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The conversion of ammonium uranate prepared via sol-gel synthesis into uranium oxides
Schreinemachers, Christian,Leinders, Gregory,Modolo, Giuseppe,Verwerft, Marc,Binnemans, Koen,Cardinaels, Thomas Korean Nuclear Society 2020 Nuclear Engineering and Technology Vol.52 No.5
A combination of simultaneous thermal analysis, evolved gas analysis and non-ambient XRD techniques was used to characterise and investigate the conversion reactions of ammonium uranates into uranium oxides. Two solid phases of the ternary system NH<sub>3</sub> - UO<sub>3</sub> - H<sub>2</sub>O were synthesised under specified conditions. Microspheres prepared by the sol-gel method via internal gelation were identified as 3UO<sub>3</sub>·2NH<sub>3</sub>·4H<sub>2</sub>O, whereas the product of a typical ammonium diuranate precipitation reaction was associated to the composition 3UO<sub>3</sub>·NH<sub>3</sub>·5H<sub>2</sub>O. The thermal decomposition profile of both compounds in air feature distinct reaction steps towards the conversion to U<sub>3</sub>O<sub>8</sub>, owing to the successive release of water and ammonia molecules. Both compounds are converted into α-U<sub>3</sub>O<sub>8</sub> above 550 ℃, but the crystallographic transition occurs differently. In compound 3UO<sub>3</sub>·NH<sub>3</sub>·5H<sub>2</sub>O (ADU) the transformation occurs via the crystalline β-UO<sub>3</sub> phase, whereas in compound 3UO<sub>3</sub>·2NH<sub>3</sub>·4H<sub>2</sub>O (microspheres) an amorphous UO<sub>3</sub> intermediate was observed. The new insights obtained on these uranate systems improve the information base for designing and synthesising minor actinide-containing target materials in future applications.