JP2007132728A - Method of manufacturing fuel compact for high-temperature gas-cooled reactor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method capable of forming a fuel compact for a high-temperature gas-cooled reactor, without charging a large load on material. <P>SOLUTION: Overcoat particles 1 are loaded into a die 10, then alcohol 2 is supplied into the die 10, and the die 10 is given vibration. Overcoat layers 1L of the overcoat particles 1 are melted, and graphite matrix which is melted, overcoat layers is filled into a clearance between the overcoat particles 1, the material is pressed by small pressing pressure, in a state where the overcoat particles are surrounded by the graphite matrix to manufacture a product fuel compact. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a method for manufacturing a fuel compact for a HTGR, and more particularly to a method for manufacturing a fuel compact that can manufacture a fuel compact without destroying a coating layer of coated fuel particles.

  The HTGR consists of graphite that has a large heat capacity and maintains soundness at high temperatures, and uses helium gas that does not cause chemical reactions at high temperatures as a cooling gas. Therefore, inherent safety is high, and helium gas having a high outlet temperature of about 900 ° C. can be recovered, and this high-temperature heat can be used in a wide range of fields such as power generation, hydrogen production, and chemical plants.

The fuel of this HTGR is formed by coating four layers around a fuel core having a diameter of about 350 to 650 microns obtained by sintering uranium dioxide into a ceramic form. The first layer is a low density pyrolytic carbon coating with a density of about 1 g / cm ³ , which absorbs the function of the gaseous fission product (FP) as a reservoir and fuel nuclei swelling. It also has a function as a buffer. The second layer is a coating of high density pyrolytic carbon having a density of about 1.8 g / cm ³ , which has a retention function for gaseous FP. The third layer is a coating of silicon carbide (SiC) having a density of about 3.2 g / cm ³ , which has a function of holding the solid FP and a function as a main reinforcing member of the coating. Finally, the fourth layer, like the second layer, is a high-density pyrolytic carbon coating with a density of about 1.8 g / cm ³ , which is a gaseous FP retention function and third layer protection. It functions as a layer.

  Typical coated fuel particles have a diameter of about 500 to 1000 microns. The coated fuel particles are dispersed in a graphite matrix, and then molded into a fixed fuel compact shape. A fixed amount of the fuel compact is placed in a graphite tube, and the top and bottom are closed with plugs to form fuel rods. . This fuel rod is inserted into a plurality of insertion ports of the hexagonal column type graphite block and becomes fuel for the high temperature gas furnace. A large number of hexagonal columnar graphite blocks are stacked in multiple stages on a honeycomb array to constitute a core.

  Generally, the fuel for the HTGR is manufactured as follows. First, uranium oxide powder is dissolved in nitric acid to form a uranyl nitrate stock solution, and pure water and a thickener are added to the uranyl nitrate stock solution and stirred to prepare a dropping stock solution. The thickener acts so that the dropped solution of the uranyl nitrate stock solution dropped into a spherical shape with its own surface tension during dropping. As such a thickener, a resin having a property of solidifying under an alkaline condition, for example, a polyvinyl alcohol resin, polyethylene glycol, or metroise can be used. The dripping stock solution prepared in this manner is cooled to a predetermined temperature and the viscosity is adjusted, and then dropped into ammonia water using a method of vibrating a small-diameter dropping nozzle.

  The droplet liquid is prevented from being deformed at the time of landing by spraying ammonia gas in a space until it reaches the surface of the aqueous ammonia solution to gel the surface. Uranyl nitrate is sufficiently reacted with ammonia in ammonia water to form particles of ammonium biuranate. These particles are roasted in the atmosphere to become uranium trioxide particles, and further reduced and sintered to become fuel nuclei of high-density ceramic uranium dioxide.

The fuel nuclei are loaded on the fluidized bed, and the reaction gas (coating gas) supplied into the fluidized bed is thermally decomposed to coat the fuel nuclei. The first layer of low density pyrolytic carbon is coated by pyrolyzing acetylene (C ₂ H ₂ ) at about 1400 ° C. The high density pyrolytic carbon of the second and fourth layers is coated by pyrolyzing propylene (C ₃ H ₆ ) at about 1400 ° C. The third layer of SiC is coated by pyrolyzing methyltrichlorosilane (CH ₃ SiCl ₃ ) at about 1600 ° C. In this way, four layers of coating are applied to obtain coated fuel particles.

  Since the particle size and sphericity of the coated fuel particles greatly affect the production conditions of the overcoat particles, the coated fuel particles are released to the overcoat process after selecting the particle size and sphericity using a sieve. Is done.

