JP2014211309A - Control rod - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a control rod which can further reduce the potential of a stress corrosion crack of a sheath by simplifying a spacer member.SOLUTION: A control rod is constituted in such a manner that a handle is attached to an upper end of a tie rod 2, a lower support member is attached to a lower end of the tie rod 2, and the control rod has four pieces of blades 3 which extend in four directions from the tie rod 2. Each blade 3 has such a constitution that a plurality of hafnium rods 5 are arranged in U-shaped sheaths 4, and the sheaths 4 are attached to the tie rod 2, the handle and the lower support member. Each hafnium rod 5 has a large-diameter part in an upper part and a small-diameter part 5B in a lower part. A plurality of the hafnium rods 5 are aligned in one row in the sheaths 4. Wire spacers 6 having shapes formed by bending spring-steel wires 8 in 8-figures are arranged in the sheaths 4, and hold the small-diameter parts 5B of the respective hafnium rods 5 at prescribed intervals.

Description

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

  In nuclear power plants, control rods having neutron absorbers are used to control reactor power. The control rods used in boiling water reactors have a cross-shaped cross section, and are inserted between multiple fuel assemblies loaded in the reactor core placed in the reactor pressure vessel, and pulled out from the reactor core. And insert into the core to control the reactor power.

  The structure of a conventional control rod used in a boiling water reactor will be briefly described. This control rod has four blades extending in four directions from a tie rod located on the central axis of the tie rod, with a handle joined to the upper end of the tie rod and a drop speed limiter joined to the lower end of the tie rod. Each blade has a sheath having a U-shaped cross section attached to a tie rod and a plurality of hafnium rods that are neutron absorber rods (see, for example, Japanese Patent Laid-Open No. 10-2983). The sheath is made of stainless steel. These hafnium rods are arranged in the sheath in parallel to the width direction of the blade. Each hafnium bar is a solid round bar, and the lower diameter is smaller than the upper diameter in the axial direction of the hafnium bar. The lower portion of the hafnium rod having a small diameter is referred to as a thin portion for convenience, and the upper portion of the hafnium rod having a diameter larger than that of the thin portion is referred to as a large portion for convenience.

  In each blade, a plurality of rectangular spacers extending in the width direction of the blade are arranged in the sheath with a predetermined interval in the axial direction of the control rod. Each spacer arranged in the axial direction of the control rod has a plurality of through-holes into which the small diameter portion of the hafnium rod is inserted in the longitudinal direction, and is fixed to the inner surface of the sheath by welding. The small diameter portion of the hafnium rod arranged in the sheath is inserted into the through hole of each spacer arranged in the axial direction and is held by the spacer. Thus, the space | interval of the small diameter part of each hafnium rod is hold | maintained by the spacer. The spacer prevents buckling of the small diameter portion of the hafnium rod during emergency insertion of the control rod into the core.

JP-A-10-2983

  Due to the fact that the spacer that holds the narrow diameter part of the hafnium rod is attached to the sheath by welding, the welding residual stress generated at the welded portion of the spacer and the sheath, and the rectangular spacer, the sheath, and the neutron absorbing rod (specifically Therefore, it is desired to reduce the stress corrosion cracking potential of the sheath due to crevice corrosion at the contact portion with each of the hafnium rods). In addition, this spacer needs to form a plurality of through-holes into which the small-diameter portion of the neutron absorber rod is inserted, so that the structure becomes complicated, and there is a problem that the time required for manufacturing the spacer is long.

  An object of the present invention is to provide a control rod capable of simplifying the spacer member and further reducing the stress corrosion cracking potential of the sheath.

