JP2006300762A - Fuel compact and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve a confining function of a fission product required following sophistication of combustion of a fuel compact for a high-temperature gas-cooled reactor. <P>SOLUTION: A binder 12 including silicon carbide is applied onto the outer surface of a compact molded article 10 before a calcination process after a degreasing process of the fuel compact, and then the compact molded article 10 is inserted into a cylindrical tube 14 comprising silicon carbide or graphite, and thereafter the compact molded article 10 and the cylindrical tube 14 are calcined, to thereby manufacture the fuel compact wherein the compact molded article 10 and the cylindrical tube 14 are integrated. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a fuel compact for a HTGR and a method for manufacturing the same, and more particularly to a fuel compact having an improved function of confining fission products in a HTGR and a method for manufacturing the same.

  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 coating of low density pyrolytic carbon with a density of about 1 g / cm ³ , which absorbs the function of the gaseous fission product (FP) as a reservoir and fuel nuclear 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 solid FP and functioning 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. Thus, four layers of coating are applied to obtain coated fuel particles.

  The fuel compact for the HTGR is a green compact (compact molded body) formed by dispersing coated fuel particles in a graphite matrix composed of graphite powder and a binder and molding it into a cylindrical or cylindrical shape by pressing or molding. Thereafter, the green compact is obtained by degreasing and baking for the purpose of removing the binder.

  This fuel compact can confine the fission product by a coating layer applied on the fuel nucleus of each coated fuel particle. However, with the recent advancement of combustion of the fuel compact, the fission product confinement function is improved. Further improvement is desired.

  One problem to be solved by the present invention is to provide a fuel compact capable of further improving the fission product confinement function required with the advancement of combustion of the fuel compact.

  Another problem to be solved by the present invention is to provide a method for manufacturing a fuel compact capable of further improving the fission product confinement function required with the advancement of combustion of the fuel compact.

  The first problem-solving means of the present invention includes a compact molded body obtained by dispersing and molding coated fuel particles in a graphite matrix, a graphite or silicon carbide cylindrical tube into which the compact molded body is inserted, a compact molded body, and a cylinder. An object of the present invention is to provide a fuel compact characterized in that it is composed of silicon carbide provided between the tubes and the compact molded body and the cylindrical tube are baked and integrated.

  The second problem-solving means of the present invention is a method for producing a fuel compact having a function of confining fission products in a high-temperature gas reactor by dispersing coated fuel particles in a graphite matrix, molding, degreasing, and firing. A compact containing a silicon carbide is applied to the outer surface of the compact molded body after the compact degreasing process and before the firing process, and then the compact molded body is inserted into a cylindrical tube made of silicon carbide or graphite. An object of the present invention is to provide a method for producing a fuel compact, characterized in that a compact body and a cylindrical tube are integrated by firing the body and the cylindrical tube.

  In the second problem-solving means of the present invention, the inner diameter of the cylindrical tube is preferably 50 mm or less, and preferably has an inner diameter of 1.000 times or more the outer diameter of the compact molded body after degreasing. It has a length equal to or longer than one compact molded body, and a plurality of compact molded bodies can be inserted into one long cylindrical tube for stacking.

  Thus, the compact molded body obtained by dispersing and molding the coated fuel particles in the graphite matrix, the cylindrical tube of graphite or silicon carbide into which the compact molded body is inserted, and the compact molded body and the cylindrical tube are provided. When the compact molded body and the cylindrical tube are integrated by firing, the fission product confinement function is further improved by the silicon carbide around the compact molded body. It is possible to reliably cope with the sophistication.

  Moreover, if silicon carbide around the compact molded body is applied in the form of a binder to the outer surface of the compact molded body after the degreasing process and before the firing process, silicon carbide is easily interposed between the compact molded body and the cylindrical tube. If the compact molded body is not before degreasing but after degreasing, an undesirable binder does not remain between the compact molded body and the cylindrical tube, and a high-quality fuel can be obtained. .

  In addition, the compact molded body is covered with and integrated with a cylindrical tube of graphite or silicon carbide. Therefore, when the fuel compact is inserted into a multi-hole type graphite block to form a fuel body, A high confinement function of the fission product can be maintained without causing a situation in which the outer peripheral portion of the compact is worn and damages the coating layer of the coated fuel particles therein.

