JP2006258799A - Manufacturing method of fuel for high-temperature gas-cooled reactor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To uniformly disperse coated fuel particles in a fuel compact. <P>SOLUTION: When overcoated particles 3 each formed by coating a graphite matrix material on the surface of a coated fuel particle 1 are pressed by using press dies to mold into a molded fuel, the overcoated particles 3 are sequentially fed to sorting processes S1 to S4 constituting a plurality of stages of different sorting particle sizes, and the overcoated particles 34S not selected in the sorting process of the final stage and having a specified particle size are molded by press work. The overcoated particles 31S to 33S selected in the sorting processes S1 to S3 before the final process are returned to an overcoating process, and are recoated with the graphite matrix powder. After this recoating, the re-overcoated particles are fed to the corresponding sorting processes. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a method for producing a molded fuel composed of coated fuel particles having four coating layers, such as a fuel compact for a HTGR, and in particular, a molded fuel capable of uniformly dispersing the coated fuel particles with high productivity. It relates to a method that can be manufactured.

  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 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 500-1000 microns. These coated fuel particles are dispersed in a graphite matrix, and then molded into a fixed-shaped molded fuel, for example, a fuel compact. A fixed amount of this fuel compact is placed in a graphite tube, and the top and bottom are sealed with stoppers. It is assumed to be a fuel rod. 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 is added so that the dropped uranyl nitrate stock solution becomes a true sphere with its own surface tension during the fall. 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 ammonium heavy uranate 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 overcoating the surface of the coated fuel particles with a graphite matrix material made of graphite powder, a binder or the like. The overcoat of the graphite matrix material has an action of preventing the coated fuel particles from being damaged by the pressure during molding.

  The fuel compact is formed by loading overcoat particles together with a graphite matrix material in a press mold and press-molding into a hollow cylindrical shape or a cylindrical shape. The fuel compact thus obtained is pre-fired to remove the binder contained in the graphite matrix material, and further fired for degassing graphitization of the graphite matrix material.

  In the fuel compact, coated fuel particles and graphite matrix material thereon (including graphite matrix material coated with overcoat particles and graphite matrix material loaded together with overcoat particles during press molding) are mixed at a predetermined ratio. In this case, considering the combustion characteristics in the fuel compact reactor, the coated fuel particles are evenly distributed in the fuel compact to prevent local combustion. It is necessary to be.

  The mixing ratio of the coated fuel particles and the graphite matrix material is measured by measuring the weights of both, but measuring these weights for each fuel compact increases the number of processes and reduces productivity. Have

  Further, in order to uniformly disperse the coated fuel particles and the graphite matrix material, they may be stirred using a jig or the like after they are loaded into a mold for molding, which is also referred to as a stirring step. Since a special process is added, productivity similarly decreases. In addition, the density of the coated fuel particles and the graphite matrix material are greatly different, and the shape to be molded is complicated, such as a columnar shape or a hollow cylindrical shape, so that it is difficult to uniformly disperse the material.

  Overcoat particles used to manufacture HTGR fuel compacts are made by coating graphite matrix powder that contains phenolic resin and other components uniformly on the outside of the coated fuel particles. It is difficult to make uniform, and there is a drawback that the variation increases as the outer diameter (particle diameter) of the overcoat particle increases. That is, the thicker the overcoat, the easier it is to overcoat. For this reason, the variation in the particle size of the overcoat particles tends to further increase.

  If there is variation in the particle size of the overcoat particles, the distance between the coated fuel particles varies during press molding performed to form a fuel compact, and if the distance between the coated fuel particles is small, the distance between the coated fuel particles This causes mechanical interference with each other, and thus the coating layer is damaged, that is, the fuel failure rate increases.

  In order to avoid such drawbacks, it is desired to select overcoat particles for each particle size and use overcoat particles having a predetermined particle size. A method of sorting by measurement and a method of sorting by using the sedimentation force of particles or the centrifugal force acting on the particles can be considered, but all of them are difficult to sort in large quantities, and the size of the equipment is increased, and a specific particle size is ensured It is anticipated that it will be difficult to sort out.

  The problem to be solved by the present invention is for a high-temperature gas reactor that can easily select overcoat particles having a constant particle diameter and obtain high-quality fuel in which coated fuel particles are uniformly dispersed with high productivity. It is in providing the manufacturing method of a fuel.