  A feature of the present invention that achieves the above-described object is that four blades extending in all directions from the tie rod have a U-shaped cross section and both ends are attached to the tie rod, respectively, and a tie rod within the sheath. It has a plurality of neutron absorber bars arranged in a row toward the curved surface of the sheath, and the neutron absorber bar is located on the lower support member side with respect to the large diameter part located on the handle side and the large diameter part, and the diameter is A spacer member that includes a thin portion that is smaller than the diameter of the large diameter portion and that holds the thin portions of each of the plurality of neutron absorber rods disposed in the sheath is disposed in the sheath, and the wire However, they are disposed between the small diameter portions of all the neutron absorber rods arranged in a line in the sheath and between the small diameter portion and the inner surface of the sheath.

  A space member that is arranged in the sheath and holds a plurality of neutron absorber rods in the sheath is constituted by a wire, and each of the small diameter portions of all the neutron absorber rods arranged in a row in the sheath is mutually connected. Since it is disposed between the narrow-diameter portion and the inner surface of the sheath, the spacer member can be simplified, and the stress corrosion cracking potential of the sheath can be further reduced.

  According to the present invention, the spacer member disposed in the sheath can be simplified, and the stress corrosion cracking potential of the sheath can be further reduced.

It is a perspective view of the control rod of Example 1 which is one suitable Example of this invention. It is II-II sectional drawing of FIG. FIG. 2 is a detailed perspective view of the hafnium rod shown in FIG. 1. It is IV-IV sectional drawing of FIG. It is a cross-sectional view of the control rod of Example 2, which is another preferred embodiment of the present invention. It is a cross-sectional view of the control rod of Example 3, which is another preferred embodiment of the present invention. It is a cross-sectional view of the control rod of Example 4, which is another preferred embodiment of the present invention. It is a cross-sectional view of the control rod of Example 5, which is another preferred embodiment of the present invention. It is a cross-sectional view of the control rod of Example 6, which is another preferred embodiment of the present invention.

  Examples of the present invention will be described below.

  A control rod according to Embodiment 1, which is a preferred embodiment of the present invention, will be described with reference to FIGS.

  The control rod 1 of the present embodiment is a control rod used in a boiling water reactor, and includes a tie rod 2, a plurality of blades 3, a handle 9, and a lower support member 10. The control rod 1 has a cross-shaped cross section, and four blades 3 extend in four directions from the tie rod 2. Each blade 3 includes a stainless steel sheath 4 having a U-shaped cross section and a plurality of hafnium rods 5 which are neutron absorbing rods. The sheath 4 bent into a U-shape forms a curved surface at the bent portion.

  A handle 9 is attached to the upper end of the tie rod 2 and is present on each of the four blades 4. A lower support member 10 is attached to the lower end of the tie rod 2 and exists under each of the four blades 4. A drop speed limiter 11 is provided at the lower end of the lower support member 10, and a lower handle 12 is provided at the lower support member 10. The side end portions on both sides of the U-shaped sheath 3 are attached to the side surfaces of the tie rod 2 (see FIG. 2), and the upper end portion of the sheath 3 is attached to the handle 9. A lower end portion of the sheath 3 is attached to the lower support member 10.

  A plurality of hafnium bars 5 are disposed between the lower support member 10 and the handle 9 in each sheath 4. These hafnium rods 5 which are neutron absorber rods are arranged in a row in the cross section of the control rod 1 from the tie rod 2 toward the bent portion of the sheath 4 which is the side end of the blade 3. As shown in FIG. 3, the hafnium rod 5 is divided into two regions having different diameters in the axial direction of the hafnium rod 5. In the hafnium rod 5, the region located on the insertion end side of the control rod 1 among these two regions is the large diameter portion 5A, and the other region located on the withdrawal end side of the control rod 1 is the small diameter portion 5B. The diameter of the small diameter part 5B is smaller than the diameter of the large diameter part 5A. The small diameter portion 5B occupies 1/2 of the total length of the hafnium rod 5 in the axial direction. A projection 7 is formed at the insertion side end (upper end) of the hafnium rod 5, and the diameter of the projection 7 is smaller than that of the large diameter portion 5A. A groove 8 is formed at the base of the protrusion 7 over the entire circumference of the protrusion 7. Each hafnium rod 5 is suspended from the handle 5 in a state where the projection 7 of each hafnium rod 5 is inserted into a projection insertion groove (not shown) formed at the lower end of the handle 9. The protrusion insertion groove has a vertical cross-sectional shape that is the same as that of the curtain rail groove.

