JPH04301793A - Control rod for atomic reactor - Google Patents

Abstract

PURPOSE:To improve the cooling characteristics and to reduce the cost for materials and manufacture, in a control rod for an atomic reactor using hafnium as neutron absorber. CONSTITUTION:A control rod is constructed so as that in a sheath 2 of U-shaped cross section, plate-frame-like neutron absorber elements 11 are arranged along both side inner walls of the sheath 2 facing to each other, and plate-like neutron absorber plates 12 made of hafnium metal or hafnium alloy are arranged between respective absorber elements 11 in a direction parallel to the axis of a tie rod 1, and respective neutron absorber plates 12 facing to each other are supported at a constant distance in the facing area with connecting spacer rods 13 made of nearly the same materials to the neutron absorber plates 12. And the above members are supported in the sheath 2 with rod-like supporting members 14 made of nearly the same materials to the sheath 2 and fixed to the inner wall of the sheath 2 at both end portions.

Description

[Detailed description of the invention]

[Object of the invention]

0002

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a control rod for a nuclear reactor applied to a boiling water reactor, and more particularly to a control rod for a nuclear reactor in which a neutron absorbing element is made of hafnium metal or a hafnium alloy.

0003

[Prior Art] Generally, a control rod for a nuclear reactor consists of a tie rod that connects a tip structural member and a terminal structural member, and a blade that protrudes radially from the tie rod and has a sheath whose outer shell has a deep U-shaped cross section. and a plurality of neutron absorbing elements provided inside the sheath and arranged parallel to the axis of the tie rod.

[0004] Conventionally, neutron absorbing elements have often been constructed by filling a tube made of stainless steel or the like with powder of a boron compound such as boron carbide (B4C) as a neutron absorbing material. In addition, boron has B-1
There are two types, 0 and B-11, but B-10 is used as a neutron absorber. This B-10
reacts with neutrons and changes into helium (He-4) and lithium (Li-7), after which it loses its neutron absorption ability.

[0005] This means that when a boron compound is used as a neutron absorbing material, the neutron absorbing ability of the control rod is attenuated relatively quickly, leading to a short nuclear life.

On the other hand, recently there has been a demand for long-life control rods, and consideration has been given to making neutron absorbing elements from hafnium metal or hafnium alloys. In addition, as a hafnium alloy, hafnium (Hf) is used as zirconium (Zr) or titanium (Ti).
) etc. is applied.

[0007] Hafnium contains many nuclides, most of which can absorb neutrons many times with a single atomic nucleus. Therefore, with a neutron absorbing element made of hafnium, the neutron absorbing ability is less likely to be attenuated. Moreover, hafnium is chemically extremely stable even when directly immersed in reactor water.
No covering material required.

[0008] However, hafnium has a specific gravity of 13.1g/
cm3, but under the same volume as boron carbide, its neutron absorption ability is inferior to that of boron carbide, and its cost is high.

Therefore, the inventors proposed a so-called trap-type device in which a pair of curved plates made of hafnium metal or alloy are joined facing each other to form a tube, and this is housed in a metal sheath as a neutron absorbing element. A hafnium control rod was proposed. In this neutron absorption element, by introducing reactor water into the element, the function of the reactor water as a neutron moderator and the neutron absorption capacity are increased, thereby making it possible to overcome the above-mentioned drawbacks. Other improvements have also been proposed, and related techniques are disclosed, for example, in Japanese Patent Laid-Open No. 2-10299.

FIGS. 7 and 8 schematically show the structure of such a trap type hafnium control rod. That is, the sheath 2 having a deep U-shaped cross section and forming the outer shell of the blade 2A is fixed to each protrusion 1a of the tie rod 1 having a cruciform cross section, thereby forming the control rod as a whole to have a cruciform cross section. ing. In the space surrounded by the tie rod 1 and the sheath 2, a plurality of long flat tubes 3 made of hafnium metal or alloy are provided as neutron absorption elements.
are arranged in parallel.

