[en] Ammonium Diuranate (ADU) is the nominal name for ammonium poliuranate. It is a very complex compound whose polymeric structure may have different chemical composition and crystallographic structure according to the precipitation conditions. ADU is used as precursor of uranium oxides in the manufacture of fuel elements. In order to find the composition of ADU its thermal decomposition was studied. Thermogravimetric analysis were performed and samples were taken in the singular points. These samples were analyzed by X-ray diffraction and infrared spectroscopy. Electron scanning photographs were taken. Besides an experience was designed in order to identify the evolved products during heating. When ultrasound has been applied in the precipitation stage, the ADU has the composition 6 U O₃.3 N H₃. 9 H₂ O. The ADU precipitated without ultrasonic treatment has the formula 6 U O₃. 4 N H₃. 8 H₂ O. The difference between both ADU is the adsorbed water or lightly coordinated water, which is present in more quantity in the ADU without ultrasonic treatment. (author)

[en] At present, obtention of U₃O₈ used in the manufacturing of MTR plates nuclear fuels, is performed by hydrolysis of UF6 to obtain uranyl fluoride. Uranyl fluoride is precipitated with ammonium hydroxide to get ammonium polyuranate (ADU). Afterwards ADU is calcinated to U₃O₈ and mechanical and thermally treated in order to obtain a powder in a determined specification. In the present work, ultrasound has been applied in the stage of precipitation of ADU and for different times in the stage of digestion in order to fasten the stages of ADU filtering and eliminate the U₃O₈ milling and sieving. Experiences on UO₂ have also been performed. The aspect of ADU changes considerably when they have been ultrasonically treated, its filtering rate is faster and it is easier to dry as it contains less humidity. U₃O₈ obtained after 800degreeC calcination of treated ADU results in an easy to desagregate powder. Only a soft mechanical treatment is needed to be performed on it before starting thermal treatment at 1400degreeC. After thermal treatment at 1400degreeC treated U₃O₈ has shown adequate characteristics of size, shape and density (8.2 g/cm³). Regarding UO₂, the shape of the agglomerates is almost spherical, leading to a free-flowing powder, whose apparent and TAP density showed to be adequate. The characteristics of the different compounds were followed by electron scanning micrographies, X-Rays, specific area measurements and differential thermal analysis. The great advantage of ultrasound appliance is that hard mechanical treatment is avoided in the obtention of U₃O₈, saving time and effort. Furthermore, UO₂ proves to be adequate to make pellets, the same precursor could be used in the obtention of both uranium oxides. (author). 5 refs., 6 figs