JP2007040744A - Fuel compact and manufacturing method of fuel same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent deformation or breakage of a fuel compact caused by a temperature difference in the fuel compact simply at low cost without having an influence on performance of the fuel compact. <P>SOLUTION: This fuel compact 10 is formed by performing integral molding of coated fuel particles 12 into a solid cylindrical shape. Each circular dish 14 having a curved shape is formed on the center part of upper and lower end faces 10A, 10B of the fuel compact 10 having the solid cylindrical shape. Assuming that the diameter of the fuel compact 10 is D, the diameter of the dish 14 is ΔD, the height of the fuel compact 10 is H, and the height of the dish 14 is ΔH, the dish 14 satisfies following conditions: 1/3≤ΔD/D≤4/5 and 1/50≤ΔH/H≤1/10. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention is a coated fuel particle that is used as a fuel in a nuclear reactor such as a high-temperature gas reactor, and in which a microsphere (fuel nucleus) of an oxide of a nuclear fuel material such as uranium or thorium is coated with a pyrolytic carbon layer, a silicon carbide layer, In particular, the present invention relates to a fuel compact formed by integrally molding in a graphite matrix and improvement of the manufacturing method thereof, and in particular, to preventing damage to the fuel compact, fuel sleeve, and graphite block easily and at low cost. Is.

  The HTGR is composed of graphite, which has a large heat capacity and good high-temperature soundness, and uses a gas such as helium gas that does not cause a chemical reaction even at high temperatures. Is a nuclear reactor that can extract helium gas with high safety and high exit temperature, and uses high-temperature heat of around 900 ° C in a wide range of fields such as hydrogen production and chemical plants as well as power generation. It is possible to do that.

(Coated fuel particles)
As a fuel for this HTGR, generally, a total of four coating layers, the first layer to the fourth layer, are applied around a fuel core having a diameter of about 350 μm to 650 μm obtained by sintering uranium dioxide, thorium or the like into a ceramic form. The coated fuel particles having a diameter of about 500 μm to 1000 μm are used. Specifically, it is the following four coatings.

That is, the innermost first layer generally called a buffer layer is a layer made of low-density pyrolytic carbon (PyC) having a density of about 1 g / cm ³ , and stores gas of gaseous fission products (FP), It also has the function of absorbing the swelling of nuclear fuel. The second layer applied over this first layer is then generally an inner pyrolytic carbon (PyC) layer formed from high density pyrolytic carbon having a density of about 1.8 g / cm ³ and is gaseous. It has a function of holding the gaseous fission product (FP) as a barrier to the diffusion of the fission product (FP). Further, the third layer called a silicon carbide (SiC) layer is made of silicon carbide having a density of about 3.2 g / cm ³ , and mainly serves as a barrier for diffusion of solid fission products. And has a function as a main strength member of the entire coated fuel particle. The outer pyrolytic carbon layer, which is the outermost fourth layer, is made of high-density pyrolytic carbon having a density of about 1.8 g / cm ³ as in the case of the second layer. It has a function of generating a compressive stress to maintain the strength of the entire coated fuel particle under irradiation and to hold a gaseous fission product (FP).

  Such coated fuel particles are generally produced through the following steps. That is, first of all, it is the generation of fuel nuclei. Specifically, by adding pure water and a thickener to the uranyl nitrate stock solution produced by dissolving uranium oxide powder in nitric acid, the dropping stock solution is stirred. Generate. In this case, the thickener is added so that the dropped uranyl nitrate stock solution drops into a true sphere due to its surface tension during dropping. As this thickener, for example, a polyvinyl alcohol resin, a resin having a property of solidifying under an alkaline condition, polyethylene glycol, Metrose, or the like can be used. Subsequently, after the dripping stock solution adjusted in this way is cooled to a predetermined temperature to adjust the viscosity, it is dropped into the ammonia aqueous solution by vibrating a dropping nozzle having a small diameter. In this case, the droplet is prevented from being deformed at the time of landing by spraying ammonia gas in the space until the droplet reaches the surface of the aqueous ammonia solution to gel the surface of the droplet.