  In each blade 3, a spacer (spacer member) 6 that holds each small-diameter portion 5 </ b> A of all the hafnium rods 5 disposed in the sheath 4 is configured by a single spring steel wire (for example, a circular cross section). The This spacer 6 is called a wire spacer for convenience. The wires of the wire spacer 6 are disposed along the side surfaces on both sides of the small diameter portion 5B of each hafnium rod 5 disposed in the sheath 4, and intersect each other between the small diameter portions 5B. In the wire spacer 6, such a wire arrangement is repeated for each small diameter portion 5 </ b> B arranged in the sheath 4. For this reason, the space | interval between the small diameter parts 5B of these hafnium rods 5 arranged in a line in the sheath 4 is held by the 8-shaped wire spacer 6. By the wire spacer 6 having the shape of figure 8, the small diameter portions 5B are arranged at predetermined intervals in the width direction of the blade 3, and are arranged in the center in the thickness direction of the blade 3 between the inner surfaces facing each other of the sheath 6. Is done. In the thickness direction of the blade 3, the widths of the gaps formed between the small diameter portion 5B and each of the opposing inner surfaces of the sheath 4 are equal. In addition, the both ends of each wire of the plurality of wire spacers 6 are wound around the small-diameter portion 5B of the hafnium rod 5 located at one end of the array of the hafnium rods 5 arranged in a row in the sheath 4, Each wire spacer 6 is held by the small diameter portion 5B of the hafnium rod 5. The wire of the wire spacer 6 passes through a gap formed between the small diameter portion 5B and the inner surface of the sheath 4. The plurality of 8-shaped wires 6 are arranged at predetermined intervals in the axial direction of the control rod 1 at the position of the small diameter portion 5B.

  A method for manufacturing the control rod 1 to which a plurality of wire spacers 6 are attached will be described below.

  The handle 9 is attached to the upper end of the tie rod 2 by welding. The lower support member 10 provided with the drop speed limiter 11 is attached to the upper end of the tie rod 2 by welding. With the plurality of 8-shaped wires 6 attached to the small diameter portion 5B, the protrusions 7 formed on the upper ends of the plurality of hafnium rods 5 arranged in the sheath 4 of one blade 3 are handles. 9 is inserted into a protruding portion insertion groove formed at the lower end of 9. As a result, these hafnium rods 5 are attached to the handle 9.

  Attachment of the plurality of 8-shaped wire spacers 6 to the small diameter portions 5B of a predetermined number of hafnium rods 5 arranged in the sheath 4 is performed as follows using a jig.

  This jig has one flat plate and a restraining member that is disposed on the upper surface of the flat plate with a predetermined interval and attached to the upper surface. The distance between the pair of restraining members is such that the number of hafnium rods 5 arranged in a line in one blade 3 toward the curved surface of the sheath 4 bent in a U shape from the tie rod 2 is equal to the large diameter portion 5A. The dimension obtained by multiplying the diameter of Further, the jig has one presser bar having both ends attached to one end of each of the pair of restraining members.

  When a plurality of 8-shaped wires 6 are attached to the small-diameter portions 5B of the predetermined number of hafnium rods 5, the hafnium rods 5 are placed on the flat plate of the jig placed on the support base. Then, the large-diameter portions 5A of the hafnium rods 5 are arranged between the pair of restraining members so that they contact each other. At this time, the end surface of the protrusion 7 formed on each large diameter portion 5A is in contact with the presser bar of the jig. A predetermined number of hafnium rods 5 arranged in the sheath 4 are arranged between the pair of restraining members. Since the jig has the presser bar, the end surfaces of the protrusions of the hafnium bars 5 can be aligned at the same position. In order to prevent the large diameter portion 5A of the hafnium rod 5 placed on the flat plate from floating upward from the flat plate, a plurality of pressing members are attached to the pair of restraining members from above the large diameter portions 5A. Each large diameter portion 5A may be pressed against the flat plate side of the jig by each pressing member.

