On the role of americium in the reduction process in plutonium-americium oxides


On the role of americium in the reduction process in plutonium-americium oxides

Belin, R. C.; Martin, P. M.; Gavilan, E.; Reynaud, M.; Scheinost, A. C.

Innovative mixed oxide (MOX) (U,Pu)O2-x fuels for sodium fast neutron reactors (SFRs) systems are currently studied within the framework of the fourth generation (GEN-IV) nuclear reactors development. SFRs will be able to burn long-lived minor actinides (MA) such as Americium by adding them homogeneously to the fuel in a small amount (2 to 6%). Because oxygen to metal ratio (O/M) has a direct impact on irradiation performance, a thorough knowledge of its correlation with oxygen potential during manufacturing and especially sintering is of major concern.
A study by Osaka et al. [ ] highlights a conflicting redox behaviour of Am and Pu cations responsible for an unusual oxygen potential vs. O/M ratio relationship in Am-doped MOX. The reason might be that interactions between U and Am predominate and those between Pu and Am can be neglected [ ]. According to the authors, this should result in U being oxidized to a pentavalent state since Am is likely to be trivalent [ ].
We recently performed a coupled X-Ray Diffraction (XRD) and X-Ray Absorption Spectroscopy (XAS) characterization on substoichiometric Am-containing MOX [ ]. Our results show that Am reduction to Am+III is completed before any reduction of Pu is yet observed. Surprisingly, no pentavalent U was evidenced and the behaviour of Am tends to solely influence that of Pu.

In the present work, to go deeper into the underlying phenomenon involved in the correlated redox behavior of Pu and Am, we focused on U-free sub-stoichiometric (Pu,Am)O2-x compounds. Three compositions were prepared via conventional solid-state reaction under Ar/5%H2 (1800K; 50h): (Pu0.992Am0.008)O2-x, (Pu0.9Am0.1)O2-x and (Pu0.8Am0.2)O2-x. XRD and XAS analyses were combined to selectively probe the long-range and the short-range order and the local environment of both cations. In particular, X-ray Absorption Near Edge Structure (XANES) allows to quantitatively determining the oxidation states for each cation. XRD shows a monophasic system for each composition. Interestingly, the variation of the cell parameter vs. Am concentration is not linear.
In the presentation, we shall detail and compare XRD and XAS results. Especially, we will focus on the correlation between Am content, cell parameter and ratios of Am+3/Am+4 and of Pu+3/Pu+4. We believe our results will improve comprehension of the role of Americium in the overall reduction process occuring in Americium-doped MOX fuels. They may also contribute to the development of the chemical thermodynamic models used to describe such materials.

Keywords: americium; plutonium; MOX; nuclear fuel; SFR; XAS; XRD

Related publications

  • Lecture (Conference)
    GLOBAL 2011, 04.-09.09.2011, Nagoya, Japan

Permalink: https://www.hzdr.de/publications/Publ-15184
Publ.-Id: 15184