JP4596876B2 - Method for preparing stock solution for production of ammonium deuterated uranate particles - Google Patents

Description

  The present invention relates to a method for preparing a stock solution for producing ammonium heavy uranate particles, and more specifically, a method for preparing a stock solution for producing ammonium heavy uranate particles that can produce fuel nuclei particles having good sphericity with high yield. About.

  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 fuel nuclei (coated fuel particles) covered with the four coating layers are dispersed in a graphite matrix and molded into a fixed fuel compact shape. The fuel compact is a cylinder made of graphite. A fixed quantity is accommodated in the container and plugged up and down to form a fuel rod. Eventually, 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 number of these hexagonal column type graphite blocks in a honeycomb shape and stacking multiple 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, a thickener and, if necessary, water are added to the uranyl nitrate solution and mixed by stirring to prepare a stock solution for producing ammonium heavy uranate particles. The thickener is added to adjust the viscosity of the ammonium heavy uranate particle production stock solution so that the ammonium heavy uranate particle production stock solution can be dripped from the nozzle under the original droplet in the dropping device. Is done.

  The prepared undiluted solution for ammonium biuranate particles is cooled to a predetermined temperature to form a dripping undiluted solution whose viscosity has been adjusted. It is dripped in the ammonia aqueous solution in the ammonia aqueous solution storage tank by vibrating.

  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 ammonia aqueous solution in the ammonia aqueous 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, a process of reacting uranyl nitrate and ammonia completely to the center of the particles by heating to produce ammonium heavy uranate (aging process), a process of washing the ammonium heavy uranate particles with warm water ( Washing process) and drying process (drying process) are performed.

  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 particles having a predetermined particle size.

The fuel core particles obtained in this way are loaded into a fluidized bed and coated by pyrolyzing 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 made of graphite powder and a binder, and then 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

  By the way, in the conventional method described above, it has been difficult to produce uranium dioxide particles having good sphericity. This is because ammonium heavy uranate particles are produced from a stock solution for producing ammonium heavy uranate particles, the resulting ammonium heavy uranate particles are roasted to produce uranium trioxide particles, and the uranium trioxide particles are reduced. In the multistage process of producing ceramic uranium dioxide particles even when sintered, there are many factors that affect the sphericity of the uranium dioxide particles. Therefore, it is not practical as an industrial production method to implement all the steps in consideration of all technical factors that affect the sphericity of uranium dioxide particles in all steps.

  This invention makes it the subject to provide the preparation method of the stock solution for ammonium heavy uranate particle production which can manufacture the fuel core particle which has favorable sphericity with a sufficient yield in view of such a situation.

  In order to solve the above-mentioned problems, the present inventors have studied. In order to produce uranium dioxide particles having good sphericity with high yield, ammonium biuranate particles with good sphericity are produced with high yield. In order to produce ammonium uranate particles having a high sphericity, the present inventors have found that the thickener in the stock solution has a great influence and reached the present invention.

  That is, the means for solving the above-mentioned problems of the present invention is to prepare a dry thickening agent as the thickening agent in preparing a stock solution for producing ammonium biuranate particles comprising uranyl nitrate and a thickening agent. It is a method for preparing a stock solution for producing ammonium biuranium particles, characterized by using a sticking agent.

  In a preferred embodiment of the means of the present invention, the dry thickener is a method for preparing a stock solution for producing ammonium biuranate particles obtained by heat-treating a thickener that has absorbed moisture, and the dried thickener, The preparation method of the stock solution for ammonium biuranate particle manufacture which is a thickener preserve | saved with the desiccant can be mentioned.

  The method for preparing a stock solution for producing ammonium biuranium particles (hereinafter sometimes referred to as “stock solution”) according to the present invention is a thickener that is mixed with uranyl nitrate, and thus is a thickener. The viscous agent can be accurately weighed. As a result, weighing and adding the wet thickener does not result in the preparation of a stock solution having a viscosity lower than the set viscosity, and it is also formed by impact when the stock solution is dropped into an aqueous ammonia solution. There is no deformation of the ammonium heavy uranate particles. Accordingly, it is possible to produce ammonium heavy uranate particles having a good sphericity, thereby producing fuel core particles having a good sphericity with a good yield. Therefore, the place which contributes to the manufacturing field of HTGR fuel is extremely large.

