JPH026037B2 - - Google Patents

Description

[Detailed Description of the Invention] [Object of the Invention] (Industrial Application Field) The present invention relates to a control rod for a boiling water type nuclear reactor, and in particular to a control rod for a boiling water type nuclear reactor, which has a long life without increasing the weight. Regarding control rods for nuclear reactors.

(Prior art) A conventional control rod for a boiling water nuclear reactor has a cross-shaped wing formed by attaching an opening in a sheath having an elongated U-shaped cross section to each protruding leg of a tie rod having a cross-shaped cross section. It is constructed by loading a large number of neutron absorption rods vertically and parallel to each other within each wing.

The neutron absorbing rod is constructed by filling a cladding tube with a neutron absorbing material, such as B ₄ C powder or pellets.

When a control rod loaded with this neutron absorption rod is inserted into a nuclear reactor core and used, B ₄ C powder, pellets, etc. absorb neutrons and are worn out, gradually losing their neutron absorption ability. Generates gas, tritium ( ³ T ₂ ) gas, etc. Here He gas is
Boron (B-10) with a mass number of 10 in B ₄ C is irradiated with neutrons and is generated by the so-called (n, α) reaction, and ³ T ₂ gas is generated by the above (n, α) reaction. It is produced by a nuclear reaction between ⁷ Li produced by neutrons and fast neutrons. In addition, when a neutron absorbing rod made of B ₄ C is used until the end of its life,
The amount of ^3T2 atoms generated is approximately 0.03% of the amount of He atoms generated, and most of _the gas generated is He gas.

Therefore, as the neutron irradiation time increases, the neutron absorption capacity decreases and the gas pressure within the cladding tube increases. The neutron irradiation time required until the neutron absorption capacity reaches a predetermined value is called the nuclear life, and the neutron irradiation time required until the cladding internal pressure reaches an allowable value is called the mechanical life. According to recent research, the mechanical lifetime is shorter than the nuclear lifetime, and the lifetime of a neutron absorbing rod is determined by the mechanical lifetime.

On the other hand, Hf metal, Eu ₂ O ₃ , Ag―In―Cd
Neutron absorbing materials such as Hf, Eu, Ag, In, Cd, etc. do not generate gas by absorbing neutrons, and Hf, Eu, Ag, In, Cd, etc. It forms isotopes, etc., or is converted into atoms that have neutron absorbing properties. Therefore, it is possible to absorb neutrons for a long period of time, and is called a long-life neutron absorbing material, and is used to construct a long-life neutron absorption rod.

(Problem to be solved by the invention) However, Hf metal, Eu ₂ O ₃ , Ag-In-Cd
Long-life neutron absorbers such as neutron absorbers generally have a high density and are expensive, so if they are used in large quantities, the weight of the control rod becomes excessive and it becomes expensive. When using control rods with significantly increased weight, it is necessary to change the design to further strengthen the structural strength of the control rod drive mechanism, which is a practical problem that cannot be easily transferred to existing nuclear reactor equipment. The point was hot. Therefore, in order to have versatility, it has been desired to have a structure that minimizes the amount of long-life neutron absorbing material used and does not increase the weight of the control rod as a whole.

The present invention has been made in consideration of the above circumstances, and provides a control rod for a boiling water reactor that minimizes the amount of expensive and heavy long-life neutron absorbing material used and has a long life. The purpose is to

[Structure of the invention]

(Means for Solving the Problems) In order to achieve the above object, the present invention configures a wing by attaching an opening of a sheath having an elongated U-shaped cross section to each protruding leg of a tie rod having a cross-like cross section. , a control rod for a boiling water reactor consisting of a large number of neutron absorption rods loaded in the width direction within the wing, with a plenum formed at the upper end and filled with B ₄ C powder as a neutron absorption material. is arranged near the tie rod, while a long-life neutron absorption rod made of hafnium material, which does not generate gas by absorbing neutrons, is arranged near the wing side edge. In the middle part, there is a hybrid neutron absorbing rod consisting of the neutron absorbing rod with a plenum, a long-life neutron absorbing material made of hafnium material near the upper end, and B ₄ C powder below the upper end. It is characterized by the fact that they are arranged alternately.

(Function) According to the control rod for a boiling water reactor having the above configuration, long-life neutron absorption rods are arranged only on the side edges and upper ends of the wings that are exposed to strong neutron irradiation, so that the control rod as a whole In addition to extending the life of the control rods, a significant increase in the weight of the control rods is suppressed, making it easier to use them in existing nuclear reactor equipment, and reducing manufacturing costs.

On the other hand, there is a plenum near the tie rod.
Because neutron absorption rods with plenums filled with B ₄ C powder are installed, the output increase rate of the fuel rod closest to the tie rod when the control rod is withdrawn is suppressed, ensuring the integrity of the fuel.