  The stock solution dropped into the aqueous ammonia solution is converted into ammonium deuterated uranate particles by sufficiently reacting with uranyl nitrate in the aqueous ammonia solution. The ammonium heavy uranate particles are calcined in the atmosphere to form uranium trioxide particles, which are further reduced and sintered to obtain a fuel nucleus composed of high-density ceramic uranium dioxide. Since the particle size and sphericity of the fuel core obtained in this way have a great influence on the production conditions in the subsequent coating process, the fuel core is subjected to particle size selection and sphericity selection by a sieve. Once done, it is sent to the coating process.

Next, in the fuel core coating step, the above-described coating is applied sequentially from the first layer by loading the fuel cores on the fluidized bed and thermally decomposing the gas to be coated. In this case, specifically, for the first low-density carbon layer, acetylene (C ₂ H ₂ ) is pyrolyzed at about 1400 ° C. to cover the fuel core. The high-density pyrolytic carbon layers of the second layer and the fourth layer are coated by pyrolyzing propylene (C ₃ H ₆ ) at about 1400 degrees. The silicon carbide layer as the third layer is formed by thermally decomposing methyltrichlorosilane (CH ₃ SiCl ₃ ) at about 1600 ° C. The coated fuel particles produced in this way are further overcoated by coating the surface of the coated fuel particles with a graphite matrix material made of graphite powder, a binder or the like.

(Fuel compact)
When the overcoated coated fuel particles are used as a fuel compact, the coated fuel particles are dispersed in a graphite matrix material, and then sintered, for example, after being pressed or molded into a cylindrical shape, A fuel compact with a fixed shape is used (see, for example, patent documents). This compact fuel is formed by unifying a plurality of coated fuel particles by compressing the coated fuel particles by heating a die or punch to soften the phenol resin contained in the graphite matrix material into a binder. Has been.

(Loading into the core)
There are two types of fuel compacts formed in this way: a solid cylindrical type and a hollow cylindrical type. In either case, 1) a certain amount is placed in a fuel sleeve (cylinder) made of graphite and placed vertically. Either in the form of a fuel rod with a plug in, and loaded into a plurality of insertion ports of a hexagonal column type graphite block of a high temperature gas reactor, or 2) A plurality of insertions of a hexagonal column type graphite block of a high temperature gas reactor It is loaded directly into the mouth and finally loaded into the core as fuel by stacking a plurality of hexagonal columnar graphite blocks in a honeycomb array.

(Deficiency of fuel compact)
The temperature distribution of the fuel compact at the time of use in this high-temperature gas reactor is, in particular, in the fuel compact having a solid cylindrical shape, in the central portion (outer peripheral portion) of the fuel compact close to the coolant. The temperature tends to be higher in the vicinity of the central axis of the fuel compact. Due to the temperature difference resulting from the difference in cooling efficiency between the central part and the outer peripheral part in the fuel compact, the thermal expansion coefficient at the central part is larger than the peripheral part (outer peripheral part) due to the difference in thermal expansion coefficient with respect to temperature. As a result, the fuel compact may be deformed into a drum shape.

  When the fuel compact is deformed into a drum shape in this way, the fuel compact is in mechanical contact with the inner surface of the fuel sleeve or graphite block existing on the outer periphery thereof, and stress is applied to both, so that the fuel compact or the fuel sleeve Or the graphite block may be damaged.

  In addition, when a defect occurs in the fuel compact, fuel sleeve, or graphite block in this way, when the fuel compact becomes hot and thermally expands in the high-temperature gas furnace, the fragment is between the fuel compact and the inner surface of the fuel sleeve or graphite block. As a result, a high stress is generated at the location, and the fuel compact, the fuel sleeve, and the graphite block are damaged.

Therefore, it is necessary to sufficiently prevent these fuel compacts from being damaged. At that time, the uranium charge (the amount of fissile material) contained in the fuel compact is reduced more than necessary so that it can be used as nuclear fuel. It is also necessary to consider so as not to affect the original function.
JP 2000-284084 A

  In view of the above problems, the problem to be solved by the present invention is that the damage due to the deformation of the fuel compact due to the temperature difference in the fuel compact and the damage of the fuel sleeve and the graphite block affect the performance of the fuel compact. It is an object of the present invention to provide a fuel compact that can be prevented easily and at low cost without giving it, and a method for manufacturing such a fuel compact.

