JPH09230080A - High performance neutron absorbing material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To elongate the life of a control rod in a reactor by hot pressing mixed powder of boron carbide, hexagonal boron nitride and obligatory impurities until the anisotropy of mechanical strength and thermal conductivity are revealed. SOLUTION: The mixing ration of boron carbide and boron nitride is set to such a range that the anisotropy of the mechanical strength and thermal conductivity is 1.5 or more, and the neutron absorbing power is not lowered to an extreme. The mixed powder is filled in a graphite mold, and a predetermined preliminary forming is performed according to need to be not-pressed in a high frequency induction hot press furnace. Hot pressing is performed at temperature of 1,700-2,200 deg.C and with pressure of 5-50Mpa. By the neutron absorbing material made by the thus obtained boron carbide/hexagonal boron nitride composite sintered compact, breakage due to thermal stress and swelling and damage of a covering pipe caused by the above can be greatly reduced, and as compared with the conventional material, the life of a reactor control rod can be elongated so as to improve the safety and the economic efficiency.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a neutron absorbing material for a reactor control rod, which has improved safety in a neutron irradiation environment.

[0002]

Boron (B) has ¹⁰ B and ¹¹ B isotopes, and natural boron contains 19.8% of ¹⁰ B. ¹¹ B absorbs almost no neutrons, but ¹⁰ B has a very large neutron absorption cross section. Among boron compounds, boron carbide (B ₄ C) has a high boron content per unit volume, is stable at high temperatures, and has an established industrial manufacturing method. ¹⁰ B is a large neutron absorption cutoff in the thermal neutron region. It has an area of ¹⁰ B because it has a large absorption capacity up to the fast neutron region.
A boron carbide material having a concentration of about 19.8% to about 90%, which is natural, is widely used as a neutron absorbing material for a control rod of a nuclear reactor.

In a fast reactor, a control rod generally has a structure in which cylindrical pellets of boron carbide serving as a neutron absorbing material are stacked and filled in a stainless steel cladding tube. Here, a sintered body of boron carbide is generally manufactured by a hot pressing method, but has features such as extremely hard and low toughness and low thermal shock resistance. Also, ¹⁰
The neutron absorption reaction of B is an (n, α) reaction, and due to the absorption of neutrons, helium (He) is retained in the crystal of boron carbide, so swelling occurs in the boron carbide sintered body. There is an essential problem of doing. The boron carbide sintered body that has undergone volume expansion due to swelling mechanically interacts with the cladding tube, and therefore shortens the original life of the control rod in the furnace.

Further, since the boron carbide pellets generate a large amount of heat when absorbing neutrons, the thermal stress generated causes the boron carbide sintered body to be finely cracked, which adversely affects the cladding tube and reduces the original design life of the control rod. It will be shortened further.

As measures against this, for example, the following techniques have been developed.

A method of controlling the crystal grain growth in the sintering process by using a boron carbide raw material having a specified B / C atomic ratio and primary particle size (Japanese Patent Publication No. 4-78159).

A method of providing a buffer portion made of solid boron nitride in the cladding tube (Japanese Patent Laid-Open No. 59-150369).

A method of filling a glass tube or a boron nitride powder in a cladding tube to prevent the sintering phenomenon of boron carbide (Japanese Patent Laid-Open No. 59-57195).

A method for improving the thermal shock resistance of a boron carbide sintered body by adding 30% by volume or less of boron nitride to boron carbide (Proc. 11th Int. Symp. Boron, B
oridesand Related Compounds, Tsukuba, 1993 JJAP Se
ries 10 (1994) pp. 216-219).

[0010]

However, the above-mentioned prior art has the following problems.

In the technique (1), the swelling property is lower than that of the conventional boron carbide sintered body, and the possibility of damage to the cladding tube is reduced, but the problem that the boron carbide sintered body collapses due to the generation of thermal stress is encountered. Still exists.

According to the technique of (1), a control rod is obtained which is hard to damage the coating pipe even if the coating pipe is damaged by swelling of boron carbide or the boron carbide sintered body is destroyed by thermal stress. However, the structure of the control rod is complicated. become.

In the technique of (1), since the boron carbide powder is used, damage to the boron carbide sintered body due to thermal stress and accompanying damage to the cladding tube are reduced, but the packing density of the neutron absorbing material is higher than that of the sintered body. Will be low. Also,
The powder, which is the neutron absorbing material, scatters only when a small crack is generated in the cladding tube, and the neutron absorbing ability is reduced.

