Fabrication of zirconia based ceramics by internal gelation for conversion of trivalent actinides

Reducing the radiotoxicity of spent nuclear fuel is an important objective to ensure the sustainability of the nuclear energy. This objective can be attained by separation of the long-lived actinides from the fuel constituents and their conversion into short-lived radionuclides by nuclear reactions, the so-called partitioning and transmutation strategy. An important step of this method is the co-conversion of the actinides solutions issued from partitioning into solid fuels precursors for transmutation. On the other hand, incorporating the actinides into an inert matrix allows an easier handling of these fuels and is particularly useful for high burn-up transmutation. Yttria-stabilized zirconia is such an inert matrix, with a high stability. Among the co-conversion methods, the sol-gel processes such as internal gelation present an interest due to their mild conditions, the fact they are easy to handle remotely and the absence of radioactive dust formation during fabrication. Internal gelation has been often used for the preparation of uranium and plutonium fuels, but no studies focused on its application to zirconium and minor actinides (neptunium, americium, curium). Therefore, the objectives of this study were to prepare and characterize solid microspheres of yttria-stabilized zirconia loaded by cerium as a surrogate for trivalent actinides by internal gelation, but also to perform basic studies on this gelation. It occurs by decomposition of organic precursors (hexamethylenetetramine HMTA, and urea) under heat, which produce ammonia. This causes an increase of pH which ensures the gelation. The studies performed aim to better understand the phenomena occurring during gelation, and the role of the organic additives used. It was found from these studies that the role of urea was to catalyze the decomposition of HMTA into NH3, and to bring porosity to the solid material. It was also found that a complete gelation could not be attained, and that an aging step in an alkaline medium was required to prevent losses. The microspheres characterization by thermal analysis, X-ray diffraction and microscopy indicated that the material crystallized into a solid solution with a fluorite structure at approximately 400°C, but also that the mixture of urea and HMTA caused a strongly exothermic decomposition. This resulted in cracks formation during thermal treatment. Optimizations were attempted to reduce this, mainly the quantity of urea and HMTA used, but also on the parameters of thermal treatment. Alternatively, microspheres were directly pressed into pellets, without milling. A repressing method allowed densities up to 86% of TD to be reached