JPH01287499A - Control rod for nuclear reactor - Google Patents

Abstract

PURPOSE:To prevent corrosion by disposing hafnium metallic plates opposite to each other via the spacers maintained therebetween by spot spacers and disposing such metallic plates opposite to the inside surface of a sheath via the spacings therebetween maintained by ring spacers. CONSTITUTION:Each of neutron absorbing elements 17a to 17d loaded into the sheath 15 is formed of split pieces 21a split in the transverse direction of a wing 16 which are supported by the plural spot spacers 22 having a cruciform section and two opposite leg pieces penetrated through the sheath 15 to form a set of the opposite hafnium metallic plates 21, 21 having the complementary shape of the inside surface of the side wall of the sheath 15. The ring spacers 22a are mounted to the above-mentioned leg pieces penetrating the sheath 15 and the respective hafnium metallic plates 21 are supported afloat from the inside surface of the sheath 15. The spot spacers 22 and the ring spacers 22a are thereby prevented from coming into contact with the hafnium metallic plates 21 or the sheath 15 over the entire surface. The corrosion thereof is thus prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Industrial Application Field) The present invention relates to a control rod for a nuclear reactor that adjusts and controls the reactor power of a nuclear reactor.

(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 roncarbide (84C) powder.

Partition edges for preventing powder movement are arranged at regular intervals within the cladding tube.

The B and C powders filled in the neutron absorption rod absorb neutrons and their neutron absorption capacity gradually decreases. During this time, boron-10 ("B) reacts with the neutrons to generate Ha gas, and the inside of the cladding tube is The life determined by the above-mentioned neutron absorption capacity is called the nuclear life, and the life determined by the gas pressure in 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 end of each wing are exposed to intense 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.

It reaches its nuclear lifetime earlier than other absorbers. Therefore,
The control rods had to be disposed of as radioactive waste, even though the rest of the neutron absorbers had sufficient nuclear lifetime remaining. Although there are usable parts in this way, disposing of radioactive waste as radioactive waste is not only uneconomical but also undesirable as it unnecessarily increases the amount of radioactive waste.

In order 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. For this neutron absorption plate, a small amount of plate material is removed in areas where the axial distribution of reactor shutdown margin is small.
On the other hand, in areas where the holes are large, through holes or recesses are provided, taking into account their size and distribution, so that a large amount of plate material is removed.

(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 plate-shaped hafnium (llf) metal, which is expensive and has a large specific gravity. However, the control rods were expensive and very heavy.

As the weight increases, the control rod drive mechanism that handles the control rods needs to be redesigned in order to withstand more weight.

It was not possible to use the conventional control rod drive mechanism 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 tends to increase over a large area. 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 the hafnium metal plates 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 plates and the inner surface of the sheath. 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 spacer with the grooves and projections does 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 is relatively lightweight and inexpensive, without fear of contact between the hafnium metal plate and the inner surface of the sheath, and in which a conventional control rod drive mechanism can be used 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 a deep U-shaped central tie rod on each protruding leg of the central tie rod. It has a wing configured by attaching a sheath at its opening, and a long-life neutron absorber loaded in each wing, and the long-life neutron absorber is divided into a plurality of sections in the control rod axis direction and the wing width direction. The neutron absorption characteristics of the neutron absorption elements in each division are determined according to the neutron irradiation amount in that division, and the opposing hafnium metal plates constituting the neutron absorption elements of each division are The hafnium metal plates and the inner surface of the sheath are separated by ring spacers attached to the leg pieces, and the spot spacers and ring spacers By introducing sufficient grooves, a sufficient distance is maintained between the spot spacer and the hafnium metal plate, between the ring spacer and the sheath, and between the ring spacer and the hafnium metal plate.

(effect) In the control rod for a nuclear reactor of the present invention having the above configuration.