In the technique of (1), the thermal shock resistance of the neutron absorbing material is improved, and the damage of the neutron absorbing material due to the thermal stress and the damage of the cladding due to it are reduced. However, the swelling of the neutron absorbing material damages the cladding. May occur.

The present invention has been made in view of the above problems, and an object thereof is a neutron absorbing material which is less likely to shorten the life of the reactor control rod in the reactor due to cracking of the neutron absorbing material due to swelling or thermal stress. More specifically, during neutron absorption, cladding of the neutron absorbing material due to swelling of the neutron absorbing material, damage of the neutron absorbing material due to thermal stress and accompanying damage of the cladding are less likely to occur, and neutron irradiation stability It is to provide a neutron absorbing material having excellent properties and economical.

[0016]

Means for Solving the Problems The present inventors have reduced the mechanical interaction with a cladding tube due to swelling of conventional neutron absorbing materials and cracking due to thermal stress, and improved safety and economic efficiency. As a result of earnestly examining the provision of materials, the following findings were found.

1) When considering the method of use as a reactor control rod, even if swelling occurs in a cylindrical neutron absorbing material, the swelling in the radial direction is reduced to suppress damage to the cladding tube. You can

2) The swelling in the radial direction is suppressed by increasing the mechanical strength in the radial direction of the cylindrical neutron absorbing material and decreasing the mechanical strength in the height direction.

3) By increasing the radial thermal conductivity of the cylindrical neutron absorbing material, the thermal stress generated in the neutron absorbing material is reduced.

4) By increasing the mechanical strength in the radial direction of the cylindrical neutron absorbing material, even if the neutron absorbing material is destroyed by swelling or thermal stress, the neutron absorbing material is not easily destroyed in the radial direction and the cladding tube is damaged. Possibility is reduced.

5) Hexagonal boron nitride has a strong anisotropy in thermo-mechanical properties like graphite, and is excellent in thermal shock resistance and chemical stability. Hexagonal boron nitride has a very high thermal conductivity in the plane of the layer as compared with that between the layers, and when the columnar pellets are manufactured, the structure such that the basal plane of the hexagonal boron nitride is aligned in the direction perpendicular to the cylinder axis. If controlled, high strength and thermal conductivity can be imparted in the radial direction of the pellet.

The above will be described in more detail below.

As shown in FIG. 1A, in the case of hexagonal boron nitride, its crystal form is a multi-layer structure. When hot pressing this hexagonal boron nitride powder,
As shown in (B), the hexagonal layers are arranged in the direction perpendicular to the hot pressing direction. Here, the hexagonal boron nitride having a multilayer structure has good thermal conductivity in the lateral direction (direction (a) of FIG. 1 (A)) and in the vertical direction (direction (b) of FIG. 1 (A)). The thermal conductivity is not good. Further, it is easy to expand and contract in the vertical direction (direction (B) of FIG. 1A), but it is difficult to expand and contract in the horizontal direction (direction (A) of FIG. 1A).

Therefore, when the powder of the mixed powder of hexagonal boron nitride and boron carbide is hot-pressed until sufficient anisotropy appears, a direction perpendicular to the hot-pressing direction (see FIG. 1).
(B) (direction (a)) has good thermal conductivity and is less likely to expand and contract in that direction, while it is parallel to the hot press direction (direction (b) in FIG. 1 (B)). A neutron absorbing material having poor thermal conductivity and expanding / contracting in that direction can be obtained. Then, if a neutron absorbing pellet is produced using this neutron absorbing material so that the hot pressing direction is the axial direction, a material having good radial thermal conductivity and high mechanical strength can be obtained. Since such a pellet is unlikely to cause vertical cracking and does not adversely affect the cladding, it does not shorten the life of the neutron control rod using the pellet.

Based on the above findings, the present inventors provide the following neutron absorbing material, neutron absorbing pellets, and control rods for nuclear reactors.

A neutron absorbing material obtained by hot pressing a mixed powder of boron carbide, hexagonal boron nitride and unavoidable impurities until the anisotropy of mechanical strength and thermal conductivity is exhibited.

Anisotropy is exhibited in which a mixed powder of boron carbide, hexagonal boron nitride and unavoidable impurities is filled in a pellet-forming mold so that the resulting pellet has better thermal conductivity in the radial direction than in the axial direction. A neutron absorbing pellet produced by hot pressing until the anisotropy is manifested in the direction.

A control rod for a nuclear reactor in which a cladding tube is filled with the above-mentioned neutron absorbing pellets.

