METHOD OF PREPARING URANIUM DIOXIDE POWDER FROM URANIUM HEXAFLUORIDE

Abstract: A method of fabricating uranium dioxide (UO₂) powder from uranium hexafluoride (UF₆) is disclosed, which comprises (1) reacting UF₆ gas with steam with controlling the temperature of reaction between said UF₆ gas and said steam at a predetermined temperature within the range of 200 to 700 °C, to form solid uranyl fluoride (UO₂F₂) and/or uranium oxide with an 0/U ratio (oxygen-to-uranium atomic ratio) of 2.7 to 3, (2) dissolving said UO₂F₂ and/or uranium oxide in water or nitric acid to form an aqueous uranyl solution containing UO₂F₂ and/or uranyl nitrate (UO₂(NO₃)₂), (3) reacting said aqueous uranyl solution with ammonia to precipitate ammonium diuranate (ADU), (4) filtering said precipitate, (5) drying said precipitate, (6) calcining said dry precipitate, and (7) reducing said calcined precipitate, whereby controlling the characteristics of said UO₂ powder.

Description: This invention relates to a method of fabricating uranium dioxide powder useful as nuclear fuel by conversion of uranium hexafluoride, more particularly an improvement in the ammonium diuranate (ADU) method. Making a general classification of methods of converting uranium hexafluoride (UF₆) into uranium dioxide (UO₂) powder useful as nuclear fuel, there are two types of methods, i.e., wet methods and dry methods. Representative examples of the wet methods include a so-called ammonium diuranate (ADU) method in which UF₆ gas is reacted with water to form an aqueous uranyl fluoride (UO₂F₂) solution, which is then reacted with ammonia to precipitate ammonium diuranate (ADU) and the precipitate is subjected to various steps such as filtration, drying, calcination, reduction, etc. to obtain UO₂ powder. Ammonium diuranate is prepared according to the following reaction schemes (I) and (II). UF₆ + 2H₂O → UO₂F₂ + 4HF (I) UO₂F₂ + 4HF + 7NH₄OH → 1/2(NH₄)₂U₂O₇ + 6NH₄F + 11/2H₂O (II) In the above-described method, there are four moles of hydrogen fluoride (HF) per mole of uranium (U) in the aqueous UO₂F₂ solution formed by the hydrolysis of UF₆ as shown in formula (I) above. This is disadvantageous in two respects. Firstly, the above-described method is difficult to give rise to UO₂ powder having a very high activity. On the other hand, fluorine is not utilized effectively and is wasted in large amounts. On the first point, in the formation reaction of ADU represented by reaction scheme (II), firstly ammonium fluoride (NH₄F) is formed by neutralization reaction with HF, resulting in that primary grains of ADU become relatively large. UO₂ powder obtained by subjecting ADU composed of large primary grains to various steps such as calcination, reduction, etc. is composed of large primary grains as in the case of ADU. This means that the activity of the powder is relatively low. When sintered pellets, i.e., nuclear fuel elements, are produced from such UO₂ powder having a relatively low activity, the sintered density of pellets produced under ordinary conditions is about 95% TD (theoretical density) and the pellets are composed of crystals having a grain size of an order of 10 micrometers. When it is desired to obtain pellets with sintered density and grain size greater than those described above, attempts to carry out sintering at elevated temperatures or for a prolonged period of time would be successful to some extent although it is practically difficult to obtain pellets with a grain size of 40 to 50 micrometers, for example. On the second point, fluorine (F) is at first converted into ammonium fluoride (NH₄F) and is contained in the filtrate after filtering off ADU precipitate. The liquid waste which contains NH₄F is treated with a precipitant such as slaked lime (Ca(OH)₂). With this treatment fluorine component is precipitated as calcium fluoride (CaF₂) and ammonia (NH₃) is recovered and recycled. On the other hand, CaF₂, which is formed in a very large amount e.g., as large as about 1 ton per ton of uranium treated, is left as solid waste without being further recycled. In the conventional ADU methods, it has been difficult to obtain UO₂ powder having a high activity due to