JPH01148995A - Control rod for nuclear reactor - Google Patents

Abstract

PURPOSE:To plan a long term of nuclear life while heightening reactivity value of a control rod and enlarging a stop margin by arranging a large quantity of neutron absorbing substance at the part of high reactivity effect and placing long life type neutron absorbers at the part of high neutron irradiation quantity. CONSTITUTION:A sheath 16 made of metal which has U-shaped cross section at a tie rod 14 joining an edge structure material and an end structure material together is anchor to form a wing 17 and a long life type neutron absorber 22 is housed in the sheath 16 at the inserting edge area of the wing 17 to form a first area. A second area is formed at the side of an inserting end neighboring the first area, neutron absorbers 24 are arranged in a row in the width direction of the wing 17 in the sheath 16 of the wing of the second area and neutron absorbing rods 27, 28, 29 of rectangular section area are placed at the outside neighboring part of the width direction of at least the wing 17 of the neutron absorbing rods 24.

Description

Detailed Description of the Invention [Objective of the Invention] (Industrial Application Field) The present invention relates to a control rod for a nuclear reactor that controls the reactor power of a light water reactor such as a boiling water reactor, and in particular to a control rod for controlling the reaction of the control rod. This invention relates to a control rod for a nuclear reactor with high reactivity and long life, which increases the reactor value, improves reactor shutdown margin, and extends its life.

(Prior art) Conventional control rods for boiling water reactors have a long, thin U-shaped sheath fixed to a central tie rod to form wings with a cross-shaped cross section, and each wing has a large number of neutron absorption rods. Loaded and configured. For example, a neutron absorbing rod is made of boron carbide (B4C) as a neutron absorbing material in a stainless steel cladding tube.
) Prepared by filling powder.

When this nuclear reactor control rod is inserted into the core of a boiling water reactor, etc., the neutron absorbing material filled in the sheath is irradiated with neutrons and gradually loses its neutron absorption ability. They are replaced periodically after reaching the end of their service life and being operated for a predetermined period of time.

(Problem to be Solved by the Invention) Control rods that are inserted into and removed from the core of a nuclear reactor are not uniformly irradiated with neutrons over the entire surface of each wing. The edge region receives intense neutron irradiation. Therefore, the neutron absorbing material filled in that area absorbs a large amount of neutrons and is consumed faster than other areas of the wing, ending its nuclear life earlier.

Therefore, even though the neutron absorbing material filled in other areas of the wing still had sufficient nuclear life left, it was uneconomical to dispose of the entire reactor control rod as radioactive waste. . On the other hand, if reactor control rods are replaced frequently, there may be a problem that the radiation exposure of the workers who perform the replacement work will also increase.

In order to solve such problems, we are developing nuclear reactor control rods in which a long-life neutron absorbing material such as hafnium, which has a long nuclear life, is partially placed in the area of the control rod that is exposed to intense neutron irradiation. The inventor has developed. As disclosed in Japanese Patent Application Laid-Open No. 53-74697, this control rod for a nuclear reactor has a hybrid structure in which long-life absorbers are arranged at the tip and outer end of the wing, which is exposed to intense neutron irradiation. This hybrid type nuclear reactor control rod has achieved a lifespan approximately twice that of a conventional control rod.

On the other hand, in conventional control rods for nuclear reactors, the entire wing area is filled with neutron absorbing material at a uniform density, and the neutron absorption capacity, or reactivity, in each area in the axial direction is adjusted to be equal. However, unevenness in the amount of neutron irradiation causes variations in reactivity over time, which may partially reduce the reactor shutdown margin at the end of the reactor operating cycle.

In other words, the axial distribution of the reactor shutdown margin (subcriticality) when the reactor is operated for a predetermined period using the above reactor control rods is determined by the design specifications of the fuel assembly or the reactor operating method. Although there are some differences depending on the distribution, the distribution basically becomes as shown in FIG. 6(A). However, while the reactor shutdown margin increases at the upper and lower ends of the core, it takes a minimum value at a position slightly below the upper end.

Possible causes of this are as follows.

