XAS Study of dense U1-YAmYO2-X (Y=0.10; 0.15; 0.20) compounds

U1-yAmyO2-x blankets are promising fuels for the transmutation of Minor Actinides (MA) in Fast Neutron Reactors. The fabrication of dense U1-yAmyO2-x pellet is challenging due to the high oxygen potential of the AmO2-x. As a consequence, a thermodynamical modelling is currently developed to determine optimum sintering conditions for given O/M ratios and MA content. Due to the lack of experimental data in the U-Am-O system [1] to refine the modelling, an experimental determination of the O/M ratio is mandatory as also its eventual consequences on the homogeneity of the solid solution. The present work focuses on the XAS characterization of U1-yAmyO2-x obtained by a conventional powder metallurgy process [2] for different sintering conditions and Am contents (10, 15, 20%). Thanks to XANES measurements, relative content of different U and Am oxidation states (+III, +IV, +V) have been obtained giving access to an experimental determination of O/M ratio as a function oxygen potential fixed during sintering. It was shown that the Am oxidation state remains unchanged at +III while there is a U(IV)/U(V) mixed valence depending on the oxygen potential. As expected, the calculated O/M ratio increase with the oxygen potential is only supported by a partial oxidation of uranium cations to U(+V). EXAFS data collected at Am and U edges showed, as suggested by laboratory XRD measurements, that a solid solution at molecular scale is obtained for all the Am content. From the calculated molar fraction of U4+, U5+ and Am3+ this solid solution can be written as U4+(1-Ay)(1-y)U5+Ay(1-y)Am3+O2-x where A is a constant.

[1] W. Bartscher and C. Sari, J. of Nuc. Mat., 118, 220-223(1983).
[2] D. Prieur and al, Powd. Tech., (2010).