  As a first means for solving the above-mentioned problems, the present invention is characterized in that a dish having a curved shape is formed on an end surface in a fuel compact formed by integrally molding coated fuel particles. A fuel compact is provided.

  The present invention provides, as a second means for solving the above-mentioned problems, a fuel compact characterized in that, in the first solution means, the dish is formed at the center of the end face of the fuel compact. To do.

  According to the present invention, as a third means for solving the above-described problem, in either of the first and second solving means, the fuel compact has a solid cylindrical shape, and the dish has a solid cylindrical shape. The present invention provides a fuel compact that is formed on an end face of the fuel compact.

  According to the present invention, as a fourth means for solving the above problems, in any one of the first to third solving means, the dish is formed on both upper and lower end faces of the fuel compact. A fuel compact is provided.

  As a fifth means for solving the above-mentioned problems, the present invention is characterized in that, in any of the first to fourth solving means, the dish has a curved shape with a deeper depth at the center. A fuel compact is provided.

  According to the present invention, as a sixth means for solving the above-described problem, in any one of the first to fifth solutions, the dish has a fuel compact diameter D, a dish diameter ΔD, and a fuel compact. Where 1/3 ≦ ΔD / D ≦ 4/5 and 1/50 ≦ ΔH / H ≦ 1/10, where H is the height of the dish and ΔH is the height of the dish. A fuel compact is provided.

  The present invention also provides the following means suitable for manufacturing a fuel compact as any one of the first to sixth means. That is, according to the present invention, as a seventh means for solving the above-described problem, in a fuel compact manufacturing method in which coated fuel particles are integrally molded by a mold to manufacture a fuel compact, the mold has a curved shape. The present invention provides a method for manufacturing a fuel compact characterized in that a dish portion is formed and a dish is formed on an end face of the fuel compact.

  According to the present invention, as an eighth means for solving the above-described problem, in the seventh solving means, the dish portion is formed in the mold by mounting the dish member having a curved shape on the mold. The present invention provides a method for producing a featured fuel compact.

  According to the present invention, as described above, the dish is formed in the central portion of the upper and lower end surfaces (upper surface and lower surface) of the cylindrical fuel compact that has larger thermal expansion than the edge portion. However, it is possible to offset the difference in the coefficient of thermal expansion between the inside and outside of the fuel compact and suppress the deformation of the fuel compact into an unnatural drum shape, so that the machine between the fuel compact and the fuel sleeve or graphite block There is an actual advantage that the mutual interference can be reduced and the damage can be sufficiently prevented by reducing the load.

  According to the present invention, as described above, since the diameter and height of the dish are appropriately set in the ratio to the diameter and height of the fuel compact, the amount of fissile material such as uranium is reduced. There is an advantage that deformation due to thermal expansion can be sufficiently absorbed while preventing the deformation, damage and the like without affecting the original performance of the fuel compact.

  According to the present invention, as described above, a dish having a deeper depth is formed in the center portion of the fuel compact having the largest thermal expansion, and accordingly, the difference in thermal expansion is appropriately adjusted according to the difference in thermal expansion coefficient. There is an actual advantage that it is possible to cancel and suppress the deformation of the fuel compact into a drum shape.

  According to the present invention, as described above, the dish portion having a curved shape is formed in the mold, and the dish is formed on the end surface of the fuel compact. Therefore, it is possible to easily prevent the fuel compact from being damaged. There are real benefits that can be made.

  In this case, according to the present invention, as described above, the dish portion of the mold is formed by the dish member having a curved shape attached to the mold to form the dish in a compact fuel. Since the existing manufacturing equipment can be used without making any significant changes simply by mounting the components on the mold, it is possible to easily and inexpensively prevent damage to the fuel compact etc. There is.

