JPH01196595A - Control rod for nuclear reactor - Google Patents

Abstract

PURPOSE:To prevent the mutual contact of hafnium metal plates and the contact of said metal plates with the inner surface of a sheath, by using a spot spacer and a ring spacer. CONSTITUTION:A sheath 15 made of high purity stainless steel having a deep U-shape cross-section is fixed to each of the protruding legs of the center tie rod 14 having a cruciform section of a control rod 10 for a nuclear reactor and a long life type neutron absorbing body 17 composed of an Hf metal plate 21. The central tie rod 14 is divided into a plurality of neutron absorbing elements 17a-17d in the axial direction thereof so that a neutron absorbing characteristic is successively lowered from an upper stage to a lower stage. The divided pieces 21a of a wing 16 in the width direction thereof are supported by a plurality of the spot spacers 22 pierced through the sheath 15 and ring spacers 22a are mounted to the leg piece piercing through the sheath 15. The thickness of the Hf metal plate in the vicinity of a side edge is made larger than that of the other part in the width direction of the wing 16. By this method, the wt. of the control rod can be reduced and the corrosion due to the contact of the Hf metal plate with the inner surface of the sheath can be also prevented.

Description

[Detailed Description of the Invention] [Object of the Invention] (Industrial Application Field) The present invention relates to a nuclear reactor control rod that adjusts and controls the reactor power of a nuclear reactor, and in particular a long-life nuclear reactor control rod. Pertains to.

(Prior art) A control rod for a nuclear reactor is formed by attaching a sheath with a deep U-shaped cross section to each protruding leg of a central tie rod with a cross-shaped cross section.
It consists of a large number of neutron absorption rods loaded into each wing. The neutron absorption rod is constructed by filling a SUS cladding tube with boron carbide (B4C) powder. Partition balls for preventing powder movement are arranged at regular intervals within the cladding tube.

The B4C powder filled in the neutron absorption rod absorbs neutrons and its neutron absorption capacity gradually decreases. During this time, boron-10 ("B) reacts with the neutrons to generate He gas, which increases the pressure inside the cladding tube. The life determined by the above-mentioned neutron absorption capacity is called the nuclear life, and the life determined by the gas pressure inside the cladding tube is called the mechanical life.

By the way, control rods that are inserted into and removed from the core of a nuclear reactor are not uniformly irradiated with neutrons; for example, the side edges and upper ends of each wing are exposed to strong neutron irradiation. For this reason, the neutron absorbers near the side edges and top ends of the control rods absorb a large amount of neutrons and reach their nuclear lifetime earlier than absorbers in other parts. Therefore, the control rods had to be disposed of as radioactive waste, even though the other neutron absorbers had sufficient nuclear lifetime remaining. Even though there are usable parts, disposing of radioactive waste as radioactive waste is not only uneconomical but also undesirable because it unnecessarily increases the amount of radioactive waste.

To solve the above problem, a nuclear reactor control rod has been developed in which a long-life neutron absorbing material with a long nuclear life is placed in the part of the control rod that is exposed to strong neutron irradiation (Japanese Patent Laid-Open No. 53
-74697).

However, the life of this type of nuclear reactor control rod is only about twice as long as that of a conventional type, which is not necessarily sufficient to extend the life of a nuclear reactor control rod.

In order to solve the above-mentioned problems of the nuclear reactor control rod disclosed in JP-A-53-74967, a nuclear reactor control rod disclosed in JP-A-58-55887 was developed which can exhibit an even longer life. This control rod for a nuclear reactor has a solid neutron absorbing plate made of a long-life neutron absorbing material loaded in each wing of the control rod. This neutron absorption plate removes a small amount of plate material in areas where the axial distribution of the reactor shutdown margin is small, and conversely, in areas where the axial distribution of the reactor shutdown margin is large, a large amount of plate material is removed. This has been taken into consideration.

(Problem to be Solved by the Invention) However, the neutron absorbing material used in the control rod for a nuclear reactor disclosed in JP-A-53-74967 is a plate-shaped hafnium (Hf) metal that is expensive and has a large specific gravity. However, the control rods were expensive and very heavy.