  The problem-solving means of the present invention is to form a molded fuel by pressing overcoated particles formed by coating graphite powder on the surface of the coated fuel particles together with a binder with a press die, and pre-firing the molded fuel. In the method for producing a fuel for a HTGR to be fired, the overcoat particles are sequentially supplied to a multistage sorting process having different sorting particle diameters, and the overcoat particles having a predetermined particle diameter sorted in the final sorting process The overcoat particles selected in the selection step before the final step are returned to the overcoat step, overcoated with graphite matrix powder, and supplied again to the corresponding selection step. It is an object of the present invention to provide a method for producing a fuel for a HTGR characterized by recovering overcoat particles that have not been sorted in the sorting step.

  In the problem-solving means of the present invention, the multi-stage sorting step preferably sorts overcoat particles using sieves having different sieve meshes, but in addition, optical measurement method, particle sedimentation force measurement method, centrifugal force Any of the measurement methods may be used.

  In the problem-solving means of the present invention, the overcoat particles selected in each selection step before the final step are overcoated with graphite overcoat powder again, and then overcoated particles that have not been selected in the corresponding selection step (non-selection). (Referred to as overcoat particles) or subsequent to non-screened overcoat particles may be supplied to the next screening step, or may be overcoated with graphite matrix powder again and then supplied to the corresponding screening step. Along with new unsorted overcoat particles, they may be supplied to the corresponding sorting step and sorted again.

  In this way, the overcoat particles are sequentially supplied to a multistage sorting process with different selection particle diameters, and the overcoat particles having a predetermined particle diameter selected only in the final stage selection process are press-molded. The coated fuel particles are uniformly dispersed in the fuel, and a high quality fuel can be obtained.

  In addition, after the overcoat particles selected in the selection step before the final step are returned to the overcoat step and overcoated with the graphite matrix powder again, the non-selected overcoat particles are used together with or after the non-selected overcoat particles. If it is supplied to the sorting process or supplied again to the corresponding sorting process together with new unsorted overcoat particles supplied to the corresponding sorting process, the particle size is sorted until it is supplied to the final sorting process. Since it is repeated, it is possible to reliably obtain overcoat particles having a predetermined particle diameter to be molded.

  Furthermore, since the overcoated particles having a large particle size that were not selected in the final selection process are recovered, the overcoated particles having an excessively large particle size are not included in the molded fuel. Dispersibility is not impaired.

  An embodiment of the present invention will be described in detail with reference to the drawings. FIG. 1 shows a method of manufacturing a fuel compact as an example of a HTGR fuel according to the present invention in the order of steps. In the method of the present invention, as shown in FIG. 3, the surface of the coated fuel particles 1 is coated with a graphite matrix material 2 containing graphite powder and a binder by spraying with a liquid as usual. As shown in FIG. 4, the overcoat particles 3 are loaded into a press die 10 and warm-pressed to form a fuel compact 4 having a predetermined shape (for example, a hollow cylindrical shape) (see FIG. 5A). ). In FIG. 4, reference numeral 10L denotes a lower mold of the mold 10, 10U denotes an upper mold, and 10R denotes a rod that forms a hollow portion.

  In the method of the present invention, before the overcoat particles 3 are loaded into the press mold 10, as shown in FIG. 1, the overcoat particles 3 are sequentially supplied to a multistage sorting process having different particle sizes to be sorted. The In the example shown in the figure, the selection is sequentially performed through four sieves A, B, C, and D having different sieve meshes, and these sieves A, B, C, and D are set so that the sieve meshes gradually increase. The sieve A is set to have a mesh corresponding to the lower limit value of the predetermined particle diameter, and the final sieve D is set to have a mesh corresponding to the upper limit value of the predetermined particle diameter.

  In the first sorting step S1, the overcoat particles 3 are sorted based on the first particle size Ad, and the overcoat particles 31L larger than the particle size Ad do not pass through the sieve A (without being sorted) as they are. After being supplied to the sorting step S2, the overcoat particles 31S smaller than the particle size Ad are passed through the sieve A, sorted, returned to the overcoat step, and overcoated with the graphite matrix material again, and then the corresponding sorting step S2. To be supplied. The overcoat particle 31L having a large particle size is not immediately supplied to the sorting step S2, but temporarily held, and after the overcoat particle 31S having a small particle size is overcoated again, the overcoat particle 31L is re-overcoated to increase the particle size. It is supplied to the sorting step S2 together with the overcoat particles 31S.

  In the second sorting step S2, the overcoat particles 31L and 31S thus supplied directly or re-coated from the first sorting step S1 are sorted based on the second particle size Bd (> Ad). Similarly, the overcoat particles 32L (non-sorted overcoat particles) larger than the particle size Bd are temporarily suspended and then supplied to the next sorting step S3 as they are, and the overcoat particles 32S smaller than the particle size Bd. (Selected overcoat particles) are selected, returned to the overcoat step, and again overcoated with the graphite matrix material, and then supplied to the selection step S3 together with the non-selected overcoat particles 32L.

