JPS5843715B2 - control rod - Google Patents

Description

[Detailed description of the invention] The present invention relates to a control rod for a nuclear reactor.

Control rods are the main components of the reactor core together with fuel, and contain nuclides with a large neutron absorption cross section, such as boron-10.
(B10) A plate-shaped or rod-shaped solid member that is inserted from outside the core as necessary and used to control the reactivity and power distribution of the core.

The structure differs depending on the type of reactor, but as an example, one used in a boiling water reactor is shown in FIGS. 1 and 2.

Figure 1 is a perspective view, and Figure 2 is a horizontal sectional view. 1 is a cross-shaped control rod blade, 2 is a poison tube disposed inside the control rod blade 1, and is coated with stainless steel. Boron carbide (B4C) powder 4 is uniformly filled and sealed inside the tube 3.

When a control rod having such a configuration is inserted into the reactor core from the lower part of the reactor core in order to control the reactivity and power distribution of the reactor core during power operation, boron 10 (B
lo) changes to helium and lithium, and the amount of B10 decreases.

Therefore, the volume expansion of the B4C powder 4 and the increase in the internal pressure of the poison tube 2 occur.

Generally, the life of a control rod is evaluated from both the nuclear life and mechanical life. The nuclear life is evaluated by the control rod's neutron absorption capacity, that is, the amount of B10, and the mechanical life is evaluated by the control rod's neutron absorption capacity, that is, the amount of B10. Determined by stress evaluation.

In conventional control rods that use poison tubes uniformly filled with B4C powder, the mechanical life is approximately 1/2 of the nuclear life due to helium generation, and the mechanical life is the same as the control rod life. was the determining factor.

The present invention aims to eliminate the drawbacks of the prior art and provide a control rod with a long mechanical life, and the present invention includes a neutron absorbing material in the cladding tube that is harder than the cladding tube and generates fissile gas when absorbing neutrons. In a control rod using multiple poison tubes filled with poison, at least a part of the inner surface of the cladding tube is made of a neutron absorbing material that is softer than the constituent materials of the cladding tube and does not generate fissile gas when absorbing neutrons. It is characterized in that it includes a poison tube on which a coating is formed.

That is, in conventional control rod poison tubes, when the neutron absorbing material, for example B4C, is irradiated with neutrons,
B4C powder absorbs neutrons and generates helium,
The volume of the B4C powder expands and comes into direct contact with the coated cladding tube.

When the volume of B4C powder continues to expand, B4
Since the C powder is harder than the material of the cladding tube, stress concentration occurs locally at the contact area between the B4C powder and the cladding tube.

Furthermore, when the helium generated when the B4C powder absorbs neutrons comes out of the B4C powder and the internal pressure inside the cladding tube increases, if there is a manufacturing defect on the inner surface of the poison tube, stress concentration occurs in this defective part.

That is, when B4C is irradiated with neutrons, the pressure of helium and the volumetric expansion of the neutron absorbing material directly exert a force on the cladding of the poison tube, causing an increase in stress in the cladding.

As a result, the control rod life was determined by the mechanical life based on the increase in stress in the cladding, even though the nuclear life remained.

The present invention is based on the results of studies on poison tubes for conventional control rods, and uses a cladding tube material such as a metal softer than stainless steel, For example, cadmium (Cd) is interposed as the inner coating of the cladding tube, and when a soft metal is interposed in the inner coating of the cladding tube in this way, the B4C powder expands in volume due to neutron irradiation. Also, since it is the soft metal that comes into direct contact with the B4C powder, the soft metal is depressed at the contact point and the contact area increases, so no local stress concentration occurs in the cladding tube.

Also, even if there is a manufacturing defect on the inside of the poison tube,
Since soft metal is present in the defective area, stress concentration in this area can be alleviated.

This makes it possible to extend the mechanical life due to stress increase, thereby making it possible to extend the life of the control rod.

Examples will be described below.

