X-ray absorption spectroscopy study of solid solutions

This talk, presented by Carol Valot for a team led by Philippe Martin, focused on characterization of the solid solutions generated by a new procedure for mixed-oxide (MOX) fuel manufacturing under development at Commissariat à l’Energie Atomique (CEA Atalante) in Marcoule, France.

Currently, MOX fuels are manufactured by cocrushing, palletizing, and sintering uranium oxide (UO₂) and plutonium oxide (PuO₂) powders, yielding a fluorite-type solid solution.

In the new CEA technique, oxalic acid (H₂C₂O₄4) coprecipitation of uranium and plutonium with subsequent conversion to the oxides substitutes for mechanical mixing and is proposed to yield a better homogeneity of the uranium and plutonium in the solid. The goal is to obtain uranium oxide and plutonium oxide solid solutions with minimal impurities and an oxygen-to-metal ratio of 2:1.

To assess the success of their method, this CEA team used X-ray absorption fine structure (XAFS) spectroscopy as a local probe of atomic and electronic structure in the crystalline

solid solution—in which the different chemical species occupy lattice points (characteristic of a solid) somewhat at random (characteristic of a solution). Four different molar ratios of Pu(III)/U(IV) were investigated (final plutonium atomic percents of 50, 30, 15, and 7).

A known feature of these MOX solid solutions, referred to as “Vegard’s Law,” is that the lattice parameter of U_1-yPu_yO₂ changes linearly with y from pure UO₂ to pure PuO₂. In this case X-ray crystallographic data were consonant with the law’s predictions.

On the other hand, the local atomic environments revealed by XAFS exhibited some distinct variations from prediction. For each of the four plutonium atomic percents, extended X-ray absorption fine structure (EXAFS) revealed a face-centered-cubic structure, and the deduced cell parameters suitably followed Vegard’s Law. For the 50-percent plutonium solid solution, the correspondence was ideal—for both actinides, the overall shape of the spectra remaining the same

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