91, so that the reactivity as shown in FIG. 10 is achieved. Effect E
can be realized, and the neutron irradiation dose distribution in the width direction of the wing 16 can be dealt with.

In this way, in this example, taking into consideration the characteristics shown in FIGS. 7 to 9, the octium content in the range and portion where the neutron multiplication factor and neutron irradiation amount are relatively low is reduced as much as possible, so that the atomic The overall weight of the reactor control rod 10 can be reduced.

In this case, the reactivity can be improved by making the wing-side end of the rod-shaped hafnium insertion space 25 into a solid hafnium rod 18a or by guiding water as a moderator into each hollow hafnium rod 18b. The amount of hafnium m can be reduced. In addition, since hafnium can be effectively placed in important positions from the standpoint of reactor shutdown margin, the reactivity can be effectively increased with the minimum amount of hafnium ports, ensuring reactor shutdown margin. .

Furthermore, hollow hafnium rods 18b are arranged in areas and parts where fuel combustion is likely to be delayed, and the hollow hafnium rods 18b
Since water is introduced as a moderator, it is possible to recover from the excessive decrease in fuel output. the result,! , even when a lJ control rod is inserted, local delays in combustion are suppressed, the rate of local increase in output during control rod handling can be reduced, and fuel integrity is improved.

In this way, in the above embodiment, the reactor control rod 1
Since the overall ff1m has been reduced, the existing control rod drive R structure can be used as is without any design changes. Further, a hollow hafnium rod 18b is incorporated as the rod-shaped hafnium rod 18, and the hollow hafnium rod 18
Since water is introduced as a moderator into the fuel cell b, it is possible to prevent an excessive drop in fuel output, alleviate the blade history effect of the fuel, and maintain the integrity of the fuel. Furthermore, the strength problem caused by thinning the sheath 15 can be solved by using the stiffener 26, the stick 28, the spacer 30, etc., and the mechanical replaceability of the reactor control rod can be maintained. .

FIG. 11 is a diagram showing another embodiment of the rod-shaped hafnium 18 in the present invention. However, Figure 11 (A) is the fifth
This is an example of a combination of rod-shaped hafnium 18 corresponding to FIG. 5(A), and FIGS. 11(B), (C), and (D) are examples of a combination of rod-shaped hafnium 18 corresponding to FIG. 5(B).

The number of solid hafnium rods 18a is generally smaller in FIG. 11 (A>, (B)) than in FIG. 5 (Δ), (B). It is relatively small and suitable for cases where reactor shutdown margin can be secured.

Further, in FIG. 11(C), the stiffener 26 is not provided, and a plurality of hollow hafnium rods 18b are skewered by the sticks 28 and joined in a row to form a plate-like structure. By forming the plate-like tΔ structure in this way,
The arrangement of the hollow hafnium rods 18b is prevented from being disturbed, and the U-shaped sheath 15 is prevented from expanding.

Further, in FIG. 11(D), a stiffener 26 is provided and a plurality of hollow hafnium rods 18b are inserted into the spacer 3.
They are skewered together with 0 and connected in rows to form a plate-like structure. The spacer 30 includes a bolt-shaped spacer 30a having a screw rod, and a nut-shaped spacer 3 screwed to the screw rod.
0b is used. This spacer 30 is provided to prevent the arrangement of the hollow hafnium rods 18b from being disturbed and to open a space. In other words, this spacer 30 portion is actually occupied by water as a moderator and constitutes a rod-shaped moderator section. Since the thermal neutron flux in this area increases, the burnup of the fuel adjacent to this area can be increased, preventing an abnormal increase in output in the axial direction of the control rod, and maintaining the integrity of the fuel. .

In addition, the embodiment shown in FIG. 11 also has the same effects as the embodiment described above.

In the above embodiments, the case where the bar-shaped hafnium 18 is a round bar has been described, but the present invention is not limited to this, and it may have an outer shape that is a square bar or a combination of a rectangle and a circle.

Furthermore, in the above embodiment, a case was described in which the average hafnium density of a plurality of rod-shaped hafnium rods inserted into the insertion space within a range of about 3/4 of the total length of the rod-shaped hafnium insertion space from the control rod end side was reduced. , the present invention is not limited to this, but a plurality of rod-shaped rods inserted into the insertion space within a range of about 3/4 to 2/3 of the total length of the rod-shaped hafnilla insertion space from the end side of the υ1111 rod. The hafnium average density of hafnium may be reduced.