  The method for preparing a stock solution for producing ammonium heavy uranate particles according to the present invention is a method for preparing a stock solution for producing ammonium heavy uranate particles containing uranyl nitrate and a thickener. It is characterized by using an agent.

The stock solution for producing ammonium deuterated uranium particles of the present invention can be prepared by various preparation methods as long as a stock solution containing uranyl nitrate and a thickener is obtained.For example, (1) uranium oxide is converted into nitric acid. A method of mixing a uranyl nitrate solution obtained by dissolving with a thickener aqueous solution containing a thickener; (2) a uranyl nitrate solution obtained by dissolving uranium oxide in nitric acid and a thickener; (3) a method of mixing a uranyl nitrate solution obtained by dissolving uranium oxide in nitric acid and a thickener, and (4) a method of mixing uranium oxide, nitric acid and an aqueous solution of thickener. It is prepared by a method of mixing and synthesizing uranyl nitrate in the system. There is no particular limitation on the preparation conditions for preparing the uranyl nitrate solution by any of the preparation methods (1) to (4), but the molar ratio of nitric acid (HNO ₃ ) / uranium (U) is It is usually 2.1 to 2.6, preferably 2.3 to 2.5, and is usually prepared by stirring and mixing at 90 to 140 ° C, preferably 95 to 130 ° C.

  Examples of the thickener include water-soluble polymer substances such as polyvinyl alcohol, carboxyvinyl polymer, polyvinylpyrrolidone, methylcellulose, hydroxyethylcellulose, hydroxypropylcellulose, polyethylene glycol, metroses, and water-soluble cyclic ether, under alkaline conditions. Examples thereof include a resin having a property of solidifying with a resin. Two or more of these thickeners may be used in combination. Of these various thickeners, water-soluble polymer substances such as polyvinyl alcohol are preferred.

  Since the suitable concentration of the thickener in the thickener aqueous solution varies depending on the type of the thickener, the preferred concentration range cannot be uniquely determined. However, when the thickener is polyvinyl alcohol, the preferred concentration of the thickener in the aqueous solution of the thickener is usually 70 to 85 g / [(polyvinyl alcohol + water) L] of polyvinyl alcohol powder. When the thickener is a thickener other than polyvinyl alcohol, the suitable concentration range can be determined as appropriate empirically depending on the type of the thickener. However, the concentration of the thickener in the aqueous solution of the thickener is appropriately determined so that the viscosity of the stock solution becomes a value described later.

  When mixing uranyl nitrate solution and thickener aqueous solution, especially when mixing uranyl nitrate solution and thickener, the ratio of uranyl nitrate and thickener, temperature during mixing and mixing time are particularly limited However, since these mixing conditions are not uniform depending on the type of thickener used, in short, the mixing is performed according to the type of thickener so that the viscosity of the stock solution becomes a value described later as described above. What is necessary is just to determine conditions suitably. Moreover, what is necessary is just to determine the compounding quantity of uranyl nitrate so that the uranium (U) density | concentration in stock solution may be 0.6-0.9 molU / L. If this concentration is less than the lower limit, it is possible to obtain highly sphericity ammonium heavy uranate particles, but it is difficult to promote the reaction between ammonia and uranyl nitrate, and cracks are formed inside the ammonium heavy uranate particles. May occur. In addition, if the concentration of uranyl nitrate in the stock solution is larger than the upper limit, the viscosity of the stock solution decreases, and when this stock solution is dropped into an aqueous ammonia solution, the probability of deformation increases. The sphericity of the resulting fuel core particles may be lowered. There is no particular limitation on the mixing device for realizing the mixing, for example, stirring for stirring the contents in the storage tank into which the uranyl nitrate solution and the thickener solution or thickener and water as required are added. An apparatus etc. can be mentioned.

  In this invention, in preparing the stock solution for producing ammonium biuranate particles, in this invention, a dried thickener is used as the thickener. The dry thickener is used to accurately weigh the thickener, but it prepares a stock solution for producing ammonium biuranate particles that can produce uranium dioxide particles with good sphericity with good yield. It is to do.

