JP2004132758A - Control rod comprising boron mixed carbon fiber reinforced carbon composite material - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve the problem wherein a control rod formed by enclosing a neutron absorber such as boron carbide or boronated graphite in a control rod housing which is a conventional structure wherein a heat-resistant alloy such as alloy 800 is used for the control rod housing which is the structure thereof does not have heat resistance in the range over 800°C up to 1,000°C or higher. <P>SOLUTION: This control rod comprising a boron mixed carbon fiber reinforced carbon composite material is a control rod in a nuclear reactor formed by integrating the control rod housing constituted from the boron mixed carbon fiber reinforced carbon composite material formed by combining a carbon fiber reinforced carbon composite material with boron, with the neutron absorber enclosed inside. The control rod is formed by winding and laminating carbon fiber cloth around the mandrel, and then by winding and laminating carbon fiber cloth into which a boron source is impregnated, and by performing heat treatment thereof. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a control rod made of a boron-mixed carbon fiber reinforced carbon composite material in which a neutron absorber and a structure in a nuclear reactor are integrated.
[0002]
[Prior art]
2. Description of the Related Art Conventionally, in a multipurpose high-temperature gas reactor or a gas-cooled fast breeder reactor, a neutron absorbing material such as boron carbide or boron boride is stored in a control rod container as a structure and used as a control rod. A heat-resistant alloy such as Alloy 800 is currently used for the control rod container as this structure. However, in the next furnace, heat resistance exceeding 800 ° C. and 1000 ° C. or more is required.
[0003]
As a countermeasure, the use of a carbon fiber reinforced carbon composite material as the above structure has been studied, which is disclosed in Patent Document 1 below. Regarding this, even within the Japan Atomic Energy Research Institute, for example, As described in Document 1, research on the irradiation characteristics and the like of carbon fiber reinforced carbon composite materials has been advanced. The control rod is one of the most important members in a nuclear reactor, and further improvement in performance and safety are required. On the other hand, improvement in cost performance is also required.
[0004]
[Patent Document 1]: Japanese Patent No. 2784304 [Non-Patent Document 1]: JAERI-Research 2001-028
[0005]
[Problems to be solved by the invention]
An object of the present invention is to further improve the performance and safety of such a control rod, and to pursue further cost reduction.
[0006]
[Means for Solving the Problems]
As a means for solving the above problems, as a result of intensive studies, a control rod in which a control rod container as a structure and a neutron absorbing material are integrated has been invented. The control rod of the present invention is solved by integrating boron having a neutron absorbing capacity and a structure. Hereinafter, the control rod according to the present invention is referred to as a boron-mixed control rod. In addition, the boron having a neutron absorption ability is, precisely, a boron isotope having a mass number of 10 has its neutron absorption action, and a certain amount of it is contained in ordinary boron and boron compounds. .
[0007]
Further, the conventional control rod has a cylindrical shape because a neutron absorbing material is separately added to the control rod container. On the other hand, since the control rod of the present invention is formed by integrating the control rod container and the neutron absorbing material, the shape of the control rod can be a cylinder, a cylinder, a plate, a cross-shaped combination of plates, or the like. Good shape can be used.
[0008]
BEST MODE FOR CARRYING OUT THE INVENTION
As a boron-mixed control rod, a borated graphite (for example, described in Patent Literature 2 and Non-Patent Literature 2) used in a high-temperature gas furnace may be used as it is as a heat-resistant material as it is. Some are theoretically possible.
[0009]
[Patent Document 2]: No. 1820502 [Non-Patent Document 2]: IAEA-TECDOC-690 “The status of graphite development for gas cooled reactors” issued in February 1993.
However, borated graphite has a sufficient heat resistance but has a drawback of poor mechanical toughness, so that it is actually used only as a neutron absorber contained in a control rod container.
