Method of controlling the crystal grain size of uranium dioxide pellet

Abstract: A method of controlling the crystal grain size of UO₂ pellets is disclosed, which comprises (1) providing an aqueous uranyl solution which is free of hydrofluoric acid and nitric acid and which contains uranyl fluoride (UO₂F₂) and/or uranyl nitrate (UO₂(NO₃)₂) as a uranium component, (2) reacting with ammonia said aqueous uranyl solution to precipitate ammonium diuranate (ADU), while adjusting the ratio of said uranium components to a predetermined value within the range varying from the ratio 100% of uranyl fluoride and 0% of uranyl nitrate to the ratio 0% of uranyl fluoride and 100% of uranyl nitrate, adjusting the uranium concentration of the reaction mixture to 50 to 1,000 gU/liter and also adjusting the rate of contact between said uranium component in said aqueous uranyl solution with ammonia to at least 2 moles NH₃/min/mole U, (3) calcining and reducing said ADU precipitate to form UO₂ powder, and (4) molding and sintering said UO₂ powder, thus producing UO₂ pellet having a predetermined crystal grain size in the range varying from 5 to 100 micrometers.

Description: This invention relates to improvement in a method of fabricating uranium oxide powder suitable for the production of nuclear fuel starting from uranium hexafluoride more particularly to improvement in an ammonium diuranate method. The ammonium diuranate (ADU) method is widely known as a method for producing from uranium hexafluoride (UF 6 ) uranium dioxide (UO 2 ) powder which is suited for the production of nuclear fuel. Most popular example of the ADU method is a method which comprises reacting UF 6 gas with water to form an aqueous uranyl fluoride (UO 2 F 2 ) solution, reacting the aqueous UO 2 F 2 solution with ammonia (NH 3 ) to precipitate ADU, filtering, drying, calcining and reducing the ADU precipitate to convert the thus processed ADU precipitate to UO 2 powder. In the above-mentioned conventional method ADU is produced by reaction steps as shown in the following reaction schemes. UF 6 + 2H 2 O → UO 2 F 2 + 4HF UO 2 F 2 + 4HF + 7NH 4 OH → (1/2)(NH 4 ) 2 U 2 O 7 + 6NH 4 F + (11/2)H 2 O In the above method, as shown in the reaction scheme (I), there is 4 moles of hydrogen fluoride (HF) per mole of uranium (U) in the aqueous UO 2 F 2 solution prepared by the hydrolysis of UF 6 . In this case, in the production of ADU as shown in the reaction scheme (II), at first ammonium fluoride (NH 4 F) is formed by neutralization with HF and as the result the ADU thus formed is in the form of primary grains whose grain size is relatively large. The greater the grain size of the primary grains of ADU the greater the grain size of the primary grains of UO 2 powder prepared from the ADU, which means that the UO 2 powder has a relatively low activity. When sintered pellets for nuclear fuel are produced from such UO 2 powder with a relatively low activity the sintered density of the pellets which can be produced under generally employed conditions is about 95 % TD (theoretical density) and the grain size of the sintered pellet is about 10 micrometers. Examples of the conventional ADU methods include, in addition to the above-described method in which ADU is produced from an aqueous UO 2 F 2 , a method in which ADU is produced from an aqueous uranyl nitrate (UO 2 (NO 3 ) 2 ) solution. This method comprises hydrolyzing UF 6 gas in a nitric acid solution of a defluorinating agent to form an aqueous UO 2 (NO 3 ) 2 solution, purifying the solution by solvent extraction, reacting the purified solution with ammonia to produce ADU, and then converting ADU to UO 2 powder in the same manner as in the above-described ADU method. With this method it is possible to obtain UO 2 powder having a very high activity and in this case it is possible to obtain sintered pellets having a high sintered density as high as 99 % TD. However, with this highly active UO 2 powder the grain size of the sintered pellets to be obtained is at most in the order of 20 micrometers. In order to obtain sintered pellets with greater grain size from UO 2 powder obtained by this type of ADU method it is necessary to take various measures such as use of elevated sintering temperature, prolonged sintering