The control rod insertion distal end side 2a of the sheath 2 is fixed to a distal end structure member 5 having a handle 4, and the control rod insertion distal end side 2a is fixed to a distal end structure member 5 having a handle 4.
b is an end structure member 7 integrated with the speed limiter 6;
is fixed to.

[0012] Inside the sheath 2, reactor water, which serves as a neutron moderator and coolant, flows through a large number of water passage holes 8, and water is passed inside and outside the flat tube 3. There is.

By the way, when observing the flow of reactor water in the flat tube 3, it is found that there is almost only an axial flow (vertical flow);
No inflow or outflow (lateral flow) occurs between the flat tubes 3. In this flow state, the flat tube 3 on the outer edge side of the blade 2A that is farthest from the tie rod 1 is irradiated with a high neutron flux, so the reactor water flowing inside the flat tube 3 in this area becomes the highest temperature. It is also possible that boiling may occur. On the other hand, the temperature of the reactor water in the other flat tube 3 on the side closer to the tie rod 1 is lower than the temperature of the reactor water in the flat tube 3 on the outer edge side of the blade 2A, and there remains sufficient room for cooling. .

However, while high-efficiency cooling is originally desired for parts of control rods with a high heat generation rate from the viewpoint of material integrity and neutron absorption characteristics, high-efficiency cooling is not required for parts of a control rod with a low heat generation rate. The trap-type hafnium control rod described above has the opposite tendency and is not preferable in terms of cooling characteristics.

The flat tube 3 is configured to be engaged with and held by the distal end structural member 5 and the distal structural member 7 together with the sheath 2, and is approximately the same length as the sheath 2 (for example, 1 m long). The hafnium material that is the flat tube 3 needs to be thicker at the control rod insertion tip side (upper side in Figure 7) from the viewpoint of neutron absorption function, while at the same time it needs to be thicker at the insertion end side (lower side in Figure 7). Therefore, it is desirable to reduce the wall thickness so as not to exceed the weight limit and to suppress material costs. However, in order to provide such a difference in wall thickness to a long object, troublesome processing steps such as shaving in the direction of the plate thickness are required, leading to problems such as increased manufacturing costs.

[0016]

[Problems to be Solved by the Invention] Conventional hafnium control rods still have problems with their cooling characteristics, such as poor cooling performance in parts of the control rod with a high heat generation rate, and conversely, high cooling performance in parts of the control rod with a low heat generation rate. ing.

[0017] Further, there are also problems such as the need for troublesome processing steps such as changing the wall thickness of a long product, which increases the manufacturing cost.

The present invention has been made in view of the above circumstances, and provides a control rod for a nuclear reactor that uses hafnium as a neutron absorber and is capable of improving cooling characteristics and reducing material and manufacturing costs. The purpose is to provide [Structure of the invention]

[0019]

[Means for Solving the Problems] In order to achieve the above object, the present invention provides a tie rod that connects a tip structure member and an end structure member, and a tie rod that protrudes radially from the tie rod and has a deep outer shell. It has a blade configured by a sheath with a letter-shaped cross section, and a neutron absorbing element provided inside the sheath and arranged parallel to the axis of the tie rod, the neutron absorbing element being made of hafnium metal or hafnium alloy. In a control rod for a nuclear reactor, which has a reactor water flow space inside, the neutron absorbing element is shaped like a plate frame, and the plate frame-shaped neutron absorbing elements are attached to each other along the inner surfaces of both side walls of the sheath. Flat neutron absorbing plates made of hafnium metal or hafnium alloy are arranged facing each other and spaced apart in a direction parallel to the axis of the tie rod, and each neutron absorbing plate is made of substantially the same material as the neutron absorbing plates. and a rod-shaped spacer connected within the range of the opposing surfaces so as to maintain a constant distance between the opposing parts, and the rod-shaped spacer is made of substantially the same material as the sheath and has both ends fixed to the inner surface of the sheath. Each support member is supported within the sheath by a supporting member.