  A wire spacer 6 having a figure-eight shape is attached to the small diameter portion 5B from the end of the small diameter portion 5B of each hafnium rod 5 placed on the flat plate of the jig. The wire spacer 6 having the shape of figure 8 has a plurality of insertion spaces (not shown) for inserting the small diameter portion 5B between the intersecting wires. After the small diameter portion 5B of each hafnium rod 5 placed on the flat plate of the jig is inserted into each insertion space formed in the wire spacer 6, the wire spacer 6 is thickened along the small diameter portion 5B. The movement is stopped toward the diameter portion 5A, and the movement is stopped at a predetermined position in the axial direction of the hafnium rod 5 in the small diameter portion 5B. In this way, the plurality of wire spacers 6 are sequentially attached to the respective small diameter portions 5B. It is necessary to make the pitch of the hafnium rods 5 at the projections 7 of the hafnium rods 5 arranged in a line and the pitch of the hafnium rods 5 at the narrow diameter portions 5B of the hafnium rods 5 equal. For example, by adjusting the tightening degree of each wire spacer 6 attached to each small-diameter portion 5B, the pitch of both is adjusted to be the same.

  A plurality of hafnium rods 5 arranged in a line and bundled by a plurality of mounted wire spacers 6 are taken out from the jig. The projections 7 of these hafnium rods 5 to which a plurality of wire spacers 6 are attached are inserted in order into the grooves formed in the handle 9 joined to the tie rod 2 by welding. When the hafnium rod 5 located closest to the tie rod 2 in the array of hafnium rods 5 to which the wire spacers 6 are attached is moved to the vicinity of the tie rod 2, each hafnium rod 5 of this arrangement arranged on one blade 3. Is attached to the handle 9.

  Next, the U-shaped bent sheath 4 is joined to the tie rod 2, the handle 9, and the lower support member 10 by welding. Prior to welding the sheath 4 to these members, the hafnium rods 5 mounted with a plurality of wire spacers 6 and arranged in a row move the sheath 4 bent in a U shape toward the tie rod 2. As a result, the sheath 4 is inserted between the opposing inner surfaces. In a state where each hafnium rod 5 is completely inserted into the sheath 4, both end portions of the sheath 4 bent in a U shape are attached to the tie rod 2 by welding. The end of the sheath 4 on the handle 9 side is attached to the handle 9 by welding. Furthermore, the end of the sheath 4 on the lower support member 10 side is attached to the lower support member 10 by welding.

  Each hafnium rod 5 arranged in the other three blades 3 is similarly attached to the handle 9. Each sheath 4 constituting each blade 3 is joined to the tie rod 2, the handle 9, and the lower support member 10 by welding.

  The control rod 1 is manufactured through the above steps. In the manufactured control rod 1, each wire spacer 6 formed in a shape of 8 with one wire is arranged in the sheath 4 as follows. The wires arranged in the shape of a letter 8 in the sheath 4 are arranged so as to intersect each other of the small diameter portions 5B of all the hafnium rods 5 arranged in a line in the sheath 4. Further, two gaps are formed between each of the opposing inner surfaces of the sheath 4 and the small diameter portion 5B. The two gaps are formed for each small diameter portion 5B arranged in the sheath 4. The wires arranged in the shape of figure 8 in the sheath 4 are arranged in two gaps formed for each of the small diameter portions 5B arranged in the sheath 4.

  Since each wire spacer 6 is made of spring steel, the tightening force of the wire spacer 6 acts on the small diameter portion 5B of each hafnium rod 5. Each wire spacer 6 is held by each small diameter portion 5 </ b> B by this tightening force and does not fall toward the lower support member 10.