When the effective axial length of the reactor core is L, in the upper end region from the position 3/4 L from the lower end to the upper end, the bubble rate (void rate) during operation is high, and the power density of the reactor is low. Due to the relative decrease, the mass number of fissile material 2
The amount of uranium 35 (U-235) remaining is relatively large. Also, the generated air bubbles (voids) cause a phenomenon of hardening of the neutron spectrum. As a result, the plutonium production reaction (
As a result, the concentration of fissile material in the upper part of the reactor core increases after the reactor is in operation, reducing the margin for reactor shutdown in that area.

On the other hand, it is inevitable that future nuclear reactors will shift to higher burn-up of nuclear fuel and longer operating cycles due to the demand for improved operating economy. As a concrete response to this, the adoption of highly enriched nuclear fuel is progressing, and as a result, there is a strong demand for control rods for nuclear reactors that have a long life and have a large margin for reactor shutdown.

However, when conventional reactor control rods are used in reactors loaded with highly enriched nuclear fuel, the margin for reactor shutdown is relatively reduced, and the reactor control rods must be replaced frequently every short operating cycle. Must be.

When replacing control rods for a nuclear reactor, it is necessary to shut down the reactor and further remove a large number of fuel assemblies arranged around the control rods to be replaced from the reactor core, which is a complicated process.

Therefore, reactor shutdowns for control rod replacement occur frequently and the shutdown period becomes long, which can significantly reduce the operating efficiency and economic efficiency of the reactor, while also significantly increasing management labor.

The present invention was made in consideration of the above-mentioned circumstances, and it increases the reactivity value of control rods by arranging a large amount of neutron absorbing material in areas with a high reactivity effect, increasing the reactor shutdown margin,
Another object of the present invention is to provide a high reactivity, long-life nuclear reactor control rod that can prolong the nuclear life by arranging a long-life neutron absorbing material in a region exposed to high neutron irradiation.

(Structure of the Invention) (Means for Solving the Problems) In order to achieve the above-mentioned object, the control rod for a nuclear reactor according to the present invention includes a U A wing is formed by fixing a metal sheath having a letter-shaped cross section, and a long-life neutron absorbing material is accommodated in the sheath at the insertion tip region of the wing to form a first region, and the first region is provided with a long-life neutron absorbing material. A second region is formed on the adjacent insertion end side, and neutron absorbing rods are arranged in a row in the width direction of the wing inside the sheath of the wing of the second region, and at least the width of the wing among the neutron absorbing rods is It is characterized in that a neutron absorption rod with a rectangular cross section is arranged near the outer end in the direction.

(Function) According to this control rod for a nuclear reactor, a large amount of neutron absorbing material is placed in the parts that make a large contribution to the neutron reactivity, so the reactivity value of the control rod is high, and as a result, there is less margin for reactor shutdown. increase In addition, long-life neutron absorbing materials are placed in areas that receive high neutron irradiation, resulting in a long nuclear lifetime.

In addition, the use of expensive long-life neutron absorbing materials is limited to the minimum number of holes necessary, which reduces the manufacturing cost of the entire reactor control rod. We can provide control rods for long-life nuclear reactors.

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

A nuclear reactor control rod 10 as shown in FIG. 1 is moved in and out of the core of a light water reactor such as a boiling water reactor.

This control rod 10 includes a tip structure member 1 having a handle 11.
2 and the end structural member 13 are integrally or integrally connected by a central tie rod 14 having a cross-shaped cross section. The tip structure member 12 is provided with a guide roller 15 that guides the control rod 10 in and out.

In addition, each protruding portion (protruding leg) of the central tie rod 14 includes:
A metal sheath 16 with a U-shaped cross section is fixed to form wings 17 . The sheath 16 is made of, for example, a stainless steel plate curved into a deep U-shape, and has a large number of water passage holes 18 formed on its surface to allow the moderator to freely flow therethrough, and a neutron absorbing material 20 is housed inside. The insertion portion of this neutron absorbing material 20 is set to the effective length L of the control rod 10.

On the other hand, among the neutron absorbing materials housed within the sheath 16,
A long-life neutron absorbing material 21, 22, typically a hafnium metal material, is housed in the insertion tip region X of the wing 17 as the first region and its outer end. Among these, the insertion tip portion w1 of the wing 17. X is, for example, 5-32cII
It has a length jl of about I, usually 15 to 16 cIII. The insertion tip region material 21 is accommodated. wing 1
7, a second region Y is formed on the end structure member 13 side adjacent to the insertion region X.

