JPH022984A - Nuclear reactor control rod - Google Patents

Abstract

PURPOSE:To contrive nuclear long life by forming a wing in the form of a box by long life neutron absorber composed of hafnium metal and the like and constituting it so that the total quantity of the neutron absorber is gradually decreased from the inserting edge toward the end thereof. CONSTITUTION:The end materials 9a, 9b composed of an alloy in which hafnium metal or hafnium is made a main component are welded on the upper end or the lower end of a wing 1. A neutron absorbing rod 10 is arranged in the inside of the inserting edge side of the wing 1 and the lower part thereof is supported by a support member 11. The member 11 is made of the alloy in which hafnium metal or hafnium is made a main constituent. The absorbing rod 10 is filled with neutron absorber 14 such as boron carbide in which natural boron (B4C) and boron-10 are enriched in a coated tube 13 made of an alloy in which hafnium metal or hafnium is made a main constituent. Thereby long life of a control rod can be contrived.

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, and in particular to a control rod for controlling the reactor power of a light water reactor such as a boiling water reactor. This invention relates to a highly reactive, long-life nuclear reactor control rod that increases value, improves reactor shutdown margin, and extends its life.

(Prior technology) Conventional control rods for boiling water reactors have a thin U-shaped sheath fixed to a central tie rod to form wings with a cross-shaped cross section, and each wing is loaded with a large number of neutron absorption rods. It is composed of A neutron absorbing rod is prepared, for example, by filling a stainless steel cladding tube with boron carbide (B, C) powder as a neutron absorbing material.

When this reactor control rod is inserted into the core of a boiling water reactor, etc., the neutron absorbing material filled in the sheath is irradiated with neutrons and gradually loses its neutron absorption ability. The rods reach the end of their lifespan and are replaced periodically after a specified period of operation.

(Problem to be Solved by the Invention) Control rods that are inserted into and removed from the core of a nuclear reactor are not irradiated with neutrons uniformly over the entire surface of each wing; The area is subjected to intense neutron irradiation. Therefore, the neutron absorbing material filled in that area absorbs a large amount of neutrons and is consumed faster than other areas of the wing, ending its nuclear life earlier.

Therefore, even though the neutron absorbing material filled in other areas of the wing still has sufficient nuclear life left, it is uneconomical that the entire reactor control rod must be disposed of as radioactive waste. On the other hand, if reactor control rods are replaced frequently, there may be a problem that the radiation exposure of the workers who perform the replacement work will also increase.

In order to solve this problem, we are developing nuclear reactor control rods in which a long-life neutron absorbing material such as hafnium, which has a long nuclear life, is partially placed in the area of the control rod that is exposed to intense neutron irradiation. The inventor has developed. As disclosed in Japanese Patent Application Laid-Open No. 53-74697, this control rod for a nuclear reactor has a hybrid structure in which long-life absorbers are arranged at the tip and outer end of the wing, which is exposed to intense neutron irradiation.

This hybrid type nuclear reactor control rod has achieved a lifespan approximately twice that of a conventional control rod.

On the other hand, in conventional control rods for nuclear reactors, the entire wing area is filled with neutron absorbing material at a uniform density, and the neutron absorption capacity, or reactivity, in each area in the axial direction is adjusted to be equal. The unevenness of the neutron irradiation amount causes variations in reactivity over time, and at the end of the reactor operating cycle, there is a possibility that the margin for shutting down the x-nuclear reactor is partially reduced.

In other words, reactor shutdown margin (subcriticality) when the reactor is operated for a specified period using the above reactor control rods.
The distribution in the core axial direction varies slightly depending on the design specifications of the fuel assembly or the operating method of the reactor, but specifically, the distribution is shown in FIG. 7(A). That is, while the reactor shutdown margin increases at the upper and lower ends of the core, it takes a minimum value at a position slightly below the upper end.

Possible causes of this are as follows.

When the axial effective length of the reactor core is L, in the upper end region from the position 3/female from the lower end to the upper end,
Because the bubble rate (void rate) during operation is high and the power density of the reactor is relatively reduced, the mass number 235 of the fissile material is
The remaining amount of uranium (U-235) is relatively large. Also, the generated air bubbles (voids) cause a phenomenon of hardening of the neutron spectrum.

As a result, the plutonium production reaction (neutron absorption reaction) is promoted, which increases the concentration of fissile material in the upper part of the reactor core in the operating equipment of the reactor, reducing the margin for reactor shutdown in that area. .

On the other hand, it is inevitable that future nuclear reactors will shift to higher burn-up of nuclear fuel and longer operating cycles due to the demand for improved operating economy. As a concrete response to this, the adoption of highly enriched nuclear fuel is progressing, and as a result, there is a strong demand for control rods for nuclear reactors that have a long life and have a large margin for reactor shutdown.

However, when conventional reactor control rods are used in reactors loaded with highly enriched nuclear fuel, the margin for reactor shutdown is relatively reduced, and the reactor control rods must be replaced frequently every short operating cycle. Must be.

