Apport de la spectroscopie Raman pour la caractérisation des combustibles nucléaires défectueux en condition d'entreposage sous eau

Une signature spécifique des dommages d’irradiation dans le dioxyde d’uranium, le combustible nucléaire le plus utilisé, dénommé « triplet de défauts » a été récemment mis en évidence par spectroscopie Raman. Ce travail vise à savoir comment cette signature peut être utilisée afin de caractériser les combustibles nucléaires irradiés qui sont entreposés sous eau. Pour cela, trois études à effets séparés sont menées. Tout d’abord, une expérience d’irradiation aux électrons montre que le triplet de défauts est dû à des interactions balistiques et est associé aux déplacements dans le sous-réseau d’uranium. Après l’irradiation aux électrons, l’échantillon d’UO2 s’oxyde de manière accélérée, ce qui a été attribué à la migration des lacunes d’oxygène créées par l’irradiation vers la surface. Ensuite, la cinétique de formation du triplet de défauts dans de l’UO2 exposé à des environnements inerte (Ar) et réactif (eau aérée) a été mesurée grâce à un dispositif Raman in-situ. Dans tous les cas, la cinétique peut être décrite par un modèle d’impact direct, mais avec des coefficients numériques différents. Enfin, de manière à simuler le combustible irradié industriel en laboratoire, l’étude de différents composés d’oxydes mixtes a montré le rôle du dopage chimique sur la formation du triplet de défauts. Ces informations seront mises à profit dans les études futures de combustibles défectueux entreposés sous eau.A specific signature characteristic of irradiation damages in uranium dioxide, the most used nuclear fuel, referred as « triplet defect bands» has recently been evidenced by Raman Spectroscopy. The objective of this study is to determine how this signature can be used to characterize actual spent nuclear fuel stored in pools. For that purpose, three separate effect studies were carried out. Firstly, an electron irradiation experiment shows that the triplet defect bands are due to ballistic interactions and result from the formation displacements in the uranium sub-lattice. Post electron irradiation, the enhanced oxidation of UO2 samples is observed and attributed to the migration of irradiation induced oxygen vacancies towards the surface. The formation kinetics of the triplet defect bands in UO2 when exposed to an inert (Ar) and a reactive (aerated water) contact medium is then investigated through the use of an in-situ Raman installation. Both kinetics can be fitted using a direct impact model, but with different numerical values. Finally, to simulate actual spent nuclear fuels in laboratory conditions, the study of different mixed oxide compounds shows that chemical doping impacts the apparition of the Raman triplet defect bands. The experimental results obtained in this work will be used as complementary data for the interpretation of Raman results of actual defective spent nuclear fuels stored in pool conditions

Assessment of positrons for defect studies in CeO₂ materials

Disposal of spent nuclear fuel poses significant challenges, as the UO2 and fission products are under constant irradiation and must be safely stored for millennia. CeO2 is a non-radioactive analog for UO2 for studying microstructure and its evolution. Many techniques have been applied to uranium and cerium oxides to investigate point defects. Positron annihilation spectroscopies (PAS) are sensitive to neutral and negatively charged vacancy-like point defects and impurity vacancy complexes. PAS has been applied previously to UO2+x to investigate nuclear fuels, but virtually no PAS work exists on CeO2. Here, the basics of positron annihilation spectroscopy is reviewed, and preliminary work on undoped and doped CeO2 is shown and compared to the literature results from UO2. To simulate fission product incorporation in spent nuclear fuels, CeO2 samples were doped at different concentration with yttrium. Select samples were irradiated with heavy ions at different doses. Doping and irradiation are shown to give rise to different defect characteristics.</p

Investigating the role of irradiation defects during UO2 oxidative dissolution

International audienceIn this study, the behavior of alpha irradiation-induced defects in UO2, when exposed to different interfaces, is investigated. Raman spectroscopy is used to measure the formation kinetics of irradiation defects in UO2 leached under oxidizing water environment and the data are then compared to a reference UO2/Ar system. The results reveal that the presence of either aerated water or inert argon gas modifies the formation kinetics of irradiation defects. The UO2 alteration in aerated water leads to the precipitation of secondary phases in the form of studtite and water chemical analysis reveals that the UO2 dissolution mechanism proceeds without the formation of an oxidized UO2 layer

Hot Isostatic Pressing (HIP):A novel method to prepare Cr-doped UO₂ nuclear fuel

The addition of Cr2O3 to modern UO2 fuel modifies the microstructure so that, through the generation of larger grains during fission, a higher proportion of fission gases can be accommodated. This reduces the pellet-cladding mechanical interaction of the fuel rods, allowing the fuels to be “burned” for longer than traditional UO2 fuel, thus maximising the energy obtained. We here describe the preparation of UO2 and Cr-doped UO2 using Hot Isostatic Pressing (HIP), as a potential method for fuel fabrication, and for development of analogue materials for spent nuclear fuel research. Characterization of the synthesised materials confirmed that high density UO2 was successfully formed, and that Cr was present as particles at grain boundaries and also within the UO2 matrix, possibly in a reduced form due to the processing conditions. In contrast to studies of Cr-doped UO2 synthesised by other methods, no significant changes to the grain size were observed in the presence of Cr.</p

Cr2+ solid solution in UO2 evidenced by advanced spectroscopy

Chromia is an important additive used in uranium dioxide fuel fabrication, but its incorporation mechanism is still not fully understood. Here, the authors use X-ray absorption spectroscopy, including both near edge and extended fine structure regions, to resolve the local structure and valence state of chromium, as divalent, in uranium dioxide

Raman and PAS Characterization of Electron Irradiated UO2 to Determine U Displacement Threshold

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Identification of vacancy-type defects in UO2

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Raman and PAS Characterization of Electron Irradiated UO2 to Determine U Displacement Threshold

International audienc