The long-life neutron absorber 21 in the insertion tip region 23 is formed in the width direction of the wing 17, and this gap 23
A cushioning material made of hafnium or other metal wool is interposed between the cushions.

This gap formation keeps the area without neutron absorbing material to a minimum. When the gap 23 is not formed,
Insertion tip II! A relatively large region where the neutron absorbing material 20 does not exist is formed in X, and a neutron flux peak occurs in this region, or the integrity of the top of the neutron absorbing material 24 in the second region X adjacent to the insertion tip region X is affected. may occur.

The plate-shaped long-life neutron absorber 21 accommodated in the upper region of the insertion tip region X is supported by support protrusions 25 that protrude laterally from each protrusion of the central tie rod 14 via a predetermined clearance.

Further, the length ρ3 of the lower section of the insertion tip region X is, for example, about 2 to 3 crR.

On the other hand, since the outer end of the wing 17 in the second region Y receives high neutron irradiation from the reactor core during reactor operation, a long-life neutron absorbing material 22 in the form of a rod such as a hafnium metal material is used.
is placed. This long-life neutron absorber 22 does not need to be disposed over the entire effective length of the control rod 10 (the axial length 1 from the insertion tip of the control rod 10, the wing 17
It is placed in a high neutron irradiation area with a length of 15 in the width direction.

In this case, if the control rods 10 are not used for reactor power operation control but are used only for reactor operation shutdown,
There is no need to arrange the long-life neutron absorber 22 at the outer end of the wing 17, and a boron carbide (B4C) neutron absorber may be used.

In the second region Y of the wing 17, neutron absorbing rods 24 filled with boron carbide as a neutron absorbing material are arranged in areas other than the outer region. The neutron absorption rods 24 are arranged in rows in the width direction of the wing 17, as shown in FIG. Of these neutron absorption rods 24, a range of length 15 from the lower end of the control rod 10 is an area in which a gas prem (space) may be formed within each neutron absorption rod 24. This length 1
, is approximately 1/2 or less of the effective length of the control rod, the remaining region is a high reactivity region (L-J15), and no gas plenum is formed within the neutron absorption rod 24.

In addition, the B4C neutron absorption rods 24 arranged in the width direction of the wing 17 include a neutron absorption rod 26 with a circular cross section arranged in the widthwise center of the wing 17, as shown in FIG. Rectangular neutron absorption rods 27, 28, 29 are arranged in the inner and outer parts. The rectangular neutron absorbing rod 24 has a square cross section near the center in the width direction of the wing 17, and neutron absorbing rods 28 and 29 with a rectangular cross section are arranged near both ends of the wing 17 in the width direction.

Among the rectangular neutron absorbing rods 24, a portion near the outside in the width direction of the wings 17 has a high reactivity effect, so a larger amount of neutron absorbing material is disposed in this portion. For this reason, the rectangular neutron absorption rod 29 near the outside in the width direction of the wing 17 is filled with square pelletized B4C neutron absorption material M instead of powder to improve the packing density of 84C and increase the reactivity value. It's increasing.

Among the neutron absorption rods 24 arranged in a row inside the sheath 16 of the wing 17 in the second region Y, the neutron absorption rods other than the neutron absorption rods arranged near the outside of the wing 17 in the width direction are coated with 84C powder. A neutron absorbing substance P is filled. At this time, since the vicinity of the central tie rod 14 side also has a relatively high reactivity effect, the rectangular neutron absorption rod 24 disposed in the vicinity thereof is made into a neutron absorption rod 30 with a rectangular cross section, and the inside of this neutron absorption rod 30 is As shown in FIG. 3, a B4C neutron absorbing material M formed into square pellets may be filled.

Further, the number of neutron absorbing substances P and M per unit length in the width direction of the wing 17 is the smallest in the circular neutron absorbing rod 26, and the largest in the rectangular neutron absorbing rods 27, 28, and 29. Also, among the rectangular neutron absorption rods, the square neutron absorption rod 2
7 is small, and rectangular neutron absorbing rods 28, 29, 30 are large, so by changing the cross-sectional shape of the neutron absorbing rod 24, more neutron absorbing substances P and M can be filled and arranged in important areas in terms of reactivity. The reactivity of the control rod 10 can be 1illiIill! can be effectively increased.