  Similarly, in the third sorting step S3, the overcoat particles 32L and 32S supplied directly or re-coated from the second sorting step S2 are based on the third particle size Cd (> Bd). The overcoat particles 33L larger than the particle size Cd are temporarily suspended and then supplied to the next selection step S3 as they are, and the overcoat particles 33S smaller than the particle size Cd are selected and returned to the overcoat step. After the graphite matrix material is overcoated again, it is supplied to the sorting step S4 together with the overcoat particles 33L.

  Finally, the fourth sorting step S4 sorts the overcoat particles 33L and 33S supplied directly or overcoated from the third sorting step S3 based on the fourth particle size Dd (> Cd), The non-separated overcoat particles 34L larger than the particle diameter Dd are recovered, and the selected overcoat particles 34S smaller than the particle diameter Dd are supplied to the fuel compact molding process of FIG.

  The fuel compact molded in this way is pre-fired to remove the binder contained in the graphite matrix material, and further fired for degassing graphitization of the graphite matrix material to be the final product fuel. Is obtained.

  In the above-described sorting steps S1 to S4, the first particle size Ad is the lower limit value of the diameter of the overcoat particle 3, and the fourth particle size Dd is the upper limit value of the diameter of the overcoat particle 3. The overcoat particles 31S, 32S, and 33S that have been selected to have a particle size smaller than the set particle size in the first to third selection steps S1 to S3 are again overcoated by spraying the graphite matrix. 3 approaches the diameter upper limit Dd with high accuracy.

  In the fuel compact 4 obtained by press molding the overcoat particles obtained through such a selection step, the coated fuel particles 1 in the adjacent overcoat particles 3 are as shown in FIG. Since the overcoat layer 2 of the overcoat particles 3 is maintained at a uniform distance from each other, the coated fuel particles 1 are uniformly dispersed in the fuel compact 4.

  It was confirmed that sufficient combustion characteristics could be exhibited when only about 70000 fuel compacts were irradiated in a nuclear reactor by press molding only the overcoat particles 3 obtained by the selection of the method of FIG. It was.

  As already described, the overcoat particles 3 are loaded into the press mold 10 together with the graphite matrix material and press-molded, and the graphite to be loaded into the press mold 10 together with the overcoat particles 3. When the entire matrix material is coated as an overcoat layer of the overcoat particles 3 (when the particle size of the overcoat particles 3 is the upper limit), only the overcoat particles can be loaded into the press mold. Therefore, coupled with the uniformity of the particle diameter of the overcoat particles, the coated fuel particles 1 and the entire graphite matrix material are uniformly dispersed in the fuel compact, and a high quality fuel compact can be obtained.

  Of course, a part of the graphite matrix material to be loaded in the press mold 10 together with the overcoat particles 3 may be coated as an overcoat layer of the overcoat particles 3 (the particle size of the overcoat particles 3 is set to the upper limit value). In this case, the press mold 10 is loaded with the overcoat particles 3 and the remaining graphite matrix material and press-molded, but is independent of the overcoat particles. Since the amount of the graphite matrix loaded in this way is small, it is possible to maintain the uniformity of dispersion between the coated fuel particles 1 and the entire graphite matrix material.

  In the above embodiment, the overcoat particles 31S, 32S, and 33S selected in each selection step before the final step are overcoated particles that were not selected in the corresponding selection step after the graphite powder was overcoated. (Non-sorted overcoat particles) 31L, 32L, 33L are sent to the next sorting step, but non-sorted overcoat particles) are not together with 31L, 32L, 33L, and then the next sorting step You may make it send to.

  Another embodiment of the present invention is shown in FIG. In the embodiment shown in FIG. 1, the overcoat particles 31S, 32S, 33S selected in each of the selection steps before the final step are returned to the overcoat step and overcoated with the graphite matrix powder again, and then the corresponding selection step S1. 2 and the overcoat particles 31K, 32L, 33L that were not selected in S2, S3, were supplied to the next selection steps S2, S3, S4, but in the embodiment of FIG. 2, the overcoat particles 31S, 32S, 33S is returned to the overcoat process and overcoated again with the graphite matrix powder, and then with the corresponding unsorted overcoat particles supplied to the corresponding sorting processes S1, S2, S3, the corresponding sorting processes S1, S2, It is supplied to S3 and sorted again.

  In this case, most of the overcoat particles will be overcoated particles 31L, 32L, and 33L that will not be sorted due to the increase in particle size due to the overcoating of the graphite matrix powder again. The same operation is repeated until it is not selected in steps S1, S2, and S3. Thus, the overcoat particles selected in each step are repeatedly subjected to graphite matrix powder overcoat and particle size selection until they are supplied to the final selection step. Can be definitely obtained.

