JP2006038793A - Method for manufacturing fuel kernel particle - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for manufacturing a fuel kernel particle having large strength and high sphericity without generating any gaps or cracks inside and without deforming the particle. <P>SOLUTION: In the method for manufacturing the fuel kernel particles, ammonium diuranate particles are calcinated for obtaining uranium oxide particles expressed by an expression UO<SB>2+X</SB>(however, x is an integer or a decimal for satisfying 0<x≤1). Then, the uranium oxide particles are reduced at 400-700°C at least for two hours before sintering. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a method for manufacturing fuel core particles, and more particularly, to a method for manufacturing fuel core particles having high strength and high sphericity.

  The core into which the fuel of the high-temperature gas reactor is charged is formed of graphite having a large heat capacity and excellent high-temperature soundness. In this high-temperature gas furnace, a gas such as helium gas that does not cause a chemical reaction even at high temperatures and has high safety is used as the cooling gas, so even if the outlet temperature is high, the cooling gas can be safely used. It can be taken out. Therefore, even if the temperature of the core rises to about 900 ° C., the cooling gas heated to a high temperature can be used safely in a wide range of fields such as power generation, hydrogen production equipment, and other chemical plants. Yes.

  Moreover, the fuel for a high temperature gas reactor to be charged into the high temperature gas reactor generally includes a fuel core and a coating layer that covers the periphery of the fuel core. The fuel core is, for example, fine particles having a diameter of about 350 to 650 μm formed by sintering uranium dioxide into a ceramic form.

The coating layer has a four-layer structure, and has a first layer, a second layer, a third layer, and a fourth layer from the fuel core surface side. The first layer is formed of low-density pyrolytic carbon having a density of about 1 g / cm ³ , functions as a gas reservoir for gaseous fission products (FP), and serves as a buffer for absorbing fuel nuclear swelling. It also has the function of The second layer is formed of high-density pyrolytic carbon having a density of about 1.8 g / cm ³ and has a function of holding a gaseous FP. The third layer is formed of silicon carbide (SiC) having a density of about 3.2 g / cm ³ , has a function of holding a solid FP, and is a main strength member of the coating layer. The fourth layer is formed of high-density pyrolytic carbon having a density of about 1.8 g / cm ³ and has a function of holding a gaseous FP and also a function of a protective layer of the third layer. . The diameter of the coated particles forming these coating layers is usually about 500 to 1500 μm.

  The coated fuel particles formed by forming the four coating layers on the surface of the fuel core are dispersed in a graphite matrix and formed into a fixed shape of a fuel compact. The fuel compact is formed of graphite. A certain amount is accommodated in the cylinder and plugged up and down to form a fuel rod. Ultimately, this fuel rod is inserted into a plurality of insertion holes of the hexagonal column type graphite block, and a core is formed by arranging a plurality of the hexagonal column type graphite blocks in a honeycomb shape and stacking a plurality of stages. The

  Such a HTGR fuel can generally be manufactured through the following steps. First, a uranium nitrate solution is prepared by dissolving uranium oxide powder in nitric acid. Next, an aqueous solution of thickener is added to the uranyl nitrate solution and mixed to prepare a stock solution for producing ammonium biuranate particles (hereinafter sometimes simply referred to as “stock solution”). A thickener is a substance added to adjust the viscosity of a droplet of uranyl nitrate, which is a stock solution dropped into an aqueous ammonia solution described later.

  Examples of the thickener include polyvinyl alcohol, a resin that has a property of solidifying under alkaline conditions, polyethylene glycol, and metroses. The stock solution prepared in this manner is cooled to a predetermined temperature, and then dropped from the small-diameter original droplet lower nozzle of the original droplet lowering device into the aqueous ammonia solution in the ammonia aqueous solution storage tank by vibrating the nozzle. The

