JP2007121128A - Ammonium diuranate particle containing gadolinium and manufacturing method thereof, fuel kernel for high-temperature gas-cooled reactor fuel, coated particle for high-temperature gas-cooled reactor, and high-temperature gas-cooled reactor fuel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high-temperature gas-cooled reactor fuel for adjusting neutron flux during combustion for increasing the combustion efficiency in the reactor core of the high-temperature gas-cooled reactor and extending its lifetime; to provide an ammonium diuranate particle that becomes the raw material of the fuel, a fuel kernel, and a coated particle; and to provide a manufacturing method of the ammonium diuranate particle. <P>SOLUTION: The manufacturing method of the ammonium diuranate particle containing gadolinium gives: the ammonium diuranate particle containing gadolinium for a high-temperature gas-cooled reactor fuel where gadolinium is dispersed uniformly in the particle; the fuel kernel that is a uranium dioxide containing gadolinium obtained by baking the ammonium diuranate particle; the coated particle for the high-temperature gas-cooled reactor fuel where the fuel kernel is coated with a thermal decomposition carbon layer, a silicon carbide layer, or the like; the fabricated high-temperature gas-cooled reactor fuel after the coated particle is overcoated by a graphite matrix material; and the ammonium diuranate particle containing gadolinium by dripping a uniformly mixed solution drop of uranyl nitrate and gadolinium nitrate into an ammonium solution. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to gadolinium-containing ammonium heavy uranate particles suitable for a HTGR fuel, a method for producing the same, a fuel core for a HTGR fuel, coated particles for a HTGR, and a HTGR fuel. Is a gadolinium-containing ammonium heavy uranate particle for HTGR fuel in which uranium and gadolinium are homogeneously mixed, fuel core for HTGR, coated particle for HTGR, and high temperature such as fuel compact and pebble sphere The present invention relates to a gas furnace fuel and a method for producing gadolinium-containing ammonium heavy uranate particles.

Usually, uranium fuel for HTGR is extracted from uranyl nitrate solution as ammonium deuterated uranium particles, then baked into uranium trioxide particles, and further baked and reduced to uranium dioxide particles (fuel nuclei). Make.) These fuel nuclei are coated with a pyrolytic carbon layer, a silicon carbide layer or the like to form coated fuel particles, from which fuel compacts and pebble balls are manufactured.
The fuel for the HTGR is usually manufactured through the following processes. First, a powder such as uranium oxide is dissolved in nitric acid, and pure water, a viscosity adjusting thickener, and the like are added to make a uranyl nitrate solution. This uranyl nitrate solution is dropped into an aqueous ammonia solution using a small-diameter dropping nozzle. Before the droplet reaches the surface of the aqueous ammonia solution, ammonia gas is sprayed, the surface of the droplet gels and hardens, and deformation due to impact is prevented at the time of collision with the surface of the aqueous ammonia solution. Uranyl nitrate droplets in ammonia aqueous solution sufficiently react with ammonia and solidify, and then washed and dried to form ammonium deuterated uranate (also referred to as ADU) particles. Here, spherical ADU particles having a uniform and high sphericity are required. Various methods for producing ADU particles having high sphericity have been studied. For example, Patent Document 1 discloses a method for producing spherical ADU particles having a diameter of several tens to 1,000 μm and high sphericity from a uranyl nitrate solution. Has been.
The ADU particles are roasted in the atmosphere, and further reduced and sintered, thereby forming high-density uranium dioxide particles (fuel nuclei). The fuel nuclei (diameter 350 to 650 μm) are introduced into a thermal decomposition apparatus such as a hydrocarbon gas serving as a carbon source to form a coating layer to form coated fuel particles. The coating layer is formed in the order of a low density pyrolytic carbon layer, a high density pyrolytic carbon layer, a SiC layer or a ZrC layer, and a high density pyrolytic carbon layer. Fuel compacts and pebble spheres, which are fuels for general high-temperature gas reactors, include the above-mentioned coated fuel particles, together with matrix materials such as graphite powder and binder, in a hollow cylindrical shape, a solid cylindrical shape, or a substantially spherical shape (tadon shape) ) And heat-treated.
The above-described fuel compact is placed in a graphite sleeve and mounted at appropriate intervals in a graphite block that also serves as a moderator. The aggregate of the graphite blocks constitutes a core for a high-temperature gas reactor. In order to control the reactivity of the core, a flammable poison such as a boron compound is arranged in places on the graphite block to absorb neutrons.