  In the above embodiment, four sorting steps S1 to S4 for sorting different particle sizes are used. However, three sorting steps or five or more sorting steps can be used, and particularly as the number of sorting steps increases. This is desirable because the diameter of the overcoat particles is made more uniform.

  Since the thickness of the overcoat layer on the coated fuel particle 1 of the overcoated particle 3 is about 1/3 to the particle size of the coated fuel particle 1, the overcoated particle selected in the final sorting step The particle size of 3 is considered to be about 1.5 to 3 times that of the coated fuel particle 1. However, the particle size of the overcoat particles 3 to be finally selected depends on the production conditions of the fuel compact to be molded, that is, the number of the coated fuel particles 1 contained in the fuel compact and the ratio of the coated fuel particles to the graphite matrix material. The value is not determined by a specific value.

  In the above embodiment, the case where the molded fuel is a hollow cylindrical fuel compact has been described. However, the fuel is not limited to the fuel compact as long as it is obtained by press molding. The present invention can be similarly applied to a spherical fuel.

  Thus, since the overcoat particles are sorted through a multi-stage sorting process with different particle sizes to be sorted, uniform overcoat particles can be obtained. Therefore, when these overcoat particles are press-molded, the coated fuel It can be seen that a high quality molded fuel in which the particles are uniformly dispersed can be obtained.

  In particular, since the screening process using a sieve can perform screening of a large amount of particle diameters, overcoat particles having a uniform particle diameter can be obtained efficiently.

  In particular, when all or part of the graphite matrix material to be loaded into the press die 10 is coated as an overcoat layer of the overcoat particles 3, the uniformity of the particle diameter of the overcoat particles is combined with the inside of the fuel compact. This is advantageous because the dispersibility between the coated fuel particles 1 and the entire graphite matrix material can be made more uniform.

  According to the present invention, a high-quality high-temperature gas that can uniformly disperse coated fuel particles in a fuel obtained by press-molding overcoat particles having a uniform particle size and can improve combustion characteristics. It can be used beneficially in the production of fuel for furnaces.

It is a block diagram of the process of the manufacturing method of the fuel by one embodiment of this invention. It is a block diagram of the process of the manufacturing method of the fuel by other embodiment of this invention. It is an expanded sectional view of overcoat particles. It is sectional drawing of the state which press-molds overcoat particle | grains. FIG. 2A is a perspective view of the fuel compact, and FIG. 2B is an enlarged sectional view of a distribution state of coated fuel particles in a part of the tissue.

Explanation of symbols

1 Coated fuel particles 2 Graphite matrix material 3 Overcoat particles 4 Fuel compact (molded fuel)
10 Press mold 10L Lower mold 10U Upper mold 10R Rod 31L to 34L Overcoat particles with large particle size 31S to 34S Overcoat particles with small particle size S1 to S4 Sorting step

Claims (4)

For high-temperature gas reactors, where overcoated particles formed by overcoating graphite powder with a binder on the surface of coated fuel particles are pressed with a press mold to form a molded fuel, and the molded fuel is pre-fired and fired. In the method for producing a fuel, the overcoat particles are sequentially supplied to a multi-stage sorting process having different selection particle diameters, and the overcoat particles having a predetermined particle diameter selected in the final selection process are press-molded. The overcoat particles selected in the selection step before the final step are returned to the overcoat step, overcoated with the graphite matrix powder again, and supplied again to the corresponding selection step, and the final selection step. A method for producing a fuel for a HTGR, wherein overcoat particles that are not selected in step 1 are collected. The method for producing a HTGR fuel according to claim 1, wherein the multistage sorting step sorts overcoat particles using sieves having different sieve meshes. Production method. 3. The method for producing a fuel for a HTGR according to claim 1 or 2, wherein the overcoat particles selected in each selection step before the final step are overcoated with graphite matrix powder again, and then the corresponding selection is performed. The HTGR fuel is characterized in that it is supplied to the next sorting step together with the overcoat particles not selected in the process (referred to as unselected overcoat particles) or subsequently to the unselected overcoat particles. Production method. 3. The method for producing a HTGR fuel according to claim 1 or 2, wherein the overcoat particles selected in each of the selection steps before the final step are overcoated with graphite matrix powder again and then subjected to a corresponding selection. A method for producing a molded fuel for a high-temperature gas reactor, wherein the fuel is supplied to the corresponding sorting step and sorted again together with new unsorted overcoat particles supplied to the step.

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