[0020]

[Operation] According to the configuration of the present invention, the vertical and lateral flow of reactor water, which is a neutron moderator and coolant, occurs in the opposing surface gap and axial gap of the neutron absorbing plates constituting the plate frame-shaped neutron absorbing element. The flow is free. Therefore, the neutron moderation function of the reactor water is more effectively exhibited, and neutron absorption is performed more actively on the inner surface of the neutron absorption element than in the conventional flat tube structure, improving the reactivity effect. At the same time, the neutron absorption plate, which generates heat due to neutron absorption, can be sufficiently cooled. As a result, there is no need to particularly increase the thickness of the neutron absorption plate, and the amount of hafnium material that is expensive and has a high specific gravity can be reduced, making it possible to reduce material costs.

Furthermore, the flow of reactor water is appropriately agitated by the arrangement of the spacers. Therefore, the temperature of the cooling water is made uniform, and the cooling characteristics of the high heat generation portion are further improved, so that the generation of voids in the cooling water is suppressed. Since this reactor water also functions as a neutron moderator, suppression of void generation leads to prevention of reduction in the reactivity effect of control rods. As a result of effectively cooling the neutron absorption plate, the integrity of the neutron absorption plate is improved.

Furthermore, since the spacer is configured not to protrude from the outer surface and outer edge of the neutron absorbing plate, the installation width of the neutron absorbing plate within the sheath is not restricted, and the distance between the neutron absorbing plates is not narrowed. . If the spacer is configured to protrude from the outer surface and outer edge of the neutron absorbing plate, unnecessary gaps will be created between the neutron absorbing plates or between the neutron absorbing plate and the tie rod or sheath, reducing the reactivity. However, the present invention does not cause such a reduction in value.

[0023] In addition, the amount of neutron irradiation increases at the tip of the reactor control rod and the outer side of the blade, and when the reactor is fully inserted, about 1/3 from the insertion tip has a high reactivity value. I need. In this case, in the nuclear reactor control rod of the present invention, the neutron absorption plate is divided into a plurality of parts in the axial direction, so by varying the thickness or spacing of each neutron absorption plate, the neutron irradiation amount can be adjusted. It can be easily adapted to the required distribution of reactivity values. Therefore, there is no need for cumbersome machining processes such as machining a part of a long member as in the past, and there is no loss of material due to machining, so manufacturing costs can be reduced and the material can eliminate waste.

[0024] Furthermore, in the nuclear reactor control rod according to the present invention,
Since the neutron absorption plate as a neutron absorption element and the spacer for maintaining the opposing gap are made of substantially the same hafnium-based material, electrochemical corrosion is unlikely to occur.

Furthermore, the support material is rod-shaped, and is fixed to the sheath by welding or the like through, for example, a hole made in the neutron absorption plate. If the supporting material and the neutron absorbing plate are made of different materials, electrochemical crevice corrosion may occur, but the holes etc. drilled in the neutron absorbing plate should be made with a diameter that is smaller than the external shape of the supporting material, taking into consideration the difference in thermal expansion coefficient. Set large enough. Therefore, the cooling water stagnation phenomenon can be easily avoided and crevice corrosion (
There is almost no risk of clevis corrosion.

Furthermore, the supporting material and the sheath are made of substantially the same material, such as stainless steel, but since they can be welded together, electrochemical corrosion between the two can be avoided. There is no danger.

[0027]

Embodiments Hereinafter, embodiments of a control rod for a nuclear reactor according to the present invention will be described with reference to FIGS. 1 to 6.

In the configuration of this embodiment, the components other than the neutron absorbing element are substantially the same as those of the conventional structure, so a part of FIG. 7 will also be referred to as the embodiment.

FIGS. 1 to 3 show one embodiment. Figure 1
2 is a cross-sectional view showing the main components, FIG. 2 is a perspective view showing a neutron absorption element, and FIG. 3 is a sectional view taken along the line A--A in FIG. 1.