As the weight increases, the design of the control rod drive mechanism that handles the control rods needs to be changed in terms of its weight capacity, and conventional control rod drive mechanisms cannot be used as is.

In addition, when using a hafnium metal plate as a neutron absorbing material for a control rod, the hafnium metal plate and the stainless steel that is the structural material of the control rod tend to come into contact over a large area, and as a result, the width of the hafnium metal plate is likely to be large. There is a possibility that a narrow gap may be formed, which is unfavorable from the viewpoint of corrosion resistance.

The present applicant has disclosed a new long-life nuclear reactor control rod to solve the above problem.

This control rod has a neutron absorber divided in the wing axis direction and width direction, and the thickness of the hafnium metal plate in each division is determined according to the neutron irradiation amount in each division. The core life of the absorber is made uniform.

However, in the control rod disclosed above, the neutron absorbing element is divided into a plurality of cylinders in the wing width direction, and the hafnium metal plate of each divided section faces a groove that supports them by floating them from the inner surface of the seat. The spacer is supported by a spacer having a projection that maintains the distance between the hafnium metal plates, and a moderator flow path is formed in the space between the two hafnium metal plates and between the hafnium metal plate and the inner surface of the sheet. Then,
The thickness of the hafnium metal plate in each divided section is set according to the neutron irradiation dose distribution of that section. That is, the thickness of the hafnium metal plate near the side edge of the wing may be made larger than the thickness of the hafnium metal plate in other parts.

In this control rod, the hafnium metal plate is supported floating from the inner surface of the sheath, and since they do not come into direct contact over a large area, problems such as corrosion will not occur.

However, the grooves and spacers of the projections do not necessarily support the hafnium metal plate reliably, and there is a risk that the hafnium metal plate may come into contact with the inner surface of the sheath.

The present invention has been made based on the above-mentioned circumstances, and is a nuclear reactor that has no fear of contact between the hafnium metal plate and the inner surface of the sheath, is relatively lightweight and inexpensive, and can use the conventional control rod interlocking mechanism as is. The purpose is to provide control rods for

[Structure of the Invention] (Means for Solving the Problems) The nuclear reactor control rod of the present invention includes a central tie rod having a cross-shaped cross section that connects a tip structural member and a terminal structural member, and each protruding leg of the central tie rod. The wing has a deep U-shaped sheath attached at its opening, and a long-life neutron absorber is loaded into each wing. A hafnium metal plate that is divided into a plurality of sections in a direction and in which the neutron absorption characteristics of the neutron absorption element in each section is determined according to the amount of neutron irradiation in that section, the opposing hafnium metal plates constituting the neutron absorption element of each section. The hafnium metal plates and the inner surface of the sheath are separated from each other by ring spacers attached to the leg pieces. shall be.

(Function) In the nuclear reactor control rod of the present invention having the above configuration, the hafnium metal plates constituting the neutron absorber are opposed to each other with a distance maintained by the spot spacer, and the distance between these metal plates and the inner surface of the sheath is Since the hafnium metal plates are held by ring spacers, there is no contact between hafnium metal plates or contact between the hafnium metal plates and the inner surface of the sheath, and the moderator flow path is of course well secured, and the gap is narrow and wide. Since contact between the hafnium metal plate and the inner surface of the sheath is completely prevented, corrosion of the two does not occur.

Furthermore, the control rod is divided into its axial and width directions, and each division is loaded with neutron absorption elements with neutron absorption characteristics corresponding to the amount of neutron irradiation. There is no waste in disposing of control rods that have reached the end of their nuclear life and have remaining nuclear life.

(Embodiment) FIG. 1 is a perspective view showing an entire embodiment of the present invention, and FIG. 2 is a layout diagram of a neutron absorber arranged inside the same. 1st
In the figure, a reactor control rod 10 includes a tip structural member 12 provided with a handle 11, an end structural member 13, and a central tie rod 14 having a cross-shaped cross section, which integrally connects both groove members.
and has.