〔Effect of the invention〕

In the nuclear reactor control rod according to the present invention, a plurality of rod-shaped hafnium rods as long-life neutron absorbers are inserted into the sheath, and 3/4 to 2/2 of the total length of the rod-shaped hafnium insertion space from the control rod end side. Among the insertion spaces in the range of 3, the average hafnium density of the plurality of rod-shaped hafniums inserted into the insertion space excluding the side ends is the hafnium average density of the rod-shaped hafnium inserted into the insertion spaces other than the insertion space. Since the weight of the reactor control rods is reduced, it is possible to effectively secure reactor shutdown margin, maintain a long life of the reactor control rods, and realize a reduction in the weight of the reactor control rods. Therefore, the control rod drive mechanism can be used without any design changes.

[Brief explanation of the drawing]

FIG. 1 is an overall perspective view showing an embodiment of the I11 ill rod for a nuclear reactor according to the present invention, FIG. 2 is a sectional view showing the control rod for a nuclear reactor of the above embodiment, and FIG. Figure 4 shows the distribution of the reactivity effect (neutron absorption characteristics) in the control rod axis direction, and Figure 4 shows the i! Figure 5 (A
) and (B) are cross-sectional views of the rod-shaped hafnium shown in Figure 4, Figure 6 is a diagram showing the axial fission nuclide concentration distribution curve of the nuclear reactor core after combustion has progressed to a certain extent, and Figure 7 is shown in Figure 6. A diagram showing the axial distribution curve of the neutron multiplication factor during reactor shutdown in the nuclear reactor core, Figure 8 is a diagram showing the axial neutron irradiation population distribution curve of the 1st control cup for the reactor, and Figure 9 is a diagram showing the axial distribution curve of the neutron irradiation factor in the reactor core. Figure 10 is a diagram showing a typical neutron irradiation m distribution curve in each wing width direction of a control rod, Figure 10 is a diagram showing the reactivity effect distribution in the wing width direction in the above example, and Figure 11 (
A) to (D) are diagrams showing other examples of rod-shaped hafnium in the present invention. 10...11JI for nuclear reactors! L rod, 11... Handle, 12... Tip structure material, 13... End structure material, 1
4...Central tie rod, 15...Sheath, 16...
・Wing, 18...rod-shaped hafnium, 18a...
Solid hafnium rod, 18b...Hollow hafnium rod, 2
2...Speed limiter, 23...Gap, 25...
- Insertion space, 26...Stiffener, 28...Stick, 30...Spacer. 1st figure, 2nd figure, 3 times Mi, 7th figure of the sheep, lower part, ChiL iL
iL-”a# O ni ryo 9bian

Claims (1)

[Claims] 1. The tip structural member and the terminal structural member are connected by a central tie rod, a sheath is axially fixed to each protrusion of the central tie rod, and a long-life neutron absorber is installed in the sheath. In the reactor control rod into which the long-life neutron absorber is inserted, the long-life neutron absorber is composed of a plurality of rod-shaped hafnium rods, and the rod-shaped hafnium insertion space extends from the end of the control rod by 3.
The hafnium average density of the plurality of rod-shaped hafnium inserted into the insertion space excluding the side ends of the insertion space in the range of /4 to 2/3 is the same as that of the rod-shaped hafnium inserted into the insertion space other than the insertion space. A control rod for a nuclear reactor, characterized in that the average density of hafnium is lower than that of hafnium. 2. The rod-shaped hafnium has a width corresponding to the dimension of the insertion space in the wing thickness direction.
Control rods for nuclear reactors as described in Section 1. 3. The control rod for a nuclear reactor according to claim 1, wherein the plurality of rod-shaped hafnium rods are densely arranged at equal pitches within the sheath. 4. The claim that the plurality of rod-shaped hafnium rods is a combination of at least two or more types of hafnium rods among a solid hafnium rod and one type or two or more types of hollow hafnium rods having different wall thicknesses. The control rod for a nuclear reactor according to item 1. 5. The control rod for a nuclear reactor according to claim 1, wherein one type or two or more types of hollow hafnium rods having different wall thicknesses are inserted into the rod-shaped hafnium insertion space with a reduced average hafnium density. . 6. At least one type of hafnium rod among one type or two or more types of hollow hafnium rods with different wall thicknesses is inserted into the rod-shaped hafnium insertion space with a reduced average hafnium density, and a neutron absorber is inserted. A control rod for a nuclear reactor according to claim 1, which is provided with an excluded rod-shaped moderator portion. 7. The nuclear reactor according to claim 1, wherein an elongated member for fixing opposing surfaces of the sheath to each other is disposed in the rod-shaped hafnium insertion space excluding the side ends thereof. control rod. 8. The control rod for a nuclear reactor according to claim 1, wherein a plurality of the rod-shaped hafnium rods are connected in a row to form a plate-like structure. 9. The control rod for a nuclear reactor according to claim 4, wherein the hollow hafnium rod is provided with a water hole through which a moderator flows through the hollow hafnium rod.