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Incorporation of tetravalent actinides in monazite-type ceramics : synthesis, sintering and long-term behavior

  • Qin, Danwen
Publication Date
Aug 25, 2020
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The management and disposal of HLW radwaste is one of the main challenges for the nuclear industry. In this field, the specific conditioning of actinides appears of strong interest, as they act as the main contributors to the long-term radiotoxicity during the storage in an underground geological repository. Monazite-type ceramics have been often considered as promising ceramic wasteform due to their ability to incorporate large amounts of tri- and tetravalent actinides, their excellent chemical durability and their exceptional high resistance to self-irradiation. Nevertheless, the incorporation of tetravalent actinides in the monazite structure via wet chemistry routes has been rarely reported. Consequently, this study was firstly aimed to synthesize single-phase thorium-bearing rhabdophane type precursors by multiparametric hydrothermal process. Their thermal conversion into monazite-cheralite ceramics was investigated and followed by a complete sintering study. As instance, sintering maps were further established in order to obtain highly densified and single phase ceramics showing controlled microstructure. Later, multiparametric dissolution tests were performed on Nd0.8Ca0.1Th0.1PO4 monazite-cheralite in acid media. They confirmed the very high chemical durability of the prepared materials as well as the very low solubility of the neoformed phases. Finally, uranium-bearing rhabdophanes were also prepared via the same wet chemistry route and then converted to single-phase monazite-cheralite solid solutions by heating. This study then provided new insights in the preparation of monazite-cheralite type wasteforms by wet chemistry route and contributed to the better understanding of the long-term behavior of such kind of ceramics in the field of the storage of radioactive waste in an underground repository site.

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