Role of Cation Interactions in the Reduction Process in Plutonium–Americium Mixed Oxides

Abstract

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.

Room-temperature oxidation of hypostoichiometric uranium–plutonium mixed oxides U1−yPuyO2−x – A depth-selective approach

“…As an illustration, a previous study shown that PuF 3 results are identical to those obtained for (U(IV),Pu(III)) mixed oxalates [35]. The latter being a similar system to that of the current study, the use of PuF 3 as the Pu(III) reference is thus adequate.…”

“…The position of white line maximum was determined by the first zero-crossing of the first derivative. UO 2 , U 4 O 9 , U 3 O 8 , PuO 2 and PuF 3 were used as references for the oxidation states of the uranium [29,31,32] and plutonium [29,[33][34][35] cations, respectively. Although the empty density of state probed by XANES is different between PuF 3 and plutonium oxides, the core-hole lifetime (8.7 eV) at…”

“…XAS using high energy light source generated with a synchrotron, the probed depth in U 1-y Pu y O 2-x compounds is equal to tens of microns [35]. The grain size of the fabricated samples being of the same scale as the synchrotron X-rays probing depth, the whole grains were then illuminated by the X-ray source.…”

“…Despite the great care taken to limit exposure to the glove-box atmosphere when preparing the XAS samples, the experiment did not reveal a difference in the O/M ratio values. Because hard X-rays penetrates up to tens of microns in the mixed oxide [35] which matches the grain size (30-40 µm), it can be deduced that spontaneous oxidation was neither avoided nor limited to the surface of the grains but propagated through the whole grains.…”

Oxygen potential measurement of (Pu0.928Am0.072)O2–xat high temperatures

“…However, recently, Belin et al [26] measured the Pu and Am valences in (Pu 1-y Am y )O 2-x by X-ray absorption near edge structure (XANES) and proved that Pu reduction from Pu 4 + to Pu 3 + started after all Am was reduced. Therefore, this model was reasonably adopted to analyze the Ḡ o 2 using the point defect chemistry.…”