Local structure and charge distribution of Am bearing nuclear oxide fuels: (U,Am)O2±x and (U,Pu,Am)O2-x


Local structure and charge distribution of Am bearing nuclear oxide fuels: (U,Am)O2±x and (U,Pu,Am)O2-x

Martin, P. M.; Belin, R. B.; Prieur, D.; Delahaye, T.; Gavilan, E.; Lebreton, F.; Robisson, A. C.; Dumas, J. C.; Scheinost, A. C.

INTRODUCTION
In the framework of recycling minor actinides (MA) in fast neutron reactors, americium can be transmuted either by adding it in a small amount to the fuel (e.g. (U,Pu,Am)O2-x) or by using dedicated blanket fuels (e.g. (U1-y,Amy)O2-x). In both strategies, the stoichiometry of the solid solution, commonly described as oxygen to metal atom ratio (O/M), is an important parameter affecting thermal, chemical, and physical properties of the fuel during irradiation. A thorough knowledge of its correlation with oxygen potential (µO2) during manufacturing and especially sintering is then of major interest. To better assess this issue, several thermodynamic descriptions have been developed1,2. Despite their differences, they all involve the valence state of actinide cations (e.g. U, Pu and Am) as an essential parameter. However, experimental data regarding actinide valence remains very limited in the literature.
As illustrated by our recent study of (U,Am)O2±x samples3, coupling X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) measurements allows to determine actinide valence states and to describe the solid solution at both long-range and short range order as a function of oxygen potential. From these data, O/M ratio is eventually calculated using the cation molar fractions determined by XAS. In the present work, we realized a complete XAS characterization on (U,Am)O2±x and (U,Pu,Am)O2-x samples.

MATERIALS AND METHODS
Mixed oxide samples (U1-y,Amy)O2±x (y=0.10, 0.15, 0.20) and (U0.750Pu0.246Am0.004)O2-x were manufactured by conventional powder metallurgy process. By adding a controlled amount of either H2O or Ar/O2 to the furnace atmosphere (Ar/5%H2), oxygen potential were ranging from -520 kJ.mol-1 to -390 kJ.mol-1 during sintering. XAS measurements were performed at the ROssendorf Beam Line (BM20) located at the European Synchrotron Radiation Facility (ESRF, Grenoble, France). For each samples, U, Pu and Am LII,III edges were collected simultaneously in fluorescence and transmission modes at 15K using a helium cryostat.

RESULTS
Concerning americium, a complete reduction of Am4+ to Am3+ is observed for both (U,Am)O2 and (U,Pu,Am)O2 samples whatever the oxygen potential and the Am content. Unexpected results were obtained in the case of (U,Am)O2 solid solutions as, even for the lowest value of µO2 (-520 kJ.mol-1), pentavalent uranium is found, highlighting a charge compensation mechanism as the cause of this partial oxidation of uranium cations3. As a consequence, the expected hypo-stoichiometry was not observed and the solid solution can be described as .
For (U0.750Pu0.246Am0.004)O2-x samples, a radical difference is observed since uranium and americium cations remain respectively tetravalent and trivalent, regardless of the sintering conditions. On the other hand, a partial reduction of Pu4+ to Pu3+ as a function of µO2 is responsible for the continuous decrease in O/M (1.97≤O/M≤1.99). Furthermore, local environments given by EXAFS confirmed that the defects in hypo stoichiometric MOX are only located around Pu cations.
In the presentation, we will detail and compare these results and discuss their consequences on thermodynamical modeling, especially the actual temperature fixing the O/M ratio of the final material.

CONCLUSION
Using XAS measurement on (U,Am)O2±x and (U,Pu,Am)O2-x samples, O/M values were obtained as a function of oxygen potential at high temperature. Comparing these two materials, we evidenced (1) a distinct cation charge mechanism and (2) how the deviation from stoichiometry is supported in the solid solution structure. These results proved useful to better assess the thermodynamical models developed for actinides mixed oxides systems.

REFERENCES
1 M. Osaka et al. (2005) J. Nucl. Mater., 344, 230-234.
2 M. Osaka et al. (2007) J. Alloys Compounds, 428, 355-361.
3 D. Prieur et al. (2011) Inorg. Chem., 50, 12437–12445

Keywords: EXAFS; XANES; MOX

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