Fabrication and microstructural characterization of pellets produced from uranium dioxide microspheres

Lucca Myngheer, Beatriz Acevedo Muñoz, Gamze Çolak, Gregory Leinders

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    Abstract

    The primary objective of this master thesis was to gain an understanding of the process parameters for producing UO2 kernels via a sol-gel production process. Additionally, the aim was to investigate the feasibility of producing mechanically strong UO2 pellets from these kernels. The successful implementation of this novel fabrication process has the potential to reduce radioactive hazards associated with conventional nuclear fuel production processes, which rely on the use of powders. Moreover, it introduces a more versatile and manipulative production process to the nuclear industry, as opposed to the current norm of the metallurgical powder-based production process. The kernels were generated using an external gelation process. This process involves precipitating uranium (U) into gelled kernels, where the U originates from a solution referred to as “sol”, composed of uranyl nitrate (UN), polyvinyl alcohol (PVA), aqueous ammonia (NH3), and tetrahydrofurfuryl alcohol (THFA). Droplets of the sol are dropped into a high molarity NH3 solution (10 M), resulting in the formation of the gelled kernels consisting of ammonium diuranate (ADU, xUO3∙yNH3∙zH2O). After retrieving the kernels from the NH3 solution subsequent heat treatments, including a drying step, a calcination (in an oxidizing atmosphere), and a reduction (in reducing atmosphere), are performed to convert the UO3, present in the ADU kernels, into U3O8, and ultimately into UO2. The kernels are then pressed into pellets, using either only UO2 kernels, or a mixture of UO2 and U3O8 kernels as material to press the pellets. The pressed pellets undergo a sintering step at high temperatures to achieve the desired densities (≥ 95% theoretical density (T.D.)) in accordance with industry standards. When it comes to the mixed (UO2 + U3O8) pellets, the goal is to obtain higher porosity values compared to the pure UO2 pellets, which is beneficial in respect of their usage in transmutation experiments. To optimize the UO2 pellet production process, various parameters were investigated, including the optimal concentrations of the precursors for the “sol” production, the most suitable procedure for conducting the external gelation process as a whole and the pressing conditions of the pellets. The combination of the following substances (including their concentrations/quantities) were found to yield the most satisfactory ADU kernels when used for to the sol production process: a UN solution (0.7 M), PVA (36 g/l), NH3-solution (1.4 M), THFA solution (10 vol.%), application of heat and stirring during preparation of the sol and using a needle diameter of 0.4 mm for creating the droplets. Subsequently, the temperature regimes for the heat treatments were examined to determine the optimal conditions for the calcination and reduction step. A temperature of 600°C and 640°C, respectively, were experimentally obtained and employed. Finally, the pressing conditions for UO2 pellets were investigated to identify the parameters that would yield pellets with the most integrity and mechanical strength. The following optimal conditions were found to be suitable: a pressure of 1.8 tons, a pressure holding time of 12 seconds, and a material weight of 460 mg. The quality of the sintered UO2 pellets was deemed satisfactory in comparison to pure UO2 pellets used in conventional nuclear fuel. The mixed pellets did not achieve the expected higher porosity values as compared to the pure UO2 pellets. In relation to the sol-gel production parameters, it is recommended to conduct further research to improve the mechanical strength and integrity of ADU kernels, with the objective of minimizing material loss during manipulation. This could involve exploring alternative concentrations of constituents, varying process conditions, or introducing additional steps to improve the overall quality of the kernels. Moreover, it is essential to further assess the usability and quality of the pressed pellets generated in this master thesis or in any future sol-gel experiments. This will provide an understanding of the pellets' performance and determine if they meet the necessary standards for practical applications in the nuclear fuel industry.
    Original languageEnglish
    QualificationIndustrial Engineer
    Awarding Institution
    • KU Leuven
    Supervisors/Advisors
    • Deschuytere, Antoinette, Supervisor, External person
    • Leinders, Gregory, SCK CEN Mentor
    • Acevedo Muñoz, Beatriz, SCK CEN Mentor
    • Çolak, Gamze, SCK CEN Mentor
    Date of Award27 Jun 2023
    Publisher
    StatePublished - 12 Jun 2023

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