[0010]
A boron mixed carbon fiber reinforced carbon composite material (hereinafter referred to as boron mixed C / C composite material) combining boron and a carbon fiber reinforced carbon composite material also having high toughness can constitute an excellent boron mixed type control rod. it can. The reasons can be given as follows.
[0011]
{Circle around (1)} Since the C / C composite material is used as the base material, the performance can be mainly improved.
(2) There is no interaction between the structure and the neutron absorber because it is an integral type. For example, when an earthquake or the like occurs, it is not necessary to consider stress generation due to mutual impact.
[0012]
{Circle around (3)} Compared to the C / C composite material alone, the boron-mixed C / C composite material has higher oxidation resistance, and is less oxidized by, for example, impurity oxygen in helium gas.
(4) Assuming that air is introduced in the event of an accident such as a pipe break, the boron-mixed C / C composite has high oxidation resistance, so unlike the C / C composite alone, higher safety can be ensured. it can.
[0013]
{Circle around (5)} The cost is lower than in the case where the structure and the neutron absorber are separately manufactured. Since the form of the C / C composite material is not particularly limited, any one can be applied. However, a one-dimensional, two-dimensional, 2.5-dimensional, three-dimensional C / C composite material is suitable for this application. Conceivable. This is because these C / C composite materials generally have particularly high mechanical strength in tension and high toughness. Therefore, it is desirable to mix boron based on these C / C composite forms.
[0014]
Further, it is desirable to use a two-dimensional (2D) C / C composite as a base. At present, the most commonly used 2D-C / C composite materials include those using a two-dimensional cloth and those alternately laminated in different directions of the one-dimensional cloth. The two-dimensional C / C composite material using a two-dimensional cloth has a wide range of manufacturability such as shape and size, is produced in a relatively large amount, and is cost-effective. Also, research is being conducted on two-dimensional C / C composites as control rods for high-temperature gas furnaces.
[0015]
The C / C composite material is a material in which bundles of carbon fibers are arranged in a certain array and have a high density in a matrix, and the carbon fibers are made of a material such as pitch, polyacrylonitrile (PAN), rayon, or the like. It becomes.
[0016]
The one-dimensional C / C composite material was manufactured by aligning long carbon fibers in one direction, the volume fraction of the carbon fibers was 20% or more, and was used in the axial direction of the carbon fibers. The characteristics of the carbon fiber are remarkably exhibited, and the carbon fiber has a large tensile strength in one direction. The two-dimensional C / C composite material is manufactured by laminating a cloth or a non-woven fabric in which long carbon fibers are woven two-dimensionally in a vertical direction and a horizontal direction, and the density thereof is generally increased by pitch or resin impregnation. And is a two-dimensionally reinforced material with high toughness. The multi-dimensional C / C composite material such as the 2.5-dimensional and 3-dimensional C / C composite materials is manufactured by orienting long fiber bundles in a multi-dimensional direction. It is made by impregnation and is multidimensionally reinforced with carbon fibers and therefore has isotropic physical properties.
[0017]
Although there is no particular limitation on the method of causing boron or its carbide as a boron source to be present in the C / C composite material, a method in which boron is more uniformly dispersed in consideration of manufacturing difficulty such as cost is desirable.
[0018]
The following three methods can be exemplified as a method for dispersing boron.
(1) A method of finely dispersing boron using a metal alkoxide,
(2) Patent Document 3 below discloses a method in which boron oxide is impregnated using a heating impregnation device (hot isostatic pressing device) and heat treatment is performed at a high temperature of 1500 ° C. or more to make boron or its carbide exist. [0019]
[Patent Document 3]: Japanese Patent No. 3034919 [3] In Patent Document 4 below, a mixture of a thermosetting resin, coal tar or pitch serving as a matrix in the production of an oxidation-resistant material, and boron carbide powder mixed with pitch is used. It shows a method of coating and a method of containing boron in a C / C composite material by thermal decomposition of a boron-containing gas.