The hafnium metal plates that make up the neutron absorber are opposed to each other with a distance maintained by spot spacers, and the distance between these metal plates and the inner surface of the sheet is maintained by a ring spacer, so that contact between the hafnium metal plates is prevented. Or, contact between the hafnium metal plate and the inner surface of the sheath does not occur, and the moderator flow path is of course well secured, and contact between the hafnium metal plate and the inner surface of the sheath in narrow gaps and large areas is also prevented. Therefore, corrosion of both does not occur. Also, by introducing sufficient grooves into the spot spacer and ring spacer, it is possible to prevent a complete gap between the spot spacer or ring spacer and the sheath or hafnium metal plate. Therefore, corrosion of the spot spacer portion and the ring spacer portion and the sheath will 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 properties 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 control rod 10 for a nuclear reactor includes a tip structure member 12 provided with a handle 11, an end structure member 13, and a central tie rod 14 having a cross-shaped cross section in which both types of materials are integrally connected.
and has. A sheath 15 made of high-purity stainless steel and having a deep U-shaped cross section is fixed to each protruding leg of the central tie rod 14 at the mouth thereof, 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.
The left half of Figure 2 shows the state in which the neutron absorption element is not loaded.
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 1δ 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 at each stage
~17d is.

The neutron absorption characteristics are made to decrease in order from the top to the bottom. Specifically, the thickness of the neutron absorption 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 characteristic, is reduced in zero steps from the upper end of the control rod to the lower end, as shown in FIG.

Further, the first stage neutron absorption element 17a adjacent to the tip structure member 12 has an area within, for example, 35 shoulders from its upper end.
Depending on the design of the control rod and how it is used, the neutron absorption characteristics can be increased to improve scram characteristics, or conversely, the neutron absorption characteristics can be decreased to reduce the range of fluctuations in reactor power caused by control rod withdrawal. Further, at least the side edge portion of the first stage neutron absorbing material 117a on the side of the central tie rod 14 and the portions near the side edge portion thereof are made to have greater neutron absorption characteristics than other portions.

By the way, in the control rod 10 for a long-life nuclear reactor, the tip structural member 12 receives a very large amount of neutron irradiation.

Since there is a risk of embrittlement due to this, the tip structure material is made of high-purity stainless steel to alleviate the problem of embrittlement. In addition, the tip structure material 12. The terminal structural member 13 and the speed limiter 19 attached to this terminal structural member are made as thin and lightweight as possible.
Furthermore, a gap 20 is provided in the lower part 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 175 to 1/3 of that in the upper part of the handle 11 or less. 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 175 to 173 or less than that of No. 1, and if the handle 11 becomes brittle, the void 20
can effectively function as a backup auxiliary 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.
is supported by a plurality of spot spacers 22 whose two leg pieces facing each other in a cross-shaped cross section are passed through 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 is 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.

The details of the spot spacer 22 are as shown in FIG.

A groove 22b is introduced so that even if the spot spacer 22 comes into contact with the hafnium metal plate 21, the moderator is in communication with the outside.

On the other hand, the details of the ring spacer 22a are as shown in FIG.
A groove 22ab is introduced so that the moderator communicates with the outside even when it contacts the sheath 15.

From the above, the spot spacer 22 and the ring spacer 22a do not come into contact with the rehafnium metal plate 21 or the sheath 15 which is spread over the entire surface.

Furthermore, the tips of the side edges of the two hafnium metal plates 21 are slightly spaced apart. The spot spacer 22 has a flat space 23 that becomes a moderator flow path within the neutron absorption element 17a.
At the same time, the mechanical strength of the neutron absorbing element 17a is improved. Furthermore, there is a flat space 24 between the hafnium metal plate 21 and the inner surface of the sheath, which becomes a moderator flow path.
is formed. Note that the hafnium metal plate 21 has a thickness of 0.
It is said to be a thin plate with a thickness of 5cm to 2.0cm.

FIG. 7 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, what is the axial fissile nuclide concentration distribution in the reactor core after combustion has progressed to some extent?

The result will be a curve A shown in FIG. Generally, the combustion management of a 1M child 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 rods 10 into four equal parts in the axial direction.

That is, curve A in FIG. 8 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, a void occurs that hardens the neutron spectrum, and this 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. 8, the neutron multiplication factor distribution at the time of reactor shutdown is as shown by curve B in FIG. 9. The larger the neutron multiplication factor, the smaller the reactor shutdown margin and the shallower the subcriticality. In curve B of FIG. 9, 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.

The curve mc in Fig. 10 shows the neutron irradiation dose distribution in the control rod axis direction when the reactor control rod 10 of the present invention is used. It can be seen that the neutron irradiation amount increases rapidly from 303 to about 303), and in other parts, the neutron irradiation amount decreases continuously and smoothly toward the lower end of the control rod.