  Since the control rod 1 of this embodiment uses the wire spacer 6 made of a wire, the structure of the spacer that holds the small diameter portion 5B of each hafnium rod 5 is simplified. For this reason, manufacture of the wire spacer 6 becomes easy and the time required for manufacture of the control rod 1 is also shortened. Moreover, since the wire spacer 6 is not welded to the sheath 4, the occurrence of stress corrosion cracking of the sheath 4 can be suppressed. The contact area between the wire spacer 6 and the inner surface of the sheath 4 is reduced as compared with the contact area between the spacer and the inner surface of the sheath in the conventional control rod. For this reason, the crevice corrosion of the sheath 4 in the control rod 1 is suppressed, and as a result, the occurrence of stress corrosion cracking of the sheath 4 is further suppressed in combination with the presence of the welded portion of the wire spacer 6 to the sheath 4. The

  A control rod according to embodiment 2 which is another preferred embodiment of the present invention will be described with reference to FIG. The control rod 1A of the present embodiment is a control rod used in a boiling water reactor.

  1 A of control rods have the structure which replaced the wire spacer 6 formed in the figure 8 shape in the control rod 1 of Example 1 with the wire spacer 6A. Other configurations of the control rod 1A are the same as those of the control rod 1. The wire spacer 6A has substantially the same configuration as the wire spacer 6, and both ends of the wire constituting the wire spacer 6A are attached to the tie rod 2 by welding. The other configuration of the wire spacer 6A is the same as that of the wire spacer 6.

  The control rod 1A of the present embodiment is different from the control rod 1 of the first embodiment only in the shape of the wire spacer, and the control rod 1A is manufactured in the same manner as the control rod 1. Also in the control rod 1A, as in the first embodiment, a plurality of wire spacers 6A are mounted on the small diameter portion 5B of each hafnium rod 5 arranged in one blade 2, and a plurality of wire spacers 6A are mounted. The hafnium rod 5 is inserted into the protrusion insertion groove of the handle 9 welded to the tie rod 2 and attached to the handle 9. In this state, both ends of the wire made of spring steel of each wire spacer 6A are attached to the side surface of the tie rod 2 by welding. Thereafter, the sheath 4 bent into a U-shape is attached to the tie rod 2, the handle 9, and the lower support member 10 by welding.

  In the present embodiment, each effect produced in the first embodiment can be obtained.

  A control rod according to embodiment 3, which is another preferred embodiment of the present invention, will be described with reference to FIG. The control rod 1B of the present embodiment is a control rod used in a boiling water reactor.

  The control rod 1B has a configuration in which the wire spacer 6 formed in the figure 8 in the control rod 1 of the first embodiment is replaced with a wire spacer 6B. Other configurations of the control rod 1B are the same as those of the control rod 1. The wire spacer 6B has substantially the same configuration as that of the wire spacer 6, and the spring steel wires constituting the wire spacer 6B are twisted once between the small diameter portions 5B of the hafnium rod 5. . It is attached to the tie rod 2 by welding. The other configuration of the wire spacer 6B is the same as that of the wire spacer 6. The control rod 1B of this embodiment, which is different from that of the control rod 1 only in the structure of the wire spacer, is manufactured substantially in the same manner as the control rod 1.

  In the present embodiment, each effect produced in the first embodiment can be obtained. In the present embodiment, the wire spacer 6B can be held in the axial direction of the control rod 1B at an arbitrary position according to the number of times the wire of the wire spacer 6B is twisted.

  A control rod according to embodiment 4, which is another preferred embodiment of the present invention, will be described with reference to FIG. The control rod 1C of the present embodiment is a control rod used in a boiling water reactor.