JP 2004-219195 A

  In the method of arranging flammable poisons (neutron absorbers) as described above, the neutron density of the graphite block as a whole can be adjusted during operation of the high-temperature gas furnace. And the neutron density is uneven in the parts that are not. For this reason, along with the operation of the nuclear reactor, fuel that burns relatively early and whose life is exhausted and fuel that is unburned and still has a sufficient lifespan coexist. However, if the overall burnup exceeds a certain value, the reactor must be shut down and the core replaced. If the degree of burnup of the fuel is not uniform, the fuel usage efficiency will deteriorate and the output will decrease, and the life of the fuel will also be shortened. The present invention realizes uniform combustion of a high temperature gas reactor fuel, and as a result, increases the combustion efficiency and extends the life of the gadolinium-containing ammonium heavy uranate particles, a method for producing the same, and a fuel for the high temperature gas reactor fuel An object is to provide nuclei, coated particles for a HTGR, and fuel for the HTGR.

The present inventor has found a method for producing a fuel capable of uniform combustion control by uniformly mixing gadolinium, which is a flammable poison, in the uranium compound in the raw material at the fuel production stage, and comprises the following means: Did.
(1) Gadolinium-containing ammonium heavy uranate particles for HTGR fuel in which gadolinium is uniformly dispersed in the particles.
(2) A fuel core for a HTGR fuel, which is gadolinium-containing uranium dioxide particles obtained by heat-treating the gadolinium-containing ammonium heavy uranate particles.
(3) Coated particles for HTGR fuel in which the fuel core is coated with a pyrolytic carbon layer, a silicon carbide layer, or the like.
(4) A high-temperature gas reactor fuel compact or a high-temperature gas reactor pebble sphere obtained by overcoating the coated particles with a graphite matrix material, followed by molding and heat treatment.
(5) The method for producing gadolinium-containing ammonium heavy uranate particles according to (1), wherein a homogeneous mixed solution of uranyl nitrate and gadolinium nitrate is dropped into ammonia water to produce gadolinium-containing ammonium heavy uranate particles.

  In the present invention, fuel compacts produced from fuel nuclei of gadolinium-containing uranium dioxide particles in which gadolinium, which is a flammable poison, is uniformly dispersed in the particles, and fuels for HTGRs such as pebble spheres are obtained. By designing the core of the HTGR using such HTGR fuel, it is possible to easily achieve uniform combustion throughout the HTGR fuel, making efficient use of the HTGR fuel, Stable operation and longer fuel life can be realized.

  The gadolinium-containing ammonium heavy uranate particles (hereinafter, “ammonium heavy uranate particles” may be abbreviated as “ADU particles”. “Ammonium heavy uranate” is also referred to as “ADU”). Gadolinium is uniformly dispersed inside. This is realized by forming gadolinium-containing ADU particles from droplets of a uniform solution uniformly containing a gadolinium compound and a uranium compound. Further, the gadolinium-containing ADU particles are roasted and reduced and sintered to obtain a fuel nucleus of gadolinium-containing uranium dioxide ceramics. The fuel nucleus is coated with a pyrolytic carbon layer, a silicon carbide layer, or the like to form coated particles. The coated particles are mixed with graphite as a matrix and overcoated with graphite, etc., then subjected to molding and heat treatment, and fuel compact or pebble spheres that are fuels for high-temperature gas reactors in which gadolinium is uniformly dispersed in the fuel core It becomes. In such a high temperature gas reactor fuel, gadolinium is uniformly dispersed in uranium. Therefore, during operation of the high temperature gas reactor, each fuel nucleus or uranium in the fuel nucleus can be uniformly burned by a fission reaction.