[0020]
[Patent Document 4]: The method disclosed in Japanese Patent No. 3058180 (1) can finely disperse boron, but has the disadvantage that alkoxides are expensive and unsuitable for allowing boron to be present in large amounts. The method (2) has an advantage that the boron can be finely dispersed, but requires a heating and impregnating device (hot isostatic pressing device), increases the cost, and limits the size of the device. . The method of (3) is the same as that of borated graphite used as a neutron absorber in the sense that boron carbide is mixed.
[0021]
The uniformity of boron when using boron carbide powder is considered as follows. The average particle size of boron carbide used in borated graphite is 3 micrometers, and it is considered sufficient to use boron carbide powder having such a particle size as a raw material.
[0022]
Further, the boron is substituted and solid-solved in the carbon part by the heat treatment at about 2000 ° C., so that the uniformity of the boron dispersion is achieved. Needless to say, in order to satisfy these conditions, boron carbide itself must be uniformly dispersed in the material. In producing the borated material, the heat treatment temperature is preferably from 1600 ° C. to 2300 ° C., and more preferably from 1900 ° C. to 2100 ° C. If the temperature is 1600 ° C. or less, boron does not form a solid solution in carbon.
[0023]
No method has been disclosed for uniformly dispersing boron carbide in a 2D-C / C composite material using a two-dimensional carbon fiber cloth so that it can be industrially and effectively mass-produced or used. Little is produced. However, in the present invention, these materials are necessary.
[0024]
Next, the boron concentration of the boron-mixed C / C composite will be described. The boron concentration is determined by the design of the furnace including the control rod. Therefore, although the concentration cannot be uniquely defined, it is considered empirically that if a boron concentration of at most about 30 to 50% by weight can be secured, it is suitable. The boron concentration in the material does not affect the essence of the invention.
[0025]
Here, the distribution state of boron is important. It is believed that boron must have the same concentration along the length of the composite control rod. This is because the nuclear reaction is controlled by moving the control rod up and down.
[0026]
It is possible to use boron-mixed control rods even if boron is uniformly distributed in the radial direction, but it is more effective to change the boron distribution in the boron-mixed control rods in the radial direction. And clarified.
[0027]
That is, during use of the boron mixed control rod, boron reacts with neutrons to generate helium. At that time, voids are generated. It is believed that the voids created in the material reduce the material strength. Therefore, if a portion not containing boron is present, for example, in the center portion of the diameter, the portion can be prevented from having a decrease in material strength, and the safety and the life of the material can be ensured. In addition, a so-called functionally graded material in which the boron concentration is increased in the radial direction of the cylinder or the cylinder to increase the concentration in the outside and decrease the concentration in the inside to sequentially change the boron concentration can be obtained. By doing so, the thermal and mechanical properties of the material change gently, and the reliability in thermal shock resistance and the like is further improved.
[0028]
In addition, it is conceivable that the portion of the boron-mixed control rod that does not contain boron is alternately provided outside the central portion and outside. In any case, boron must be present on the outer surface in order to prevent deterioration of the control rods due to oxidation due to air intrusion in the event of an accident.
[0029]
【Example】
An example in which a control rod of a boron mixed 2D-C / C composite material is manufactured is shown. In order to produce a boron-mixed 2D-C / C composite material, a boron carbide powder having an average particle size of 3 μm and a carbon fiber cloth @ PAN (polyacrylonitrile) -based 6K (carbon fiber bundle having 6000 fibers) A plain weave (a fabric in which fibers are folded over each other) was used.
[0030]
In molding, a stainless steel rod having a diameter of 5 mm was used as a mandrel, and a prepreg of a carbon fiber cloth was continuously wound to form a molded body having a diameter of 100 mm and a length of 200 mm. This molded body does not contain boron up to a diameter of 30 mm on the radially inner side.
[0031]
A method for manufacturing a prepreg of a carbon fiber cloth will be described. The carbon fiber cloth is divided into a portion containing no boron and a portion containing boron.
