JP4000382B2 - Control rod made of boron mixed carbon fiber reinforced carbon composite material - Google Patents

Description

[0001]
BACKGROUND 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]
Conventionally, in a multi-purpose HTGR or gas-cooled fast breeder reactor, a neutron absorber such as boron carbide or boronated graphite is housed in a control rod vessel that is a structure and used as a control rod. The control rod container, which is this structure, currently uses a heat-resistant alloy such as Alloy 800, but these next-generation furnaces require heat resistance exceeding 800 ° C. and 1000 ° C. or higher.
[0003]
As a countermeasure, use of a carbon fiber reinforced carbon composite material as the above-mentioned structure has been studied, which is shown in the following Patent Document 1, and also in Japan Atomic Energy Research Institute, for example, the following non-patent As described in Document 1, studies on the irradiation characteristics of carbon fiber reinforced carbon composite materials are underway. The control rod is one of the most important members in the 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]
It is an object of the present invention to further reduce costs by further improving the performance and safety of such control rods.
[0006]
[Means for Solving the Problems]
As means for solving the above-mentioned problems, as a result of intensive studies, a control rod in which a control rod container, which is a structure, and a neutron absorber are integrated has been invented. The control rod of the present invention is solved by integrating boron having a neutron absorption capability and a structure. Hereinafter, the control rod according to the present invention is referred to as a boron mixed control rod. Note that boron having neutron absorption ability is precisely a boron isotope having a mass number of 10 has its neutron absorption action, and a certain amount is contained in normal boron and boron compounds. .
[0007]
Moreover, since the conventional control rod puts a neutron absorber separately in a control rod container, the shape was a cylindrical shape. On the other hand, since the control rod of the present invention is an integrated control rod container and neutron absorber, the shape of the control rod is convenient, such as a cylinder, a column, a plate, or a combination of plates in a cross shape. A good shape can be used.
[0008]
DETAILED DESCRIPTION OF THE INVENTION
As a boron mixed type control rod, it is also possible to use borated graphite (for example, described in the following Patent Document 2 and Non-Patent Document 2) as a heat-resistant material as it is. Some are theoretically possible.
[0009]
[Patent Document 2]
: 1820502 [Non-Patent Document 2]
: IAEA-TECCDOC-690, issued in February 1993, “The status of graph development for gas cooled reactors”
However, borated graphite has a drawback that it has sufficient heat resistance but poor mechanical toughness, so that it is actually used only as a neutron absorbing material to be placed 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), which combines boron with a carbon fiber reinforced carbon composite material that also has high toughness, constitutes an excellent boron mixed control rod. it can. The reason can be given as follows.
[0011]
(1) Since the C / C composite material is used as a base material, the performance can be mainly improved.
(2) Since it is an integrated type, there is no interaction between the structure and the neutron absorber. For example, when an earthquake or the like occurs, it is not necessary to consider the generation of stress due to mutual impact.
[0012]
{Circle around (3)} The boron-mixed C / C composite material has higher oxidation resistance than the C / C composite material alone, and is less oxidized by, for example, impurity oxygen in helium gas.
(4) When air is assumed to be introduced at the time of an accident such as a pipe breakage, the boron-mixed C / C composite material has high oxidation resistance, so it is possible to ensure higher safety unlike the C / C composite material alone. it can.
[0013]
(5) Cost is lower than when the structure and neutron absorber are manufactured separately. Since the form of the C / C composite material is not particularly limited, any one can be applied, but one-dimensional, two-dimensional, 2.5-dimensional, and three-dimensional C / C composite materials are suitable for this application. Conceivable. This is because these C / C composite materials generally have high tensile mechanical strength and high toughness. Therefore, it is desirable to mix boron based on the form of these C / C composites.