  An embodiment for carrying out the present invention will be described in detail with reference to the drawings. FIG. 1 shows a fuel compact 10 manufactured by the manufacturing method of the present invention. As shown in FIG. The plurality of coated fuel particles 12 are formed by compression and integral molding while heating by press molding or molding.

  Specifically, the fuel compact 10 has a predetermined amount of coated fuel particles 12 dispersed in a graphite matrix made of graphite powder, a binder, etc., and together with this graphite matrix material, as shown in FIG. Then, it is manufactured by being put into the die 1 and compressed by the upper and lower punches 2A, 2B in the die 1.

  When the coated fuel particles 12 are compressed, the metal die 1 and the punch 2 and the coated fuel particles 12 are heated to soften the phenol resin contained in the graphite matrix material and to bind the binder between the coated fuel particles 12. And is formed into a cylindrical fuel compact 10 shown in FIG. The dies 1 and the upper and lower punches 2 can be formed from, for example, alloy tool steel.

  In addition, it is preferable to coat | cover the coated fuel particle 12 with the overcoat layer formed by coating the surface of the graphite matrix material which consists of graphite powder, a binder, etc. previously before compressing. This overcoat layer 1) prevents the coated fuel particles 12 from being damaged by pressure during press molding or the like, and 2) the coated fuel particles 12 are disposed in the fuel compact 10 as an intervening between the coated fuel particles 12. It is formed in order to disperse uniformly and prevent the coated fuel particles 12 from being damaged mechanically during sintering. For this reason, it is common to make the fuel compact 10 after uniformizing the diameter of the coated fuel particles 12 on which the overcoat layer is formed so that the coated fuel particles 12 are uniformly dispersed.

  The overcoat layer can be formed by dispersing the coated fuel particles 12 in a graphite matrix material made of graphite powder, a binder, or the like. When sieving the coated fuel particles 12 on which the overcoat layer is formed during and at the end of the overcoat process, by appropriately setting the mesh size of the sieve and adjusting the time for overcoating longer , Can be adjusted to an appropriate thickness.

  As shown in FIG. 1, the fuel compact 10 of the present invention has a solid cylindrical shape, and a dish 14 having a curved shape is formed on an end surface thereof. As shown in FIG. 1, the dish 14 is desirably formed in a circular shape on the upper and lower end surfaces (upper surface 10 </ b> A and lower surface 10 </ b> B) of the solid cylindrical fuel compact 10.

  Further, as shown in FIG. 1, the dish 14 is formed at the center of the end faces 10 </ b> A and 10 </ b> B of the fuel compact 10 (a predetermined range centered on the axis of the fuel compact 10). Since the central portion of the fuel compact 10 near the axis is larger in thermal expansion than the peripheral portion (outer peripheral portion) of the fuel compact 10 close to the coolant, the dish 14 is formed by forming the dish 14 in the central portion. The difference in the coefficient of thermal expansion between the inside and outside of the fuel compact 10 can be offset, and the deformation of the fuel compact 10 into the drum shape can be suppressed.

  For this reason, mechanical mutual interference between the fuel compact 10 and a fuel sleeve or graphite block (not shown) can be reduced, and the damage can be sufficiently prevented by reducing the load.

  Further, as shown in FIG. 1, the dish 14 has a curved shape with a deeper depth toward the center. This is because the fuel compact 10 gradually increases in temperature toward the center away from the coolant and increases in thermal expansion as it approaches the center, so that the difference in thermal expansion is appropriately offset according to the degree of thermal expansion. This is to suppress the deformation of the fuel compact 10 into the drum shape.

  Further, it is desirable that the diameter and height of the dish 14 be appropriately set in a ratio with the diameter and height of the fuel compact 10. Specifically, as shown in FIG. 1, when the diameter of the fuel compact 10 is D, the diameter of the dish 14 is ΔD, the height of the fuel compact 10 is H, and the height of the dish 14 is ΔH, 1/3 ≦ ΔD / D ≦ 4/5, and the height is formed in a range satisfying the condition of 1/50 ≦ ΔH / H ≦ 1/10.