adverse influence of fluorine, and in addition they are disadvantageous in that a large amount of fluorides are formed as wastes. Examples of wet methods include, besides the above-described ADU method, an ammonium uranyl carbonate (AUC) method, a modified ADU method, etc. The AUC method is a method in which UF₆ gas is reacted with ammonia, carbon dioxide and water to precipitate ammonium uranyl carbonate (AUC) and then subjected to steps of filtration, drying, calcination, reduction, etc. substantially in the same manner as in the ADU method to obtain UO₂ powder. On the other hand, the modified ADU method comprises hydrolyzing UF₆ gas in a nitric acid solution containing a defluorinating agent to obtain an aqueous UO₂(NO₃)₂, purifying the solution by solvent extraction, reacting the purified solution with ammonia to form ADU and subjecting the resulting ADU to the same treatments as in the ADU method, thus converting the ADU into UO₂. Although these methods enable fabrication of UO₂ powder having activity higher than that of the powder obtained according to ordinary ADU method, they not only fail to give rise to UO₂ powder with a grain size of larger than about 20 micrometers but also they suffer substantially the same disadvantage as encountered in the ordinary ADU method with respect to the occurrence of fluoride wastes. More particularly, in the AUC method fluorine remains in the final product as CaF₂ as in the ADU method while in the modified ADU method fluorides of the defluorinating agent, e.g., aluminum fluoride when aluminum nitrate is used as a defluorinating agent, are found in the final product. The dry method, which is excellent since it is free of the above-described defects such as formation of a large amount of fluoride waste in the wet ADU method, is represented by the following reaction schemes. UF₆ + 2H₂O → UO₂F₂ + 4HF (III) UO₂F₂ + H₂ → UO₂ + 2HF (IV) In the above reactions, at first UF₆ is reacted with steam to form UO₂F₂ which is solid, with HF being separated as gas therefrom. Then, UO₂F₂ is reduced with hydrogen gas to give rise to UO₂ powder. In this method, all the fluorine component is recovered as HF gas in contrast to the wet method in which the fluorine component goes to liquid waste which causes the occurrence of fluoride wastes. However, according to the dry method, it is generally difficult to control the characteristics of UO₂ to be obtained and only UO₂ powder having activity lower than that of the powder from wet ADU method is obtained. Chemical Abstracts, Vol. 108, 11. 01. 88, 12919t discloses a process for preparing UO₂ from UF₆ by initially reacting the UF₆ with steam in the gas phase to form a mixture of UO₂F₂ and HF. The UO₂F₂ is dissolved in water followed by the addition of aqueous ammonia to precipitate ADU. The ADU is then filtered, dried, calcined and reduced in turn to form UO₂ powder. FR-A-2582641 discloses a similar process to the above Chemical Abstract of converting UF₆ to UO₂ powder. However, this reference does not mention the possibility of dissolving the intermediate product UO₂F₂ in water or nitric acid prior to the step of reacting it with ammonia. SUMMARY OF THE INVENTION An object of this invention is to obviate the above-described defects of the prior arts and to provide a method of fabricating UO₂ powder which method enables fabrication of UO₂ powder having a high activity and a highly controlled characteristics. Another object of this invention is to provide a method in which the occurrence of fluoride wastes is minimized. According to this invention, the above objects are attained by a method of fabricating uranium dioxide (UO₂) powder from uranium hexafluoride (UF₆), comprising (1) reacting UF₆ gas with steam with controlling the temperature of reaction between UF₆ gas and steam at a predetermined temperature within the range of 350 to 700 °C, to form solid uranyl fluoride (UO₂F₂) and/or uranium oxide with an O/U ratio (oxygen-to-uranium atomic ratio) of 2.7 to 3, (2) dissolving said UO₂F₂ and/or uranium oxide in nitric acid to form an aqueous uranyl solution containing UO₂F₂ and/or uranyl nitrate (UO₂(NO₃)₂), (3) reacting said aqueous uranyl solution