  The coated fuel particles are overcoated by coating the surface with a graphite matrix material made of graphite powder, a binder or the like. The overcoat particles are hot-pressed into a cylindrical shape or a cylindrical shape, and become a fuel compact. This fuel compact is pre-fired to remove the binder and the like contained in the matrix material, and further fired to degas from the matrix material.

Conventionally, the overcoat process of coating the surface of the coated fuel particles with a graphite matrix material has been mainly performed in order to achieve the following two purposes.
(1) Prevent the coated fuel particles from being damaged by pressure during press molding.
(2) Disperse the coated fuel particles uniformly in the fuel compact to prevent the coated fuel particles from being thermally and mechanically damaged during combustion.

  In order to prevent the coating layer of the coated fuel particles from being damaged at the time of compact press, the overcoat layer works as a cushioning material for the coating layer, so a thicker one is preferable, but since the volume of the fuel compact is predetermined, If the amount of the matrix material in the fuel compact increases, the number of coated fuel particles contained in the fuel compact decreases, and the amount of uranium contained in one fuel compact decreases. In the conventional fuel compact, the value of the coated particle filling rate defined by the volume of the coated particle / the volume of the compact is about 35%.

  In order to increase the burnup of HTGR fuel, it is necessary to increase the amount of U235 contained in the fuel compact per unit. For this purpose, the enrichment of uranium is increased or the fuel per unit is increased. It is necessary to increase the amount of uranium contained in the compact. In order to increase the amount of uranium contained in one fuel compact, it is necessary to put as many coated fuel particles as possible into the compact. However, when the amount of matrix material decreases, the coating pressure is reduced by the pressure during press molding. There has been a problem that it is difficult to prevent the fuel particles from being damaged.

  The problem to be solved by the present invention is to provide a method capable of producing a fuel compact without damaging the coating layer of the coated fuel particles during press molding even if the overcoat layer of the overcoat particle is thin. .

  According to the means for solving the problems of the present invention, overcoat particles necessary for one fuel compact are loaded into a fuel compact molding die, an overcoat layer solvent is introduced into the die, and the die is vibrated. The overcoat layer where the overcoat particles are in contact with each other is dissolved with a solvent and is released from the overcoat particles so that the gaps between the overcoat particles are densely filled, and then the mold is heated. The present invention provides a method for producing a fuel compact for a high-temperature gas reactor, characterized in that a solvent compact is scattered and a mold is subsequently heated and pressurized to produce a fuel compact.

  In the problem solving means of the present invention, the vibration applied to the molding die can be a vertical vibration and / or a horizontal vibration.

  According to the present invention, the fuel compact is obtained by dissolving the overcoat layer where the overcoat particles are in contact with each other while applying vibration to the mold after the solvent is injected into the mold. Since the gap is densely filled with the graphite matrix which is the free overcoat layer, the overcoat layer can be thinned, so that the filling rate of the coated fuel particles can be increased, and the fuel compact can be increased in burnup. be able to.

  In addition, the graphite matrix liberated by the solvent in the gaps between the overcoat particles densely fills the gaps in the overcoat particles, so it is not necessary to heat and press the overcoat particles to the extent that the overcoat layer is deformed. The coated layer of coated fuel particles is not damaged during compact press molding.

  An embodiment of the present invention will be described in detail with reference to the drawings. FIG. 1 shows a method for producing a fuel compact according to the present invention in the order of steps, and weigh the overcoat particles 1 required for one compact, The measured overcoat particles 1 are loaded into a fuel compact molding die 10 (see FIG. 1A).

  As shown in FIG. 2A, the overcoat particle 1 is formed by overcoating a coated fuel particle 1A with a graphite matrix 1B. The graphite matrix 1B is made of artificial graphite powder, natural graphite powder, and phenol resin. It is formed by mixing a binder (binder) such as The thickness of the overcoat layer 1L is determined by the final fuel compact covering fuel particle filling rate.

  The mold 10 includes a cylindrical die 12, an upper punch 14, and a lower punch 16. The inner diameter of the cylindrical die 10 and the compact height at the time of pressing are determined in consideration of the amount of shrinkage during heat treatment such as pre-baking or baking. The upper punch 14 is used for pressing to a predetermined compact height. The lower punch 16 has a function of pushing the fuel compact as a product upward after press molding and ejecting the product from the mold 10.

