JP2006267010A - Fuel compact manufacturing method, fuel compact, and overcoating method for coated fuel particle - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To enhance thermal and mechanical strength of coated fuel particles while maintaining production efficiency on fuel compacts and their performance, even in manufacturing fuel compacts out of a plurality of kinds of coated fuel particles having different enrichment. <P>SOLUTION: Surfaces of a plurality of kinds of coated fuel particles 12A and 12B having different enrichment are coated with graphite matrix material to form overcoat layers 14A and 14B, and then, the fuel compacts 10 are manufactured out of coated fuel particles 12 each having an overcoat layer 14. The overcoat layers 14A and 14B are formed of thickness varying according to the enrichment of the fuel particles 12A and 12B, and the plurality of kinds of fuel particles 12A and 12B having overcoat layers 14 different in thickness are unified into the fuel compacts 10. As to the overcoat layers 14, the overcoat layers 14A of the fuel particles 12A having a higher enrichment are formed so as to have greater thickness than that of the overcoat layers 14B of the fuel particles 12B having a lower enrichment. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an improvement in a fuel compact used as a fuel in a nuclear reactor such as a high temperature gas reactor, a manufacturing method thereof, and an overcoat method of coated fuel particles used in a fuel compact. The present invention relates to improving the strength of the coated fuel particles while maintaining the production efficiency when the fuel compact is manufactured from the fuel particles having the fuel particles.

  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, in general, a coating of a total of four layers, a first layer to a fourth layer, is 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 gaseous fission product (FP) gas. 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 composed of high-density pyrolytic carbon having a density of about 1.8 g / cm ³ , as in the second layer, and is formed into a third layer of silicon carbide by irradiation shrinkage. It has a function of generating compressive stress to maintain the strength of the entire coated fuel particle under irradiation and also to maintain 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.

(Overcoat)
The coated fuel particles produced in this way are further coated with a graphite matrix material made of graphite powder, a binder, etc. on the surface of the coated fuel particles to form an overcoat layer, and then press molded or It may be used by being molded into a hollow cylindrical or cylindrical fuel compact form by molding (see, for example, Patent Document 1). In this case, the fuel compact is subjected to heat treatment to carbonize the phenol resin contained as a binder in the green compact after press molding or the like, and further heat treatment aimed at removing gas components contained in the fuel compact. After applying the above, it is used in the form of a fuel rod in which a certain amount is put into a cylinder made of graphite and plugged up and down. The coated fuel particles in the form of fuel rods are finally loaded with a plurality of hexagonal columnar graphite blocks by loading the fuel rods into a plurality of insertion ports of the hexagonal columnar graphite block of the HTGR. Configure the core by stacking multiple layers on the honeycomb array

(Function of overcoat layer)
The formation of this overcoat layer was mainly performed for the following two purposes. That is, the overcoat layer first played the role of preventing damage to the coated fuel particles due to 1) pressure during press molding or the like. Further, as shown in FIG. 2, the overcoat layer 14 has 2) the so-called interposition between the coated fuel particles 12 so that the coated fuel particles 12 are uniformly dispersed in the fuel compact 10 so that the coated fuel particles 12 are sintered. It also has a role of preventing thermal and mechanical damage. For this reason, attempts have been made to make the fuel compact 10 after uniformizing the diameters of the coated fuel particles 12 on which the overcoat layer 14 is formed so that the coated fuel particles 12 are uniformly dispersed.

(Use of coated fuel particles with different enrichment)
Heretofore, it has been common for only one fuel compact to be loaded with coated fuel particles having one type of enrichment, but in recent years there has been an increase in burnup and reduction in manufacturing costs. As a target high-performance fuel, the concentration of coated fuel particles is increased, and a plurality of types of coated fuel particles having different concentrations are mixed to form a fuel compact, and the mixing ratio is changed to change the fuel compact. Adjustment of effective enrichment is under consideration.