On the other hand, if relatively inexpensive B4C is used as the neutron absorbing materials P and M, He gas is generated by the neutron reaction of boron-10 (B), and the gas pressure in the cladding tube of the neutron absorbing rod 24 increases. Generally, the circular neutron absorption rod 26 is the strongest against internal pressure, and the neutron absorption rod 26 with a rectangular cross section becomes weaker as the shape changes from square to rectangular.

Furthermore, since the sheath 16 constituting the wing 17 has a deep U-shaped cross section and is fixed to the central tie rod 14, the inner and outer ends in the width direction of the wing 17 are highly resistant to swelling stress caused by internal pressure. Has resistance. Therefore, even if the neutron absorption rod 24 swells due to swelling stress at the inner and outer ends of the wing 17 in the width direction, the sheath 16 suppresses this bulge, while the middle part of the wing 17 is relatively easily carved. Print.

In this reactor control rod, as shown in Figures 2 and 3, the neutron absorbing rods 26 and 27, which are difficult to expand, are placed in the central part of the wing 17 where it is easy to engrave, and the neutron absorbing rods 26 and 27 are placed in the part where it is difficult to engrave. Easily inflatable neutron absorption rods 28, 29, and 30 are arranged to suppress deformation of the wing 17.

In addition, a neutron absorption rod with a rectangular cross section 27.28.29.30
Of these, the neutron absorption rod with a rectangular cross section that is easy to expand 29.3
When the neutron absorbing materials B and C filled in 0 are formed into rectangular square pellets, the shape restraining force of these square pellets suppresses the engraving of the neutron absorbing rod 29.30 with a rectangular cross section. In addition, each neutron absorption rod 24 forms a gas plenum near the insertion end, which has little effect on the reactor shutdown margin, and if the gas generated in this gas plenum is released, the increase in internal pressure inside the neutron absorption rod is suppressed. can.

If the gas plenums are arranged at different axial positions so that adjacent neutron absorption rods are not adjacent to each other, and only small portions are adjacent to each other, reactivity mutual interference effects will occur between the gas plenums. First, the effect of reducing reactivity due to the gas plenum can be suppressed.

Furthermore, by pelletizing the neutron absorbing material of B4C, it not only has the effect of suppressing the deformation of the wing 17, but also increases the reactivity value of the control rod 10 because a larger amount of neutron absorbing material can be placed than in the case of powder. be able to.

Furthermore, if a neutron absorbing material that does not generate gas, such as FU O-Hf02, is used instead of 84C, no bulge deformation occurs due to the internal gas pressure, so no measures against bulge are required. Further, when a neutron absorbing substance such as hafnium is used in the neutron absorbing rod 24, the reactivity value of the control rod is further improved.

In this case, the neutron absorption rod 24 disposed in the wing 17 of the second region of the reactor control rod 10 is
It is not always necessary to provide a neutron absorption rod with a circular cross section near the center in the width direction, and only a neutron absorption rod with a square cross section may be provided in this portion.

Next, an embodiment of a control rod for a nuclear reactor will be described with reference to FIG.

The reactor control rod 10A shown in this embodiment has the same reference numerals because the overall arrangement of the neutron absorber 20 disposed in the wing 17 is almost the same as that of the control rod 10 shown in FIG. In addition, in the reactor control rod 10A shown in FIG. 4, a neutron absorbing material 33 is arranged also in a portion near the axis of the control rod 10A, which is a portion where the reactor has a severe shutdown margin.
This increases the neutron absorption phase of the portion perpendicular to the longitudinal axis of the control rod 10A.

In this reactor control rod 10A, a tip structure member 12 having an operation handle 11 and a terminal structure member 13 are connected by a plurality of tie rods 35, and each tie rod 35 is provided at an appropriate distance in the axial direction. A plurality of wings 17 are formed by fixing the open end of a metal sheath 16 having a deep U-shaped cross section to the tie rod 35, which are integrally or integrally connected by a tie rod connecting member 36 having a cross-shaped cross section. There is. Each wing 17 is connected in a cross shape, and each sheath 16 of these wings 17 is provided with a large number of water holes 18 through which moderator material freely flows.

The sheath 16 of each wing 17 is filled with a neutron absorber 20 depending on the characteristics of the nuclear reactor. Control rod for nuclear reactor 1
0A is a long-life neutron absorbing material 21.0A typically made of hafnium in the insertion tip region X of the wing 17 and its outer edge.
22 is accommodated, and a high neutron irradiation area is formed.