Below, according to the manufacturing process of the fuel for high temperature gas reactors, the manufacturing method of each particle | grain and fuel is demonstrated.
(Manufacture of ADU particles containing gadolinium)
The ADU particles containing gadolinium of the present invention can be produced by either a batch production method or a continuous production method, similarly to conventional ADU particles not containing gadolinium. The batch manufacturing method is easy in terms of criticality safety control, and the continuous manufacturing method is suitable for mass production, but it is necessary to pay particular attention to criticality safety control. Although these manufacturing processes are the same as the conventional manufacturing process of ADU particles not containing gadolinium, the gadolinium compound should be added to form a gadolinium nitrate-containing uranyl nitrate solution in the first uranyl nitrate solution preparation stage. Thus, ADU particles containing gadolinium of the present invention can be obtained.

  Specifically, the first step is to prepare a gadolinium-containing uranyl nitrate solution and react it with ammonia to generate gadolinium-containing ADU particles that are solid particles uniformly containing gadolinium. In the preparation of gadolinium-containing uranyl nitrate solution, the uranyl nitrate concentration, gadolinium nitrate concentration and solution viscosity are adjusted by dissolving uranium oxide and gadolinium raw materials in nitric acid and mixing with pure water and thickener to make a homogeneous solution. To do. As the uranium oxide, uranium dioxide, uranium trioxide, or triuranium octoxide that has already been enriched with radioactive isotopes for nuclear fuel, preferably uranium octaoxide may be used. As the gadolinium raw material, gadolinia or gadolinium nitrate, which is easily available and may be easily dissolved in nitric acid, may be used. At the time of dissolution, the solution is easily dissolved by heating and stirring the nitric acid solution. The uranium concentration in the uranyl nitrate solution is usually desirably 1.5 to 2.5 molU / L. If the concentration is too high, a dissolution residue is generated. If the concentration is too low, it is difficult to produce high-density ADU particles with high sphericity. The concentration of gadolinium may be a concentration calculated from the mass ratio of gadolinium to uranium required from the core design. Usually, it is 10% or less by mass ratio with respect to uranium, preferably 0.5 to 5% in many cases.

  The viscosity of the gadolinium-containing uranyl nitrate solution is large in the size, sphericity, drop on the aqueous ammonia solution, and deformation resistance at the time of collision when dropping into the aqueous ammonia solution as described below. Works. However, since the viscosity of the gadolinium-containing uranyl nitrate solution varies depending on the concentration and temperature of the uranyl nitrate, the viscosity under actual droplet forming conditions is appropriately controlled. When the gadolinium-containing uranyl nitrate solution droplet formation and the initial gadolinium-containing ammonium uranium acidation reaction of the uranyl nitrate solution droplet are performed at room temperature to 50 ° C., the viscosity of the gadolinium-containing uranyl nitrate solution is the temperature at the time of droplet formation. 0.1 to 100 mPa · s, particularly 40 to 90 mPa · s is preferable. A water-soluble polymer or the like may be used as the thickener for adjusting the viscosity. For example, polyvinyl alcohol (hereinafter abbreviated as PVA), synthetic polymers such as sodium polyacrylate and polyethylene oxide, cellulose polymers such as carboxymethyl cellulose, hydroxyethyl cellulose, methyl cellulose, and ethyl cellulose, soluble starch, and carboxymethyl starch Water-soluble natural polymers such as starch-based polymers, dextrin, and galactan can be exemplified. Among these, a synthetic polymer is preferable, and PVA is particularly preferable. These various water-soluble polymers may be used alone or in combination of two or more. A gadolinium-containing uranyl nitrate solution to which a thickener or the like is added as described above (hereinafter, “a gadolinium-containing uranyl nitrate solution to which a thickener or the like is added” may be abbreviated as “a uranyl nitrate solution”) may contain gadolinium. It becomes a raw material solution for ADU particle production.

  As a specific production example, a batch reaction apparatus is shown in FIG. 1 and will be described with reference to FIG. The uranyl nitrate solution is introduced into the uranyl nitrate solution dropping device 1. Usually, the uranyl nitrate solution dropping device 1 uses a container provided with a thin nozzle that opens toward the bottom. In order to form and drop a droplet having a uniform size, it is preferable to drop the droplet by giving a fine vibration of a uniform frequency to the nozzle. In order to obtain a desired droplet diameter, the viscosity of the uranyl nitrate solution, the shape of the nozzle, the frequency of vibration, and the like may be controlled. The droplet size directly affects the diameter of the ADU particles produced. A plurality of nozzles are arranged in the uranyl nitrate solution dropping device 1 during mass production. When there are a plurality of nozzles, it is particularly desirable to align the tip shape and vibration state so that droplets of the same shape are formed.