When replacing control rods for a nuclear reactor, it is necessary to shut down the reactor and further remove from the core a large number of fuel assemblies arranged around the control rods to be replaced, which is a complicated process. Therefore, reactor shutdowns for control rod replacement occur frequently and the shutdown period becomes long, which can significantly reduce the operating efficiency and economic efficiency of the reactor, while also significantly increasing management labor.

The present invention was made in consideration of the above-mentioned circumstances, and aims to prolong the nuclear lifetime by using long-life neutron absorbing material in the cross-shaped wings, and increasing the number of neutron absorbing materials in areas with high reactivity. An object of the present invention is to provide a high reactivity, long-life control rod for a nuclear reactor that increases the reactivity value of the control rod and increases the reactor shutdown margin.

[Structure of the invention]

(Means for Solving the Problems) In order to achieve the above-mentioned object, a nuclear reactor control rod according to the present invention includes wings formed in a cross-like structure, in which the wings are made of hafnium. The wing is formed into a box shape with a long-range neutron absorbing material made of metal or an alloy whose main component is hafnium, and the wing is formed so that the total amount of the neutron absorbing material gradually decreases from the insertion tip side to the insertion end side. It is characterized by what it did.

(Function) When the wing is formed into a box shape using a long-life neutron absorbing material made of hafnium metal or an alloy whose main component is hafnium 11, the nuclear life is long, and the life can be extended.

In addition, water as a moderator is introduced into the wing, and this water slows down neutrons and turns them into neutrons with energy that is easily absorbed, increasing the neutron absorption effect, that is, the reactivity effect of the control rods. Furthermore, neutron absorbing material is actively and effectively placed to reduce the degree of subcriticality in the axial direction of the reactor core when the reactor control rods are fully inserted.

- A much larger reactivity value is obtained, increasing the margin for reactor shutdown.

(Example) Hereinafter, a first example of a control rod for a nuclear reactor according to the present invention will be described with reference to the accompanying drawings.

FIG. 1 is a perspective view showing one embodiment of a control rod for a nuclear reactor according to the present invention, and FIG. 2 is a cross-sectional view taken in the direction of arrows - -■ in FIG. 1.

The cruciform wing 1 is formed by welding a wing tip 2, a plate 3, and a strut 4 made of hafnium metal or a hafnium-based alloy. A stiffener 15 is provided in the wing 1 to reinforce the plate 3 and is welded to the plate 3. A tie rod 5 made of hafnium metal or an alloy mainly composed of hafnium is arranged at the center of the reactor control rod, and its cross section is cross-shaped, and wings 1 are welded to each of its four protruding parts. be done. The tie rods 5 are connected intermittently in the axial direction of the reactor control rod.

This reduces the weight of the tie rod 5.

As shown in FIG. 1, a nuclear reactor control rod is provided with a handle 6 made of stainless steel at its upper end and a speed limiter 7 made of stainless steel at its lower end.

FIG. 4 is a longitudinal sectional view of a control rod for a nuclear reactor according to the present invention. At the upper and lower ends of the wing 1 are terminal materials 9 + 9a made of hafnium metal or an alloy whose main component is hafnium.
, ! llb is welded.

Since hafnium and stainless steel cannot be welded, the handle 6 and the speed limiter 7 are connected to the wing 1 in the following manner.

The terminal member 9 is inserted into a groove provided in the handle 6 and the speed limiter 7. Also, terminal material 9, handle 6
A hole is made in the speed limiter 7, and a stainless steel pin 17 is inserted into the hole to connect the respective members.

The pin 17 is welded to the handle 6 and the speed limiter 7.

A neutron absorption rod 10 is arranged inside the wing 1 on the insertion tip side, and its lower part is supported by a support member 11. The support member 11 is made of hafnium metal or a hafnium-based alloy and is welded to the plate 3.

The neutron absorption rod 10 has natural boron (84%
C) and boron carbide enriched with boron-10, Eu
A neutron absorbing material 14 such as 203-tlfo□ is filled. When the neutron absorbing material 14 is boron carbide, He gas is generated by a neutron reaction, so it is necessary to fill it with metal wool or the like to form a gas plenum.

Further, as shown in FIG. 3, plate-shaped stiffeners 12 are provided in place of some of the neutron absorption rods 10. The plate stiffener 12 is made of hafnium metal or an alloy containing hafnium as a main component, and is welded to the plate 3 to reinforce the plate 3.

In this nuclear reactor control rod, the wing 1 is made of a long-life neutron absorbing material, so it has a long nuclear life, and the control rod has a long life. As shown in the overall diagram, since the wing 1 has cooling holes 8, the inside of the wing 1 is filled with water, which is a moderator, and this water slows down neutrons, reducing the energy that is easily absorbed by the neutron absorbing material. neutrons, increasing the neutron absorption effect and, by extension, the control rod reactivity effect.

In addition, a neutron absorption rod 10 is placed inside the insertion tip side of the wing 1.
Since the subcriticality in the core axis direction becomes shallow when the reactor control rods are fully inserted, the neutron absorbing material is actively placed in the region where the subcriticality becomes shallower. characteristics) are obtained as shown in Fig. 7 (8), and the
As shown in Figure (C), the reactor shutdown margin is increased compared to the conventional control rod indicated by the broken line.

