JP2000121769A - Control rod for reactor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent a covering pipe from being damaged by the swelling of a neutron absorber in a control rod for a reactor. SOLUTION: A control rod 20 is provided with a covering pipe 21 whose lower end is closed by a lower edge plug 3, an upper neutron absorber 23 and a lower neutron absorber 27 that are provided in the covering pipe 21, and a sleeve 25 for supporting the upper neutron absorber 23 from a lower part while being provided in the covering pipe 21 by surrounding the lower neutron absorber 27. A gap 33 for absorbing volume inflation is demarcated between an upper lid 29 of the sleeve 25 and an upper end face 31 of the lower neutron absorber 27, and a frusto-conical surface 25 where the inner surface of the sleeve 25 surrounding a gap 33 has a gentle slope is formed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a control rod for a nuclear reactor, and more particularly, to a structure of a control rod of a pressurized water reactor.

[0002]

2. Description of the Related Art In a pressurized water reactor, a canless fuel assembly having a hollow tube called a control rod guide thimble or a guide tube disposed between a number of fuel rods is used as a fuel assembly constituting a reactor core. ing. Then, a plurality of control rods movably inserted into the control rod guide tube are connected to form a control rod assembly or a control rod cluster, which is driven by a control rod driving device provided on the reactor vessel top cover. That is, the control rod is inserted into or pulled out of the core constituted by a large number of fuel assemblies to control the reactivity of the core. As described above, since the control rod is elongated, it is easy to swing, and since the control rod is inserted into the control rod guide tube, the tip portion is easily rubbed and worn. Furthermore, during normal operation of the nuclear reactor, only the tip of the control rod is located in the upper part of the core, and the volume expansion or swelling due to neutron absorption of the neutron absorber located there is large. For this reason, there is a tendency that the cladding tube of the control rod is pushed out from the inside, and the above-mentioned wear is easily promoted. That is, abrasion damage at the lower end of the cladding tube of the control rod has a great effect on the life of the control rod.

[0003] In order to prevent the above-mentioned control rod cladding tube from being worn and damaged and to prolong its life, a structure of a control rod tip portion as shown in FIG. 11 has been proposed. In brief, a lower end plug 3 is provided at the lower end of the cladding tube 1 made of stainless steel.
Are hermetically welded by circumferential welding 5. A sleeve 9 is provided in the cladding tube 1 while leaving a helium gas space 7, and a narrow neutron absorber 11 having a narrow diameter is provided therein. As the structure of the lower neutron absorber 11, although not shown, a hollow shape or a shape with an outer peripheral slit has been proposed. The lower neutron absorber 11
A gap 15 for absorbing volume expansion is defined between the upper lid 13 and the upper lid 13, and an upper neutron absorber 17 is provided on the upper lid 13. Further, a chrome plating 19 is applied to the outer surface of the cladding tube 1 as shown in an exaggerated manner.
In short, the diameter of the lower neutron absorber 11 is reduced to prevent the cladding tube from being spread and to prevent wear by chrome plating.

[0004]

The driving system of the control rod driving device of the pressurized water reactor is such that the control rod driving shaft, that is, the control rod is stepped in the axial direction using an electromagnetically driven latch mechanism. It is generally driven. The acceleration generated when the control rod is intermittently driven by this method may reach 20 G or more, and the lower neutron absorber 11 described above moves in the vertical direction due to the gap 15 to cause collision. This may cause inconveniences such as expansion in the lateral direction and generation of excessive stress in the girth welded portion, and even if the intended purpose is achieved, another problem may occur. Accordingly, an object of the present invention is to provide a control rod for a nuclear reactor, in which the relative movement of the lower neutron absorber when the control rod is driven is effectively prevented, and the cladding tube does not suffer from abrasion damage or the like.

[0005]

According to the present invention, a control rod for a nuclear reactor is provided with a cladding tube having an upper end and a lower end closed by end plugs, and provided in the cladding tube. An upper neutron absorber, a lower neutron absorber, and a sleeve provided in the cladding tube surrounding the lower neutron absorber and supporting the upper neutron absorber from below, the upper lid of the sleeve and the upper end surface of the lower neutron absorber And a gap for absorbing volume expansion is defined between them, and the gap is provided with a displacement restraining structure that restrains high-speed displacement of the lower neutron absorber but restrains low-speed displacement. As the displacement restraining structure, the inner surface of the sleeve may be formed as a frusto-conical surface having a gentle slope, or may be a spring member or a thin ring, and further, the sleeve may be filled with helium gas. Alternatively, a solid disk-shaped neutron absorber for restricting the flow of the helium gas may be provided.