  The uranyl nitrate droplet 2 is first dropped in the ammonia gas layer 4, and while falling in the ammonia gas layer 4, a part of the droplet surface reacts with the ammonia gas and gels, and a gel-like substance is formed on the droplet surface. A film is formed. The reaction in which uranyl nitrate is converted to ADU is represented by the following formula. It is speculated that a similar reaction occurs in some of the coexisting gadolinium.

  The uranyl nitrate solution droplet 2 whose surface is gelled and solidified falls into the ammonia aqueous solution layer 5 of the reaction vessel 3. It is desirable that the uranyl nitrate solution droplets 2 are gelled and solidified to such an extent that they are not broken or deformed by the impact of dropping on the surface of the aqueous ammonia solution. The dropped uranyl nitrate solution droplet 2 further sinks in the ammonia aqueous solution layer 5 while reacting with ammonia in the ammonia aqueous solution layer 5, and gelation proceeds to the inside as it is. The reaction temperature in the aqueous ammonia solution is preferably 10 to 30 ° C., the residence time of the produced solid particles 7 in the aqueous ammonia solution is preferably 20 to 120 minutes, and the ammonia concentration in the aqueous ammonia solution is preferably 20 to 35% by weight. The uranyl nitrate solution droplets that have been gelled from the surface are further reacted with ammonia of uranium and gadolinium in an aqueous ammonia solution to form gadolinium-containing ADU solid particles 7. Since the ammonia content in the aqueous ammonia solution decreases with the reaction, it is desirable to blow in ammonia gas or add a fresh aqueous ammonia solution to maintain the ammonia concentration. At this time, in designing and using the reaction vessel 3, the uranyl nitrate solution droplets 2 and the gadolinium-containing ADU particles 7 with insufficient reaction are unevenly distributed in a part of the reaction vessel 3 or partially in a high density state. Don't do it. If gadolinium-containing ADU particles that are not sufficiently reacted and solidified are deposited in an aqueous ammonia solution, they may be deformed and become non-spherical. Usually, a reaction vessel having a cylindrical shape, an elliptical column shape, a rectangular parallelepiped shape or the like is used, and the particles are allowed to fall naturally, or in some cases, the ammonia aqueous solution layer 5 may be kept in a fluid state and reacted. In addition, there is a method of preventing the stagnation by forming a circulation flow path at a place where particles or the like tend to stay and circulating a part of the ammonia water. The generated gadolinium-containing ADU particles having at least the surface gelled and solidified fall as solid particles 7 below the ammonia aqueous solution layer 5.

  When a certain amount of the falling solid particles 7 is deposited, the solid particles 7 are discharged from the solid particle outlet 8 and separated from the aqueous ammonia solution. As long as consideration is given to deformation of the solid particles, critical safety control, etc., solid-liquid separation of the aqueous ammonia solution and the solid particles may be any commonly used method. Suitable methods include separation by a filter such as decantation or mesh. Examples of the continuous method include a rotary drum filter and a hydrocyclone. The filter may be a continuously operable filter having a structure in which solid particles are separated while filtering a liquid by installing a filter obliquely under the solid particle discharge port 8. The solid-liquid separation is sufficient if it is not perfect because it is aged and washed in the next step. The separated liquid may be led to the original aqueous ammonia layer 5 again and recycled. In some cases, the ammonia water can be used as the aging ammonia water described later without separating the solid particles from the ammonia water.