  The droplets dropped into the aqueous ammonia solution are sprayed with ammonia gas in the space up to the surface of the aqueous ammonia solution. Since the surface of the droplet gels by this ammonia gas and a film is formed, the droplet particles on which the gel film is formed are prevented from being deformed by impact when falling onto the surface of the aqueous ammonia solution, and the ammonia in the aqueous ammonia solution storage tank To form ammonium deuterated uranate particles. The formed ammonium heavy uranate particles are deposited in the lower part of the ammonia aqueous solution storage tank. If the formed ammonium heavy uranate particles are left in the deposited state, the lower ammonium heavy uranate particles are deformed by the load of the ammonium heavy uranate particles located in the upper part of the deposition of ammonium heavy uranate particles. End up. Therefore, the aqueous ammonia solution in the aqueous ammonia solution storage tank is so formed that the ammonium heavy uranate particles are not deformed by the deposition of ammonium heavy uranate particles, in other words, the ammonium heavy uranate particles do not remain deposited. , A liquid flow is formed from the lower side to the upper side of the ammonia aqueous solution storage tank.

  The ammonium heavy uranate particles formed in the ammonia aqueous solution storage tank are transferred to a post-treatment device that is connected to the ammonia aqueous solution storage tank. The transfer of the heavy ammonium uranate particles to the post-treatment device is normally performed by opening a valve of a pipe connecting the ammonia aqueous solution storage tank and the post-treatment device and dropping it together with the aqueous ammonia solution by its own weight. In this post-treatment device, while rotating the post-treatment tank, by heating, the uranyl nitrate and ammonia are completely reacted to the center of the particles to produce ammonium heavy uranate (ripening treatment), heavy uranic acid with hot water, etc. It is an apparatus that performs a process for cleaning ammonium particles (cleaning process) and a process for drying (drying process).

  Aged, washed and dried ammonium heavy uranate particles are roasted in air to form uranium trioxide particles. Further, the uranium trioxide particles are reduced and sintered to become high-density ceramic-like uranium dioxide particles. The uranium dioxide particles thus formed are classified and obtained as fuel core fine particles having a predetermined particle diameter.

The fuel core particles obtained in this way are loaded onto a fluidized bed and coated by thermally decomposing the coating gas. For example, the first layer can be formed by pyrolyzing acetylene at about 1400 ° C., and the second and fourth layers can be formed by pyrolyzing propylene at about 1500 ° C. it can. For example, the third layer can be formed by thermally decomposing methyltrichlorosilane at about 1600 ° C. A normal fuel compact can be manufactured by press-molding or molding coated fuel particles into a hollow cylindrical shape or a medium-density cylindrical shape together with a graphite matrix material composed of graphite powder, a binder, and the like, followed by firing. (See Non-Patent Documents 1 and 2).
"Reactor Material Handbook" p221-p247, published October 31, 1977, published by Nikkan Kogyo Shimbun "Nuclear Power Handbook" p161-p169, issued on December 20, 1995, Ohm Corporation

In such a HTGR fuel manufacturing process, the ammonium heavy uranate particles that have been post-treated (ripening, washing and drying) are roasted to obtain the formula UO _{2 + X} (where x is 0 < uranium oxide particles (hereinafter sometimes referred to as “uranium trioxide particles, etc.”) represented by the following formula: In the process of obtaining uranium particles and then producing the fuel core fine particles by sintering the uranium dioxide particles,
conventionally,
(1) Reduction process and sintering process in the process of raising the temperature from room temperature to a sintering temperature of 1200 ° C. or higher (the relationship between temperature and time in the reduction process and sintering process of this method is shown in FIG. .)
Or
(2) A method in which the reduction treatment is performed while maintaining the temperature in the range of 800 to 1200 ° C., and then the temperature is raised and the sintering treatment is performed at 1200 ° C. or higher (the temperature and time in the reduction treatment and the sintering treatment of this method) The relationship is shown in FIG.
Was adopted.

  However, when such reduction treatment and sintering treatment are performed, there is a problem in that voids, cracks and / or deformations are produced inside the produced fuel core particles.

  FIG. 3 is a photomicrograph showing a cross section of the fuel core particle having voids formed therein. FIG. 4 is a photomicrograph showing a cross section of the deformed fuel core particle. 3 and 4, 1 is a fuel core particle, 2 is a void, and 3 is a deformed portion.

