Patent Number: 050154222
Section: claims

1. A process for fabricating UO.sub.2 pellets of controlled grain size comprising: (i) treating an aqueous solution of 50 to 500 gl.sup.-1 of UO.sub.2 F.sub.2, containing no HF, with NH.sub.3 at a molar ratio of NH.sub.3 /U of from 3 to 5, to obtain a slurry of a precipitate of ADU (NH.sub.4).sub.2 U.sub.2 O.sub.7 and an aqueous solution which also contains U;  (ii) adding NH.sub.3 to said solution to obtain an ADU (NH.sub.4).sub.2 U.sub.2 O.sub.7 precipitate at a NH.sub.3 /U molar ratio of from 6 to 12;  (iii) filtering and drying said (NH.sub.4).sub.2 U.sub.2 O.sub.7 precipitate;  (iv) calcining and reducing said (NH.sub.4).sub.2 U.sub.2 O.sub.7 under a hydrogen atmosphere forming UO.sub.2 powder;  (v) compacting said UO.sub.2 powder; and  (vi) sintering said compacted UO.sub.2 powder in a H.sub.2 atmosphere, forming pellets of UO.sub.2 of a grain size of from 10 to 100 .mu.m. 2. A process for fabricating UO.sub.2 pellets according to claim 1 wherein UO.sub.2 F.sub.2 powder is dissolved in demineralized water forming an aqueous solution which contains no HF. 3. A process for fabricating UO.sub.2 pellets according to claim 1 wherein NH.sub.3, water and an aqueous solution of UO.sub.2 F.sub.2 are first fed continuously to a first-stage settling chamber, so that the NH.sub.3 /UO.sub.2 F.sub.2 molar ratio is between 3 to 5, to carry out the first-stage ADU precipitation. 4. A process for fabricating UO.sub.2 pellets according to claim 3 wherein ADU slurry is formed in the first-stage settling chamber, so that aqueous NH.sub.3 is fed continuously to a second-stage settling chamber, and so that the NH.sub.3 /UO.sub.2 F.sub.2 molar ratio becomes within a range of from 6 to 12. 5. A process for fabricating UO.sub.2 pellets according to claim 4 wherein second-stage ADU slurry is filtered, dried, calcined and reduced under an H.sub.2 atmosphere, transforming the slurry into UO.sub.2 powder. 6. A process for fabricating UO.sub.2 pellets according to claim 1 wherein UO.sub.2 powder is compacted at a pressure of 5 t/cm.sup.2, and then sintered for 4 hours at 1750.degree. C. in an H.sub.2 atmosphere, forming pellets.