As described above, the neutron absorbing material and the like according to the present invention is made of a sintered body of a mixed powder containing boron carbide and hexagonal boron nitride, and has anisotropy of mechanical strength and thermal conductivity. The anisotropy is preferably 1.5 or more. When the mechanical strength is less than 1.5, there is almost no effect of suppressing swelling in the direction of high mechanical strength, which is not preferable. If the anisotropy of thermal conductivity is less than 1.5, the thermal stress generated at the time of neutron absorption becomes high, which is not preferable.

The boron carbide raw material used in the production of the composite sintered body according to the present invention is preferably raw material powder in which the size of the primary particles is adjusted to an average of 5 μm or less. If the average crystal grain size exceeds 5 μm, the grain boundary area per unit volume becomes small, and the time period during which He is held in the crystal grains becomes long.
This is not preferable because the amount of retention increases and swelling increases.

The hexagonal boron nitride raw material may be a general one, but since the thermal conductivity differs depending on its crystallinity,
It is preferable to perform evaluation using the GI value defined by the following formula (1).

G.I. = (I (100) + I (101)) / (I (102)) (1) where I is the integrated intensity measured by X-ray diffraction.

If a low crystalline powder having a large GI value is used, the thermal conductivity of the composite sintered body will be reduced, which is not preferable.

Regarding the particle size of hexagonal boron nitride, it is preferable to use a powder having a too small particle size because it becomes difficult to impart desired mechanical strength and thermal conductivity anisotropy to the composite sintered body. Absent.

Using the boron carbide and boron nitride raw materials as described above, the raw materials are mixed by a general mixing method such as dry mixing or a wet mixing method using an alcohol solvent or the like so as to obtain a uniform dispersion state. As the mixer, for example, a general device such as a ball mill or a ribbon blender can be used.

As for the mixing ratio of boron carbide and boron nitride, if the amount of boron carbide is too small, the neutron absorbing ability will be insufficient, while if it is too large, the mechanical strength and the anisotropy of thermal conductivity will decrease, which is not preferable. Therefore, the mixing ratio may be appropriately selected within a range in which the anisotropy of mechanical strength and thermal conductivity is 1.5 or more and the neutron absorption capacity does not extremely decrease.

The mixed powder is filled in a graphite mold and, if necessary, preformed at a predetermined preforming pressure, and then hot pressed in a high frequency induction hot press furnace. The conditions for hot pressing include general temperatures (1700 to 220).
0 ° C. and a pressure of about 5 to 50 MPa). If the temperature is too low or the pressure is too low, it takes a long time to obtain a dense sintered body, which is not preferable. If the temperature is too high, it tends to react with the graphite material used during hot pressing, which is not preferable. If the pressure sintering pressure is too high, the frequency of breakage of the graphite material (mold) used during hot pressing becomes extremely high, which is not preferable. In place of the hot press sintering method, known equipment (for example, CI
It is also possible to use an atmospheric pressure sintering method in which the molding is performed using P, a die press, etc., and then sintering is performed in an inert atmosphere.

The neutron-absorbing material of the boron carbide / hexagonal boron nitride composite sintered body obtained in this manner significantly suppresses damage to the cladding due to swelling under a neutron irradiation environment, and also causes thermal stress and swelling. The destruction of the neutron absorbing material due to the above and damage to the cladding due to it can be greatly reduced, and the life of the reactor control rod is extended as compared with the conventional material, so that the safety and economic efficiency can be improved.

[0039]

EXAMPLES The present invention will be described in detail below with reference to examples.

[Conditions for Examples 1 to 5 and Comparative Examples 1 and 2] Boron carbide powder (average particle size measured by Microtrac method: 3 μm, B / C molar ratio: 4, ¹⁰ B enrichment: natural 19.8)
%) And hexagonal boron nitride powder (GI = 1.1, average particle diameter 10 μm measured by Microtrac method) were mixed for 2 hours using a ribbon blender so that a uniform dispersion state was obtained. The obtained mixed powder was filled in a graphite mold, preformed at a preforming pressure of 10 MPa, and then set in a high frequency induction hot press furnace under the conditions shown in Table 1 (maximum temperature and pressure holding time are Hot press sintering for 1 hour all, diameter 50mm, height 40m
A sintered body of m was obtained.

As a result of X-ray diffraction of the obtained sintered body, boron carbide and boron nitride were formed. As a result of chemical analysis of the sintered body, it was confirmed that the blending ratio of boron carbide and boron nitride was in agreement with the raw material blending.