Next, a second embodiment of the nuclear reactor control rod according to the present invention will be described.

FIG. 5 shows a second embodiment of this reactor control rod. The control rod for a nuclear reactor shown in this embodiment has a neutron absorbing plate 16 of a multi-stage structure lined inside the insertion tip side of the wing 1. The neutron absorbing plate 16 is made of a long-life neutron absorbing material of hafnium metal or an alloy mainly composed of hafnium, and its thickness gradually decreases from the insertion tip side to the insertion end side of the wing 1. It is configured as follows.

This neutron absorbing plate 16 is fixed inside the wing 1 via a stiffener 15. The nuclear reactor control rod of this embodiment also has the same effects as the nuclear reactor control rod shown in FIG.

In one embodiment of the present invention, wings were formed by welding wing tips and rods to both ends of opposing plates, but these wings are made of hafnium metal or a hafnium-based alloy with a long lifespan. Any type of neutron absorbing material that has a U-shaped structure is acceptable. In FIG. 8, a U-shaped tube 16 made of hafnium metal or an alloy containing hafnium as a main component is used as the wing. This is a third example. FIG. 9 shows a fourth embodiment in which a plate made of hafnium metal or an alloy containing hafnium as a main component is bent to form a U-shaped plate 18 and used as a wing. be.

This U-shaped plate 18 is inserted into the tie rod 5 to constitute a control rod.

〔Effect of the invention〕

According to the present invention, since the wing is formed of a long-life neutron absorbing material made of hafnium metal or an alloy containing hafnium as a main component, the nuclear life of the control rod can be extended and the life of the control rod can be extended.

In addition, since a long-life neutron absorbing material is used as a sheath, water as a moderator can be introduced into the sheath, and the introduced water slows down fast neutrons and turns them into energy neutrons that are easily absorbed, resulting in a neutron absorption effect. In other words, while the reactivity effect of the control rods can be increased, neutron absorbing materials are actively placed in areas where the reactivity effect is high.
- A much larger reactivity value can be obtained, the reactor shutdown margin is increased, and a long-life nuclear reactor control rod with high reactivity can be provided.

[Brief explanation of the drawing]

FIG. 1 is a perspective view showing a first embodiment of a control rod for a nuclear reactor according to the present invention, FIG. 2 is a cross-sectional view taken along the line ■-■ in FIG. 1, and FIG. 4 is a longitudinal sectional view taken along the line ■-■ in FIG. 1; FIG. 5 is a longitudinal sectional view showing a second embodiment of the control rod for a nuclear reactor according to the present invention. A top view, FIG. 6 is a perspective view showing the entire stiffener, and FIG. 7 (A)
7(B) is a graph showing the axial distribution of the reactor shutdown margin when the reactor is operated for a predetermined period using a conventional reactor control rod, and FIG. 7(B) is a graph showing the reactor control rod according to the present invention. FIG. 7(C) is a graph showing the axial distribution of the neutron absorption characteristics in , and FIG. 7(C) shows the axial distribution of the reactor shutdown margin when the reactor is operated for a predetermined period using the reactor control rod according to the present invention. FIGS. 8 and 9 are cross-sectional views showing third and fourth embodiments of the nuclear reactor control rod according to the present invention. DESCRIPTION OF SYMBOLS 1... Wing, 3... Plate, 5... Tie rod, 7... Speed limiter, 9a, 9b... Terminal material, 11... Support member, 13... Cladding tube, 15a, 15b-Stiffener-1 17...Pin. 2... Wing tip material, 4... Rod, 6... Handle, 8... Cooling hole, 10... Neutron absorption rod, 12... Plate stiffener 14... Neutron absorption material , 16a, 16b...neutron absorption plate, 18...U-shaped plate. (8733) Representative Patent Attorney Yoshiaki Inomata (and 1 other person) Figure 3 Figure Figure Figure Figure

Claims (3)

[Claims] (1) In a nuclear reactor control rod consisting of wings formed in a cross-like structure, the wings are formed into a box shape with a long-life neutron absorbing material made of hafnium metal or an alloy containing hafnium as a main component; A control rod for a nuclear reactor, characterized in that the total amount of neutron absorbing material is formed such that it gradually decreases from the insertion tip side to the insertion end side. (2) The neutron absorbing material consists of hafnium metal or a long-life neutron absorbing material whose main component is hafnium, and a coated neutron absorbing material using a coating material, and the coated neutron absorbing material is placed inside the wing. The control rod for a nuclear reactor according to claim 1. (3) A neutron absorption plate made of hafnium metal or a hafnium-based alloy is fixed to the inside of a wing made of hafnium metal or a hafnium-based alloy, and the thickness of the neutron absorption plate is adjusted from the insertion tip side. The control rod for a nuclear reactor according to claim 1, wherein the control rod is reduced stepwise or continuously toward the terminal end.