  The fuel core particles having voids inside cause a decrease in strength, and the possibility of destruction when coated with the four coating layers increases. Further, when the deformed fuel core particles are coated with the four coating layers, a uniform coating layer cannot be formed. Therefore, it becomes a fuel core particle which cannot be provided for nuclear fuel in practical use.

  The present invention eliminates such conventional problems, and produces fuel core particles having high strength and high sphericity without causing voids or cracks in the interior and without causing deformation of the particles. The problem is to provide a method.

  In order to solve the above problems, the present inventor has examined the conditions for completely reducing the uranium trioxide particles and the like prior to the sintering treatment of the uranium dioxide particles. The inventors have found that the above problem can be solved by reduction treatment, and have completed the present invention based on this finding.

That is, the means for solving the problems of the present invention are:
After calcining ammonium deuterated uranium particles to obtain uranium oxide particles represented by the formula UO _{2 + X} (where x is an integer satisfying 0 <x ≦ 1), the uranium oxide is obtained. A method for producing fuel core particles, wherein the particles are subjected to a reduction treatment at 400 to 700 ° C. for at least 2 hours and then subjected to a sintering treatment.

  In order to avoid the simultaneous reduction and sintering of the fuel core particle manufacturing method of the present invention, the uranium oxide particles are completely reduced under the above conditions prior to the uranium dioxide particle sintering process. Therefore, there is an effect that the fuel core particles having a large strength can be manufactured without generating voids or cracks in the manufactured fuel core particles. In addition, since the uranium trioxide particles and the like are completely reduced, the sintering process of the uranium dioxide particles is not hindered by water vapor generated during the reduction treatment. Therefore, there is an effect that the fuel core particles to be manufactured can be manufactured without deformation and the fuel core particles having high sphericity can be manufactured.

  Furthermore, since the fuel core particles manufactured by the fuel core particle manufacturing method of the present invention have a high strength and a high sphericity as described above, they are not destroyed when covered with the coating layer. There is also an effect that a uniform coating layer can be formed.

The ammonium heavy uranate particles used in the present invention can be produced, for example, as follows. First, uranium oxide is dissolved in nitric acid to prepare a uranyl nitrate solution. The preparation conditions at this time are not limited, but the molar ratio of nitric acid (HNO ₃ ) / uranium (U) is usually 2.1 to 2.6, preferably 2.3 to 2.5. Is prepared by stirring and mixing at 90 to 140 ° C, preferably 95 to 130 ° C.

  The uranyl nitrate solution thus prepared and the aqueous solution of the thickener are mixed to prepare a stock solution for producing ammonium heavy uranate particles (hereinafter sometimes simply referred to as “stock solution”). Then, this undiluted solution is dropped into an aqueous ammonia solution, whereby ammonium deuterated uranate particles are formed.

  The uranium oxide is preferably dissolved in nitric acid after removing impurities in advance. The thickener is a substance added to adjust the viscosity of uranyl nitrate droplets, which is a stock solution dripped in an aqueous ammonia solution. For example, polyvinyl alcohol has a property of solidifying under alkaline conditions. Resin, polyethylene glycol, and metroses.

  The ammonium heavy uranate particles formed in the aqueous ammonia solution are then subjected to aging treatment, washing treatment and drying treatment to become ammonium heavy uranate particles used in the present invention.

In the method for producing fuel core particles according to the present invention, first, the ammonium heavy uranate particles are roasted to obtain the formula UO _{2 + X} (where x is an integer or decimal that satisfies 0 <x ≦ 1). Uranium oxide particles represented by

  Although there is no restriction | limiting in particular in the said roasting process, Usually, the said heavy ammonium uranate particle | grains are accommodated in an open container, and it roasts in an air atmosphere. Although there is no restriction | limiting in roasting temperature, Usually, it is 400-600 degreeC. There is no restriction on the heat source required for roasting.

By calcination of ammonium heavy uranate particles in this way, uranium oxide particles represented by the formula UO _{2 + X} (where x is an integer or a decimal number satisfying 0 <x ≦ 1) are obtained. Examples of the uranium oxide particles include uranium trioxide (UO ₃ ), diuranium pentoxide (U ₂ O ₅ ), triuranium octoxide (U ₃ O ₈ ), and uranium tetraoxide derived from uranium oxide as a raw material. Mention may be made of (U ₄ O ₉ ) and mixtures thereof.