Furthermore, the neutron absorption rod with a plenum has a large plenum installed in the area where neutron irradiation is strongest, so it is possible to suppress the increase in the internal pressure of the cladding tube due to He gas, ³ T ₂ gas, etc. generated in the nuclear reaction. .
Furthermore, when B ₄ C exists at the top of the neutron absorption rod,
There is a risk that the mechanical life of the absorption rod will be reduced due to nuclear reactions, reactions between chemical reactants and the cladding, and increases in internal pressure. However, in the present invention, a plenum is provided at the upper end of the neutron absorption rod filled with _B4C . There is no risk of that happening.

(Example) Next, an example of the present invention will be described with reference to the accompanying drawings. The accompanying drawing is a sectional view showing a main part of an embodiment of a control rod for a boiling water reactor according to the present invention.

That is, in the control rod for a boiling water reactor according to this embodiment, a wing 2 is constructed by attaching an opening of a sheath having an elongated U-shaped cross section to each protruding leg of a tie rod 1 having a cross-shaped cross section. A control rod for a boiling water reactor comprising a large number of neutron absorbing rods loaded in the width direction within the plenum 4a at the upper end and filled with B ₄ C powder 5b as a neutron absorbing material. stick 4,
A long-life neutron absorbing rod 3 made of hafnium material that does not generate gas by absorbing neutrons is arranged near the side edge of the wing 2. 2, the neutron absorbing rod 4 with a plenum and a long-life neutron absorbing material 5a made of hafnium material are filled near the upper end, and B ₄ C powder 5b is filled below the upper end. The hybrid neutron absorption rods 5 are arranged alternately.

To explain the configuration more specifically, there is a 2 cm width near the side edge of the wing 2 that spans the entire length of the wing 2.
A long-life neutron absorption rod 3 is arranged, and a plenum-equipped neutron absorption rod 4 having a plenum 4a with a length of about 10 to 30 cm at the upper end is arranged in a width of about 2 cm near the tie rod 1. Furthermore, between the neutron absorbing rods 3 and 4 in the wing 2, a long-life neutron absorbing material 5a is arranged at the upper end over a length of about 30 cm, and a normal-life neutron absorbing material 5a is arranged at the upper end over a length of about 30 cm. Hybrid neutron absorbing rods 5 filled with B ₄ C powder 5b, which is a neutron absorbing material, and neutron absorbing rods 4 with plenums having the same configuration as the neutron absorbing rods arranged near the tie rods 5 are arranged alternately. ing.

Here, as the hafnium material constituting the long-life neutron absorption rod 3 and the long-life neutron absorption material 5a, an exposed hafnium rod is used without being loaded into a cladding tube.

Furthermore, Ag- is used as a neutron absorbing material with long life.
There is also the idea of using In-Cd alloy or Eu ₂ O ₃ powder, but
Ag becomes radioactive and dissolves into the reactor water, potentially increasing the radiation level of the reactor water. Therefore, when used, it is necessary to house it in a cladding tube and keep it sealed. On the other hand, europium is a particularly expensive material among rare earths, so it has many disadvantages in terms of cost.

According to the boiling water reactor control rod according to this embodiment, long-life neutron absorbing materials are placed only on the side edges and upper ends of the wing 2, which are exposed to strong neutron irradiation, so that the control rod as a whole Not only will the lifespan be significantly extended, but a significant increase in the weight of the control rods will be avoided, and it will be easier to convert it into existing nuclear reactor equipment.

Furthermore, since the amount of expensive long-life neutron absorbing material used is kept to the minimum necessary amount, the manufacturing cost of the control rods does not increase much.

On the other hand, a plenum 4a is provided near the tie rod 1, and a neutron absorbing rod 4 with a plenum filled with normal B ₄ C powder is arranged, so that when the control rod is withdrawn, the fuel rod closest to the tie rod 1 is The output increase rate is suppressed and the soundness of the fuel can be ensured.

In addition, at the upper end of the control rod, hafnium material, which has a longer life than B ₄ C but has a lower reactivity, is installed, so the reactivity value partially decreases at the upper end, but from the perspective of the entire control rod. In some cases, it is negligible.

However, there is also the effect that the amount of increase in the output of the adjacent fuel assembly when the control rod is withdrawn can be reduced by the amount that the reactivity of the upper end is reduced.

Furthermore, the neutron absorbing rod 4 with a plenum filled with B ₄ C as a normal-life neutron absorbing material has a large plenum 4a in the area where neutron irradiation is strongest, so He gas generated by nuclear reaction, ³ T ₂₎ It is possible to suppress the increase in the internal pressure of the cladding tube due to gas, etc. In other words, in conventional control rods in which B ₄ C is present at the upper end of the neutron absorption rod, the mechanical life of the absorption rod may be shortened in a short period of time due to nuclear reactions, reactions between chemical reactants and the cladding, and increases in internal pressure. However, in this embodiment, a large plenum 4a is provided at the upper end of the neutron absorption rod filled with B ₄ C powder 5b, so the generated gas is stored in the plenum 4a one after another, and the gas pressure is maintained even at the end of the operation cycle. There is little chance of it becoming abnormally high.