[0006]

Embodiments of the present invention will be described below with reference to the accompanying drawings. The same reference numerals are given to the same parts throughout the drawings. Referring first to FIG. 1, the overall structure of the lower end of a control rod 20 according to the present invention is shown. In the figure, the lower end of a cladding tube 21 made of stainless steel such as SUS304 is fitted to a lower end plug 3, and both are hermetically joined by a circumferential weld 5. Although not shown, the cladding tube 21 is an elongated tube having a length of about 3 to 4 meters in an actual machine, and a similarly long upper neutron absorber (hereinafter referred to as an upper absorber) 23 is provided therein. And rests on a sleeve 25 with a lid. A relatively short lower neutron absorber (hereinafter, referred to as a lower absorber) 27 is provided in the sleeve 25. A gap 3 for absorbing volume expansion between the upper lid 29 of the sleeve 25 and the upper end surface 31 of the lower absorber 27.
3 are defined. The inner surface of the sleeve 25 surrounding the gap 33 forms a gently inclined frusto-conical surface 25a. Although not shown, the outer surface of the control rod 20 including the cladding tube 21 is plated with chrome to a required extent.

The mutual structural relationship between the sleeve 25 and the lower absorber 27 is shown enlarged in FIG. Sleeve 2
A small circumferential gap 35 is formed between 5 and the lower absorber 27, and the above-mentioned gap 33 is defined above.
The truncated conical surface 25a surrounding the gap 33 is shown with an exaggerated taper angle for easy understanding.

When the control rod 20 having the above-described configuration is inserted into the core, the lower absorber 27 and the upper absorber 23 absorb neutrons and control the reactivity of the core. The lower absorber 27 in the core for a relatively long period of time has a relatively large amount of swelling due to a large accumulated amount of absorbed neutrons, but since the upward displacement of the upper end surface 31 due to this swelling is slow, The frusto-conical surface 25a does not prevent this, and the gap 33 absorbs swelling. do not do. However,
When the control rod 20 is moved up and down, as shown in the partially enlarged view of FIG. 3, when the lower absorber 27 is to be relatively displaced in the direction of the arrow, it comes into contact with the truncated conical surface 25a at the point C and receives frictional resistance. In this way, the rapid displacement of the lower absorber 27 is suppressed by the frictional force, and the generation of an impact load and the generation of excessive stress in each part due to the impact load are reduced.

Next, another embodiment of the present invention will be described with reference to FIG. In this embodiment, the sleeve 45 defines a gap 43 for absorbing volume expansion above the lower absorber 27, but its inner surface is a cylindrical surface and is not inclined. A spring 47 having a shape similar to a washer is provided in the gap 43.
Is interposed. The spring 47 has a circular washer portion 47a as shown in the bottom view of FIG. 5, and further has a tongue piece 47b which is bent and raised. The washer 47a rests on the upper end face 31 as shown in FIG. 4, and the tip of the tongue piece 47b contacts the lower surface of the upper lid 49 to exhibit a spring action. Thus, the spring 4
7 deforms to absorb the displacement of the upper end face 31 due to volume expansion, and further holds the lower absorber 27 against a stepping load when the control rod is driven. At this time, since the elastic force of the spring 47 is set to be larger than the stepping load,
The lower absorber 27 is securely held without any rattling,
In addition, while holding the stepping load from a low cycle to a high cycle, the holding effect is particularly large for low frequency components.

Further, a thin ring or collar 57 may be used instead of the spring 47 as shown in FIG.
In FIG. 6, the thickness of the thin ring 57 is exaggerated for convenience of description, but the inner diameter of the thin ring 57 is slightly smaller than the outer diameter of the lower absorber 27. For this reason,
It acts as a complete stopper against the relative displacement of the lower absorber 27 when the control rod is driven, and the upper end surface 3 due to swelling.
The gap 43 effectively absorbs volumetric expansion, with no substantial restraint effect for a slow displacement of one. The effect of the thin ring 57 on the stepping load displacement does not change depending on the frequency, and further, even if it is subjected to neutron irradiation, its functional characteristics do not decrease, and it works effectively for a long period of time.

Still another embodiment will be described with reference to FIGS. In FIG. 7, a lower absorber 61 provided in a sleeve 45 is formed in a hollow cylindrical shape, and a neutron absorbing disk 63 having a solid disk shape is provided on an upper end surface thereof. Then, the gap 43 for absorbing volume expansion is absorbed between the neutron absorbing disk 63 and the upper lid 49. The gap 43 is filled with a helium gas 65. In the control rod, since the space between the cladding tube and the upper absorber and the sleeve 45 is generally filled with helium gas, the sleeve 4 which merely contacts the lower end plug (not shown) is used.
By forming a groove at the lower end of 5, the gap 43 is filled with helium gas 65 without doing anything special.
The lower absorber 61 has a hole 61a as shown in FIG. 9, and the neutron absorbing disk 63 closes the hole 61a upward as shown in FIG.

In the structure as described above, since no solid substance exists in the gap 43, it is necessary to absorb the volume expansion of the lower absorber 61 and the neutron absorbing disk 63 in multiple words.
Now, when the control rod is driven and the lower absorber 61 and the neutron absorbing disk 63 are rapidly displaced in the direction of the arrow as shown in FIG.
Attempts to flow between the inner surface of the sleeve 45 and the neutron absorbing disk 63, but since the gap is narrow, a throttling action occurs here, and a fluid damping force is generated. According to this embodiment,
The fluid damping force of the helium gas filled in the volume expansion absorbing gap of the lower absorber 61 generates a fluid damping force proportional to the velocity, and suppresses the vibration displacement of the lower absorber 61 and the like, especially in the frequency components in the middle region. And suppresses the generation of excessive impact force and stress. In the present embodiment, the same function and effect can be obtained even when the lower absorber 61 is a solid lower absorber having a slit formed in the outer peripheral portion with a slit extending in the axial direction.