The insertion tip area X is approximately 5 to 320 mm long from the insertion tip.
Since this is a region where the neutron flux changes significantly, particularly within a range of, for example, 5 cm from the insertion tip, it is necessary to fill at least this range with a long-life neutron absorbing material.

On the other hand, in the combustion management of a nuclear reactor, adjustment of the relative position between the fuel assembly and control rods is carried out every 15 to 16 cm, which is the effective length of the core divided into 24 equal parts. The length fJl is preferably set to a unit length of 15 to 16 cm or 30 to 32 cm, which is twice the unit length.

However, if the length significantly exceeds 32 cm, the weight of the control rod will increase significantly due to heavy use of long-life neutron absorbers with high density, so it is usually set to 15 to 16 cm.

As shown in FIG. 5(A), the long-life neutron absorbing material 21 filled in the insertion tip region It is constructed by a hafnium plate 21a provided. A gap 37 through which a moderator flows is formed between the opposing hafnium plates 2ia and between the inner surface of the sheath 16 and the hafnium plate 21a.

Further, the inside of the sheath located at the outer edge of each wing 17 is filled with a rod-shaped long-life neutron absorbing material 22 . The width I5 of the region filled with this long-life neutron absorbing material 22 is about 1 to 2 cIII, and the length fJ4 is from the insertion tip to the axial direction of the reactor core, which is used in a nuclear reactor control rod that is always inserted. The effective length is at least 172 or more.

Here, as the long-life neutron absorbing material 22, a hafnium rod 22 in the shape of a round rod or a square rod with one end surface processed to match the shape of the inner surface of the sheath as illustrated in FIG. 5(B) is employed.

Furthermore, within the sheath in the region from the lower end of the first region They are filled in parallel in the width direction, and a gap 37 through which the moderator flows is provided in the axial center of the reactor control rod 10A, as shown in FIG. 5(C).

According to the above configuration, during nuclear reactor operation, the neutron absorption ability is maintained for a long period of time even in the insertion tip region X where the amount of neutron irradiation is particularly large, and the nuclear life of the control rod is long. Therefore, it becomes possible to significantly extend the life of the reactor control rod 10A as a whole.

Furthermore, in FIG. 4, a second region Y is formed adjacent to the lower end of the insertion tip of the insertion tip region X as the first region and extends vertically to the insertion end, and the second region Depending on the characteristics, for example, by concentrating boron-10 or pelletizing it, a highly reactive neutron absorption rod 28 or a normal life type neutron absorption rod 26 has a large neutron absorption capacity.
is filled. As this normal life type neutron absorption rod 26, a neutron absorption rod whose cladding tube is filled with boron carbide (B4C) is employed.

Also, among the tie rod binding materials arranged at intervals in the axial direction of the center of the reactor control rod 10A, the first region
A neutron absorbing material 33 is disposed between the wing bonding material 36 disposed in a range of at least 1/4 or more of the effective length of the wing in the axial direction from the lower end of the insertion tip of the wing (N6-J1). be done. As illustrated in FIG. 5(B), the neutron absorbing material 33 includes a rectangular member 33a made of stainless steel or hafnium material at the axial center, and neutron absorbing rods 33b arranged in a cross shape around the rectangular member 33a. configured. wing 17
The length J2 from the insertion tip to the lower end of the installed neutron absorber 33 is usually 1/4 to 1/4 of the effective length of the reactor core.
Set to 3.

In this case, the length 13 of the region in which the cavity 37 through which the moderator flows without interposing the neutron absorber 33 at the lower part of the control rod axis is 3/4 to 2/3 of the effective length of the core. . The moderator that has flowed into the gap 37 prevents the output from decreasing at the corner of the fuel assembly, thereby preventing the blade history phenomenon when the control rod is withdrawn.

On the other hand, when securing a larger reactor shutdown margin than preventing the blade history phenomenon described above, the above 16 is 4/4
・Set to . However, even in this case, a gap 37 remains in the length 11 portion corresponding to the first region X, and the blade history phenomenon is suppressed by the action of the moderator flowing into the gap 37.