In the method for producing fuel core particles according to the present invention, the uranium oxide particles thus obtained are subjected to reduction treatment at 400 to 700 ° C. for at least 2 hours to obtain uranium dioxide (UO ₂ ) particles. This is a method of sintering uranium dioxide particles.

  In the reduction method and the sintering treatment method of the conventional method (1) and the conventional method (2), the present inventor reduced and sintered some uranium trioxide particles and the like and uranium dioxide particles. As a result of considering that the phenomenon of simultaneous progress occurs, the reduction temperature is relatively high, the reduction temperature is close to the sintering temperature, or the reduction temperature may partially overlap the sintering temperature. I guessed it was due to this.

  Although the reason why such a relatively high reduction temperature condition has conventionally been used is not clear, it seems that the reduction treatment and the sintering treatment were intended to be performed more quickly and efficiently.

  Thus, prior to the sintering of the uranium dioxide particles, the conditions for completely reducing the uranium trioxide particles and the like, in particular, the temperature conditions, were repeatedly investigated. As a result, the sintering temperature of 1200 ° C. or higher was 400 to 700 ° C. As a result of selecting a temperature condition in a large low temperature region, it was found that uranium trioxide particles and the like can be completely reduced.

  Here, as a mode for completely reducing the uranium trioxide particles and the like, for example, all particles such as uranium trioxide particles are reduced as much as possible, and from the surface layer of individual uranium trioxide particles etc. It can be mentioned that the process is reduced without leaving an unreduced part.

  The reduction treatment is performed in a reducing atmosphere. Examples of the reducing atmosphere include a hydrogen gas atmosphere or an atmosphere containing hydrogen gas. However, as long as uranium trioxide particles and the like can be reduced, the hydrogen gas atmosphere or an atmosphere containing hydrogen gas is used. You will not be bound by

  In the present invention, the reduction treatment is carried out at 400 to 700 ° C., preferably 500 to 600 ° C., for at least 2 hours, usually 2 to 10 hours, preferably 3 to 7 hours. If the reduction treatment temperature is less than 400 ° C, the reduction reaction rate may be reduced or the reduction reaction may not be promoted. If the reduction treatment temperature exceeds 700 ° C, it will be close to the sintering temperature. May occur at the same time. Further, when the reduction treatment time is less than 2 hours, the reduction reaction may not be completed.

  By adopting the conditions for the reduction treatment, it is possible to avoid the reduction and sintering from proceeding at the same time, and no voids or cracks are produced in the produced fuel core particles. In addition, since the uranium trioxide particles and the like are completely reduced, the sintering process of the uranium dioxide particles is not hindered by water vapor generated during the reduction treatment. Therefore, the fuel core particles to be manufactured are not deformed, and the fuel core particles having high sphericity can be manufactured.

  The uranium dioxide particles obtained after finishing the reduction treatment are subjected to a sintering treatment. This sintering treatment is usually carried out by heating at 1200 to 1800 ° C., preferably 1400 to 1700 ° C., usually for 1 to 5 hours. There is no restriction on the heat source required for sintering. FIG. 5 shows the relationship between temperature and time in the reduction treatment and sintering treatment of the method for producing fuel core fine particles of the present invention.

  The ceramic-like uranium dioxide particles obtained by sintering in this way are then classified and can be obtained as fuel core fine particles having a predetermined particle diameter. FIG. 6 is a photomicrograph showing a cross section of the fuel core particles manufactured by the method for manufacturing fuel core fine particles of the present invention. From FIG. 6, the fuel core particle 1 manufactured by the method for manufacturing fuel core fine particles according to the present invention is a particle having no voids or cracks therein, no deformation of the particle, and high sphericity. I understand.

The sphericity of the fuel core particle is obtained by measuring the diameter of the fuel core particle from an arbitrary direction with respect to a certain particle and using the following equation.
Sphericality = (maximum diameter during measurement) / (minimum diameter during measurement)
The sphericity of the fuel core particle is an index representing how close the particle is to the true sphere, and the closer to the true sphere, the closer to 1.00.

Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited to the examples.
(Example)

[Production Example of Ammonium Heavy Uranate Particles]
Uranium oxide powder was dissolved in nitric acid and stirred at 100 ° C. for 1.5 hours to prepare a uranyl nitrate solution (0.76 mol-U / L). A thickener (polyvinyl alcohol aqueous solution) was added to this uranyl nitrate solution and stirred, and tetrahydrofurfuryl alcohol and water were further added to prepare a stock solution for producing ammonium biuranate particles (polyvinyl alcohol concentration 12.5 g / L). Prepared. The viscosity of this stock solution was 53 × 10 ⁻³ P · s (53 cP) at 20 ° C.

Next, the stock solution 24L prepared as described above was supplied to the stock droplet lowering device at a stock solution flow rate of 240 cm ³ / min via a stock solution flow rate adjustment valve by a stock solution feed pump. The undiluted solution supplied to the original droplet lowering device is dropped as a droplet from a nozzle under the original droplet vibrated at a vibration frequency of 75 Hz into a tank storing a 28% aqueous ammonia solution while passing ammonia gas. Ammonium particles were formed.

Subsequently, the ammonium heavy uranate particles formed as described above are accommodated in a post-treatment tank, and the post-treatment tank is rotated and aged at 80 ° C. for 1 hour, and then washed with 80 ° C. water. Then, it was further dried at 100 ° C. for 3 hours to produce dry ammonium biuranate particles.
[Roasting of heavy ammonium uranate particles]

The dried ammonium ammonium uranate particles were roasted at 550 ° C. for 3 hours in the air to produce uranium trioxide particles.
[Reduction treatment of uranium trioxide particles]

The uranium trioxide particles were reduced at 600 ° C. for 3 hours in a hydrogen gas atmosphere to produce uranium dioxide particles.
[Sintering of uranium dioxide particles]

The uranium dioxide particles were sintered at 1500 ° C. for 1 hour to produce ceramic-like uranium dioxide particles.
[Classification and evaluation of uranium dioxide particles]

The uranium dioxide particles were classified (sphericity selection) to obtain fuel core particles having an average sphericity of 1.05 shown in FIG. The uniaxial fracture strength of the fuel core particles was 2.5 kgf.
(Comparative Example 1)

[Example of production of fuel core particles by the conventional method (1)]
Fuel core particles were produced in the same manner as in Example except that the temperature was raised from room temperature to 500 ° C. over 4 hours, and reduction treatment and sintering treatment were performed. The fuel core particles had voids inside as shown in FIG. The average sphericity was 1.05, but the uniaxial fracture strength was 1.0 kgf.
(Comparative Example 2)

[Example of production of fuel core particles by the conventional method (2)]
Fuel core particles were obtained in the same manner as in Example except that the reduction temperature in the reduction treatment of uranium trioxide particles was 900 ° C. and the reduction time was 1 hour. The fuel core particles had been deformed as shown in FIG.

It is a figure which shows the relationship between the temperature and time in the reduction | restoration process and sintering process of the conventional method (1). It is a figure which shows the relationship between the temperature and time in the reduction process and sintering process of the conventional method (2). It is a microscope picture showing the cross section of the fuel core particle which produced the space | gap inside. It is a microscope picture showing the cross section of the deformed fuel core particle. FIG. 5 shows the relationship between temperature and time in the reduction treatment and sintering treatment of the method of the present invention. It is a microscope picture showing the cross section of the fuel core particle manufactured by the method of this invention.

Explanation of symbols

1 Fuel core particle 2 Void 3 Deformation part

Claims (1)

After calcining ammonium deuterated uranium particles to obtain uranium oxide particles represented by the formula UO _{2 + X} (where x is an integer satisfying 0 <x ≦ 1), the uranium oxide is obtained. A method for producing fuel core particles, wherein the particles are subjected to a reduction treatment at 400 to 700 ° C. for at least 2 hours and then subjected to a sintering treatment.

Images