In the portion not containing boron, first, 100 parts by weight of a resol type phenol resin was dissolved in ethyl alcohol and applied to 100 parts by weight of the carbon fiber cloth.
[0032]
In the subsequent portion containing boron, 100 parts by weight of boron carbide powder and 100 parts by weight of an ethyl alcohol slurry of a resol-type phenol resin were applied to the carbon fiber cloth surface. In order to manufacture a composite material in which the concentration of boron is sequentially changed, a slurry in which the concentration of boron carbide to be added is changed may be sequentially applied.
[0033]
A cloth coated with the resin alone or with boron carbide and the resin was laminated and air-dried.
Next, the molded body was subjected to a heat treatment at 1000 ° C. while flowing nitrogen gas. The heat-treated material was impregnated with a resol-type phenol resin, and the same heat treatment was performed again. This operation was repeated three times in total.
[0034]
Next, heat treatment was performed at 2000 ° C. under an argon gas atmosphere of 5 Torr to obtain a boron-mixed 2D-C / C composite material. The measurement was performed using a chemical titration technique such as the Mannitour titration method. The boron concentration was 30% by weight. The concentration of boron oxide was 0.02% by weight. The bulk density of the entire boron mixed 2D-C / C composite material was 1.62 g / cm ^3.
[0035]
Tensile test specimens were taken from the boron-free portion and the portion containing no boron in the manufactured boron-mixed 2D-C / C composite material, and the tensile strength in the axial direction of the control rod was measured. The portion containing boron was 141 MPa. The portion not containing boron was 115 MPa. Any of the boron-containing 2D-C / C composites of the present invention show a crosshead displacement as shown by a commercially available 2D-C / C without any brittle fracture, and exhibit the original 2D-C / C composite. It was confirmed that the toughness of the material was maintained.
[0036]
When the material was designed with reference to the conditions in the HTTR multipurpose high temperature gas furnace, the following results were obtained. If the tensile strength is 141 MPa, it is calculated that if it has a diameter of 23 mm, it can be used in design. Therefore, the boron-mixed 2D-C / C composite of this example has a sufficient strength.
[0037]
The calculation is 95% reliable with 99% non-destructive probability, the standard deviation difference of the material is 15.3 MPa, the safety factor is 3, and the design strength is 33.6 MPa. The load applied to one control rod in the HTTR is 1360 kg in consideration of the seismic load. From these values, a diameter of 23 mm was obtained.
[0038]
【The invention's effect】
A control rod containing a neutron absorbing material such as boron carbide or boride graphite in a control rod container that is a conventional structure uses a heat-resistant alloy such as Alloy 800 in the control rod container that is the structure. It does not have heat resistance exceeding 800 ° C. and 1000 ° C. or more.
[0039]
On the other hand, the control rod of the present invention made of a boron-mixed carbon fiber reinforced carbon composite material in which a neutron absorbing material in a nuclear reactor and a structure are integrated satisfies the above heat resistance sufficiently.

Claims (6)

A control rod for a nuclear reactor, in which a control rod container composed of a boron-mixed carbon fiber-reinforced carbon composite material in which carbon fiber-reinforced carbon composite material and boron are combined, and a neutron absorbing material stored therein are integrated. The control rod according to claim 1, wherein the control rod is formed by winding and laminating a carbon fiber cloth impregnated with a boron source, winding and laminating a carbon fiber cloth impregnated with a mandrel. The control rod according to claim 1 or 2, wherein the carbon fibers of the carbon fiber reinforced carbon composite material are arranged one-dimensionally, two-dimensionally, 2.5-dimensionally, or three-dimensionally. The control rod according to claim 2, wherein the boron source uses boron carbide powder. The control rod according to any one of claims 1 to 4, wherein the boron concentration gradually decreases from the outer surface to the inner side in the radial direction of the control rod. The control rod according to any one of claims 1 to 5, wherein the shape of the control rod is a cylinder, a cylinder, a plate, or an appropriate shape obtained by combining plates in a cross shape.