[Conditions for Comparative Example 3] As boron nitride powder
GI = 3.6, was prepared and evaluated in the same manner as in Examples 1 to 4 except that the average particle size measured by the Microtrac method was 4 μm.

[Evaluation Method of Physical Properties] The following physical properties of the obtained composite sintered body were evaluated.

Mechanical strength: A test piece of 3 × 4 × 30 mm was processed and sampled from the obtained sintered body as shown in FIG.
Three-point bending strengths at room temperature in the directions parallel and perpendicular to the hot press direction were measured.

Thermal conductivity: As shown in FIG. 3, a test piece having a diameter of 10 mm and a thickness of 1.5 mm was processed and sampled from the obtained sintered body, and heat was applied at room temperature in the directions parallel and perpendicular to the hot press direction. The conductivity was measured by the laser flash method.

Evaluation of cracking property by thermal stress: From the obtained sintered body, as shown in FIG. 4, diameter 12.5 mm, length 2
A 5 mm test piece is processed and sampled, heated at 400 ° C. for 10 minutes, rapidly cooled in water, dried in the air at 90 ° C. for 2 hours and 120 ° C. for 1 hour, and then heated again. The heating / quenching cycle is repeated 10 times. Thermal shock was applied to evaluate the number of cycles in which cracks were generated in the molded body.

[Results] Table 1 shows the evaluation results together with the preparation conditions.

[0048]

[Table 1] As shown in Table 1, it can be seen that the sintered bodies according to the examples have improved thermal shock resistance and excellent resistance to cracking due to thermal stress as compared with those of the comparative examples. Further, the sintered bodies according to the examples all have anisotropy of mechanical strength and thermal conductivity of 1.5.
As described above, when the pellets were formed in the sintered body according to the example so that the hot pressing direction was the axial direction (FIG. 5), the thermal conductivity was good in the radial direction, and the hot pressing direction was in the axial direction. A pellet is formed which expands but hardly expands in the radial direction. When the neutron is absorbed, the pellet expands in the axial direction by swelling but hardly expands in the radial direction, so that it does not apply a mechanical force to the cladding tube surrounding the pellet. Further, since heat generated during neutron absorption is well conducted in the radial direction, a temperature difference in the pellet radial direction is unlikely to occur, and therefore vertical cracking is less likely to occur. Of course, when a temperature difference occurs in the axial direction, lateral cracking occurs, but since the pellets are stacked in the axial direction, lateral cracking has less influence on the entire control rod than longitudinal cracking. As described above, according to the sintered bodies of the examples, it is possible to manufacture pellets that are less likely to adversely affect the cladding tube, and the furnace life of the control rod filled with the pellets is not shortened.

As described above, the boron carbide / hexagonal boron nitride composite sintered body (neutron absorbing material according to the present invention) obtained by the present invention can absorb neutrons caused by swelling or thermal stress under a neutron irradiation environment. It is clear that the mechanical interaction with the cladding tube due to the cracking of the material can be significantly reduced, and it is clear that the neutron absorbing material can achieve the safety and economical improvement of the control rod as compared with the conventional material.

[Brief description of drawings]

FIG. 1 is a diagram for explaining the principle of the present invention.

FIG. 2 is a diagram for explaining a test method of mechanical strength and thermal conductivity.

FIG. 3 is a diagram for explaining a thermal conductivity test piece and a test method.

FIG. 4 is a diagram for explaining a test piece and a test method for evaluation of cracking property due to thermal stress.

FIG. 5 is a diagram for explaining a pellet according to the present invention.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Kenichi Adachi Inventor Kenichi Adachi 1 Shinkai-cho, Omuta-shi, Fukuoka Electric Chemical Industry Co., Ltd. Omuta Plant

Claims (3)

[Claims] 1. A neutron absorbing material obtained by hot pressing a mixed powder of boron carbide, hexagonal boron nitride and inevitable impurities until the anisotropy of mechanical strength and thermal conductivity is exhibited. 2. A pellet-forming mold is filled with a mixed powder of boron carbide, hexagonal boron nitride and unavoidable impurities, and the resulting pellet has better mechanical strength and thermal conductivity in the radial direction than in the axial direction. A neutron absorbing pellet produced by hot pressing in a direction in which anisotropy appears until the anisotropy appears. 3. A control rod for a nuclear reactor, wherein a cladding tube is filled with the neutron absorbing pellet according to claim 2.