  After a predetermined amount of an overcoat layer solvent for overcoating particles 1, such as alcohol 2, is poured into the mold 10 loaded with the overcoat particles 1 from above the die 12, a small amplitude is applied to the mold 10. Apply vertical and / or horizontal vibration. Due to this vibration, the alcohol 2 permeates below the deposited layer of the overcoat particles 1, and in this process, as shown in FIG. 2 (B), the overcoat layer 1L at the place where the overcoat particles 1 are in contact with each other. Is dissolved by the alcohol 2 to release the graphite matrix 1B from the coated fuel particles 1A, and the free graphite matrix 1B moves so as to fill the gap 1S between the overcoat particles 1 (the portion where the overcoat particles 1 are not in contact). Then, the space between the coated fuel particles 1A is densely filled (see FIG. 1B).

  In this state, the mold 10 is heated in the heating furnace 20 to blow off the alcohol 2 (see FIG. 1 (C)), and then the upper punch 14 is lowered while heating in the same heating furnace 20 to make graphite. Fuel compact 3 is manufactured by press-molding coated fuel particles 1A (a mixture of coated fuel particles 1A and graphite matrix 1B) surrounded by matrix 1B (see FIG. 1D).

  In a specific example, the heating of the mold 10 for alcohol splashing was performed at about 40 ° C. At this time, it is preferable to prevent a portion where the alcohol 2 is scattered from becoming a cavity by applying a load with the upper punch 14. This load (pressure) was set at a low level so as not to destroy the coated fuel particles 1A.

  Pressurization of the mixture of the coated fuel particles 1A and the graphite matrix 1B was performed while raising the temperature to about 80 ° C. and softening the phenol resin contained in the graphite matrix 1B. Although the heating time depends on the amount of the phenol resin in the graphite matrix 1B, 30 minutes or less was sufficient.

  Thereafter, this temperature was raised to about 110 ° C. while a load was applied by the upper punch 14, and the phenol resin contained in the graphite matrix 1B was cured. Since the gap 1S between the overcoat particles 1 is filled by dissolution of the overcoat layer 1L, it is not necessary to carry out the deformation by heating and pressurizing the overcoat layer 1L of the overcoat particle 1, Therefore, since the press pressure of the upper punch 14 may be low, the coating layer of the coated fuel particles 1A was not damaged.

  After the fuel compact 3 is taken out from the mold 10 by the lower punch 16, the fuel compact 3 is pre-fired by keeping it at 800 ° C. in a nitrogen atmosphere to carbonize the phenol resin as the binder in the graphite matrix 1B. In this way, the pre-fired fuel compact 3 was sintered at 1800 ° C. in a vacuum to remove gas components contained in the graphite matrix 1B, thereby obtaining a product fuel compact.

  When the failure rate and penetration failure rate (exposed uranium rate) of the SiC coating layer of the fuel compact 3 obtained in this example were measured, no damage of the coated fuel particles due to the production of the fuel compact was observed.

  According to the present invention, the fuel compact dissolves the overcoat layer of the overcoat particles by loading the overcoat particles into the mold and then injecting a solvent into the mold to impart vibration to the mold. The space between the overcoat particles is filled with the graphite matrix which is the released overcoat layer, so that it is possible to form a fuel compact without performing press molding under a large pressure. Even so, the coating layer of the coated fuel particles will not be damaged, and the filling rate of the coated fuel particles can be improved to reliably cope with the advanced combustion of the fuel compact, improving the industrial utility. .

It is a schematic explanatory drawing which shows the manufacturing method of the fuel compact of this invention in order of a process. It is an expanded sectional view explaining the state by which the clearance gap between overcoat particles is filled in a metal mold | die.

Explanation of symbols

1 Overcoat particle 1A Coated fuel particle 1B Graphite matrix 1L Overcoat layer 2 Alcohol 3 Fuel compact
10 Mold for Compact Molding 12 Dies 14 Upper Punch 16 Lower Punch 20 Heating Furnace

Claims (2)

Overcoat particles necessary for one fuel compact are loaded into a fuel compact molding die, an overcoat layer solvent is introduced into the die, and vibrations are applied to the die so that the overcoat particles The overcoat layer in contact with the overcoat layer is dissolved by the solvent and released from the overcoat particles to fill the gaps between the overcoat particles, and the gaps between the overcoat particles are densely filled, and then the gold A method for producing a fuel compact for a HTGR, wherein the solvent is scattered by heating the mold and the mold is pressurized while continuing to heat the mold to produce a fuel compact. 2. The high temperature gas reactor fuel compact manufacturing method according to claim 1, wherein the vibration applied to the molding die is vertical and / or horizontal vibration. Compact manufacturing method.

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