Patent Number: 051397099
Section: description

EXAMPLE 1 UF.sub.6 in an 8A cylinder was loaded in an electrically heated water bath equiped with a stirrer. The temperature of water in the bath was automatically controlled at 90.degree. to 95.degree. C. and was kept homogeneous by stirring. The UF.sub.6 in the cylinder was melted and vaporized to give a final pressure of ca. 30 psig. Then, the vapor was introduced into the top part of an Inconel tube reactor of 4" diameter through a mass flowmeter. Steam having a pressure of 10 psig was also introduced to the reactor from the reactor wall side at a position below and near to the inlet point of UF.sub.6. The flow rate of steam was regulated with a needle type metering valve. The whole piping system, as well as the tube reactor body, was heated with heating mantles to maintain a constant temperature of 125.degree. C. The steam entering the reactor was thus super-heated and became completely dry. The flow rate of UF.sub.6 vapor was 300 g/hr and steam was 60 g/hr. The UO.sub.2 F.sub.2 powder formed in the reactor was finely divided and settled at the bottom of the reactor. Hydrogen fluoride gas produced in the reaction was filtered with a sintered-metal (Inconel 600) filter assembly and was sent to an alkali scrubbing system. After 5 hours, feeds of UF.sub.6 and steam were ended. The feed line of UF.sub.6 and the reactor were purged with nitrogen gas, and the UO.sub.2 F.sub.2 powder produced was discharged to a container. The foregoing UO.sub.2 F.sub.2 powder was dissolved in de-ionized water to give a solution containing 1138 g/l of uranium with a density of 2.26 g/ml. Four liters of the solution was put in an Inconel pot, which was then pressurized to 75 psig with nitrogen gas. The UO.sub.2 F.sub.2 solution coming from the pot through a bottom tube was atomized to form very small liquid droplets with an impingement type nozzle at the top of a spray column. The column had been prepurged with ammonia gas before use. An excess amount of ammonia gas was supplied continuously from the bottom of the column simultaneously. The liquid UO.sub.2 F.sub.2 droplets were converted to ADU particles having a brown yellow color as soon as it contacted with ammonia gas. The excess ammonia gas and the water vapor leaving the column through a top exit pipe were sent to a water scrubber. The UO.sub.2 F.sub.2 solution was used up completely in 4 minutes, then, the supply of ammonia gas was stopped immediately, and the column was purged with nitrogen gas. The product collected in a bottom tray in the column were wet but loosely divided granules, which, identified with x-ray diffractometry, were found to contain essentially ADU, AUF, and ammonium fluoride. No liquid filtrate was generated and no filtration operation was involved in the operation. A sample weighted 169.5 g taken from the foregoing ADU product, having a moisture content of 25.6 wt % and an uranium content of 68.34 wt % (dry basis), was put in an Inconel tray with a bed depth of 1 cm. The tray was loaded inside a retort in an electrically heated furnace. The ADU mixture was converted to UO.sub.2 with the following steps: (1) The bed temperature of ADU was increased from room temperature to 600.degree. C. in 145 minutes; a nitrogen gas with a flow rate of 30 SCFH was introduced from the beginning of heating. (2) A steam with a mass flow rate of 12.79 g/min and a hydrogen gas with a volume flow rate of 30 SCFH were introduced immediately as temperature reached 600.degree. C.; then, the bed temperature was kept isothermally for 90 minutes, and was then decreased to 540.degree. C. in 40 minutes; the supply of the hydrogen gas was ended at this temperature. (3) The bed was cooled to 50.degree. C., and the steam supply was ended at the moment when the temperature was lowered to 200.degree. C. (4) Kept the temperature isothermally at 50.degree. C., and a nitrogen gas containing 10% air was introduced for 60 minutes to stabilize the UO.sub.2 powder. Then, the furnace was shut down and the product was cooled to room temperature and discharged. The UO.sub.2 powder thus obtained was 97.5 g, which gives a recovering rate of 99.7%. The powder was found to have a good flowability, a fluorine content 32 ppm, an O/U ratio 2.034, a bulk density 2.15 g/ml, and a specific surface area 2.2 m.sup.2 /g. EXAMPLE 2 The procedure of example 1 was repeated except that the concentration of uranium in the UO.sub.2 F.sub.2 solution for preparing ADU was 500 g/l. The ammoniation product thus prepared was found to contain essentially ADU, ammonium fluoride, and a small amount of AUF. The UO.sub.2 powder thus obtained has a fluorine content 22 ppm, an O/U ratio 2.041, a bulk density 1.71 g/ml, and a specific surface area 2.2 m.sup.2 /g. EXAMPLE 3 The procedure of example 1 was repeated with the exceptions that: (1) uranium concentration in the UO.sub.2 F.sub.2 solution for preparing ADU was 634 g/l; (2) the drying and the calcining of the ADU product were carried out with a temperature profile of increasing from room temperature to 550.degree. C. in 130 minutes, and then kept the temperature isothermally at 550.degree. C. for 60 minutes; (3) the reduction was carried out at a constant temperature of 550.degree. C. for 60 minutes and, then, by decreasing the temperature from 550.degree. C. to 500.degree. C. in 30 minutes. The UO.sub.2 powder thereof made has a fluorine content 45 ppm, an O/U ratio 2.048, a bulk density 2.22 g/cm.sup.3, and a specific surface area 3.7 m.sup.2 /g. EXAMPLE 4 An uranyl nitrate solution containing 502 g/l uranium was prepared by dissolving pure uranyl nitrate in a de-ionized water. The procedures of example 1 were repeated to convert the uranyl nitrate to UO.sub.2 powder with the exceptions that: (1) the impingement nozzle was replaced by an ultra sonic atomizer in atomizing the solution; (2) the drying and the calcining of ADU were carried out with a temperature profile of increasing from room temperature to 500.degree. C. in 85 minutes, and then kept this temperature at constant for 60 minutes; (3) the reduction was carried out with a temperature profile of increasing from 500.degree. C. to 600.degree. C. in 35 minutes, then kept the temperature isothermally for 100 minutes, and finally decreased the temperature from 600.degree. C. to 500.degree. C. in 50 minutes. The UO.sub.2 powder obtained is free flowing, it was found to have an O/U ratio 2.106, a specific surface area 4.9 m.sup.2 /g, and a bulk density 0.4 g/ml. EXAMPLE 5 A solution containing 502 g/l uranium and 30.12 g/l gadolinium was prepared by dissolving uranyl nitrate in a de-ionized water and dissolving gadolinium oxide in a nitric acid solution, and then mixed up. Following the procedures of example 4, the mixed solution of uranyl nitrate and gadolinium nitrate was converted to a gadolinium-uranium oxide with the exceptions that: (1) the drying-calcining operation was done with a temperature profile of increasing from room temperature to 500.degree. C. in 100 minutes, and then kept the temperature at constant for 60 minutes; (2) the reduction operation was done with a temperature profile of increasing the temperature from 500.degree. C. to 650.degree. C. in 45 minutes, then kept this temperature at constant for 100 minutes, and decreased to 500.degree. C. in 80 minutes; (3) the cooling profile comprised decreasing the temperature from 500.degree. C. to 60.degree. C. in 195 minutes; and (4) the stabilization of UO.sub.2 powder was carried out at 60.degree. C. The U-Gd oxide thus obtained was found to have an oxygen/metal molar ratio 2.187, a bulk density 0.32, and a specific surface area 10.5 m.sup.2 /g.