title: Peculiar Behavior of (U,Am)O(2-δ) Compounds for High Americium Contents Evidenced by XRD, XAS, and Raman Spectroscopy.
abstract: In U(1-x)Am(x)O(2±δ) compounds with low americium content (x ≤ 20 atom %) and oxygen-to-metal (O/M) ratios close to 2.0, Am(III+) cations are charge-balanced by an equivalent amount of U(V+) cations while the fluorite structure of pure U(IV+)O2 is maintained. Up to now, it is unknown whether this observation also holds for higher americium contents. In this study, we combined X-ray diffraction with Raman and X-ray absorption spectroscopies to investigate a U(0.5)Am(0.5)O(2±δ) compound. Our results indicate that americium is again only present as Am(III+), while U(V+) remains below the amount required for charge balance. Unlike lower americium contents, this leads to an overall oxygen hypostoichiometry with an average O/M ratio of 1.92(2). The cationic sublattice is only slightly affected by the coexistence of large amounts of reduced (Am(III+)) and oxidized (U(V+)) cations, whereas significant deviations from the fluorite structure are evidenced by both extended X-ray absorption fine structure and Raman spectroscopies in the oxygen sublattice, with the observation of both vacancies and interstitials, the latter being apparently consistent with the insertion of U6O12 cuboctahedral-type clusters (as observed in the U4O9 or U3O7 phases). These results thus highlight the specificities of uranium-americium mixed oxides, which behave more like trivalent lanthanide-doped UO2 than U(1-x)Pu(x)O(2±δ) MOX fuels.
authors.affiliation: CEA, DEN, DTEC/SDTC/LEMA , F-30207 Bagnols-sur-Cèze Cedex, France.
authors.forename: Florent
authors.initials: F
authors.lastname: Lebreton
authors.affiliation: CEA, DEN, DTEC/SDTC/LEMA , F-30207 Bagnols-sur-Cèze Cedex, France.
authors.forename: Denis
authors.initials: D
authors.lastname: Horlait
authors.affiliation: CEA, DEN, DTCD/SECM/LMPA , F-30207 Bagnols-sur-Cèze Cedex, France.
authors.forename: Richard
authors.initials: R
authors.lastname: Caraballo
authors.affiliation: CEA, DEN, DEC/SESC/LLCC , F-13108 Saint-Paul-lez-Durance Cedex, France.
authors.forename: Philippe M
authors.initials: PM
authors.lastname: Martin
authors.affiliation: Helmholtz Zentrum Dresden Rossendorf, Institute of Resource Ecology , P.O. Box 10119, 01314 Dresden, Germany.
authors.forename: Andreas C
authors.initials: AC
authors.lastname: Scheinost
authors.affiliation: Helmholtz Zentrum Dresden Rossendorf, Institute of Resource Ecology , P.O. Box 10119, 01314 Dresden, Germany.
authors.forename: Andre
authors.initials: A
authors.lastname: Rossberg
authors.affiliation: CEA, DEN, DTCD/SECM/LMPA , F-30207 Bagnols-sur-Cèze Cedex, France.
authors.forename: Christophe
authors.initials: C
authors.lastname: Jégou
authors.affiliation: CEA, DEN, DTEC/SDTC/LEMA , F-30207 Bagnols-sur-Cèze Cedex, France.
authors.forename: Thibaud
authors.initials: T
authors.lastname: Delahaye