The nuclear reactor control rod 10 of the present invention is configured so as to obtain a satisfactory control effect in accordance with the neutron multiplication factor distribution shown in FIG. 9 and the neutron irradiation dose distribution shown in FIG. There is. In other words, the tip part (1/
In order to cope with the fact that the reactor shutdown margin tends to decrease due to an increase in the neutron multiplication factor (reduction in the reactor shutdown margin) and an increase in the neutron irradiation amount, the neutron absorption point 117a is The thickness of the constituting hafnium metal plate 21 is made larger than that of the other neutron absorption elements 1117b 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 1/4 L are as follows:
/4 area. This means that although the neutron irradiation amount is considerably smaller in the lower region (17 females from the lower end) than in the next adjacent region (174 to 2/4 L from the lower end), as shown in curve C in Figure 10, This is to cope with the fact that the multiplication factor becomes relatively large as shown by curve B in FIG.

The curved line in FIG. 11 shows the neutron irradiation dose distribution in the wing width direction of the reactor control rod. From this curved line, the neutron irradiation dose increases rapidly on the outer side of the wing 16, that is, on the side line, and on the inner side, that is, on the central tie rod 14 side. You can see that it is slightly higher.

In order to deal with this, it is sufficient to set the distribution of the reactivity effect of the control rod in the wing width direction as shown by 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 they do not come into contact with each other, problems such as corrosion will not occur.Furthermore, there are grooves 2 in the spot spacer 22 and ring spacer 22a for floating support.
2b is sufficiently introduced, these spacers do not come into contact with the rehafnium metal plate 21 or the sheath 15 which are spread over the entire surface, and problems such as corrosion do not occur in these parts as well.

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 divided division. This is because the board is also divided in the width direction, and the plate thickness of each width direction division section is determined according to the neutron irradiation amount for each division section.

The nuclear lifetimes of each neutron absorption element can be made approximately equal, and 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. I can do it.

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.

〔Effect 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 the distance between these metal plates and the inner surface of the sheath is Since the hafnium metal plates are held by ring spacers, the moderator flow path can be well secured without causing contact between the hafnium metal plates or between the hafnium metal plates and the inner surface of the sheath.

Further, 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 of them will not occur.

Furthermore, sufficient grooves are introduced into the spot spacer and ring spacer, and the moderator communicates with the outside in the groove, forming a moderator flow path, and the partial contact area between these spacers and the hafnium metal plate or sheath However, corrosion is prevented.

[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 interior of FIG. 1, FIG. 3 is a diagram showing the axial reactivity distribution according to the present invention, and FIG. The figure is the first according to the invention.
A sectional view taken along the line IV-IV in FIG. 5 is a perspective view of a spot spacer according to the present invention, FIG. 6 is a perspective view of a ring spacer according to the present invention, FIG. 7 is a sectional view taken along the line v-■ in FIG.
The figure is a diagram showing the fissile nuclide concentration distribution in the axial direction of the reactor core according to the present invention, FIG. 9 is a diagram showing the neutron multiplication factor distribution in the axial direction of the reactor core according to the present invention, and FIG. Diagram showing such core axial neutron irradiation dose distribution, No. 11
The figure is a diagram showing the neutron irradiation dose distribution in the width direction of the control rod wing according to the present invention, and FIG. FIG. 10...Reactor control rod 11...Handle 12
... Tip structure material 13 ... End structure material 14 ...
Central tie rod 15...Sheath 16...Wing 17...Long-life neutron absorber 17a to 17d
...Neutron absorption element 18...Neutron absorption element support piece 19...Speed limiter 20...Gap 21.
... Hafnium metal plate 21a... Divided piece 22...
・Spot spacer 22a...Ring spacer 22
b... Groove 23, 24... Space (velocity material channel)
25... Moderator inlet agent Patent attorney Nori Ken Chika Yudo Ken Daishimaru Figure 1 Figure 2 Figure 32 Figure 6 Figure 8 Figure 9 □
-11-Inside wing Outside wing-m-
-1-1

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 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, and grooves are introduced into the spot spacer and the ring spacer, thereby improving corrosion resistance as a whole. control rod.