with ammonia to precipitate ammonium diuranate (ADU), (4) filtering said precipitate, (5) drying said precipitate, (6) calcining said dry precipitate, and (7) reducing said calcined precipitate, whereby controlling the characteristics of the UO₂ powder. This invention thus overcomes the above-described disadvantages of the prior arts by reducing or minimizing the occurrence of fluoride wastes which would be formed in large amounts in the conventional wet method and increasing the activity of the product and enabling control of the characteristics of the product. BRIEF DESCRIPTION OF THE DRAWING Single figure is a diagrammatic illustration of a system for carrying out the method according to an embodiment of this invention. DETAILED DESCRIPTION OF THE INVENTION Hereinafter, this invention will be described with reference to single figure which is a diagrammatic illustration of the apparatus used in an embodiment of this invention. In the figure, UF₆ as a starting material is vaporized in a vaporizer 1 and the resulting gas is introduced by injecting or jetting it into a fluidized bed reactor 2 through a side nozzle 2a provided on the side wall of the reactor 2. On the other hand, steam, which is used as a reaction gas and at the same time as a fluidizing gas, is introduced into the reactor 2 through a bottom nozzle 2b provided on the bottom of the reactor 2 through a conduit 3 communicating to the bottom nozzle 2b. If desired, nitrogen gas can be used as a fluidizing gas and introduced into the reactor 2 together with steam in order to improve the fluidization of the fluidized bed. In the fluidized bed reactor 2, UF₆ gas is contacted with steam or a mixed gas composed of steam and nitrogen gas and allowed to react with each other at a temperature of 350 to 700 °C and three types of uranium compounds are formed depending on the reaction temperature. When the reaction temperature is in the range of higher than 200 °C and not higher than 350 °C, UO₂F₂ powder is formed. When the reaction temperature is below 200 °C, UO₂F₂ is formed, but it takes a long time for the reaction to proceed and thus such low reaction temperature should not be used. At a little higher reaction temperature, i.e., higher than 350 °C and not higher than 500 °C, mixed powder composed of UO₂F₂ and a uranium oxide with an O/U ratio of 3 is formed. When the reaction temperature is in the range of higher than 500 °C and lower than 700 °C a uranium oxide with an O/U ratio of 2.7 to 3 is formed. In the third case, the higher the reaction temperature the smaller the O/U ratio. Use of reaction temperatures above 700 °C should be avoided because such high temperatures accelerate corrosion of the apparatus used. The powder produced in the reactor 2 is once stored in a hopper 4 and then is introduced into a dissolving apparatus 5 in which the powder is dissolved in water, preferably deionized water or distilled water, supplied through a water supply conduit 6 when the powder is composed of UO₂F₂, dissolved in nitric acid supplied through a nitric acid supply conduit 7 when the powder is mixed powder composed of UO₂F₂ and uranium oxide specified above or when the powder is composed of the specified uranium oxide exclusively. In the above three cases, an aqueous UO₂F₂ solution, an aqueous mixed solution of UO₂F₂ and UO₂(NO₃)₂, and an aqueous UO₂(NO₃)₂ solution are produced correspondingly. The amounts of nitric acid used in the second and third cases, respectively, are as small as possible as far as the uranium oxide is dissolved. HF gas by-produced in the fluidized bed reactor 2 is recovered as an aqueous HF solution in an HF condenser 8a and received by an HF reservoir 8b. The aqueous uranyl solution containing UO₂F₂ and/or UO₂(NO₃)₂ produced in the dissolving apparatus 5 is sent to a precipitation apparatus 9 where it is reacted with ammonia supplied through an ammonia supply conduit 10 to form ADU precipitate. The ADU precipitate is filtered and dried in a filtration and drying apparatus 11 and then sent to a calcination and reduction apparatus 12 where the precipitate is calcined and reduced with a reducing gas (H₂ or a mixed gas composed of H₂ and