[0014]
Furthermore, it is desirable to use a two-dimensional (2D) C / C composite as a base. In addition, as the 2D-C / C composite materials that are most commonly used at present, there are those using a two-dimensional cloth or those laminated alternately by changing the direction of the one-dimensional cloth. Two-dimensional C / C composites using two-dimensional cloth have a wide range of manufacturability such as shape and size, are produced in relatively large quantities, and are advantageous in terms of cost. Also, research is being conducted mainly on two-dimensional C / C composites as control rods for HTGRs.
[0015]
The C / C composite material is a bundle of carbon fibers arranged in a certain array and densified with a matrix, and the carbon fibers are made from pitch, polyacrylonitrile (PAN), rayon, etc. It will be.
[0016]
The one-dimensional C / C composite material is produced by aligning long carbon fibers in one direction, and the carbon fiber volume fraction is 20% or more, and used in the axial direction of the carbon fibers. The characteristic of the carbon fiber appears remarkably, and it has a large tensile strength in one direction. The two-dimensional C / C composite material is produced by laminating cloth or non-woven fabric in which long carbon fibers are woven in two dimensions in the vertical and horizontal directions, and the densification is generally performed by pitch or resin impregnation. It is a two-dimensionally reinforced material and has high toughness. A multidimensional C / C composite material such as the 2.5-dimensional, three-dimensional C / C composite material is produced by orienting long fiber bundles in a multidimensional direction. Since it is carried out by impregnation and reinforced with carbon fibers in a multidimensional manner, it has isotropic physical properties.
[0017]
There is no particular limitation on the method of causing boron or its carbide, which is a boron source, to exist in the C / C composite material, but a method in which boron is more uniformly dispersed while considering the difficulty of production such as cost is desirable.
[0018]
Examples of the method for dispersing boron include the following three methods.
(1) A method of finely dispersing boron using a metal alkoxide,
(2) A method of impregnating boron oxide using a heating impregnation apparatus (hot isostatic pressing apparatus) and performing heat treatment at a high temperature of 1500 ° C. or higher to make boron or its carbide exist is shown in Patent Document 3 below. [0019]
[Patent Document 3]
: Patent No. 3349919 Specification (3) Further, in Patent Document 4 below, a method of applying a mixture of a boron carbide powder to a thermosetting resin, coal tar or pitch, which is a matrix in the production of an oxidation resistant material, and boron It shows a method of incorporating boron into the C / C composite material by thermal decomposition of the contained gas.
[0020]
[Patent Document 4]
: The method described in Japanese Patent No. 3058180 (1) can finely disperse boron, but has a drawback that an alkoxide is expensive and unsuitable for the presence of a large amount of boron. The method (2) has the advantage that fine dispersion of boron is possible, but it requires a heating impregnation device (hot isostatic pressing device) and increases the cost, and the size of the device is limited. . The method {circle around (3)} is the same as that for boronated graphite used as a neutron absorber in the sense that boron carbide is mixed.
[0021]
The uniformity of boron when boron carbide powder is used is considered as follows. The average particle size of boron carbide used in borated graphite is 3 micrometers, and it seems sufficient to use boron carbide powder having such a particle size as a raw material.
[0022]
Further, since boron is substituted and dissolved in the carbon portion by heat treatment at about 2000 ° C., the uniformity of boron dispersion can be achieved. Needless to say, boron carbide itself must be uniformly dispersed in the material in order to satisfy these conditions. In the production of the borated material, the heat treatment temperature is 1600 to 2300 ° C, preferably 1900 to 2100 ° C. This is because if it is 1600 ° C. or lower, boron does not dissolve in carbon, and if it is 2300 ° C. or higher, graphitization develops too much and mechanical strength deteriorates.
[0023]
There is no disclosed method 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 effectively mass-produced or used. Almost no production. However, in the present invention, these materials are necessary.
[0024]
Next, the boron concentration of the boron mixed C / C composite material will be described. The boron concentration is determined by the furnace design including the control rod. Therefore, although the concentration is not limited to anything, it is considered that if the concentration of boron up to about 30 to 50% by weight can be secured empirically, it can be adapted. 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 in the length direction of the composite control rod. This is because the nuclear reaction is controlled by moving the control rod up and down.