  As a result, it is possible to increase the initial enrichment of the coated fuel particles to increase the burnup, and conventionally, in order to adjust the enrichment setting of the entire fuel compact, there are various kinds of enrichments having 10 or more enrichments. It is necessary to use 10 coated fuel particles (in order to form a fuel compact from only coated fuel particles having the same enrichment, when 10 types of fuel compact are set, 10 types of coated fuel particles are used. Particles need to be used), but depending on the design, if you use several types of coated fuel particles of high and low enrichment (two examples are the least), just adjust the mixing ratio Therefore, since the labor for managing the concentration is drastically reduced, the production cost can be expected to be reduced.

  However, in this case, a plurality of types of coated fuel particles having different enrichment levels are mixed in one fuel compact, and the temperature of the coated fuel particles is high for the highly concentrated particles, Since particles having a low particle size are low, especially when the coated fuel particles having a high concentration are close to each other, the coated fuel particles may be thermally and mechanically damaged.

  In this case, it is conceivable to increase the thermal mechanical strength and improve the combustion soundness by increasing the thickness of the overcoat layer of all the coated fuel particles regardless of the enrichment level. In order to increase the thickness of the overcoat layer, it is necessary to set the time required for coating the graphite matrix material to be long, so that the production efficiency is lowered, and in particular, it is necessary to increase the thickness of the overcoat layer so much. The processing of the low-enriched coated fuel particles, which is not performed, also causes problems that require extra time and effort.

  In addition, if the overcoat layer thickness of all coated fuel particles is increased, the number of coated fuel particles constituting one fuel compact decreases, and in some cases, the amount of uranium necessary for nuclear design can be secured. The problem of being unable to do so can also arise.

Furthermore, if the overcoat layer having the same thickness is neglected regardless of whether the enrichment is high or low, the enrichment of the coated fuel particles cannot be identified by the particle outer diameter, and workability may be reduced. is there.
JP 2000-284084 A

  In view of the above problems, the problem to be solved by the present invention is to maintain the production efficiency and performance of a fuel compact even when a fuel compact is manufactured from a plurality of types of coated fuel particles having different enrichments. Another object of the present invention is to provide a fuel compact manufacturing method and fuel compact capable of improving the thermal mechanical strength of the coated fuel particles, and a method for overcoating the coated fuel particles.

  In the present invention, as a first means for solving the above-mentioned problems, a graphite matrix material is coated on the surfaces of a plurality of types of coated fuel particles having different concentrations to form an overcoat layer. In a method for producing a fuel compact from coated fuel particles having a coating layer, a plurality of types of coated fuel particles having overcoat layers with different thicknesses formed according to the concentration of the coated fuel particles, A fuel compact manufacturing method is provided, characterized in that the fuel compact is integrated into a fuel compact.

  As a second means for solving the above-mentioned problems, the present invention provides a fuel compact characterized in that, in the first solving means, the coated fuel particles having a higher concentration are formed with a thicker overcoat layer. The manufacturing method of this is provided.

  As a third means for solving the above-mentioned problems, the present invention provides an overcoat having an overcoat layer formed by coating the surface of a plurality of types of coated fuel particles having different concentrations with a graphite matrix material. In the fuel compact composed of particles, the overcoat layer provides a fuel compact characterized by having different thicknesses depending on the enrichment of the coated fuel particles.

  The present invention provides, as a fourth means for solving the above problems, a fuel compact characterized in that, in the third solution means, the coated fuel particles having a higher concentration have a thicker overcoat layer. Is.

  The present invention provides, as a fifth means for solving the above-mentioned problems, coated fuel particles in which an overcoat layer is formed by coating a surface of a plurality of types of coated fuel particles having different enrichments with a graphite matrix material. In this overcoat method, an overcoat method for coated fuel particles is provided, wherein overcoat layers having different thicknesses are formed according to the concentration of the coated fuel particles.

