JP2007278997A - Method for manufacturing fuel compact for high-temperature gas-cooled reactor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To efficiently manufacture fuel compacts by using overcoat particles with different quantities of uranium without varying a graphite matrix density. <P>SOLUTION: While fuel compacts are manufactured by pressing overcoat particles 3 formed by coating the surface of coated fuel particles with a graphite matrix by a press metal mold, the overcoat particles 3 are formed so as to have an overcoating layer whose thickness is fitted to the specification of the maximum content of uranium in a fuel compact and, when a fuel compact (referred to as a fuel compact outside the maximum specification) which has a uranium content other than the maximum uranium content in a fuel compact is manufactured, the granular graphite matrix particles of a compensating quantity corresponding to the uranium content of the fuel compact outside the maximum specification are mixed into the overcoat particles 3 before the granular graphite matrix particles are molded by the press metal mold. <P>COPYRIGHT: (C)2008,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 from overcoat particles having various uranium contents while simplifying the overcoat process.

  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 basically composed of coated fuel particles formed by coating a ceramic layer such as carbon or silicon carbide around a fuel core having a diameter of about 350 to 650 microns obtained by sintering uranium dioxide into a ceramic form. It has a structure.

The coated fuel particles generally used in a HTGR have four coating layers. 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 500-1000 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 additives are added to this uranyl nitrate stock solution and stirred to prepare a dropping stock solution. The additive is added so that the dropped uranyl nitrate stock solution becomes a true sphere with its own surface tension during dropping. As such an additive, 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.

  Since the particle size and sphericity of fuel nuclei greatly affect the production conditions of coated fuel particles, the fuel nuclei are released to the coating process after performing particle size selection and sphericity selection by a sieve.

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 a coated particle fuel.

  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 overcoat particles are formed by coating the surface of the coated fuel particles with a graphite matrix material made of graphite powder, a binder or the like.

  The amount of graphite matrix material coated with the coated fuel particles is determined by the amount of uranium and core determined by the amount of nuclear fuel material required to generate the planned amount of heat when used as a fuel compact in a HTGR. A calcination process that is performed for the purpose of degassing during the production of a fuel compact, while the coolant (mainly helium gas) has a graphite matrix density determined so as to have a thermal conductivity that can efficiently remove generated heat. It is necessary to make the amount necessary to uniformly coat the number of coated fuel particles determined in consideration of the density change at the same time.

  Since the particle size of the overcoat particles greatly affects the production conditions of the fuel compact, the overcoat particles are released to the compact press process after the particle size is selected with a sieve.

  As described above, the fuel compact is manufactured by accurately weighing and press-molding an amount of the overcoat particles in which the amount of uranium and the density of the graphite matrix are both compatible, and then using the binder in the graphite matrix material. It is pre-fired for removal, and further fired for degassing from the graphite matrix material.

  In the prior art, it was necessary to control the thickness of the overcoat layer in order to change the uranium content of the overcoat particles without changing the graphite matrix density, but the thickness of the overcoat layer was changed. In order to achieve this, it is necessary to perform trial manufacture and inspection for each change in thickness, set conditions, and prepare a sieve suitable for the conditions, which has the disadvantage that the production efficiency of overcoat particles, and thus fuel compacts, is reduced. It was.

  A problem to be solved by the present invention is a method for producing a fuel compact for a HTGR capable of efficiently producing a fuel compact using overcoat particles having various uranium contents without changing the graphite matrix density. Is to provide.

  The basic problem-solving means of the present invention is to manufacture a fuel compact for a high temperature gas reactor in which overcoated particles formed by coating a graphite matrix on the surface of the coated fuel particles are pressed with a press mold to produce a fuel compact. In this method, the overcoat particles are formed so as to have an overcoat layer having a thickness in accordance with the specification of the maximum uranium content of the fuel compact, and the fuel compact having a uranium content other than the maximum uranium content of the fuel compact. (When referred to as non-maximum specification fuel compact), after mixing the overcoated particles with an amount of granular graphite matrix particles so as to compensate for the uranium content of the non-maximum specification fuel compact. Provides a method for producing a fuel compact for a HTGR characterized by molding with a press die In the door.

  In the problem-solving means of the present invention, the ratio of the size of the graphite matrix particles to the overcoat particles is preferably 1:20 to 1: 2.

  In the means for solving the problems of the present invention, only the graphite matrix particles are preheated and mixed with the overcoat particles, and the graphite matrix particles and the overcoat particles are arranged so that the graphite matrix particles are arranged at the upper and lower ends in the press mold. Alternately insert the graphite matrix particles and overcoat particles in the radial direction in the press mold so that the graphite matrix particles are placed on the outermost side in the press mold alternately in the axial direction in the press mold It is preferable to throw it in.