A sheath 15 made of high purity stainless steel and having a deep U-shaped cross section is secured to each protruding leg of the central tie rod 14 at its opening, and four wings 16 are formed. A long-life neutron absorber 17 made of a hafnium (Hf) metal plate is inserted into the sheath 15.

As shown in the left half of FIG.
It is divided into ~17d. In addition, the state in which the neutron absorption element shown in FIG. 2 is not loaded is shown. Among the neutron absorption elements, 17a to 17c excluding 17d at the bottom end
are supported by neutron absorbing element support pieces 18 installed at appropriate intervals on each protruding leg of the central tie rod 14, and the neutron absorbing elements 17a to 17c are prevented from moving in the axial direction. Each of the above-mentioned neutron absorbing element support pieces 18 is provided protruding from the central tie rod 14 at necessary intervals. Note that the lowest neutron absorbing element 17d is the terminal structural member 1.
It is supported by 3. Neutron absorption elements 17a in each stage
~17d, the neutron absorption characteristics are made to decrease in order from the upper stage to the lower stage. Specifically, the thickness of the neutron absorbing elements 17a to 17d is gradually reduced from the top. As a result, the reactivity effect of the control rod, that is, the neutron absorption characteristics, is reduced stepwise from the upper end of the control rod to the lower end, as shown in FIG.

In addition, the first stage neutron absorption element 17a adjacent to the tip structure member 12 may improve the scram characteristic by increasing the neutron absorption characteristic in a region within 35 nm from the upper end, depending on the design of the control rod and its usage. Or, conversely, it can be made smaller to reduce fluctuations in reactor output due to control rod withdrawal. Further, at least the first
The side edge portion of the stage neutron absorbing element 17a on the side of the central tie rod 14 and a portion near the side edge portion are made to have greater neutron absorbing characteristics than other portions.

By the way, in the control rod 10 for a long-life nuclear reactor, the tip structure material 12 is exposed to a very large amount of neutron irradiation, and there is a risk of embrittlement due to this, so the tip structure material is constructed of high-purity stainless steel. We are trying to alleviate the problem of brittleness. Further, the tip structural member 12, the terminal structural member 13, and the speed limiter 19 attached to the terminal structural member are made as thin and lightweight as possible. moreover,
A cavity 20 is provided at the bottom of the tip structure 12 to be used as an auxiliary handle. Since the region where the void is provided requires almost no neutron absorbing material in view of the neutron absorption performance of the control rod, the weight of the control rod can be reduced without reducing the neutron absorption performance. - On the other hand, it has been experimentally confirmed that the amount of fast neutron irradiation in the upper part of the cavity 20 is about 115 to 1/3 or less than that in the upper part of the handle 11. Therefore, the embrittlement of the upper part of the cavity 20 due to fast neutron irradiation will cause the handle 1 to become brittle.
It can be considered that the gap is about 115 to 173 or less than that of No. 1, and if the handle 11 becomes brittle, the void 20
There is a backup auxiliary handle that can act effectively as a back-up handle.

In addition, a neutron absorbing element 17a loaded in the sheath 15
~17d is a divided piece 21a divided in the width direction of the wing 16, as shown for the neutron absorption element 17a in FIG.
are supported by a plurality of spot spacers 22 whose two leg pieces facing each other with a cross-shaped cross section penetrate the sheath 15, and a pair of opposing hafnium metal plates 21, 21 having a shape complementary to the inner surface of the side wall of the sheath 15 are formed. Let it form. A ring spacer 22a is attached to the leg that passes through the sheath 15, and each hafnium metal plate 21 is supported so as to float from the inner surface of the sheath 15. Furthermore, the tips of the side edges of the two hafnium metal plates 21 are slightly spaced apart. The spot spacer 22 forms a flat space 23 serving as a moderator flow path within the neutron absorption element 17a, and improves the mechanical strength of the neutron absorption element 17a. Furthermore, a flat space 24 that serves as a moderator flow path is formed between the hafnium metal plate 21 and the inner surface of the sheath. There is.