  Next, the obtained solid particles 7 are transferred to an aging and washing process. When the gadolinium-containing ADU reaction is insufficient, the solid particles 7 thus solid-liquid separated are preferably aged by providing an aging tank for further reaction with ammonia to form complete gadolinium-containing ADU particles. . If the gadolinium-containing ADU reaction of the solid particles is sufficiently advanced, a dense and high-quality fuel nucleus can be obtained when a fuel nucleus which is a gadolinium-containing uranium dioxide sintered body is manufactured later. In the ripening, the solid particles are transferred to an aging tank that is a second ammonia aqueous solution tank, and the gadolinium-containing ADU reaction is advanced by suspending the solid particles by circulating, stirring, or the like. In this case, the reaction temperature is 30 to 100 ° C, preferably 40 to 90 ° C. The aging time of the solid particles is 10 to 180 minutes, preferably 20 to 120 minutes. The ammonia concentration of the aqueous ammonia solution in the aging tank is 5 to 40% by weight, preferably 10 to 30% by weight. In the ripening step, it is desirable to prevent uneven distribution of solid particles as described above. The solid particles completely solidified as gadolinium-containing ADU particles are transferred to the same separation step as described above to separate the solid particles from the aqueous ammonia solution.

  The crude solid particles separated into solid and liquid are guided to a washing and drying process. The washing step usually has a pure water washing tank and an organic solvent washing tank. In the pure water washing tank, ammonia, unreacted uranyl nitrate, gadolinium nitrate, and thickener contained in the coarse solid particles are removed. As washing conditions, the washing temperature is 5 to 120 ° C., preferably 10 to 100 ° C., and the washing time of the solid particles is 10 to 240 minutes, preferably 20 to 200 minutes. In the organic solvent washing tank, moisture contained in the coarse solid particles and organic substances such as a residual thickener are removed. As the organic solvent, low-boiling water-soluble solvents such as ethanol, methanol, and acetone, volatile hydrocarbon solvents, and ethers that are easily volatilized or evaporated may be used. The washing temperature when washing the solid particles with an organic solvent is 5 to 100 ° C., preferably 10 to 80 ° C. The washing time of the solid particles is 10 to 240 minutes, preferably 20 to 200 minutes. Solid-liquid separation after washing may be performed in the same manner as the above-described solid-liquid separation. Note that cleaning in the organic solvent cleaning tank may be omitted.

The solid particles that have been washed are transferred to a drying process. You may dry by what kind of method in a drying process. There is no problem with hot air drying, heat drying, fluidized bed drying, or natural drying, but hot air drying may be performed on a conveying means such as a belt conveyor. These easy-to-control methods are preferable in terms of critical safety management and radiation protection management. In the drying process, care should be taken because if the solid particles are heated too rapidly, the solid particles may be damaged or deformed. Drying is preferably performed at 100 ° C. or lower. In this way, gadolinium-containing ADU particles are obtained. In addition, the process from aging to drying can also be carried out sequentially in one tank.
The ADU particle production apparatus shown in FIG. 1 is an example of producing gadolinium-containing ammonium heavy uranate particles according to the present invention, and various commonly known batch-type and continuous-type ADUs other than those shown in FIG. The particle production apparatus can be the ADU particle production apparatus in the present invention.