In this embodiment, a blade is constructed of a tie rod 1 that connects the tip structure member 4 and the end structure member 7, and a sheath 2 that projects radially from the tie rod 1 and whose outer shell portion has a deep U-shaped cross section. 2A (see FIG. 7). The sheath 2 is made of stainless steel or the like, and has a large number of water holes 8 formed in its constituent wall.

As shown in FIG. 1, a plurality of neutron absorbing elements 11 are provided inside the sheath 2 in an array parallel to the axis of the tie rod 1. Neutron absorption element 11
has a plate-frame shape as shown in FIGS. 1 to 3, and this plate-frame-shaped neutron absorption element 11 connects a flat neutron absorption plate 12 and opposing neutron absorption plates 12. It is constituted by a rod-shaped spacer 13.

The neutron absorption plate 12 is made of hafnium metal or hafnium alloy. As the hafnium alloy, an alloy obtained by diluting hafnium (Hf) with zirconium (Zr), titanium (Ti), or the like is used.

The neutron absorbing plates 12 are arranged opposite to each other along the inner surfaces of both side walls of the sheath 2, and are arranged at intervals in a direction parallel to the axis of the tie rod 1. In this embodiment, as shown in FIG. 1, three neutron absorbing elements 11 are installed in the sheath 2 constituting one blade 2A in the horizontal direction (between the tie rod 1 and the outer end of the blade 2A).
are arranged. Further, in the vertical direction, a large number of neutron absorption elements 11 are arranged as shown in FIG.

[0034] The spacer 13 is made of substantially the same material as the neutron absorption plates 12, and is connected within the range of the opposing surfaces of each neutron absorption plate 12 so as to maintain a constant distance from each other. This spacer 13 has a round bar shape, for example, and is inserted into the mounting hole 12A of the neutron absorption plate 12. It is convenient for manufacturing purposes to make the insertion end of the spacer 13 slightly smaller in diameter than the other portions, such as for accurate gap formation. In addition, a mounting hole 12 for inserting a spacer in the neutron absorption plate 12 is provided.
It is preferable that A and the outer diameter of the spacer 13 be approximately equal, so that no space for retained water is formed.

Further, the spacer 13 is provided within the range of the opposing surface of the neutron absorption plate 12, so that the spacer 13
is configured so that it does not protrude from the side end surface of the neutron absorption plate 12. This avoids the formation of unnecessary gaps within the sheath 2 and prevents the control rod reactivity value from decreasing.

[0036] Such a neutron absorbing element 11 is supported within the sheath 2 by a rod-shaped support member 14. That is, the support material 14 is made of the same material as the sheath 2, that is, made of stainless steel. A portion is fixed to the inner surface of the sheath 2 by welding or the like. Note that the diameter of the water passage hole 15 through which the support material 14 penetrates is larger than the diameter of the support material 14, and
The difference in thermal expansion between the sheath 2 and the sheath 2 and the difference in irradiation growth due to neutron irradiation can be sufficiently absorbed. As a result, reactor water is easily exchanged at the contact portion between the neutron absorption plate 12 and the support member 14, and the generation of stagnant water is prevented, and electrochemical corrosion problems such as clevis corrosion do not occur.

The neutron absorbing plate 12 (12a) located at the outer end of the blade 2A is set to be thicker than the other two neutron absorbing plates 12. This makes it possible to extend the life of the neutron absorption plate 12a, which is disposed in a region where the neutron irradiation amount is high and whose life tends to be shortened, and to effectively increase the reactivity value.

In addition, the ends of the pair of opposing neutron absorption plates 12 are at different levels in the vertical and horizontal directions (a≠b, a'≠b', c≠d, c'≠), except for a part. d'),
Similarly, the water passage holes 15 of the neutron absorbing plate 12 are also arranged at opposite positions at different levels.

Note that the portions of the end of the neutron absorbing plate 12 that are not stepped are, for example, the tie rods 1 and the blades 2.
A, a lateral end facing the outer end, and a longitudinal end facing the tip structure 5 and the end structure 6, etc.

