The slides were presented at the Uranium Science Conference on July 1, 2021 (T21). Abstract Uranium dioxide is used as conventional fuel for the production of energy by nuclear fission. Even though the front-end of the nuclear fuel cycle is well known, studies to investigate alternative fabrication routes to prepare precursors for oxidic uranium-based fuels are ongoing. The precipitation induced by thermal decomposition of urea has been demonstrated for several metals (e.g. Ti, Ni, Cu, Zn, Ce, Th), and a modified hydrothermal approach has been applied to precipitate ammonium diuranate (ADU) from a solution containing uranyl ions. Within this study, we investigated the hydrolysis behaviour of uranyl and lanthanide mixtures to support the development of alternative fabrication routes for transmutation fuel, such as sol-gel processes. The lanthanides Nd and Ce acted as surrogates for the actinides Am and Pu, respectively. We specifically sought out parameters for the hydrolysis of uranyl ions induced by thermal decomposition of urea at ambient pressure. Moreover, the hydrolysis behaviour of Nd(III), Ce(III) and Ce(IV), as well as mixtures of the lanthanide- and uranyl ions, was investigated using the conditions determined for uranyl. Hydrolysis experiments were carried out at 90 °C and 100 °C for n(urea) : n(UO22+) ratios of 26 and 52. The solution was sampled during the precipitation reaction to monitor its pH and certain samples were analysed applying UV/VIS spectroscopy and inductively coupled plasma mass spectrometry, while powder X-ray diffraction and scanning electron microscopy were applied to characterise the precipitates. Uranyl ions hydrolysed between pH 5.1 and pH 5.5 and the experimental conditions impacted the reaction kinetics significantly. A temperature increase from 90 °C to 100 °C reduced the time to finish the precipitation by about 75 %, whereas a doubling of the urea content decreased the reaction time by about 50 %. ADU precipitates of different composition (x UO3 · y NH3 · z H2O) formed under the applied conditions. For trivalent Nd and Ce, a comparable pH evolution and lanthanide carbonate hydroxide (LnCO3OH) products were observed, whereas tetravalent Ce hydrolysed at a lower pH forming CeO2. The precipitation behaviour was confirmed for solutions containing binary mixtures of uranyl and lanthanide cations, while a simultaneous precipitation of Nd(III) and Ce(III) was observed for ternary U/Nd/Ce compositions. For the latter, a partial incorporation of the Ln phase into the ADU phase was observed, whereas the precipitation in the presence of Ce(IV)/CeO2 led to the formation of three separate phases.

Financial support for this research was provided by the European Commission (project: GENIORS - GEN IV Integrated Oxide fuels Recycling Strategies (755171)) and the Belgian FPS Economy (project: ASOF - Advanced Separation for Optimal management of spent Fuel), as well as the SCK CEN academy.