time, etc., which are however practically difficult to achieve. When using UO 2 pellets as nuclear fuel it is necessary for the fuel to burn stably during irradiation. In this case, one of the indices for showing the state expressed by the term "burn stably" is that fission product gas (FP gas) is retained within the pellet as much as possible, or as little as possible an FP gas is released from the pellet. From the results of irradiation tests a tendency has been confirmed that the greater the grain size of the pellet the more excellent the ability of the pellet to retain FP gas. However, when the pellet has too great a grain size there is a possibility that there occurs harmful effects such as reduction in the mechanical strength, and therefore the grain size must be selected or adjusted appropriately. Although it has not yet been clarified completely as to which range of crystal grain size is appropriate it is presumed practically sufficient to use pellets with a grain size of up to 100 micrometers which is set up as a standard for the present. When UO 2 powder obtained by the conventional ADU method is used it is difficult to produce UO 2 pellets with a crystal grain size of greater than 20 micrometers in ordinary sintering methods and it is further difficult to produce UO 2 pellets having an appropriate crystal grain size. Japanese published application 62-297 215 discloses a method for controlling the crystal particle diameter of UO 2 pellets. The method comprises reacting an agueous solution containing UO 2 F 2 and optionally HF with ammonium to precipitate ADU. The U concentration of the aqueous solution lies in the range 50-1000 g of U/litre. The ADU precipitate is filtered, dried, baked and reduced to prepare UO 2 pellets having a crystal particle diameter of 10-100 µm. AIChE Symp. Ser. 1984 pp 123-126 discloses that ADU particles formed from a uranyl fluoride solution are much coarser than those formed from a uranyl nitrate solution at comparable concentrations. Accordingly ADU particle size distribution can be controlled by adjusting the uranyl fluoride to uranyl nitrate solution in the feed stream. SUMMARY OF THE INVENTION An object of this invention is to obviate the above-described disadvantages of the conventional methods and provide a method of controlling the crystal grain site of UO 2 pellets to 20 to 100 micrometers. As a result of intensive investigation it has now been found that the above-described disadvantages of the prior arts can be overcome by using an aqueous uranyl solution which is free of hydrofluoric acid and nitric acid and adjusting the composition of the uranium component, the concentration of uranium and the speed at which ammonia is contacted with the aqueous uranyl solution to specific ranges. Therefore, this invention provides a method of controlling the sintered density and crystal grain size of UO 2 pellets comprising: (1) providing an aqueous uranyl solution which is free of hydrofluoric acid and nitric acid and which contains both uranyl fluoride (UO 2 F 2 ) and uranyl nitrate (UO 2 (NO 3 ) 2 ) as uranium components, (2) reacting with ammonia said aqueous uranyl solution to precipitate ammonium diuranate (ADU), while adjusting the ratio of said uranium components to a predetermined value, adjusting the concentration of uranium in the reaction mixture to a predetermined value within the range 50 to 1,000 gU/liter and also adjusting the rate of contact between said uranium components in said aqueous uranyl solution with ammonia to a predetermined value of at least 2 moles NH 3 /min/mole U, the crystal grain size of the pellets being adjusted to the desired value upwards by decreasing the relative amount of uranyl nitrate in the uranyl solution and/or increasing the rate of contact between the uranium components and ammonia or downwards by increasing the relative amount of uranyl nitrate in the uranyl solution and/or decreasing the rate of contact between the uranium components and ammonia, (3) calcining and reducing said ADU precipitate to form UO 2 powder, and (4) molding and sintering said UO 2 powder, thus producing UO 2 pellets having a predetermined crystal grain size in the range varying from more than 20 to 100 micrometers and a sintered density of 99% of the theoretical