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- Hälldahl, Lars, 1950-
(author)
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Studies of reactions occurring in the AUC-process : from UF₆ to sintered UO₂ pellets
- 1985
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Doctoral thesis (other academic/artistic)abstract
- This study has been carried out to investigate the influence of process parameters upon the material characteristics during the conversion of UFg to UO2 powder, and the importance of variation in these charcteristics for the final quality of the sintered pellets. The first process step is a precipitation of ammonium uranyl carbonate (AUC) from UFg, ammonia and carbon dioxide. In this step the most important characteristic is the particle size distribution, which is determined by the concentration of the reacting species and by the temperature.The reducing capacity of the atmosphere, the heating rate and the final temperature influence the specific surface area, the stoichiometry and the crystallite size, obtained in the reduction of AUC to UO2 powder.The studies also reveal that the reaction path from AUC to UO2 is very complex. Intermediates such as amorphous UO3, CÏ-UO3, uranium oxide hydrates, U4O9 and U30g can under certain conditions exist, transiently during this operation. The reaction path actually followed very strongly influences the powder characteristics.Methods to study sinterability and sintering kinetics have also been employed. Quasi-isothermal dilatometry (QID), has been used to find the rate controlling mechanism in the initial stage of sintering, and to evaluate the activation energy in the different atmospheres and diffusion coefficients.Full-scale experiments have verified the findings; coarse, inactive powders accumulate high defect concentations during pressing, leading to a relatively high defect frequency after sintering. Finer powders, which are still coarser than a certain minimum, however give the strongest green pellets with the best sinterablity. This type of powders also yields high quality sintered pellets with low defect frequencies.
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