The present invention is not limited to the above-mentioned embodiments. For example, in the neutron absorption rod 4 with a plenum, the plenum 4a
A member such as a hollow body or metal wool may be placed inside. It is desirable that these members adsorb nuclear reaction and chemical reaction products.

Further, a getter, which is an active metal that adsorbs and removes moisture and oxygen, may be placed in the plenum. Also,
In the hybrid neutron absorption rod 5, a long-life neutron absorption material 5a and a normal-life neutron absorption material are used.
A wool-like member 5c such as metal wool may be interposed between the B ₄ C powder 5b and the B 4 C powder 5b. That is, when inserting the long-life neutron absorber 5a, the wool-like member 5c is brought into contact with the lower end of the long-life neutron absorber 5a, and the wool-like member 5c is pushed in at the same time as the long-life neutron absorber 5a is inserted. The B ₄ C powder 5b that had adhered to the inner surface of the tube is pushed down together with the wool-like member 5c, and the wool-like member 5c is pushed downward.
B ₄ C powder 5b is prevented from moving upward, so there is a gap between the long-life neutron absorber 5a and the inner surface of the cladding tube.
B ₄ C powder 5b does not enter. Therefore
There is no possibility that a pressing force will act on the cladding tube through the B ₄ C powder 5b and cause local stress. Further, when the long-life neutron absorbing material 5a is inserted, there is no risk that the lower end corner portion of the material will catch on the inner surface of the cladding tube and cause damage or galling.

In addition, the length of the plenum 4a is determined within a range that does not reduce the reactivity value of the control rod and avoids a region where neutron irradiation is extremely strong, that is, from the upper end of the control rod.
It is desirable to form in an area of 10 to 30 cm.

Furthermore, the length of the long-life neutron absorbing material 5a is set to the length of the part of the control rod that receives strong neutron irradiation, that is, from 30 cm from the upper end of the control rod to about 1/4 of the effective length of the wing.

Further, it is not necessary to arrange the long-life neutron absorbing material 5a over the entire effective length of the wing near the side edge of the wing 2, and it may be arranged only over the upper half of the entire effective length.

〔Effect of the invention〕

As explained above, according to the control rod for a boiling water reactor according to the present invention, long-life neutron absorption rods are arranged only on the side edges and upper ends of the wings, which are exposed to strong neutron irradiation, so that the control rod as a whole The lifespan of the control rods will be extended, a significant increase in the weight of the control rods will be suppressed, it will be easier to use the control rods in existing reactor equipment, and manufacturing costs will be reduced.

On the other hand, there is a plenum near the tie rod.
Because neutron absorption rods with plenums filled with B ₄ C powder are installed, the output increase rate of the fuel rod closest to the tie rod when the control rod is withdrawn is suppressed, ensuring the integrity of the fuel.

Furthermore, the neutron absorption rod with a plenum has a large plenum installed in the area where neutron irradiation is strongest, so it is possible to suppress the increase in the internal pressure of the cladding tube due to He gas, ³ T ₂ gas, etc. generated in the nuclear reaction. .
Furthermore, when B ₄ C exists at the top of the neutron absorption rod,
There is a risk that the mechanical life of the absorption rod will be reduced due to nuclear reactions, reactions between chemical reactants and the cladding, and increases in internal pressure. However, in the present invention, a plenum is provided at the upper end of the neutron absorption rod filled with _B4C . There is no risk of that happening.

[Brief explanation of drawings]

The figure is a sectional view showing a main part of an embodiment of the present invention. 1... Tie rod, 2... Wing, 3... Long-life neutron absorption rod, 4... Neutron absorption rod with plenum, 4
a... Plenum, 5... Hybrid neutron absorption rod, 5a...
Long-life neutron absorber, 5b...B ₄ C powder, 5c...
Wool-like material.

Claims (1)

[Claims] 1 A boiler consisting of a wing formed by attaching an opening of a sheath having an elongated U-shaped cross section to each protruding leg of a tie rod having a cross-shaped cross section, and loading a large number of neutron absorption rods in the width direction within the wing. In control rods for water reactors, a neutron absorption rod with a plenum formed at the upper end and filled with B ₄ C powder as a neutron absorbing material is installed near the tie rod, but does not generate gas by absorbing neutrons. A long-life neutron absorption rod made of hafnium material is arranged near the side edge of the wing, and the neutron absorption rod with plenum is placed between the tie rod and the wing side edge, and the neutron absorption rod is made of hafnium near the upper end. A control rod for a boiling water reactor, characterized in that hybrid neutron absorbing rods filled with a long-life neutron absorbing material made of a neutron absorbing material made of carbonaceous material and a hybrid neutron absorbing rod filled with B ₄ C powder below the upper end are arranged alternately. .