  As a sixth means for solving the above-mentioned problems, the present invention provides the coated fuel characterized in that, in the fifth solving means, the coated fuel particles having a higher concentration form a thicker overcoat layer. A method for overcoating particles is provided.

  According to the present invention, as described above, an overcoat layer having a necessary thickness according to the concentration of the coated fuel particles is formed, and the coated fuel particles having a high concentration are coated with the thick overcoat layer. Even if the overcoat layer is mixed with low-concentration coated fuel particles with a small thickness, the thick overcoat layer can be said to have a sufficient distance between the coated fuel particles that are high in concentration and high in temperature. There is an actual advantage that high coated fuel particles can be prevented from approaching and thermal mechanical damage of the coated fuel particles can be suppressed.

  At the same time, according to the present invention, as described above, the overcoat layer having a necessary thickness according to the concentration is formed, and the thickness of the overcoat layer is varied depending on the concentration of the coated fuel particles. An overcoat of low-concentration coated fuel particles has the practical advantage of suppressing a decrease in production efficiency and an increase in cost without requiring more time and effort than necessary.

  Furthermore, according to the present invention, the overcoat layer having a necessary thickness corresponding to the concentration of the coated fuel particles is formed, and the thickness of the overcoat layer is varied depending on the concentration. On the other hand, even if the overcoat layer is made thick, the outer diameter of the low-concentration coated fuel particles, which are not so necessary, can be kept small, so that the coated fuel particles can be introduced per one fuel compact. There is an actual benefit that the amount of uranium necessary for nuclear design can be secured sufficiently.

  In addition, according to the present invention, as described above, since the thickness of the overcoat layer is varied according to the enrichment, the enrichment of the coated fuel particles can be identified by the particle outer diameter, which is different. There is an advantage that the handling efficiency of the coated fuel particles having the enrichment is improved, and the production efficiency of the fuel compact composed of the coated fuel particles having different enrichments is increased.

  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. The fuel compact 10 includes a plurality of fuel-coated particles 12. It is manufactured by compressing and integrating while heating by press molding or mold molding.

  In the present invention, a plurality of types of coated fuel particles 12 having different enrichments are used. In this case, the number of types of enrichment set can be determined as needed. However, if too many types of coated fuel particles 12 are set, labor is required to manufacture and manage the coated fuel particles 12, while If only two types of high and low (two types of coated fuel particles 12A of high enrichment and coated fuel particles 12B of low enrichment) are set as in the embodiment of the above, it is possible to manufacture by adjusting the mixing ratio, etc. Since the enrichment setting of the entire fuel compact 1 can be changed as appropriate, it is desirable that the number of settings is as small as possible with at least two types.

  Further, as shown in FIG. 1, the surface of these coated fuel particles 12 is coated with an overcoat layer 14 formed by coating a graphite matrix material made of graphite powder, a binder or the like.

  In the present invention, as shown in FIG. 1, the overcoat layer 14 is formed in different thicknesses according to the enrichment of the coated fuel particles 12. Specifically, it can be dealt with by forming an overcoat layer 14 having a necessary thickness corresponding to the concentration of each of the various coated fuel particles 12. In this case, normally, the higher the enrichment of the coated fuel particles 12, the higher the possibility of thermal mechanical damage, and a stronger protective treatment is required. For this reason, in the illustrated embodiment using the coated fuel particles 12A and 12B having two kinds of high and low enrichments, as shown in FIG. 1, the overcoat layer of the coated fuel particles 12A having a high enrichment is provided. About 14A, the overcoat layer 14 is formed thicker than the overcoat layer 14B of the coated fuel particles 12B having a low enrichment.