FIG. 5 is a cross-sectional view of the embodiment at another position. In this figure, a moderator inlet 25 that passes through the sheath 15 and communicates with the flat space 24 is provided at a position where the spot spacer 22 is not present. Note that these moderator inflow ports 25 are provided so that, in principle, those on both sides of the wings 16 face each other. Thus, the moderator enters the flat space 24 from the moderator inlet 25, flows from there into the similarly flat space 23 between the hafnium metal plates 21, and flows within the control rod wing.

Hereinafter, the general operation of a nuclear reactor control rod and the operation of the nuclear reactor control rod of the present invention will be explained.

In a boiling water reactor, the axial fissile nuclide concentration distribution in the reactor core after combustion has progressed to a certain extent is as shown by curve A shown in FIG. Generally, combustion management of a nuclear reactor core is performed by dividing the core into four equal parts in the axial direction, and it is convenient to divide the reactor control rod 1o into four equal parts in the axial direction.

That is, curve A in FIG. 6 shows the following. First, assuming that the axial length of the nuclear reactor core is L, the progress of combustion during combustion is delayed at the lower end (174 mm or less) of the reactor core, so the distribution of fissile nuclides is large. Furthermore, from the center (2/4L) to the upper end, the voids that occur cause a hardening phenomenon of the neutron spectrum, which accelerates the plutonium production reaction, resulting in the generation of many voids and a decrease in thermal neutron flux. This causes a combustion delay and increases the concentration distribution of fissile nuclides.

When the nuclear reactor core has a fissile nuclide concentration distribution shown by curve A in FIG. 6, the neutron multiplication factor distribution at the time of reactor shutdown is as shown by curve B in FIG. 7. The larger the neutron multiplication factor, the smaller the reactor shutdown margin and the shallower the subcriticality. Song 14 in Figure 7! In B,
The reason why the neutron multiplication factor decreases at the upper and lower ends of the core is due to leakage of neutrons from the upper and lower ends of the core.

Curve C in FIG. 8 shows the neutron irradiation dose distribution in the control rod axis direction when the nuclear reactor control rod 10 of the present invention is used. This figure shows that the neutron irradiation amount increases rapidly in a very limited area of the reactor control rod (usually about 30 am from the tip), and in other parts, the neutron irradiation amount continues and smoothly towards the lower end of the control rod. It can be seen that it has decreased.

The nuclear reactor control rod 10 of the present invention is configured to obtain a satisfactory control effect in accordance with the neutron multiplication factor distribution shown in FIG. 7 and the neutron irradiation dose distribution shown in FIG. There is. That is, at the tip of the reactor control rod 10 (1/4 L length, for example, about 90 to 95 AO), the reactor shutdown margin is reduced due to an increase in the neutron multiplication factor (decreased reactor shutdown margin) and an increase in the neutron irradiation amount. In order to cope with this tendency, the thickness of the hafnium metal plate 21 constituting the neutron absorbing element 17a is made larger than that of the other neutron absorbing elements 17b to 17d. As described above, the thickness of the hafnium metal plates constituting the neutron absorbing elements 17b to 17d is also made to gradually decrease. In particular, the neutron absorption characteristics of the lower region from the lower end of the reactor control rod 10 (the upper end of the terminal structural member 13) to 1/4L are slightly smaller than the region adjacent above it from 1/4L to 2/4. ing. Although the neutron irradiation dose is considerably smaller in the lower region (1/4L from the lower end) than in the next adjacent region (1/4 to 2/4L from the lower end), as shown by curve C in Figure 8, This is to cope with the fact that the neutron multiplication factor becomes relatively large as shown by curve B in FIG.

The curved line in FIG. 9 shows the neutron irradiation dose distribution in the wing width direction of the reactor control rod. From this curve, it can be seen that the neutron irradiation amount increases sharply on the outer side of the wing 16, that is, on the side edge, and slightly increases on the inner side, that is, on the side of the center tie rod 14. To deal with this, the first step is to
It is sufficient to set the distribution of the reactivity effect of the control rod in the wing width direction as shown in the curve E shown in FIG.