FIG. 3 is a cross-sectional view of a poison tube of one embodiment of the control rod of the present invention, where 30 is a stainless steel cladding tube, and 50 is a Cd film coated on the inner surface of the stainless steel cladding tube 30. 40 is B4C powder filled into the stainless steel cladding tube 30 via a Cd film 50.

The Cd film 50 has a thickness of about 0.5 to 0.6 mrt.
Thickness approx. 30-40mm for stainless steel cladding
μm deposited.

In a poison tube with such a configuration, B4C powder 4
When 0 absorbs neutrons and generates helium and expands in volume, the force does not directly reach the stainless steel cladding tube 30, but is transmitted through the Cd coating 50 that is present in direct contact with the B4C powder 40. Stainless steel cladding tube 30
However, since Cd is less hard than stainless steel, helium and B4C powder 40
The effect of the volumetric expansion of the cladding tube can be alleviated, and therefore, the increase in stress in the cladding tube can be alleviated.

Since the stress increase can be alleviated in this way, the life of the cladding tube can be extended, and a control rod with a long mechanical life can be obtained.

In addition, the stay time in the reactor is longer than that of conventional control rods, and the amount of spent radioactive waste can be reduced.

Furthermore, since the Cd coating is a neutron absorbing material, the amount of neutron absorbing material in the poison tube does not decrease due to the formation of this coating, and the efficiency as a control rod remains unchanged. Sometimes no fissile gas is produced.

In the above example, an example was shown in which a Cd film was deposited on the inner surface of a stainless steel cladding tube, but silver (Ag),
Any material, such as indium (In), which has lower hardness than the constituent material of the cladding tube, does not generate fissile gas when absorbing neutrons, and has a large neutron absorption cross section can be used in the same manner.

In addition, in the above embodiment, the Cd film was formed over the entire length of the poison tube, and the explanation was given assuming that a plurality of poison tubes constituting the control rod had such a configuration. The relative neutron dose distribution for poison tubes is not uniform; in general, the neutron dose distribution for poison tubes in any horizontal direction of the control rod is greatest for poison tubes at the outermost ends of the blades, and The neutron irradiance distribution in the axial direction of one poison tube is highest at the leading edge of the blade, decreases as you move downward, and is lowest at the poison tube adjacent to the center. least.

The magnitude of the internal pressure of each poison tube has a distribution shape similar to the horizontal direction relative neutron irradiation distribution, and the internal pressure of the poison tube located at the outermost end of the blade is the largest.

In this way, since the amount of neutrons irradiated differs depending on the location of the poison tube in the control rod, Cd film is coated only on the portion of the poison tube that is irradiated with neutrons. Alternatively, poison tubes having different Cd coating locations and coating amounts may be used depending on the position of the poison tube in the control rod.

With this configuration, C which is more expensive than B4C
d or A g I I n etc. can be reduced.

As described above, the control rod of the present invention can provide a control rod with a long mechanical life, and is an industrially effective control rod.

[Brief explanation of the drawing]

FIG. 1 is a perspective view showing the external appearance of a control rod, FIG. 2 is a horizontal cross-sectional view of the main parts of a conventional control rod, and FIG. 3 is a cross-section of the main parts of an embodiment of the control rod of the present invention. It is a diagram. 2... Poison tube, 30... Stainless steel cladding tube, 40... B4C powder, 5
0...Cd film.

Claims (1)

[Scope of Claims] 1. In a control rod using a plurality of poison tubes in which the cladding tubes are filled with a neutron absorbing material harder than the cladding tubes, which generates fissile gas when absorbing neutrons, the inner surface of the cladding tubes A control rod comprising a poison tube on at least a part of which is coated with a neutron absorbing material that is softer than the constituent material of the cladding tube and does not generate fissile gas when absorbing neutrons. 2. The control rod according to claim 1, wherein the neutron absorbing material that generates fissile gas is a boron compound, and the neutron absorbing material that does not generate fissile gas is cadmium.