  Furthermore, if the cylindrical tube has a length corresponding to the growth of a fuel-compact stack knitting and a plurality of compact molded bodies are inserted and integrated into this cylindrical tube, the graphite sleeve used in the pin-in-block type becomes unnecessary, making it It is economical.

  When silicon carbide is present on the outer peripheral surface of the compact molded body, the high oxidation resistance of this silicon carbide can eliminate the disadvantages of having a coating with low oxidation resistance on the inner coated fuel particles. For example, even when the coating of the third layer of the coated fuel particles inside the compact molded body is made of zirconium carbide (ZrC) that does not have oxidation resistance, the coating is oxidized and the fission product confinement function is lowered. And is useful for use in fuel compacts containing coated fuel particles having such a zirconium carbide (ZrC) coating.

  An embodiment of the present invention will be described in detail with reference to the drawings. FIG. 1 shows a method for manufacturing a fuel compact according to the present invention in the order of steps. The compact molded body (green compact) 10 in FIG. 1 is not shown, but the coated fuel particles are dispersed in a graphite matrix composed of graphite powder and a binder, and formed into a cylindrical shape or a cylindrical shape by pressing or molding. can get.

  In the method of the present invention, the binder 12 containing silicon carbide is applied to the outer surface of the compact molded body 10 after the degreasing process of the compact molded body 10 and before the firing process (see step S1 in FIG. 1). The binder 12 is applied by an appropriate means such as a spray so as to have a thickness of, for example, 0.04 mm.

  The compact molded body 10 thus coated with the binder 12 is inserted into a separately prepared cylindrical tube 14 made of silicon carbide or graphite, as shown in step S2 of FIG. The inner diameter of the cylindrical tube 14 is preferably 50 mm or less, and preferably has an inner diameter of 1.000 times or more the outer diameter D10 of the compact molded body 10 after degreasing. In this case, the binder 12 containing silicon carbide is interposed between the compact molded body 10 and the cylindrical tube 14. The height of the cylindrical tube 14 is preferably equal to or greater than the length of one compact molded body 10.

  Thereafter, as shown in step S3 of FIG. 1, the compact molded body 10 and the cylindrical tube 14 are fired at a temperature of 1400 ° C. to 1900 ° C. in a vacuum atmosphere with the compact molded body 10 and the cylindrical tube 14 fitted. 14 to produce a compact fuel.

  Thus, the binder 12 containing silicon carbide is applied to the outer surface of the compact molded body 10 after degreasing, and the compact molded body 10 is inserted into the cylindrical tube 14 made of silicon carbide or graphite. And the cylindrical tube 14 are fired to integrate the compact molded body 10 and the cylindrical tube 14, the fission product generated by the operation of the high temperature gas furnace by the silicon carbide binder 12 around the compact molded body 10. It can be seen that the confinement function of the fuel is improved, so that even if the combustion of the fuel compact is advanced, it can be surely coped with.

  The compact molded body 10 is not degreased, but after degreasing, it is necessary to apply the binder 12 to the outer surface and insert it into the cylindrical tube 14, but if this is before degreasing, Since the binder contained in the compact molded body 10 is inserted into the cylindrical tube 14 without being removed, the binder remains between the compact molded body 10 and the cylindrical tube 14 and the fuel itself is cracked or cracked. This is because there is a fear.

  Moreover, the compact molded body 10 after degreasing expands about 1.000 to 1.0001 in the cylindrical tube 14 after firing, whereby the compact molded body 10 and the cylindrical tube 14 are in close contact and integrated. Therefore, the processing of applying the binder to the compact molded body 10 and inserting the cylindrical tube is performed before firing the compact molded body 10, not after firing.

  When the compact molded body 10 is covered with the cylindrical tube 14 of graphite or silicon carbide, the outer periphery of the fuel compact is worn when the fuel compact is inserted into the multi-hole type graphite block to form the fuel body. There is no possibility of damaging the coating layer of the coated fuel particles therein, so that the high confinement function of the fission product can be maintained.