density. DETAILED DESCRIPTION OF THE INVENTION The method of this invention differs in the step of precipitating ADU from the conventional ADU method. In the conventional ADU method, the aqueous uranyl solution used is an aqueous UO 2 F 2 solution containing HF or a UO 2 (NO 3 ) 2 solution, with the result that the crystal grain size of pellets produced from ADU through UO 2 powder has been at most 20 micrometers. On the contrary, this invention uses as an aqueous uranyl solution an aqueous UO 2 F 2 solution free of HF and nitric acid. Use of UO 2 F 2 substantially containing no HF as the aqueous uranyl solution enables production of pellets having a very large crystal grain size. When an aqueous UO 2 F 2 solution not containing HF is reacted with ammonia the resulting ADU is composed of very fine primary grains because of lack of neutralization reaction which would occur in the presence of HF. The production of ADU involves formation of NH 4 F (cf. reaction scheme (II) above). NH 4 F thus formed serves as a binder for causing the very fine primary grains of ADU to adhere to each other, and therefore, the crystal grain size of UO 2 pellet produced from the fine primary grain ADU containing NH 4 F through UO 2 powder is very large; it is possible to produce pellets whose crystal grain size is as large as 100 micrometers if the production conditions are adjusted or controlled appropriately. Production of pellets with different crystal grain sizes starting from an aqueous UO 2 F 2 solution which is free of HF can be attained by controlling, on one hand, the concentration of uranium in the aqueous UO 2 F 2 solution and, on the other hand, the speed of contact at which the uranium component is contacted with ammonia. More particularly, the lower the concentration of uranium, on one hand, and the higher the speed of contact of the uranium component with ammonia, on the other hand, the smaller the primary grain of ADU, resulting in the greater grain size of pellets produced from UO 2 powder through ADU. Thus, sintered UO 2 pellets having a desired crystal grain size in the range of 20 to 100 micrometers can be obtained by controlling the concentration of uranium to a predetermined value within the range varying from 50 to 1000 gU/liter, preferably 100 to 500 gU/liter, and the speed of contact of the uranium component with ammonia to a predetermined value not lower than 2 moles NH 3 /min/mole U, preferably not higher than 30 moles NH 3 /min/mole U. Use of precipitation conditions outside the above-described ranges should be avoided from the following reasons. That is, the concentration of uranium of below 50 gU/liter is practically unacceptable since it leads to increase in the size of apparatus system used and in the amount of liquid waste. The concentration of uranium of above 1,000 gU/liter is undesirable since it causes the viscosity of the aqueous uranyl solution to become too high to be practical, with the result that homogeneous ADU is difficult to produce. It is practically undesirable to set up the speed of contact at which the uranium component and ammonia are brought in contact with each other to a level below 2 moles NH 3 /min/mole U because such a low speed of contact results in drastic decrease in the speed of production of ADU, which is practically unacceptable. When an aqueous UO 2 F 2 solution free of HF is used as a starting material it is generally easy to produce pellets having a high sintered density and a large crystal grain size simultaneously. However, it is difficult to produce pellets having a high sintered density but a small crystal grain size. This problem can be overcome by using an aqueous UO 2 (NO 3 ) 2 solution. That is, when the aqueous UO 2 (NO 3 ) 2 solution is used as a starting material pellets having a high sintered density as high as 99 % TD and a relatively small crystal grain diameter as small as about 20 micrometers can be produced; both the sintered density and crystal grain size of sintered pellets can be controlled by using as a starting material an aqueous mixed solution containing UO 2 F 2 and UO 2 (NO 3 ) 2 as a uranium component. When it is desired to obtain pellets with smaller crystal grain density it is sufficient to use an aqueous uranyl solution containing