(Manufacture of gadolinium-containing fuel nuclei and coated particles)
The gadolinium-containing ADU particles obtained as described above are roasted at 500 to 650 ° C. to become oxide particles such as uranium trioxide. This is sintered at 1500 ° C. or higher in a reducing atmosphere to obtain uranium dioxide particles uniformly containing gadolinium oxide. When the particles are classified and selected as necessary, gadolinium-containing ADU particles having a uniform particle size and particle shape are obtained, which are suitably used as fuel nuclei. In general, the fuel core is preferably a fine particle having a diameter of about 350 to 650 μm.
The coated particles are obtained by coating fuel nuclei with a pyrolytic carbon layer, a silicon carbide layer, or the like. Normally, it has four coating layers on the surface of the fuel core. From the fuel core surface side, the first layer is a low-density pyrolytic carbon layer, the second layer is a high-density pyrolytic carbon layer, and the third layer is silicon carbide. The layer and the fourth layer are composed of a high-density pyrolytic carbon layer, and the particle size is about 500 to 1000 μm. The first layer is made of low-density pyrolytic carbon having a density of about 1 g / cm ³ , functions as a gas reservoir for gaseous fission products (hereinafter sometimes referred to as “FP”), and fuel nucleus. It functions as a buffer that absorbs swelling. The second layer is made of high-density pyrolytic carbon having a density of about 1.8 g / cm ³ and has a function of holding the gaseous FP. The third layer is made of silicon carbide having a density of about 3.2 g / cm ³ (hereinafter sometimes abbreviated as “SiC”), has a function of holding a solid FP, and maintains the main strength of the coating layer. It is a material. The fourth layer, like the second layer, is made of high-density pyrolytic carbon having a density of about 1.8 g / cm ³ , has a function of holding the gaseous FP, and functions as a protective layer for the third layer. Have The method for producing coated particles is, for example, in which a fuel core is fluidized at a high temperature in a fluidized bed reactor, a carbon source gas for forming a coating layer is introduced into the fluidized bed, and this is pyrolyzed, A coating layer of pyrolytic carbon is formed on the surface of the core particle. For example, in the case of coating the first layer with low-density pyrolytic carbon, acetylene is introduced at about 1300 ° C. and pyrolyzed. In the case of coating the second layer and the fourth layer with high-density pyrolytic carbon, propylene is introduced at about 1400 ° C. and pyrolyzed. Furthermore, when the third layer is a coating layer made of SiC, it is thermally decomposed by introducing methyltrichlorosilane at about 1500 ° C. These coating layer forming reactions can be carried out by continuously forming each layer on each particle in the same fluidized bed reactor, or by forming a coating layer one by one on each particle in a separate fluidized bed. Thus, the coated particles of the present invention are obtained.

(Manufacture of fuel for HTGR)
The coated particles are molded by an injection molding method or an overcoat press to obtain a fuel for a HTGR called a fuel compact or pebble sphere. In the injection molding method, the coated particles are mixed with a graphite matrix material composed of graphite powder and a binder such as a fat thermosetting resin such as pitch and phenol resin, and after being overcoated and molded by an injection molding machine, Heat to 1000 ° C. or higher to carbonize the binder. Thereby, a fuel compact having a relatively high coating particle filling rate can be manufactured. In the overcoat press, the coated particles are mixed with graphite powder and a binder of a thermosetting resin such as a phenol resin, subjected to primary pressing at around 150 ° C., and then further heated to cure the binder. The surface of the molded body is again coated with graphite powder, loaded into a mold, and subjected to secondary pressing to form an outer shell (shell) on the outer surface. This is heat-treated at 1000 ° C. or higher to obtain a high temperature gas reactor fuel. Those formed into a cylindrical or cylindrical shape having a diameter and height of about 20 to 50 mm are called fuel compacts, and those made into a substantially spherical shape of about 30 to 50 mm are called pebble balls. Used as fuel for HTGR.

  The gadolinium-containing ammonium heavy uranate particles, fuel nuclei, and fuel compacts or pebble spheres for the HTGR fuel of the present invention are novel fuels for the HTGR, increasing the freedom of design and operation of the HTGR, It is used as an economical and safe fuel for HTGR.

FIG. 1 shows an example of a reaction vessel for producing gadolinium-containing ADU particles of the present invention.

Explanation of symbols

  1: Uranyl nitrate solution dropping device, 2: Uranyl nitrate solution droplet, 3: Reaction tank, 4: Ammonia gas layer, 5: Ammonia aqueous solution layer, 6: Ammonia gas supply port, 7: Solid particles (gadolinium-containing ADU particles) 8: Solid particle outlet

Claims (5)

  Gadolinium-containing ammonium heavy uranate particles for HTGR fuel in which gadolinium is uniformly dispersed in the particles.   A fuel core for a HTGR fuel, which is gadolinium-containing uranium dioxide particles obtained by heat-treating the gadolinium-containing ammonium heavy uranate particles according to claim 1.   Coated particles for HTGR fuel, wherein the fuel core according to claim 2 is coated with a pyrolytic carbon layer and a silicon carbide layer.   A fuel compact for a HTGR, or a pebble sphere for a HTGR obtained by overcoating the coated particles according to claim 3 with a graphite matrix material, followed by molding and heat treatment.   The method for producing gadolinium-containing ammonium heavy uranate particles according to claim 1, wherein a homogeneous mixed solution of uranyl nitrate and gadolinium nitrate is dropped into ammonia water to produce gadolinium-containing ammonium heavy uranate particles.

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