[0013]

As described above, according to the present invention,
A space for absorbing volume expansion is provided between the upper lid of the sleeve provided at the lower end inside the control rod and the lower absorber therein, and a restraining structure for restraining rapid relative displacement of the lower absorber, such as a frusto-conical surface A friction structure, a spring member, a thin ring, or a fluid damping mechanism are provided to effectively prevent the expansion of the cladding tube due to volume expansion and the generation of excessive stress due to stepping drive. The life of the control rod can be extended by preventing damage to the pipe.

[Brief description of the drawings]

FIG. 1 is a partial sectional view of a lower end portion of a control rod showing a main part of an embodiment according to the present invention.

FIG. 2 is a partially enlarged cross-sectional view showing a main part of the embodiment in an enlarged manner.

FIG. 3 is a partially enlarged cross-sectional view for explaining the operation of the embodiment.

FIG. 4 is a partial sectional view showing a main part of another embodiment according to the present invention.

FIG. 5 is a bottom view showing a part of FIG. 4 separately;

FIG. 6 is a partial sectional view showing a main part of still another embodiment according to the present invention.

FIG. 7 is a partial sectional view showing a main part of still another embodiment according to the present invention.

FIG. 8 is a plan sectional view taken along VIII-VIII in FIG. 7;

FIG. 9 is a plan sectional view along IX-IX in FIG. 7;

FIG. 10 is a partially enlarged sectional view for explaining the operation of the embodiment in FIG. 7;

FIG. 11 is a partial cross-sectional view showing a conventional structure of a lower end portion of a control rod.

[Explanation of symbols]

 Reference Signs List 3 lower end plug 5 circumferential welding 20 control rod 21 cladding tube 23 upper neutron absorber 25 sleeve 25a truncated conical surface 27 lower neutron absorber 29 upper lid 31 upper end face 33 gap 35 circumferential gap 43 gap 45 sleeve 47 spring 49 upper lid 57 Thin Ring 61 Lower Absorber 61a Hole 63 Solid Absorbing Disk 65 Helium Gas

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Ryu Fukuda 1-1-1 Wadazakicho, Hyogo-ku, Hyogo-ku, Kobe-shi, Hyogo Mitsubishi Heavy Industries, Ltd. 1-1-1 Tazakicho Mitsubishi Heavy Industries, Ltd., Kobe Shipyard

Claims (4)

[Claims] 1. A cladding tube having an upper end and a lower end closed by an end plug, an upper neutron absorber, a lower neutron absorber provided in the cladding tube, and the lower neutron absorber provided in the cladding tube so as to surround the lower neutron absorber. In a control rod for a reactor having a sleeve that supports an upper neutron absorber from below, a volume expansion absorption gap is defined between an upper lid of the sleeve and an upper end surface of the lower neutron absorber, and surrounds the gap. A control rod for a nuclear reactor, wherein the inner surface of the sleeve forms a frusto-conical surface having a gentle slope. 2. A cladding tube having an upper end and a lower end closed by end plugs, an upper neutron absorber, a lower neutron absorber provided in the cladding tube, and the cladding tube provided in the cladding tube so as to surround the lower neutron absorber. In a control rod for a nuclear reactor having a sleeve for supporting an upper neutron absorber from below, a gap for absorbing volume expansion is defined between an upper lid of the sleeve and an upper end surface of the lower neutron absorber. A control rod for a nuclear reactor, wherein a spring member is provided on the control rod. 3. A cladding tube having an upper end and a lower end closed by an end plug, an upper neutron absorber and a lower neutron absorber provided in the cladding tube, and the cladding tube provided in the cladding tube so as to surround the lower neutron absorber. In a control rod for a nuclear reactor having a sleeve that supports an upper neutron absorber from below, a volume expansion absorption gap is defined between an upper lid of the sleeve and an upper end surface of the lower neutron absorber, and the lower neutron absorber is formed. A control rod for a nuclear reactor, wherein a thin ring having an inner diameter slightly smaller than an outer diameter of a body is provided in said gap. 4. A cladding tube having an upper end and a lower end closed by an end plug, an upper neutron absorber, a lower neutron absorber, and the lower neutron absorber provided in the cladding tube. In a control rod for a reactor having a sleeve that supports an upper neutron absorber from below, the lower neutron absorber is formed to have a hollow portion inside or at an outer peripheral portion, and an upper lid and a lower neutron absorber of the sleeve are formed. A space for absorbing volume expansion is defined between the upper surface and the upper surface, and the sleeve including the space is filled with helium gas, and a solid disk-shaped neutron absorber is provided on the upper surface of the lower neutron absorber. A control rod for a nuclear reactor.