Speaker
Mr
Denis Horlait
(ICSM/LIME)
Description
The understanding of dissolution processes of actinides dioxides has become essential for the optimization of reprocessing operations associated to the back-end of nuclear fuel cycle. As several GenIV reactor concepts plan to operate the simultaneous reprocessing of majors and minors actinides (such as AmIII and CmIII) in new fuel elements, we focused a study on the influence of the incorporation of a trivalent lanthanide cation (NdIII or ErIII) in fluorite-type matrix CeIVO2, as model compounds before forthcoming work based on
AnIV-LnIII oxides.
In this aim, several mixed-oxides were synthesized using an initial oxalic co-precipitation of the cations in order to maximize the homogeneity of the final compounds, obtained after heat treatment at 1000°C. From a structural point of view, the substitution of CeIV by LnIII appears to be accompanied by the formation of oxygen vacancies, in good agreement with literature data [1]. From XRD study of CeIV1-xLnxIIIO2-x/2 mixed dioxides, fluorite structure (Fm3m) remains stable up to about x = 0.4. For higher x values a cubic superstructure (Ia-3) appeared, resulting of rearrangement of oxygen vacancies. Finally, for CeIV0.25NdIII0.75O1.625 and upper x values, formation of additional hexagonal Ln2O3 (space group P3mm) was observed.
Dissolution experiments of dioxides compounds were then undertaken in 4M HNO3. Small amounts of solution were regularly uptaken and cations concentrations were measured by ICP-AES. Fig. 1 reports the evolution of normalized weight loss of Cerium (NL,Ce) for several CeIV1-xNdxIIIO2-x/2 samples. The associated initial dissolution rates (RL,i) determined from this results and reported in fig. 2 clearly showed that the trivalent cations fraction (and therefore the oxygen vacancies) strongly enhance the dissolution rate. Conversely, the crystalline structure-type does not seem to have any significant influence on the dissolution rate.
Environmental SEM and BET studies were then undertaken on several samples to follow the variation of the samples morphology throughout the dissolution process. We observed in the first times of the dissolution a strong increase of the specific surface area, correlated to the breakaway of nanometric crystalites (fig. 3, 3days). For longer leaching times, the formation of a gelatinous layer was systematically observed and could be linked to the decrease of dissolution rate after few hours.
Similar experiments were carried on sintered pellets (t = 10 hours, T = 1400°C)
(fig. 4). The material appeared preferentially altered 1) through the creation of corrosion pits randomly dispersed onto grains surface, 2) along grain boundaries, which also causes surface grains break away, and 3) by an alteration of the grains surface only observable after 60 hours for this example. Thereby, it seems that there is not only one preferential site or mechanism of dissolution in sintered compounds since the material is more rapidly dissolved from the alteration of grain boundaries and surface local defects.
Author
Mr
Denis Horlait
(ICSM/LIME)
Co-authors
Mr
Nicolas Clavier
(ICSM/LIME)
Mr
Nicolas Dacheux
(ICSM/LIME)