  As described above, the overcoat particles are formed so as to have an overcoat layer having a thickness in accordance with the specification of the maximum uranium content of the fuel compact, but the fuel having a uranium content other than the maximum uranium content of the fuel compact. When producing a compact (referred to as non-maximum specification fuel compact), after mixing the overcoated particles with an amount of granular graphite matrix particles to compensate for the uranium content of the non-maximum specification fuel compact. Mold with a press die. In this way, since the overcoat process is performed by covering the graphite matrix material of a certain density and amount, the process conditions of the overcoat are fixed, so the operation is simplified, and the fuel compact is The granular graphite matrix material mixed with the overcoat particles will disperse in the graphite matrix material in an amount commensurate with the total uranium amount of the fuel compact, and a fuel compact having normal combustion characteristics can be obtained. .

  Since the graphite matrix material mixed with the overcoat particles is granular, there is little scattering when thrown into the mold, and it can be managed in the same way as the overcoat particles, and is equivalent to the overcoat particles Since it can be put into the mold under the conditions, it becomes easy to handle.

  If only graphite matrix particles are preheated and mixed with the overcoat particles, the overcoat layer of the overcoat particles can be molded while being cured, so that the molding pressure hardly affects the coated fuel particles and is of good quality. The fuel compact can be obtained.

  When graphite matrix particles and overcoat particles are alternately put in the axial direction in the press mold, or graphite matrix particles and overcoat particles are put alternately in the radial direction in the press mold, the graphite matrix particles The troublesome work of mixing the particles with the overcoat particles is not required, and the graphite matrix particles are placed on the upper and lower ends and outside of the press mold, so the molding pressure directly affects the overcoat particles during press molding. Without the need for fuel-free areas at the top and bottom and outer periphery of the fuel compact, and the outer peripheral dimensions important as fuel can be finished with high precision by machining, greatly improving fuel performance. be able to.

  An embodiment of the present invention will be described in detail with reference to the drawings. FIG. 1 shows a method of manufacturing a high temperature gas reactor fuel compact according to the present invention in the order of steps. First, the coated fuel particles 1 and the powdery graphite matrix 2 are prepared (see steps S1 and S2 in FIG. 1 and FIG. 2).

  Thereafter, the surface of the coated fuel particles 1 is coated with a graphite matrix 2 together with a binder to form overcoat particles 3 shown in FIG. 2 (see step S3 in FIG. 1 and FIG. 2). In FIG. 2, reference numeral 2L denotes an overcoat layer made of a graphite matrix material. In this overcoat step S3, the overcoat layer 2L of the overcoat particles 3 is set to have a thickness that meets the specifications for the maximum uranium content of the fuel compact. Accordingly, it can be seen that in the overcoat step S3, the amount of the powdery graphite matrix material 2 is always constant regardless of the uranium content of the fuel compact to be manufactured.

  When manufacturing the fuel compact (non-maximum specification fuel compact) 4 (see FIG. 3) having a uranium content other than the maximum uranium content by using the overcoat particles 3 having the overcoat layer 2L having a predetermined thickness in this way. That is, when the fuel compact 4 is not formed by using the overcoat particles 3 having the maximum uranium amount, but is formed by using the overcoat particles 3 having a smaller total uranium amount than the maximum uranium amount, Prepared granular graphite matrix particles in an amount so as to compensate for the uranium content of the fuel compact 4 (see step S4 in FIG. 1), and after mixing the graphite matrix particles with the overcoat particles. (Refer to step S5 in FIG. 1), these are molded by a press die (see step S6 in FIG. 1), and the green that is the basis of the fuel compact 4 Producing compact (see step S7 in Fig. 1).

  Thereafter, the green compact is pre-fired to remove the binder, and is fired to degas the graphite matrix material, thereby completing the fuel compact 4.

  At present, the fuel compact 4 has a particle filling rate (uranium content) of 12 types, of which the maximum particle filling rate is about 35%. In this case, the overcoat layer 3 of the overcoat particle 3 The thickness is about 120 μm. Therefore, in the method of the present invention, the thickness of the overcoat layer 2L of the overcoat particle 3 is also set to 120 μm even when a non-maximum specification fuel compact having a maximum particle filling rate of less than 35% is manufactured. In addition, in order to manufacture the fuel compact outside the maximum specification, the mixing amount of the graphite matrix particles to be compensated so as to correspond to the small particle filling rate increases as the particle filling rate decreases. For example, in order to manufacture a fuel compact having a particle filling rate of 25%, the amount of graphite matrix particles to be mixed with the overcoat particles 3 is about 2 when the number of coated fuel particles is 1.

