A new hermetic sample holder to be used with radioactive or air‐ and moisture‐sensitive samples has been developed; it has been designed to fit most of the commercial Siemens/Bruker diffractometers (e.g. D5000 and D8 series). Thanks to the design of the sample holder and to a sample preparation process allowing two‐containment‐barrier protection, X‐ray data can be collected using a standard uncontaminated diffractometer mounted in a Bragg–Brentano geometry. The design offers very accurate and reliable sample positioning. In order to demonstrate the high quality of the data obtained, the Rietveld analysis of plutonium dioxide is presented. Good agreement between the refinement and published data demonstrates the quality of the sample preparation and the accuracy of the sample holder. High‐quality X‐ray diffraction powder patterns can be recorded for use in Rietveld refinements, even on highly absorbing radioactive materials.

The oxygen to metal ratio (O/M) is directly related to oxygen potential, which strongly influences the sintering and irradiation performance of nuclear fuels. A better understanding of these two parameters is therefore of major interest. To further ascertain the correlation between O/M ratio and oxygen potential in Am-bearing MOX, several thermodynamic descriptions are being developed. Despite their differences, they all involve the valence of actinide cations (e.g., U, Pu, and Am) as essential parameters. However, as no experimental data on their valence are available, these models rely on assumptions. In the present work, we coupled X-ray diffraction and X-ray absorption spectroscopy to follow the behavior of Pu and Am in three hypo-stoichiometric, U-free Pu(1-y)Am(y)O(2-x) compounds. We provide for the first time a quantitative determination of Pu and Am valences, demonstrating that plutonium reduction from Pu(4+) to Pu(3+) starts only when americium reduction from Am(4+) to Am(3+) is completed. This result fills in an important gap in experimental data, thereby improving the thermodynamic description of nuclear fuels. At last, we suggest that the O/M ratio may evolve at room temperature, especially for high Am content, which is of main concern for the fabrication of Am-loaded MOX and their storage prior to irradiation.