[0040] With the above-mentioned stepped configuration, it is possible to suppress a decrease in reactivity value due to gaps formed between adjacent neutron absorbing elements 11. Note that when the gap is small (for example, about 1 mm), the decrease in reactivity value may be negligible, and in such a case, it is not necessary to use the above-mentioned stepped structure.

[0041] The material of the neutron absorption plate 12 may be varied depending on the level of neutron irradiation. For example, in areas where the neutron irradiation dose is high, hafnium metal plates are used, and in areas where the neutron irradiation dose is not high or where the reactivity value can be slightly lowered, other materials that form a solid solution alloy with hafnium, such as zirconium or titanium, are used. A hafnium alloy plate can also be obtained by using the above method. In this case, spacer 1
3 is also preferably made of a hafnium alloy having the same composition as the neutron absorption plate 12 in terms of weldability and electrochemical stability.

As described above, in the neutron absorbing element 11 outside the blade 2A, the neutron absorbing plate 12a is made thick, and the gap therebetween is narrow. Reactor water can freely flow between the gaps, and the spacer 13 and supporting material 14 provide agitation to the flow of reactor water, so a suitable cross flow occurs in the reactor water flowing through the gaps, and each neutron is absorbed. The plate 12 is effectively cooled and the integrity of the neutron absorbing plate 12 is improved.

In addition to the normally provided water holes 8, new water holes 8A and 8B are provided in the sheath 2 near the tie rods and at the outer end of the blade 2A, which allows for water cooling. Efforts are being made to further improve the effectiveness.

Further, a minute dimple ring is provided on the inner surface of the sheath 2 at a portion facing the outer surface of the neutron absorbing plate 12, if necessary.

According to the above embodiment, the neutron absorption plate 1
2 (inside the neutron absorbing element 11) and the axial gap between the neutron absorbing plates 12, the reactor water, which serves as a neutron moderator and coolant, freely flows vertically and horizontally. Therefore, the neutron moderating function of the reactor water is more effectively exhibited, and neutron absorption is performed more actively on the inner surface of the neutron absorption element 11 than in the conventional flat tube structure, and the reactivity effect is improved. At the same time, the neutron absorption plate 12, which generates heat due to neutron absorption, can be sufficiently cooled.

As a result, there is no need to particularly increase the thickness of the neutron absorbing plate 12, and the amount of hafnium material, which is expensive and has a high specific gravity, can be reduced, making it possible to reduce material costs.

[0047] Furthermore, the spacer 13 allows the reactor water to be appropriately agitated. Therefore, the temperature of the cooling water is made uniform, and the cooling characteristics of the high heat generation portion are further improved, so that the generation of voids in the cooling water is suppressed. Since this reactor water also functions as a neutron moderator, suppression of void generation leads to prevention of reduction in the reactivity effect of control rods. As a result of effectively cooling the neutron absorption plate 12, the integrity of the neutron absorption plate is improved.

Further, the spacer 13 is connected to the neutron absorbing plate 12.
Because it has a structure that does not protrude from the outer surface and outer edge of
The installation width of the neutron absorbing plates 12 within the sheath 2 is not restricted, and the distance between the neutron absorbing plates 12 is not narrowed. If the spacer 13 is configured to protrude from the outer surface and outer edge of the neutron absorbing plate 12, the neutron absorbing plate 12
An unnecessary gap would be created between each other or between the neutron absorbing plate 12 and the tie rod 1 or sheath 2, leading to a reduction in the reactivity value, but this does not occur in this embodiment.

Note that the amount of neutron irradiation increases at the tip of the reactor control rod and the outer side of the blade 2A, and when the reactor is fully inserted, the reactivity value is high up to about 1/3 from the insertion tip. Requires. In this case, in the reactor control rod of this embodiment, the neutron absorbing plate 12 is divided into a plurality of parts in the axial direction, and each neutron absorbing plate 12 is made to have a different thickness and a different spacing. It corresponds to the irradiation amount and the distribution of necessary reactivity values. Therefore, there is no need for cumbersome machining processes such as machining a part of a long member as in the past, and there is no loss of material due to machining, so manufacturing costs can be reduced and the material can eliminate waste.