  The granular graphite matrix particles are managed in the same manner as the overcoat particles 3, so that they can be put into the press mold in the same manner as the overcoat particles 3. Preferably, the ratio of the size (particle size) of the graphite matrix particles and the overcoat particles 3 is set to 1:20 to 1: 2.

  In the mixing step S5 of the graphite matrix particles and the overcoat particles, only the graphite matrix particles can be preheated and the preheated graphite matrix particles can be mixed with the overcoat particles. In this way, since the entire overcoat layer of the overcoat particles can be molded while being cured, the molding pressure hardly affects the coated fuel particles, and the coated fuel particles are protected.

  Also, when introducing graphite matrix particles and overcoat particles into the press mold, these particles are alternately inserted in the axial direction of the press mold, or alternately in the radial direction of the press mold. It is preferable to do. If it does in this way, the process of mixing a graphite matrix particle and overcoat particles will become unnecessary.

  In this case, it is preferable to introduce these particles into the press mold so that the graphite matrix particles are arranged at the upper and lower ends in the press mold and arranged at the outermost side in the press mold. In this way, the overcoat particles are not positioned at the upper and lower ends or outside of the press mold, the molding pressure does not directly affect the overcoat particles, and no fuel is added to the upper and lower and outer peripheral portions of the fuel compact. It becomes possible to provide a region and to finish the outer peripheral dimension important as a fuel with high precision by machining, and the performance of the fuel can be greatly improved.

  As described above, according to the present invention, the overcoat process is performed by coating a graphite matrix material having a constant density and amount, so that the process conditions of the overcoat are fixed, and the operation is simplified. The compact is dispersed in the graphite matrix material in an amount corresponding to the total amount of uranium, and a fuel compact having normal combustion characteristics can be obtained.

  Since the graphite matrix particles are granular, not powder, they can be managed in the same way as the overcoat particles or put into the mold, and if only the graphite matrix particles are preheated, the molding pressure is covered. There is little mechanical influence on the fuel particles, and the troublesome mixing work becomes unnecessary by alternately putting the graphite matrix particles and the overcoat particles into the mold.

  According to the present invention, it is possible to manufacture a fuel compact having normal combustion characteristics while simplifying the overcoat operation by making the density and amount of the graphite matrix constant, and thus manufacturing a fuel compact for a HTGR It can be used beneficially.

It is a block diagram of the process of the manufacturing method of the fuel compact of this invention. It is an expanded sectional view of overcoat particles. It is a perspective view of the fuel compact obtained by the method of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Coated fuel particle 2 Graphite matrix 2L Overcoat layer 3 Overcoat particle 4 Fuel compact

Claims (5)

In the method for producing a fuel compact for a high temperature gas reactor, in which the overcoat particles formed by coating the surface of the coated fuel particles with a graphite matrix are pressed with a press mold to produce a fuel compact, the overcoat particles are A fuel compact having a uranium content other than the maximum uranium content of the fuel compact (referred to as a fuel compact outside the maximum specification) formed so as to have an overcoat layer having a thickness that matches the specifications of the maximum uranium content of the fuel In the manufacturing process, a granular graphite matrix particle in an amount so as to compensate for the uranium content of the fuel compact outside the maximum specification is mixed with the overcoat particle, and then molded with the press die. A method of manufacturing a fuel compact for a HTGR characterized by the above. The method for producing a fuel compact for a HTGR according to claim 1, wherein a ratio of the size of the graphite matrix particles to the overcoat particles is 1:20 to 1: 2. Manufacturing method. A method for producing a fuel compact for a HTGR according to claim 1 or 2, wherein only the graphite matrix particles are preheated and mixed with the overcoat particles. Method. The method for producing a fuel compact for a HTGR according to any one of claims 1 to 3, wherein the graphite matrix particles and the overcoat are arranged so that the graphite matrix particles are arranged at upper and lower ends in the press mold. A method for producing a fuel compact for a HTGR, wherein coated particles are alternately charged in an axial direction in the press mold. The method for producing a fuel compact for a HTGR according to any one of claims 1 to 4, wherein the graphite matrix particles and the overcoat are arranged so that the graphite matrix particles are arranged on the outermost side in the press mold. A method for producing a fuel compact for a HTGR, wherein coated particles are alternately introduced in a radial direction in the press mold.

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