Impact of the cation distribution homogeneity on the americium oxidation state in the U0.54Pu0.45Am0.01O2−x mixed oxide
Introduction
In the prospect of future sodium-cooled fast neutron reactors (SFR), uranium–plutonium mixed oxide fuels incorporating high amounts of plutonium are currently considered. Because of their specific neutronic spectrum, SFR will be able to burn long-lived minor actinides (MAs) such as americium [1]. Homogeneous transmutation is one possible way to reach this target by introducing small amounts (2–5%) of MAs into the U1−yPuyO2−x mixed oxide fuel. Even if some specifications are still to be discussed, general trends are already identified for such nuclear fuels. For instance, the Oxygen/Metal (O/M) ratio of the SFR’s fuel will have to range from 1.94 to 2.00. This oxygen stoichiometry dictates many (or most) of the fuel properties (thermal conductivity, melting point, diffusion phenomena, …), hence studying the O/M ratio of MA-bearing mixed oxides is relevant. The O/M ratio relies mainly on the oxidation state of the cations because metal vacancies are not expected in the uranium–plutonium mixed oxides. It is well known that, in hypostoichiometric mixed oxides U1−yPuyO2−x, uranium is tetravalent whereas plutonium could exhibit either fully reduced Pu3+ or a mixed +III/+IV valence [2], [3], [4], [5], [6]. Americium has been reported to be trivalent in (U,Pu,Am)O2−x [7] and U1−yAmyO2−x [8], [9], to be of mixed valence (+III/+IV) in (U,Pu,Am)O2−x [10] and (Pu,Am)O2−x [11] and finally to follow the plutonium reduction behavior in (U,Pu,Am)O2−x [12], [13]. The americium chemistry therefore appears to be still not clearly identified within uranium–plutonium mixed oxides. Furthermore, the consequence of the cation distribution homogeneity on the americium chemistry has, to our knowledge, never been studied. Therefore, the intention of our work presented here was to investigate how the americium oxidation state is impacted by the cation homogeneity of the fuel pellet by coupling X-ray diffraction (XRD), electron probe micro analysis (EPMA) and X-ray absorption spectroscopy (XAS).
Section snippets
Sample preparation
In this study, uranium dioxide and plutonium dioxide powders were used. The uranium dioxide powder was produced by a wet fabrication route based on the formation of ammonium diuranate (ADU) from uranyl nitrate precipitated with ammonia. The obtained particles were then atomized, dried and calcined, leading to spherical-shaped agglomerates of around 20 μm in diameter. Plutonium dioxide powder was produced by precipitation of a plutonium nitrate solution within oxalic acid to form plutonium
Results and discussion
In the prospect of ranking the samples in terms of U–Pu distribution homogeneity, XRD measurements were performed on manually crushed pellets obtained (1) after sintering at 2023 K for 24 h under Ar + 5% H2 + ∼1500 vpm H2O and slowly cooled at 50 K h−1 [17], (2) after the reducing annealing at 2023 K for 4 h under Ar + 5% H2 + ∼5 vpm H2O cooled at 300 K h−1 and (3) after the re-oxidizing thermal treatment at 1173 K for 16 h under Ar + 5% H2 + ∼24,000 vpm H2O. The diffraction patterns (and lattice parameters) obtained
Conclusion
The americium chemistry in uranium–plutonium mixed oxides is discussed in the literature without being able to conclude on the trivalent or tetravalent behavior of Am. The present study points out a clear cation distribution homogeneity effect on the Am oxidation state which may explain the discrepancies between the different authors. Americium exhibits a mixed valence III/IV when the mixed oxide shows some U–Pu distribution heterogeneities. When (U,Pu,Am)O2−x is homogeneous, however, americium
Acknowledgments
The authors are pleased to acknowledge Dr. J. Léchelle, Dr. T. Truphémus, Dr. S. Berzati, Dr. R. Bes, I. Felines, Y. Marc and JC. Richaud for their precious contributions to this work. The ACTINET-I3 project is also to be acknowledged for allowing us to perform the XAS experiments at the ESRF facility, France.
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