Furthermore, in the nuclear reactor control rod according to this embodiment, the neutron absorption plate 12 and the pacer 13 are made of substantially the same hafnium-based material, so that electrochemical corrosion is less likely to occur.

Furthermore, the support member 14 is rod-shaped, and is fixed to the sheath 2 by welding or the like through, for example, a water passage hole 15 formed in the neutron absorption plate 12. Since the supporting material 14 and the neutron absorption plate 12 are made of different materials, electrochemical crevice corrosion may occur. Since the cooling water is set to be sufficiently larger than the outer diameter, the phenomenon of cooling water retention can be easily avoided, and there is almost no risk of crevice corrosion (clevis corrosion).

Furthermore, since the support member 14 and the sheath are made of the same material, that is, stainless steel, they can be welded together, and there is a risk of electrochemical corrosion between them. do not have.

FIGS. 4 to 6 show other embodiments of the invention.

In the embodiment shown in FIG. 4, inside the sheath 2,
Two neutron absorption elements 11 are arranged in a direction (horizontal direction) perpendicular to the axis of the control rod.

Furthermore, in the embodiment shown in FIG.
One neutron absorption element 11 is disposed within the control rod in a direction (horizontal direction) perpendicular to the axis of the control rod.

According to the embodiments shown in FIGS. 4 and 5,
Although the space for reactor water to flow is reduced, neutron absorption element 1
Since the number of units installed is reduced, manufacturing costs can be reduced.

As shown in FIGS. 4 and 5, in these embodiments, the neutron absorbing plate 12, which is the outer neutron absorbing element 11, is not particularly thick. This is suitable when the neutron irradiation is lower than in the previous example and the reactivity value is small, since the cooling effect is reduced due to the reduction in the flow rate of reactor water. The structure is such that the capacity is also reduced.

Note that the configurations shown in each of the above embodiments can be combined in various ways as necessary. for example,
In one control rod, the embodiment shown in FIG. 1 is applied to the insertion tip side, and the embodiment shown in FIG. 4 or 5 is applied to the distal end side. In addition, one control rod may be constructed by arbitrarily combining one, two, or three sets of the configurations of the respective embodiments described above.

FIG. 6 shows a modification of the spacer 13 that connects the neutron absorption plates 12 at regular intervals. That is, in this embodiment, the spacer 13 has a rectangular rod shape and is arranged at the outer end of the neutron absorption plate 12. Note that the end positions of the neutron absorbing plate 12 and the water passage holes 15 are arranged at different levels as described above.

[0060] Of course, even with such a configuration, substantially the same effects as those of the above-mentioned embodiments can be achieved. In this way, the spacer 13 is provided within a range that does not protrude from the side end surface of the neutron absorption plate 12, that is, within the range of the opposing surface of the neutron absorption plate 12, in order to prevent a reduction in control rod reactivity value due to the formation of unnecessary gaps. For example, its shape and arrangement can be changed in various ways.

[0061]

As described above, according to the control rod for a nuclear reactor according to the present invention, the gap between the facing surfaces and the axial gap of the neutron absorbing plate constituting the plate frame-shaped neutron absorbing element can be used as a neutron moderator. Since the reactor water, which is the coolant, can freely flow vertically and horizontally, the neutron moderation function of the reactor water is more effectively demonstrated, and the neutron absorption element is Neutron absorption is actively carried out on the inner surface side, improving the reactivity effect, and the neutron absorption plate, which generates heat due to neutron absorption, can be sufficiently cooled. As a result, there is no need to particularly increase the thickness of the neutron absorption plate, and the amount of hafnium material that is expensive and has a high specific gravity can be reduced, leading to a reduction in material costs.