[0026]
Although it is possible to use boron mixed control rods even if boron is uniformly distributed in the radial direction, it is more effective to change the boron distribution in the radial direction of boron mixed control rods. And found out.
[0027]
That is, while the boron mixed control rod is in use, boron reacts with neutrons to generate helium. At that time, voids are generated. Voids generated in the material are thought to reduce the material strength. Therefore, if a portion not containing boron is present, for example, in the central portion of the diameter, the portion can prevent the material strength from being lowered, thereby ensuring the safety and long life of the material. Further, a so-called functionally graded material can be obtained in which the boron concentration is increased in the radial direction of the cylinder or cylinder, the inner concentration is increased and the inner concentration is decreased, and the boron concentration is sequentially changed. By doing so, the thermal and mechanical properties of the material change gradually, and the reliability in terms of thermal shock resistance and the like increases.
[0028]
In addition, it is conceivable that the part containing no boron of the boron mixed control rod is alternately arranged outside the center part. In any case, boron must be present on the outer surface in order to prevent deterioration of the control rod due to air intrusion during an accident.
[0029]
【Example】
The example which produced the control rod of the boron mixing 2D-C / C composite material is shown. To prepare a boron mixed 2D-C / C composite material, boron carbide powder having an average particle diameter of 3 μm and carbon fiber cloth {PAN (polyacrylonitrile) 6K (the number of carbon fiber bundles is 6000) Plain weaves (those in which the fibers are woven together) were used.
[0030]
In forming, a stainless steel rod having a diameter of 5 mm was used as a mandrel, and a prepreg of 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 product does not contain boron up to a diameter of 30 mm inside in the radial direction.
[0031]
A method for producing a carbon fiber cloth prepreg will be described. In the production, the carbon fiber cloth is divided into a part not containing boron and a part containing boron.
In the part 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, boron carbide powder corresponding to 100 parts by weight and 100 parts by weight of 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 having a changed concentration of boron carbide to be added may be sequentially applied.
[0033]
These resin alone or cloth coated with boron carbide and resin were laminated and air-dried.
Next, the molded body was heat-treated 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 a total of 3 times.
[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. Measurement was performed using a chemical titration method such as Mannithol titration. 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 was 1.62 g / cm ³ .
[0035]
Tensile test specimens were sampled from the part not containing boron and the part containing 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. All of the boron-mixed 2D-C / C composite materials of the present invention exhibit crosshead displacement as shown by commercially available 2D-C / C without brittle fracture, and the original 2D-C / C composite material It was confirmed that the toughness of the material was maintained.
[0036]
In addition, the following results were obtained when the material was designed with reference to the conditions in the HTTR multipurpose HTGR. If the tensile strength is 141 MPa, it is calculated that it can be used in design if it has a diameter of 23 mm. Therefore, the boron mixed 2D-C / C composite material of this example has 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 HTTR is 1360 kg considering the seismic load. From these values, a diameter of 23 mm was obtained.
[0038]
【The invention's effect】
A control rod that stores a neutron absorber such as boron carbide or boron 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 made of a boron mixed carbon fiber reinforced carbon composite material in which the neutron absorber and the structure in the reactor are integrated in the present invention sufficiently satisfies the above heat resistance.

Claims (4)

A nuclear reactor characterized in that the boron concentration in the radially outer side is larger than that in the inner side by winding and heat treating a carbon fiber cloth not containing a boron source and a carbon fiber cloth containing a boron source in this order. Control rod for . Control rod according to claim 1, wherein the boron source is formed by using boron carbide powder. The control rod according to claim 1 or 2 , wherein the boron concentration decreases sequentially 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 3 , wherein the shape of the control rod is a cylinder, a column, a plate, or an appropriate shape obtained by combining plates in a cross shape.