Extracting the Americyl Hydration from an Americium Cationic Mixture in Solution: A Combined X-ray Absorption Spectroscopy and Molecular Dynamics Study

Abstract

Am(VI) solution chemistry differs from that of lighteractinoids, as U, Pu, and Np, where the actinyl [AnO2]2+is the most stableform and plays an important role in nuclear fuel technology. The behaviorof americium in solution shows the trend to stabilize lower oxidation states,mainly Am(III). Riddle and co-workers recently reported the EXAFS andfirst XANES spectra of an americium-containing aqueous solution wherethe americyl species is detected in a mixture. We have developed Am3+−H2O and [AmO2]2+−H2O intermolecular potentials based on quantum-mechanical calculations to carry out classical MD simulations of these twocations in water. Structural information extracted from the statisticaltrajectories has been used to simulate EXAFS and XANES spectra of both solutions. For the Am3+case the theoretical−experimental agreement for both EXAFS and XANES spectra is satisfactory. This is not the case for the [AmO2]2+aqueoussolutions. However, when an aqueous solution mixture of both cationic forms in a 55/45 [AmO2]2+/Am3+ratio is considered, thetheoretical−experimental agreement is recovered. EXAFS and XANES spectra which would correspond to a pure [AmO2]2+aqueous solution are proposed. In the XANES case, the main features characterizing the simulated spectrum are consistent withthose previously found in the experimental XANES spectra of stable [UO2]2+and [PuO2]2+in water.