In addition, the flow of reactor water is appropriately stirred by the arrangement of the spacers, so the temperature of the cooling water is made uniform, and the cooling characteristics of high heat generation parts are further improved, so that voids are prevented from forming in the cooling water. is suppressed. This reactor water also functions as a neutron moderator, so suppressing the generation of voids leads to preventing a reduction in the reactivity effect of the control rods, and as a result of effectively cooling the neutron absorption plate, the integrity of the neutron absorption plate is improved. can be improved.

Moreover, since the spacer is constructed so as not to protrude from the outer surface and outer edge of the neutron absorbing plate, the installation width of the neutron absorbing plate within the sheath is not restricted, and the distance between the neutron absorbing plates is not narrowed. . Therefore, unlike a structure in which the spacer protrudes from the outer surface and outer edge of the neutron absorption plates, unnecessary gaps are not created between the neutron absorption plates or between the neutron absorption plates and the tie rod or sheath. No reduction in reactivity value is caused.

Furthermore, the amount of neutron irradiation increases at the tip of the reactor control rod and the outer side of the blade, and when the reactor is fully inserted, about 1/3 from the insertion tip has a high reactivity value. However, in this case, according to the nuclear reactor control rod of the present invention, the neutron absorption plate is divided into a plurality of parts in the axial direction, so by making the thickness or spacing different for each neutron absorption plate, , it can be easily adapted to the neutron irradiation amount and the required reactivity value distribution. Therefore, there is no need for cumbersome machining processes such as machining a part of a long member as in the past, and there is no loss of material due to machining, so manufacturing costs can be reduced and the material can eliminate waste.

Furthermore, since the neutron absorption plate as the neutron absorption element and the spacer for maintaining the opposing gap are made of substantially the same hafnium material, electrochemical corrosion is less likely to occur. The support material is rod-shaped and is fixed to the sheath by welding or the like through, for example, a hole drilled in the neutron absorption plate. By taking into consideration the difference in coefficient of thermal expansion and setting the holes etc. to be sufficiently larger than the outer diameter of the support material, it is possible to easily avoid the cooling water retention phenomenon, and there is almost no risk of crevice corrosion or the like. Note that since the support material and the sheath are made of substantially the same material, there is no risk of electrochemical corrosion between them.

[Brief explanation of drawings]

FIG. 1 is a cross-sectional view showing one embodiment of the present invention.

FIG. 2 is a perspective view showing the neutron absorption element in the embodiment.

FIG. 3 is a sectional view taken along line AA in FIG. 1;

FIG. 4 is a cross-sectional view showing another embodiment of the present invention.

FIG. 5 is a cross-sectional view showing still another embodiment of the present invention.

FIG. 6 is a perspective view showing still another embodiment of the present invention.

FIG. 7 is a perspective view showing a conventional example.

FIG. 8 is a sectional view taken along line BB in FIG. 7;

[Explanation of symbols]

1 Tie rod 2 Sheath 2A Blade 5　Advanced structural material 7 Terminal structural material 11 Neutron absorption element 12 Neutron absorption plate 13 Spacer 14 Support material

Claims (1)

[Claims] 1. A tie rod that connects a tip structure member and a terminal structure member, a blade that projects radially from the tie rod and is constituted by a sheath whose outer shell has a deep U-shaped cross section, and a blade provided inside the sheath. and a neutron absorption element arranged parallel to the axis of the tie rod, the neutron absorption element being made of hafnium metal or hafnium alloy and having a reactor water flow space inside. In the control rod, the neutron absorbing elements have a plate frame shape, and the plate frame-shaped neutron absorbing elements are arranged opposite to each other along the inner surfaces of both side walls of the sheath, and are arranged at intervals in a direction parallel to the axis of the tie rod. flat plate-shaped neutron absorption plates made of hafnium metal or hafnium alloy arranged in the neutron absorption plates, and areas of their opposing surfaces made of substantially the same material as the neutron absorption plates so as to maintain the opposite sides of each neutron absorption plate at a constant distance from each other. and a rod-shaped spacer connected within the sheath, and each supported within the sheath by a rod-shaped support member made of substantially the same material as the sheath and having both ends fixed to the inner surface of the sheath. Control rods for nuclear reactors.