  This is because the diameter ΔD of the dish 14 is 1/3> ΔD / D in the ratio of the diameter D of the fuel compact 10 or the height ΔH of the dish 14 is equal to the diameter H of the fuel compact 10. This is because when the ratio is 1/50> ΔH / H, the dish 14 is too small to suppress deformation of the fuel compact 10 due to thermal expansion, and the deformation preventing action cannot be sufficiently exhibited.

  As described above, in order to suppress the deformation of the fuel compact 10, it is preferable to set the size of the dish 14 as large as possible. On the other hand, when the size is set too large, specifically, the diameter ΔD of the dish 14 is set. However, ΔD / D> 4/5 in the ratio with the diameter D of the fuel compact 10, or the height ΔH of the dish 14 is ΔH / H> 1 in the ratio with the diameter H of the fuel compact 10. At / 10, since the dish 14 is large, the amount of fissile material such as uranium is reduced. Therefore, by setting the size of the dish 14 within a range satisfying the conditions of 1/3 ≦ ΔD / D ≦ 4/5 and 1/50 ≦ ΔH / H ≦ 1/10, While preventing the performance from being affected, it is possible to prevent deformation or breakage.

  As shown in FIG. 2B, the dish 14 forms a dish portion having a curved shape on the upper and lower punches 2A, 2B and the like, and compresses the coated fuel particles 12 having an overcoat layer. Can be formed. In this case, as shown in FIG. 2B, the dish portion can be formed by mounting the dish member 3 having a curved shape on the upper and lower punches 2A and 2B.

  For this reason, it can respond to the manufacturing method of this invention only by mounting | wearing the dish member 3 without adding a significant change to the conventional general punch 2A, 2B shown to FIG. 2 (A). Therefore, since the existing manufacturing equipment can be used effectively, the fuel compact 10 of the present invention having the dish 14 can be manufactured easily and at low cost. In addition, the dish member 3 is installed in the location corresponding to the location where the dish 14 should be formed in the compression surface of the upper and lower punches 2A and 2B.

  The present invention can be applied to processing overcoated fuel particles into various forms of fuel compacts such as fuel rods.

1A and 1B show a fuel compact of the present invention, in which FIG. 1A is a sectional view thereof and FIG. 1B is a plan view thereof. FIG. 2 shows a state in which a method for manufacturing a fuel compact is carried out. FIG. 2 (A) is a schematic cross-sectional view of a state in which a conventional manufacturing method is carried out. FIG. 2 (B) shows a state in which the manufacturing method of the present invention is carried out. It is a schematic sectional drawing of the state to carry out.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Dice 2 Punch 3 Dish member 10 Fuel compact 10A, 10B End face (upper surface, lower surface) of fuel compact
12 Coated fuel particles 14 Dish ΔD Dish diameter ΔH Dish height D Fuel compact diameter H Fuel compact height

Claims (8)

A fuel compact formed by integrally molding coated fuel particles, wherein a dish having a curved shape is formed on an end surface. 2. The fuel compact according to claim 1, wherein the dish is formed at a central portion of an end face of the fuel compact. 3. The fuel compact according to claim 1, wherein the fuel compact has a solid cylindrical shape, and the dish is formed on an end surface of the solid cylindrical fuel compact. This is a fuel compact. The fuel compact according to any one of claims 1 to 3, wherein the dish is formed on both upper and lower end faces of the fuel compact. The fuel compact according to any one of claims 1 to 4, wherein the dish has a curved shape with a depth that is deeper toward a center portion. 6. The fuel compact according to claim 1, wherein the dish has a diameter D of the fuel compact, a diameter of the dish ΔD, a height of the fuel compact H, and the dish. A fuel compact characterized by satisfying the conditions of 1/3 ≦ ΔD / D ≦ 4/5 and 1/50 ≦ ΔH / H ≦ 1/10, where ΔH is ΔH. In a fuel compact manufacturing method of manufacturing a fuel compact by integrally molding coated fuel particles with a mold, a dish portion having a curved shape is formed in the mold, and a dish is formed on an end surface of the fuel compact. A method for producing a fuel compact. 8. The fuel compact manufacturing method according to claim 7, wherein a dish portion is formed in the mold by mounting the dish member having the curved shape on the mold. Method.

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