N₂) containing steam supplied through a reducing gas supply conduit 13 and converted into UO₂ powder. The UO₂ powder obtained is stored in a UO₂ reservoir or container 14 until use. Waste gas from the calcination and reduction apparatus 12 and off-gas from the HF condenser 8a are sent to an exhaust gas treatment system 15 which may be conventional one. As described above, one of the features of the method of this invention is that while in the conventional wet method the entire fluorine content derived from UF₆, starting material, finally goes into wastes in the form of fluorides the dry type hydrolysis of UF₆ utilized in this invention makes it possible to recover fluorine as HF, which is useful, in amounts ranging from about 2/3 to its entirety with minimizing or completely avoiding the occurrence of fluoride wastes. Another important feature of the method of this invention is that after the dry type hydrolysis of UF₆, the product is dissolved in water or nitric acid to form an aqueous uranyl solution containing UO₂F₂ and/or UO₂(NO₃)₂, the uranyl solution being used as a starting material for producing ADU, with the result that the characteristics of the UO₂ powder finally obtained can be controlled with ease. In other words, the characteristics of the UO₂ powder are determined mostly depending on the conditions of ADU precipitation among which particularly important is the properties of the aqueous uranyl solution. In this invention, control of the temperature of reaction between UF₆ and steam leads to control of the ratio of UO₂F₂ and uranium oxide, which ratio in turn controls the ratio of UO₂F₂ and UO₂(NO₃)₂ in the aqueous uranyl solution. When UO₂ powder is prepared from the UO₂F₂ solution or the UO₂(NO₃)₂ solution through ADU under similar conditions the UO₂ powder obtained from the UO₂(NO₃)₂ solutions has activity higher than that of the UO₂ powder derived from the UO₂F₂ solution. This is because the primary grains of ADU obtained from the UO₂(NO₃)₂ solution are smaller than those of ADU derived from the UO₂F₂ solution. On the other hand, from the viewpoint of the speed of crystal growth upon molding UO₂ powder and pellet formation by sintering the UO₂ powder, the UO₂ powder prepared using the aqueous UO₂F₂ solution shows crystal growth speed higher than that of the powder derived from the aqueous UO₂(NO₃)₂ solution. This is because NH₄F is also formed when ADU is produced from the aqueous UO₂F₂ solution and this NH₄F serves to increase agglomerability of the resulting powder. Therefore, it is possible to control fundamentally important characteristics of the UO₂ powder, i.e., activity and crystal growth speed, of the UO₂ powder at desired levels by appropriately controlling or adjusting the ratio of UO₂F₂ and UO₂(NO₃)₂ in the aqueous uranyl solution which ratio can be adjusted by appropriately selecting the temperature of reaction between UF₆ and steam. This invention will be descried in greater detail with reference to example which is by way of an example and does in no way limit this invention. EXAMPLE A fluidized bed reactor comprising a reaction portion having a diameter of 8.3 cm was used and the operation was performed under the following conditions. Speed of UF₆ treatment 5 kg U/hr. Linear speed of steam gas: 15 cm/sec. Reaction temperature: 280 °C, 400°C, 700°C When the product obtained in the fluidized bed apparatus was UO₂F₂ it was dissolved in water. On the other hand, when the product contained uranium oxide it was dissolved in nitric acid to give an aqueous uranyl solution. The aqueous uranyl solution together with ammonia water was introduced in a precipitation apparatus with adjusting pH to 10.5 to form precipitation of ADU and then the precipitate was filtered and dried. When the filtrate contained NH₄F the aqueous uranyl solution was treated with slaked lime to recover the fluorine content as CaF₂. The ADU thus obtained was subjected to calcination and reduction in H₂ stream containing steam at 650 °C to convert it to UO₂ powder. After molding it at a compacting pressure of 4 t/cm² the UO₂ powder was sintered at a temperature of 1,750 °C to obtain sintered pellets, which were then