  In the above embodiment, one compact molded body 10 is inserted into the cylindrical tube 14, but the cylindrical tube has a length corresponding to the growth of the fuel compact stack, and a plurality of compact molded bodies are inserted into the cylindrical tube. When integrated, the graphite sleeve used in the pin-in-block type becomes unnecessary, and the fuel compact can be directly inserted into the graphite block, which is preferable.

Zirconium carbide (ZrC) may be used instead of silicon carbide (SiC) for the coating of the third layer of the coated fuel particles used for molding the compact molded body 10, but zirconium carbide is oxidation resistant like silicon carbide. Since it does not have performance, zirconium carbide becomes zirconium dioxide (ZrO ₂ ) when oxygen is present at high temperatures. Zirconium dioxide does not have a fission product confinement function. Therefore, the fuel compact made from coated fuel particles with zirconium carbide as the third layer coating will cause air intrusion into the reactor for some reason. When the graphite component in the inside may oxidize, the coated fuel particles from which the zirconium carbide coating layer is exposed may remain. However, when the compact molded body 10 is covered with silicon carbide having high oxidation resistance as in the present invention, oxidation of the graphite component and zirconium carbide is suppressed, so that the fission product confinement function of the fuel compact is maintained. be able to.

  In the specific embodiment of the present invention, a cylindrical fuel compact was manufactured by the method shown in FIG. 1, but in this specific example, the compact molded body has an outer diameter of about 26.6 mm and a height of about 39 mm. Thus, it was formed by warm pressing at a temperature of 100 ° C. and a holding time of 2 minutes. Next, this compact molded body was placed on a graphite tray and placed in an inert atmosphere in an electric furnace and degreased at about 800 ° C. The compact molded body thus degreased had an outer diameter of 25.95 mm and a height of 38.1 mm. Next, a binder containing silicon carbide was applied to the outer surface of the compact molded body to a thickness of about 0.04 mm by spraying. Thereafter, the compact molded body was inserted into a graphite cylindrical tube having an inner diameter of 26.02 mm and a height of 39 mm, and fired at about 1800 ° C. in a vacuum atmosphere. After firing, a compact fuel in which the compact molded body and the cylindrical tube were integrated was obtained.

  According to the present invention, a binder containing silicon carbide is applied to the outer surface of a compact molded body, the compact molded body is inserted into a cylindrical tube made of silicon carbide or graphite, and fired. Because the silicon compact around the compact molded body further enhances the fission product confinement function, it is possible to reliably cope with the advanced combustion of the fuel compact. Improves usability.

It is process drawing of the manufacturing method of the fuel compact of this invention.

Explanation of symbols

10 Compact molded body 12 Silicon carbide 14 Graphite cylindrical tube S1 Step of applying silicon carbide to the outer surface of the compact molded body S2 Step of inserting the compact molded body into the graphite cylindrical tube S3 Step of firing the compact molded body and the cylindrical tube

Claims (5)

A compact molded body obtained by dispersing and molding coated fuel particles in a graphite matrix, a graphite or silicon carbide cylindrical tube into which the compact molded body is inserted, and silicon carbide provided between the compact molded body and the cylindrical tube The compact compact and the cylindrical tube are integrally fired and integrated, and a fuel compact for a HTGR. In a method of manufacturing a fuel compact having a function of confining fission products in a high temperature gas reactor by dispersing and molding coated fuel particles in a graphite matrix, degreasing and firing, before the firing step after the fuel compact degreasing step A binder containing silicon carbide is applied to the outer surface of the compact molded body, and then the compact molded body is inserted into a cylindrical tube made of silicon carbide or graphite, and then the compact molded body and the cylindrical tube are baked to form the compact A method for producing a fuel compact for a HTGR characterized by integrating a molded body and a cylindrical tube. It is a manufacturing method of the fuel compact of Claim 2, Comprising: The internal diameter of the said cylindrical tube is 50 mm or less, and has an internal diameter more than 1.000 times the outer diameter of the compact molded object after the said degreasing | defatting. A method for producing a fuel compact. The method for manufacturing a fuel compact according to claim 2 or 3, wherein the cylindrical tube has a length equal to or longer than a length of one compact molded body. 5. The method for producing a fuel compact according to claim 2, wherein a plurality of compact molded bodies are inserted into the cylindrical tube.