  The dried thickener can be obtained by sufficiently drying the moisture-absorbed thickener. The degree of drying is achieved, for example, by storing the moisture-absorbing desiccant in a drier containing the desiccant and maintaining the vacuum in a high vacuum until no weight loss of the thickener is observed. The degree of drying can be mentioned. Specifically, taking polyvinyl alcohol as an example, polyvinyl alcohol is contained in a desiccator or the like containing a desiccant, for example, and at least all day and night, the inside of the desiccator is maintained at a high vacuum to obtain dried polyvinyl alcohol. Can do. In the same treatment, other moisture-absorbing desiccant can be made the “dried thickener” in the present invention. As a method of drying the thickener, as described above, in addition to storing the thickener together with the desiccant under normal pressure, preferably under high vacuum, the thickened moisture absorbed under normal pressure or reduced pressure. A drying method in which the agent is heated to a high temperature, a freeze-drying method in which freezing and thawing are repeated under a high vacuum, and the like can also be employed. The thickener dried by such a drying method is preferably stored in a dry state so as not to absorb moisture until used in the method of the present invention.

  The processing temperature such as the heating temperature in the heat drying or freeze drying and the processing time such as the heating time are not uniform and are appropriately determined depending on the type of thickener to be heat-treated, the amount handled, the heating method, and the like. What is necessary is just to judge that the dried thickener was obtained in the state in which the mass of the thickener was lost. Heating conditions such that the thickener is deformed, altered or decomposed should not be employed. For example, when using polyvinyl alcohol as a thickener, the heating temperature is 70 to 90 ° C., and the heating time is 20 minutes to 2 hours.

  In the heat treatment of the moisture absorbent that has absorbed moisture, it is not necessary to use a special heating device. However, the thickener that has absorbed moisture may be accommodated in a furnace or a tube, and heat treatment may be performed. A thickener may be laid or a thickener that has absorbed moisture may be accommodated in a bowl and heated by blowing hot air.

  When using a means for using a thickener stored with the desiccant for a certain period, a container containing a desiccant such as activated carbon, activated alumina, silica gel, for example, a desiccator, after being stored for a certain period of time. A thickener is used. Silica gel is preferable as the desiccant, and the storage period is appropriately determined depending on the type of thickener stored, the storage amount, and the like. What is necessary is just to judge that the dried thickener was obtained in the state in which the mass of the thickener was lost. In any case, in the method of the present invention, specifically, a dried thickener obtained by heat-treating a hygroscopic thickener and stored together with the desiccant. A dry thickener is preferably used.

The dried thickener thus obtained is dissolved in water to form a thickener aqueous solution. For example, this thickener aqueous solution, uranyl nitrate solution, additives as necessary, and water as necessary are mixed. Thus, a stock solution is prepared. As said additive, cyclic alcohols, such as tetrahydrofurfuryl alcohol, etc. can be mentioned as a suitable example. The viscosity of a stock solution convenient for producing ammonium uranate particles having good sphericity is usually preferably 40 × 10 ^{−3 to} 65 × 10 ⁻³ Pa · s (40 to 65 cP) at 10 ° C., especially ^{45 × 10 -3 ~60 × 10 -3} Pa · s (45~60cP) are preferred.

  The prepared undiluted solution is cooled to a predetermined temperature to form a dripping undiluted solution whose viscosity is adjusted, and then dripped into the aqueous ammonia solution by vibrating this nozzle from the small-sized undiluted droplet lower nozzle of the original droplet dropping device Is done. In addition, as temperature of the cooled undiluted | stock solution, 5-20 degreeC is preferable normally, and 7-13 degreeC is especially preferable. The reason for cooling to the above temperature range is that there is a merit that the viscosity of the dripping stock solution is made appropriate and the alteration can be prevented depending on the additive used.

  Although there is no restriction | limiting in particular in the density | concentration of the ammonia aqueous solution by which a stock solution is dripped, Usually, it is 25 to 29%, Preferably it is 27 to 29%. The droplet dropped into the aqueous ammonia solution is sprayed with ammonia gas in the space until it reaches the surface of the aqueous ammonia solution, and the droplet surface gels to form a droplet with a film formed thereon. Uranyl nitrate in the stock solution dropped into the aqueous ammonia solution reacts with ammonia to form ammonium deuterated uranate particles.

  The ammonium heavy uranate particles thus produced are then subjected to aging treatment, washing treatment and drying treatment, followed by roasting, reduction and sintering, whereby high-density ceramic-like uranium dioxide particles are obtained. It becomes. The uranium dioxide particles are classified to obtain fuel core particles having a predetermined particle size.