examined for their ceramic processability to see if they are acceptable as nuclear fuel. On the other hand, an aqueous uranyl solution was prepared according to the conventional (ADU) method and the aqueous uranyl solution was treated in the same way as above to prepare UO₂ powder through ADU to finally obtain pellets for comparison. Main test conditions and test results obtained are shown in Table below. Invention Conventional Temperature of reaction between UF₆ and steam (°C) 400 700 - Reaction product UO₂F₂+UO₃ U₃O₈ - Composition of uranyl solution UO₂F₂+UO₂(NO₃)₂ UO₂(NO₃)₂ UO₂F₂+HF Amount of ammonia water (kgNH₃/hr) 3.3 3.3 7.5 Amount of CaF₂ (kgCaF₂/hr) 0.8 0 4.7 Specific area of UO₂ powder (m²/g) 8.4 10.2 2.7 Sintered density (%TD) 98.2 98.3 95.2 Crystal grain size (µm) 31.3 20.1 8.8 From the results shown in Table above, it can be seen that in the method of this invention different products were formed at different temperatures of reaction between UF₆ and steam. At 280 °C UO₂F₂ was formed while a mixture of UO₂F₂ and UO₂(NO₃)₂ was formed at 400 °C. On the other hand, U₃O₈ was formed at 700 °C. In the method of this invention, the amount of ammonia water used for preparing ADU from the aqueous uranyl solution is not larger than half the amount used in the conventional method with sufficient effect. The amount of CaF₂ recovered from the filtrate after removal of the precipitate in this invention decreased according as the temperature of reaction between UF₆ and steam was elevated and at most about 1/3 time as large as the amount obtained in the conventional method. According to the method of this invention, it is possible to obtain UO₂ powder having a superior activity over that of the UO₂ powder prepared in the conventional method. In addition, it is possible to control the activity of the UO₂ powder by controlling the temperature of reaction between UF₆ and steam. On the other hand, from the viewpoint of crystal growth, UO₂ powder having a high crystal growth speed can be obtained. As stated above, this invention enables reduction to a greater extent in the amount of by-produced fluorides which has been disposed of as wastes by recovering HF in contrast to the conventional wet method, and at the same time reduction in the amount of ammonia to half or less as much as that used in the conventional method. In addition, it is possible to obtain UO₂ powder having a very high activity, with control of the characteristics of powder such as activity and crystal growth speed being very easy to achieve.

Claims: A method of fabricating uranium dioxide (UO₂) powder from uranium hexafluoride (UF₆), comprising (1) reacting UF₆ gas with steam with controlling the temperature of reaction between said UF₆ gas and said steam at a predetermined temperature within the range of 350 to 700 °C, to form solid uranyl fluoride (UO₂F₂) and/or uranium oxide with an O/U ratio (oxygen-to-uranium atomic ratio) of 2.7 to 3, (2) dissolving said UO₂F₂ and/or uranium oxide in nitric acid to form an aqueous uranyl solution containing said UO₂F₂ and/or uranyl nitrate (UO₂(NO₃)₂), (3) reacting said aqueous uranyl solution with ammonia to precipitate ammonium diuranate (ADU), (4) filtering said precipitate, (5) drying said precipitate, (6) calcining said dry precipitate, and (7) reducing said calcined precipitate to form UO₂ powder, whereby controlling the characteristics of said UO₂ powder. The method as claimed in Claim 1, wherein said reaction between UF₆ gas and steam is carried out in a fluidized bed reactor. The method as claimed in Claim 2, wherein nitrogen gas is introduced in said fluidized bed reactor. The method as claimed in Claim 1, wherein said temperature of reaction between UF₆ gas and steam is controlled to a temperature higher than 350 °C and not higher than 500 °C. The method as claimed in Claim 1, wherein said temperature of reaction between UF₆ gas and steam is controlled to a temperature higher than 500 °C and not higher than 700 °C. The method as claimed in Claim 1, further comprising recovering hydrogen fluoride by-produced during said reaction between UF₆ and steam.

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