  The control rod 1C has a configuration in which the wire spacer 6 formed in the figure 8 in the control rod 1 of the first embodiment is replaced with a wire spacer 6C. The other structure of the control rod 1C is the same as that of the control rod 1. The wire spacer 6 </ b> C has substantially the same configuration as the wire spacer 6, and an array of hafnium rods 5 in which both ends of spring steel wires constituting the wire spacer 6 </ b> C are arranged in a row in the sheath 4. The hafnium rod 5 closest to the tie rod 2 and the tie rod 2 are twisted once. The control rod 1C of this embodiment, which is different from that of the control rod 1 only in the structure of the wire spacer, is manufactured substantially in the same manner as the control rod 1.

  In the present embodiment, each effect produced in the first embodiment can be obtained.

  A control rod according to embodiment 5, which is another preferred embodiment of the present invention, will be described with reference to FIG. The control rod 1D of the present embodiment is a control rod used in a boiling water reactor.

  The control rod 1D has a configuration in which the wire spacer 6 formed in the figure 8 in the control rod 1 of the first embodiment is replaced with a wire spacer 6D. Other configurations of the control rod 1D are the same as those of the control rod 1. The wire spacer 6D is a wire made of spring steel between the adjacent small diameter portions 5B of the plurality of hafnium rods 6 arranged in the sheath 4 of the blade 3 in the cross section of one blade 3. It has the structure arrange | positioned so that it may meander while contacting the surface of the small diameter part 5B. The wires of the wire spacer 6D are arranged between every other small diameter part 5B arranged in the sheath 4 and one inner surface of the sheath 4 in the arrangement of the small diameter parts 5B arranged in the sheath 4. The sheath 4 is disposed between the other inner surface facing the inner surface and the remaining small diameter portion 5B.

  In the present embodiment, unlike the first embodiment, the wire of the wire spacer 6D is one of two gaps formed between each of the opposed inner surfaces of the sheath 4 and the small diameter portion 5B in each hafnium rod 5. Placed in one gap. As a result, the length of the wire of one wire spacer 6D that contacts the outer surface of one thin diameter portion 5B in the circumferential direction of the thin diameter portion 5B is larger than the contact length of the wire of the wire spacer 6D in the first embodiment. (The former contact length is approximately ½ of the latter contact length).

  The attachment of the wire spacer 6D to the small diameter portion 5B of each hanium rod 5 arranged in the sheath 4 is performed in the same manner as the wire spacer 6 in the first embodiment. Both end portions of the wire spacer 6D are wound around the small diameter portions 5B of the hafnium rods 5 located at both end portions of the array of the hafnium rods 5 arranged in a line in the sheath 4 (see FIG. 8).

  The control rod 1D of this embodiment, which is different from that of the control rod 1 only in the structure of the wire spacer, is manufactured substantially in the same manner as the control rod 1.

  By the action of the wire spacer 6D, the pitches of the plurality of small diameter portions 5B arranged in a row in the sheath 4 are substantially equal, and are arranged in the center in the thickness direction of the blade 3 between the opposing inner surfaces of the sheath 6. Is done.

  In the present embodiment, each effect produced in the first embodiment can be obtained. In the present embodiment, the structure of the wire spacer 6D is further simplified than the wire spacer 6 used in the first embodiment.

  A control rod according to embodiment 6, which is another preferred embodiment of the present invention, will be described with reference to FIG. The control rod 1E of the present embodiment is a control rod used in a boiling water reactor.

  The control rod 1E has a configuration in which the wire spacer 6D is replaced with the wire spacer 6E in the control rod 1D of the fifth embodiment. Other configurations of the control rod 1E are the same as those of the control rod 1D. The wire spacer 6E is made of a spring steel wire between all adjacent small diameter portions 5B of the plurality of hafnium rods 6 arranged in the sheath 4 of the blade 3 in the cross section of one blade 3. Each hafnium rod 5 is bent in a U-shape and arranged in each gap formed between the same inner surface of the two inner surfaces facing each other of the sheath 4 and the respective small diameter portions 5B (FIG. 9). reference). The wire of the wire spacer 6E is in contact with the small-diameter portion 5B in one hafnium rod 5 with a length exceeding 1/2 of the circumferential length on the outer surface of the small-diameter portion 5B. However, this wire is not in contact with the small diameter portion 5B over the entire circumference of the small diameter portion 5B.

