A Molecular Dynamics Study of Actinide Nanoclusters

A Molecular Dynamics Study of Actinide Nanoclusters

Ken Newcomb, Edward J. Maginn

The actinides are an important class of materials to modern society, due to their role in nuclear power generation, nuclear waste treatment, and national security. Recent experimental work has led to the characterization of a new class of materials: actinide nanoclusters¹. UO₂ groups self-assemble in aqueous solution to form the closed clusters, ranging in size from 1.5 to 3.0 nm in diameter. The nanoclusters could play an important role in nuclear fuel reprocessing, since they can be readily separated from aqueous solution using ultrafiltration. We have developed a classical force field for the simulation of U₂₀ nanoclusters in water, parameterized from highly accurate quantum-mechanical calculations. The clusters carry a large negative charge, which causes cations in solution to occupy locations within and around the cluster. These cations are known to play a key role in cluster aggregation. but the exact positions and quantity of cations within the cluster are hard to determine experimentally. By performing molecular dynamics simulations, the free energy profile associated with bringing various cations (Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺) from the bulk into the clusters has been calculated, revealing a process hindered by two significant barriers. Using this method, the equilibrium distribution of cations around the cluster was determined.

References:

1) Burns, P. C., Kubatko, K., Sigmon, G., Fryer, B. J., Gagnon, J. E., Antonio, M. R., & Soderholm, L. (2005). Actinyl Peroxide Nanospheres. Angewandte Chemie Angew. Chem., 117(14), 2173-2177. doi:10.1002/ange.200462445