  Since the thickener mixed in the uranyl nitrate solution is a dried thickener, the method for preparing the ammonium biuranate particle manufacturing stock solution of the present invention is essentially a thickener to be used for the mixing. The amount can be accurately weighed. As a result, the viscosity of the stock solution does not decrease, and the formed ammonium heavy uranate particles are not deformed by impact when the stock solution is dropped into the aqueous ammonia solution. For this reason, fuel core particles having good sphericity can be produced with high yield.

The sphericity of the fuel core particle is obtained by the following equation by measuring the diameter of the fuel core particle many times from an arbitrary direction for a certain particle.
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. The said measurement is implemented about 100 particle | grains, for example, average sphericity is calculated, and this average sphericity is evaluated.

  The average sphericity of the fuel core particles produced using the stock solution prepared by the method for preparing the stock solution for producing ammonium biuranium particle according to the present invention is superior to 1.05, and has this sphericity. The yield of fuel core particles reaches 99% by mass or more.

Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited to the examples.
Example 1

Uranium oxide powder was dissolved in nitric acid and stirred at 100 ° C. for 1.5 hours to prepare a uranyl nitrate solution (2.4 mol-U / L). A 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 alcohol uranate particles having a polyvinyl alcohol powder concentration of 12.5 g / L. The viscosity of this stock solution was 53 × 10 ⁻³ Pa · s (53 cP) at 10 ° C.

  The thickener used at this time was preheated at 85 ° C. for 50 minutes, completely dried, and weighed.

Next, the stock solution 24L prepared as described above was supplied to the stock droplet dropping 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 raw solution supplied to the original liquid dropping device is dropped as a liquid droplet in an ammonia gas atmosphere from a nozzle under the original liquid droplet oscillated at a frequency of 75 Hz into a tank storing a 28% aqueous ammonia solution. Particles were produced.

  Subsequently, the ammonium heavy uranate particles produced 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. This was treated and further washed with ethyl alcohol at 70 ° C. for 30 minutes and then dried at 100 ° C. for 3 hours to obtain dry ammonium uranate particles.

  Thereafter, the dried ammonium ammonium uranate particles obtained as described above were roasted in the atmosphere at 550 ° C. for 3 hours to produce uranium trioxide particles. The uranium trioxide particles were reduced at 600 ° C. for 3 hours under a reducing atmosphere (hydrogen-nitrogen mixed gas atmosphere), and further sintered at 1550 ° C. for 1 hour to obtain ceramic-like uranium dioxide particles. Got.

As a result of classification (outer diameter selection and sphericity selection) of the uranium dioxide particles obtained as described above, the average sphericity of the fuel core particles was 1.04. The yield of fuel core particles was 99.1% by mass.
(Example 2)

Fuel core particles were produced in the same manner as in Example 1 except that polyvinyl alcohol powder stored for 3 days in a desiccator loaded with silica gel was used as a thickener. However, the viscosity of the stock solution was 51 × 10 ⁻³ Pa · s (51 cP) at 10 ° C. As a result, the average sphericity of the fuel core particles was 1.05. The yield of fuel core particles was 98.5% by mass.
(Comparative example)

Fuel core particles were produced in the same manner as in Example 1 except that polyvinyl alcohol powder containing 9% by mass of water was used as the thickener. However, the viscosity of the stock solution was 44 × 10 ⁻³ Pa · s (44 cP) at 10 ° C. As a result, the average sphericity of the fuel core particles was 1.08. Further, the yield of the fuel core particles was 92% by mass.

While the yield of the fuel core particles having high sphericity produced by Examples 1 and 2 is good, the yield in the comparative example is inferior because the thickener is not accurately weighed. It is surmised that the viscosity of the stock solution for producing ammonium heavy uranate particles was reduced by using a thickening agent in an amount smaller than the above amount.

Claims (3)

  Preparation of ammonium heavy uranate particles characterized in that a dry thickener is used as the thickener in preparing a stock solution for producing ammonium heavy uranate particles comprising uranyl nitrate and a thickener. For preparation of the undiluted solution.   The method for preparing a stock solution for producing ammonium biuranate particles according to claim 1, wherein the dried thickener is obtained by heat-treating a thickened moisture absorbent. The method for preparing a stock solution for producing ammonium biuranate particles according to claim 1, wherein the dried thickener is a thickener stored together with the desiccant.