larger amount of UO 2 (NO 3 ) 2 . This invention will be described in greater detail with reference to examples which are by way of example and do not limit in any way the scope of this invention. EXAMPLE UO 2 F 2 powder was dissolved in deionized water to obtain an aqueous UO 2 F 2 solution. On the other hand, crystalline uranyl nitrate was dissolved in deionized water to obtain an aqueous UO 2 (NO 3 ) 2 solution. The aqueous UO 2 F 2 solution was mixed with the aqueous UO 2 (NO 3 ) 2 solution in a ratio of 2 : 1 or 1 : 1 to produce two types of aqueous uranyl nitrate solutions. After adjusting the concentration of uranium of the solutions to predetermined values in the range of 50 to 1,000 gU/liter, ammonia water was added to the mixed solutions at a rate of 2 to 30 moles NH 3 /min/mole U while stirring until pH 10.5 is reached to form precipitate of ADU. After washing with water, filtration and drying, the ADU precipitate was calcined and reduced in H 2 atmosphere at 650 °C for 2 hours to form UO 2 powder. The UO 2 powder was molded at a compacting pressure of 4 t/cm 2 and then sintered in H 2 atmosphere at 1,750 °C for 4 hours to form sintered pellets. The sintered density and crystal grain size of the UO 2 pellets obtained are shown in Table below. Conditions for Forming ADU Precipitate UO 2 Pellet Aqueous Uranyl Solution Uranium Component Uranium Concentration (gU/liter) Speed of Addition of Ammonia Water (mole NH 3 /min/mole U) Sintered Density (%TD) Crystal Grain Size (µm) UO 2 F 2 50 30 99.2 97 100 30 98.8 60 500 30 96.4 16 1000 30 94.9 6 100 10 97.3 43 100 2 96.5 14 UO 2 F 2 : UO 2 (NO 3 ) 2 = 2 : 1 100 30 98.9 55 UO 2 F 2 : UO 2 (NO 3 ) 2 = 1 : 1 100 30 99.0 38 UO 2 (NO 3 ) 2 100 30 99.3 20 From the results shown in Table above it can be seen that the sintered density and crystal grain size of pellets decrease according as the concentration of uranium increases and according as the speed of addition of ammonia water decreases. In addition, it is possible to decrease only the crystal grain diameter by increasing the content of the UO 2 (NO 3 ) 2 component in the aqueous uranyl solution with retaining the sintered density at a high level as high as 99 % TD.

Claims: A method of controlling the sintered density and crystal grain size of UO 2 pellets comprising: (1) providing an aqueous uranyl solution which is free of hydrofluoric acid and nitric acid and which contains both uranyl fluoride (UO 2 F 2 ) and uranyl nitrate (UO 2 (NO 3 ) 2 ) as uranium components, (2) reacting with ammonia said aqueous uranyl solution to precipitate ammonium diuranate (ADU), while adjusting the ratio of said uranium components to a predetermined value, adjusting the concentration of uranium in the reaction mixture to a predetermined value within the range 50 to 1,000 gU/liter and also adjusting the rate of contact between said uranium components in said aqueous uranyl solution with ammonia to a predetermined value of at least 2 moles NH 3 /min/mole U, the crystal grain size of the pellets being adjusted to the desired value upwards by decreasing the relative amount of uranyl nitrate in the uranyl solution and/or increasing the rate of contact between the uranium components and ammonia or downwards by increasing the relative amount of uranyl nitrate in the uranyl solution and/or decreasing the rate of contact between the uranium components and ammonia, (3) calcining and reducing said ADU precipitate to form UO 2 powder, and (4) molding and sintering said UO 2 powder, thus producing UO 2 pellets having a predetermined crystal grain size in the range varying from more than 20 to 100 micrometers and a sintered density of 99% of the theoretical density. A method as claimed in Claim 1, wherein said rate of contact between said uranium components in said aqueous uranyl solution with ammonia is adjusted to a predetermined value of 2 to 30 moles NH 3 /min/mole U. A method as claimed in Claim 1, wherein the concentration of uranium in said reaction mixture is between 100 and 500 gU/liter.

Inventors: HAGINO SADAAKI, TANAKA HIROSHI, YATO TADAO, HAGINO, SADAAKI, TANAKA, HIROSHI, YATO, TADAO
Applicants: MITSUBISHI MATERIALS CORP, MITSUBISHI MATERIALS CORPORATION
Application year: 1987
Main classification: C01G43
IPC codes: G21C3, C01G43