According to this reactor control rod 10A, the neutron absorbing material 33 is interposed between the tie rod coupling materials 36 arranged in the region where the reactor shutdown margin decreases, and the effective wing width of the 111m rod is centered on the axis. The reactivity of the control rod increases as it expands in the direction of the control rod.

In addition, hafnium plates 21a are disposed facing each other within the sheath at the insertion tip, and since the hafnium plates 21a have sufficient reactivity in a small amount and have a long life, sufficient margin for reactor shutdown is ensured. be able to.

On the other hand, a gap 37 is provided between the wing coupling material 36 located at the insertion tip and the insertion end of the control rod, through which reactor water, which is a moderator, flows without intervening the neutron absorber 33. While the reactivity is relatively high when the control rod is inserted, the reactivity is low in the lower part because the neutron absorbing material 33 is not interposed therein.

Generally, near the upper end of a nuclear reactor control rod, the degree of subcriticality suddenly increases as shown in FIG. 6(A). One of the causes of this is a decrease in the multiplication factor due to neutron leakage from the core end, and another reason is that the core fuel economy is improved by placing natural uranium or low-enrichment nuclear fuel at the core end. This is due to core design policy aimed at improving performance. Corresponding to that policy, in this embodiment as well, as shown in FIG. 6(B), a long-life neutron absorber 21 is placed in the first region The only aim is to extend the life of the battery by filling it with

A nuclear reactor control rod having a distribution of neutron absorption characteristics in the axial direction as shown in Fig. 6(B) is installed in a nuclear reactor, and a predetermined 1
1! ] Figure 6(C) shows the distribution of subcriticality in the axial direction of the reactor core, that is, the reactor shutdown margin after operation for a period of 20 minutes. Figure 6 (A
Even compared to the conventional example shown in ), the reduction in fuel output at the corner portions of the fuel assembly is suppressed and combustion progresses.

Therefore, when handling control rods and transitioning to high-output operation, a sudden increase in output at the corner (blade history phenomenon) is suppressed, and thermal shock is significantly alleviated.

In addition, since the expensive long-life neutron absorbing material is arranged in a limited number of necessary minimum lengths, the manufacturing cost of the entire control rod can be reduced.

Next, the effects of using the nuclear reactor control rod according to this embodiment will be explained with reference to FIG. 6.

FIG. 6(B) is a graph showing the axial distribution of neutron absorption characteristics in the reactor control rod according to this example, and each region of the reactor control rod corresponds to the effective length in the axial direction of the reactor core. The relative values of reactivity in X and Y are shown. In the first region X, a long-life neutron absorbing material 21 is provided.

In addition, considering the subcriticality distribution in the core axis direction of the conventional IIJ control rod, the neutron absorbing material 33 is inserted in the axial center gap 37 of the control rod, so it is placed in the upper part of the second region Y. The subcriticality distribution in the upper half has been improved, and an almost -like subcriticality limit curve can be obtained throughout the axial direction of the reactor control rod.
It can be seen that sufficient margin for reactor shutdown is secured.

In addition, at the boundary between the long-life neutron absorbing material 22 in the second region and the neutron absorbing rod 24 having a circular cross section and a rectangular cross section, a neutron absorbing material having a reactivity between the two, such as boron-10, is applied. Boron carbide with different concentrations,
By intervening a region filled with, etc., it is possible to further flatten the subcritical limit curve. It is also possible to adjust the neutron absorption capacity by thinning or perforating the neutron absorbing material 33 or hafnium plate 21a interposed between the wing bonding materials 36, thereby making the subcriticality curve flatter. At the same time, the amount of expensive neutron absorbing material used can be reduced, and the manufacturing cost of the control rod can also be reduced.

As explained above, according to the nuclear reactor control rod of this example,
It is possible to maintain sufficient reactivity value and to extend the service life at the same time.

Therefore, even when the reactor is operated for a long period of time, the reactivity value of the entire reactor control rod is maintained, and a sufficient margin for reactor shutdown can be ensured.