  Therefore, the overcoat layer 14 having a sufficient thickness necessary for ensuring the thermal mechanical strength according to the concentration is formed for each of the coated fuel particles 12A and 12B having various concentrations. Even if a plurality of types of the coated fuel particles 12A and 12B having the overcoat layer 14 having a large and small outer diameter are mixed (the low-enriched coated fuel particles having the small-size overcoat layer 14B exist). In particular, since the coated fuel particles 12A having a high degree of concentration do not approach each other and have sufficient strength, the coated fuel particles 12 are not thermally and mechanically damaged. Even if the fuel compact 10 is composed of the coated fuel particles 12A and 12B having different enrichments, the combustion soundness can be improved.

  Further, in this case, when two types of coated fuel particles 12A and 12B formed with overcoat layers 14A and 14B having different thicknesses are mixed and compressed by press molding or the like to form a fuel compact 10, FIG. As shown, since the coated fuel particles 12B having a small diameter and a low enrichment enter between the coated fuel particles 12A having a large diameter and a high enrichment so as to enter the fuel compact 10 per one fuel compact 10. The quantity of the coated fuel particles 12 that can be input does not decrease, and a sufficient amount of uranium necessary for nuclear design can be secured.

  The overcoat layer 14 can be formed by dispersing the coated fuel particles 12 in a graphite matrix material made of graphite powder, a binder or the like. In this case, the thickness of the overcoat layer 14 is When sieving the coated fuel particles 12 on which the overcoat layer 14 is formed during and at the end of the overcoat process, the mesh size of the sieve is appropriately set and the time for overcoat is adjusted to be long. Therefore, it can be adjusted to an appropriate thickness.

  In this case, the specific thickness of the overcoat layer 14, that is, the thickness of the coated fuel particles 12A having a high enrichment and the coated fuel particles 12B having a low enrichment, is determined depending on the nuclear design and the thermal machine. Since it depends on the design, it cannot be determined uniquely, but it is necessary on the condition that at least sufficient thermal mechanical strength can be maintained according to the enrichment of the coated fuel particles 12. It can be set accordingly. In general, the low enrichment level can be ensured so that the quantity of uranium required can be secured so that the quantity of the coated fuel particles 12 introduced per one fuel compact 10 does not decrease too much. For the coated fuel particles 12 having the above, it is sufficient if the overcoat layer 14 has a thickness comparable to that of a conventional general enrichment.

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

It is a partially broken side view of the fuel compact of this invention. It is a partially broken side view of the conventional fuel compact.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Fuel compact 12 Coated fuel particle 12A Coated fuel particle 12A with high enrichment 12B Coated fuel particle with low enrichment 14 Overcoat layer 14A Overcoat layer 14B formed on coated fuel particle with high enrichment 14B Low high enrichment Overcoat layer formed on coated fuel particles having

Claims (6)

In the method for producing a fuel compact from the coated fuel particles having the overcoat layer after coating the surface of a plurality of types of coated fuel particles having different enrichments with a graphite matrix material to form an overcoat layer, The overcoat layer having different thicknesses is formed according to the concentration of fuel particles, and a plurality of types of coated fuel particles having the overcoat layers having different thicknesses are integrated to form a fuel compact. Fuel compact manufacturing method. 2. The method for producing a fuel compact according to claim 1, wherein the overcoat layer is formed thicker as the coated fuel particles having a higher concentration. In the fuel compact comprising overcoat particles having an overcoat layer formed by coating a surface of a plurality of types of coated fuel particles having different enrichments with a graphite matrix material, the overcoat layer is formed of the coated fuel particles. A fuel compact characterized by having different thicknesses depending on the degree of enrichment. 4. The fuel compact according to claim 3, wherein the coated fuel particles having a higher enrichment have a thicker overcoat layer. In a method for overcoating coated fuel particles in which an overcoat layer is formed on a surface of a plurality of types of coated fuel particles having different enrichments by coating a graphite matrix material, the thickness varies depending on the concentration of the coated fuel particles. A method for overcoating coated fuel particles, wherein the overcoat layer is formed. 6. The method of overcoating coated fuel particles according to claim 5, wherein the coated fuel particles having a higher enrichment form a thicker overcoat layer.

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