Thus, the thickness of the hafnium metal plate 21 in each divided section is set according to the neutron irradiation dose distribution in that section. In other words,
If the reactivity effect shown in curve E in FIG.
In this embodiment, the hafnium metal plate 21 is supported floating from the inner surface of the sheath 15, so that the hafnium metal plate 21 can be directly attached to a large area. Since there is no contact, problems such as corrosion will not occur.

[Effects of the Invention] As is clear from the above, in the nuclear reactor control rod of the present invention, the hafnium metal plates constituting the neutron absorber are opposed to each other with a distance maintained by the spot spacer, and Since the distance between the hafnium metal plates and the inner surface of the sheath is maintained by the ring spacer, there is no contact between hafnium metal plates or contact between the hafnium metal plates and the inner surface of the sheath, and the moderator flow path is of course well maintained. Furthermore, since contact between the hafnium metal plate and the inner surface of the sheath in a narrow gap or over a wide area is completely prevented, corrosion of both will not occur.

In addition, the neutron absorber is divided into a plurality of neutron absorbing elements in the axial direction of the control rod, and the thickness of the neutron absorbing material plate constituting each neutron absorbing element is determined according to the neutron irradiation amount in that division. In addition, it is also divided in the width direction, and the plate thickness of each width direction division is determined according to the neutron irradiation amount for each division, so the nuclear lifetime of each neutron absorption element can be made almost equal. , there is no risk of unnecessarily increasing the amount of radioactive waste.

In addition, since the thickness of the neutron absorbing material plate is determined as described above, the weight of the control rod can be further reduced, and the control rod drive mechanism designed for normal control rods can be used as is. Can be done.

Furthermore, since the coolant flow path is between the two opposing neutron absorbing material plates constituting the neutron absorbing element, the reactivity is improved. From this point of view as well, the amount of neutron absorbing material can be reduced, and the weight of the control rod can be reduced.

Further, since the thickness of the neutron absorbing material plate is selected depending on the neutron irradiation amount in the axial direction and width direction of the control rod, a high-performance and economical control rod can be obtained.

[Brief explanation of the drawing]

FIG. 1 is a perspective view of an embodiment of the present invention, FIG. 2 is a front view schematically showing the inside thereof, FIG. 3 is a diagram showing the axial reactivity distribution of the embodiment, and FIG. 4 is a diagram showing the axial reactivity distribution of the embodiment. Figure 1 is a cross-sectional view taken along the IV-IV line, Figure 5 is a cross-sectional view taken along the same line ■-V, Figure 6 is a diagram showing the fissile nuclide concentration distribution in the core axis direction of the reactor core, and Figure 7 is a diagram showing the core axis direction. Figure 8 is a diagram showing the neutron irradiation dose distribution in the axial direction of the core, Figure 9 is a diagram showing the neutron irradiation distribution in the control rod wing width direction, and Figure 10 is a diagram showing the neutron irradiation dose distribution in the width direction of the control rod wing. FIG. 3 is a diagram showing the widthwise reactivity distribution of the wing necessary to cope with the widthwise neutron irradiation dose distribution.

Claims (1)

[Claims] A central tie rod with a cross-shaped cross section that connects the tip structural member and the terminal structural member, a wing configured by attaching a deep U-shaped sheath to each protruding leg of the central tie rod at its opening, and a wing loaded in each wing. The long-life neutron absorber is divided into a plurality of sections in the control rod axis direction and the wing width direction, and the neutron absorption characteristics of the neutron absorption elements in each section are determined in that section. In the method determined according to the neutron irradiation amount, the opposing hafnium metal plates constituting the neutron absorbing elements of each section are kept spaced apart by a spot spacer having opposing leg pieces with a cross-shaped cross section penetrated through the sheath. A control rod for a nuclear reactor, characterized in that the hafnium metal plate and the inner surface of the sheath are separated by a ring spacer attached to the leg piece.