〔Effect of the invention〕

As described above, in the nuclear reactor control rod according to the present invention, the long-life neutron absorbing material is housed in the sheath at the insertion tip region of the wing to form the first region, and the first region is adjacent to the first region. A second region is formed on the insertion end side of the wing, and neutron absorption rods are arranged in a row in the wing width direction inside the sheath of the wing in this second region, and at least one of the neutron absorption rods is arranged near the outer end in the wing width direction. Since neutron absorption rods with a rectangular cross section are placed in the control rods, it is possible to place more neutron absorption materials in areas that have a greater contribution to reactivity, increasing the reactivity value of the control rods and making it easier to shut down the reactor. The margin can be increased, and since the long-life neutron absorbing material is arranged in the portion where the amount of neutron irradiation is high, it is possible to provide a long-life control rod with a long nuclear life and high reactivity.

[Brief explanation of the drawing]

FIG. 1 is a partially cutaway view showing one embodiment of a control rod for a nuclear reactor according to the present invention, FIG. 2 is a plan cross-sectional view taken along the line ■-■ in FIG. 1, and FIG. FIG. 2 is a plan cross-sectional view showing an example of a modified arrangement of the control rods shown in FIG. (
C) is a plan cross-sectional view taken along lines A-A, B-B, and C-C in Figure 4, and Figure 6 (A> is a plan view taken along lines A-A, B-B, and C-C in Figure 4). FIG. 6(B) is a graph showing the axial distribution of reactor shutdown margin during operation, and FIG. 6(C) is a graph showing the axial distribution of neutron absorption characteristics in the control rod for a nuclear reactor according to the present invention. 10 is a graph showing the axial distribution of the reactor shutdown margin when the reactor is operated for a predetermined period using the reactor control rod according to the present invention in comparison with a conventional example. 10.10A...Control Rod, 12...Tip structural material, 1
3... End structure material, 14.35... Tie rod, 1
6... Sheath, 17... Wing, 20... Neutron absorbing material, 21.22... Long-life neutron absorbing material, 2
3... Gap, 24... Neutron absorbing material (neutron absorbing rod), 26... Neutron absorbing rod with circular cross section, 27.28.
29゜30...Neutron absorption rod with rectangular cross section, 33...
Neutron absorbing material, X...first region (insertion tip region), Y
... terminal area. Applicant's agent Hisashi Hatano 4 times

Claims (1)

[Claims] 1. A metal sheath having a U-shaped cross section is fixed to a tie rod that connects the tip structure member and the end structure member to form a wing, and the insertion tip region of the wing is inserted into the sheath. accommodating a long-life neutron absorber to form a first region;
A second region is formed on the insertion end side adjacent to the first region, and neutron absorption rods are arranged in a row in the width direction of the wing inside the sheath of the wing of the second region, and among the neutron absorption rods, A control rod for a nuclear reactor, characterized in that a neutron absorption rod with a rectangular cross section is arranged at least in the vicinity of the outer end in the width direction of the wing. 2. The first area is 5 points from the insertion tip of the wing to the insertion end side.
A control rod for a nuclear reactor according to claim 1, which has a length of 32 cm to 32 cm. 3. A long-life neutron absorption rod is arranged at the outer end of the second region wing in a width of 1 to 2 cm over at least 1/2 of the effective length of the control rod from the insertion tip of the first region. A control rod for a nuclear reactor according to scope 1. 4. A control rod for a nuclear reactor according to claim 1, wherein neutron absorption rods having a circular cross section or a substantially square cross section are arranged in a row in the width direction of the wing in the widthwise central part of the wing in the second region. . 5. A rectangular neutron absorbing rod is disposed at least near the outer end of the wing in the second region, and the neutron absorbing rod contains a rectangular pelletized neutron absorbing material in a cladding tube. The control rod for a nuclear reactor according to item 1. 6. The control rod for a nuclear reactor according to claim 1, wherein a neutron absorbing material of boron carbide enriched with boron-10 is used at least near the outer end of the wing in the second region. 7. In the wing of the second region, neutron absorption rods with a circular or approximately square cross section are arranged at the center in the width direction of the wing, and neutron absorption rods with a rectangular section are arranged near the outer and inner ends of the wing in the width direction. A control rod for a nuclear reactor according to claim 1. 8. The reactor control according to claim 1, wherein the neutron absorption rod disposed in the second region has a gas plenum within 1/2 of its effective length from the insertion end of the control rod's effective part. rod. 9. Among the neutron absorption rods arranged in the second region, the gas plenums formed in adjacent neutron absorption rods are located at different axial positions so that most of them are not adjacent to each other. Control rods for nuclear reactors as described in Section 1.