  Due to the action of the wire spacer 6E, similarly to the control rod 1D, also in this embodiment, the pitches of the plurality of small-diameter portions 5B arranged in the sheath 4 and arranged in a row are substantially equal. Between the inner surfaces, the blades 3 are arranged in the center in the thickness direction.

  The attachment of the wire spacer 6D to the small diameter portion 5B of each hanium rod 5 arranged in the sheath 4 is performed in the same manner as the wire spacer 6 in the first embodiment. Both end portions of the wire spacer 6E are wound around the small diameter portions 5B of the hafnium rods 5 positioned at both end portions of the array of the hafnium rods 5 arranged in a line in the sheath 4 (see FIG. 8).

  The control rod 1E of the present embodiment, which is different from that of the control rod 1 only in the structure of the wire spacer, is manufactured substantially in the same manner as the control rod 1.

  In the present embodiment, each effect produced in the first embodiment can be obtained. In the present embodiment, the structure of the wire spacer 6E is further simplified than the wire spacer 6 used in the first embodiment.

  1, 1A, 1B, 1C, 1D, 1E ... control rod, 2 ... tie rod, 3 ... blade, 4 ... sheath, 5 ... hafnium rod, 5A ... large diameter portion, 5B ... small diameter portion, 6, 6A, 6B, 6C, 6D, 6E ... wire spacer, 9 ... handle, 10 ... lower support member.

Claims (5)

A tie rod, a handle attached to an upper end portion of the tie rod, a lower support member attached to a lower end portion of the tie rod, and four blades extending in four directions from the tie rod,
The blade has a U-shaped cross section, a sheath having an upper end attached to the handle, a lower end attached to the lower support member, and both side ends attached to the tie rod, and the tie rod in the sheath. Having a plurality of neutron absorber rods arranged in a row from the sheath to the curved surface,
The neutron absorber rod includes a large diameter portion located on the handle side and a small diameter portion located on the lower support member side than the large diameter portion and having a diameter smaller than the diameter of the large diameter portion,
A spacer member constituted by a wire that holds each small-diameter portion of the plurality of neutron absorber rods disposed in the sheath is disposed in the sheath,
The wire is disposed between each of the small diameter portions of all the neutron absorber rods arranged in a line in the sheath, and is disposed between the small diameter portion and the inner surface of the sheath. Control rod characterized by that.   The control rod according to claim 1, wherein the wire is made of spring steel.   Two gaps formed between each of the opposing inner surfaces of the sheath and the small-diameter portion are formed for each of the small-diameter portions, and the spacer member forms one wire into an 8-shape. The wires arranged in the shape of 8 are crossed between each of the small diameter portions of all the neutron absorber rods arranged in a row, and 3. The control rod according to claim 1, wherein the control rod is disposed in each of the two gaps formed for each of the small diameter portions in contact with an outer surface of the small diameter portion.   The wires are arranged between the thin diameter portions of all the neutron absorber rods arranged in the row, and meander in the sheath while being in contact with the outer surface of the thin diameter portions. Each of the other small diameter portions arranged in the sheath and one of the opposing inner surfaces of the sheath, and the remaining small diameter portions arranged in the sheath. The control rod according to claim 1, wherein the control rod is disposed between the inner surface of the sheath and the other inner surface of the sheath.   The wires are arranged between the thin portions of all the neutron absorber rods arranged in the row, and are bent in a U shape between the thin portions, respectively. And the control rod of Claim 1 or 2 arrange | positioned in each clearance gap formed between the same one inner surface of the two inner surfaces which the said sheath opposes, and each small diameter part 5B.

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