Homogeneity at molecular scale of (U,Pu)O2 solid solutions probed by XAS

Main objectives of new fuel developed for Generation III/IV systems are economy of resources, minimized volume and lower long-term potential radiotoxicity of ultimate wastes and proliferation risk reduction. To achieve these goals, one major fuel cycle option under evaluation is co-management of the actinides (An) in an integrated closed fuel cycle.1 Co-management of two (or more) actinides implies separating these actinides from the fission products, most often by hydrometallurgical processes, and then converting them to solid forms to re-fabricate fresh fuel or dedicated fuels or targets. Considering the significant amount of minor actinides and the different designs of future nuclear fuels, including mixed actinide pellets, composite materials or spherical particles, various uranium-actinide(s) wet co-conversion routes are currently investigated at the CEA-ATALANTE facility. Once the precipitation is achieved, the resulting mixed-actinide compound is calcined into a mixed oxide. As this compound will be the solid precursor of fuel or a dedicated transmutation target, controlling its composition at the molecular scale is of major importance. Usually, solid solution structures are studied using diffraction methods. But, as demonstrated in a previous study2, in order to fully investigate the ideality of a solid solution, X-ray absorption spectroscopy (XAS) using synchrotron radiation is an extremely suitable technique.
The subject of this paper is the study of (U1-x,Pux)O2 (with x=0.15, 0.28, 0.45 and 0.50) solid solutions synthesized by two co-conversion routes: the first one based on the oxalic co-precipitation of U(IV) and Pu(III) complex3 and the second on the internal gelation of a U(VI)-Pu(IV) or U(IV)-Pu(III) solution4. Before XAS experiments, samples were characterized by X-Ray diffraction and exhibit the expected cubic face centered structure with lattice parameters in agreement with Vegard Law.
XAS experiments were performed at the Rossendorf Beamline (BM20) located at the European Synchrotron Radiation Facility (ESRF, Grenoble, France). For each sample, plutonium LII and U LIII-edge were collected at 15K using a helium cryostat.