Thermodynamics and Speciation of Actinide Materials in the Condensed Phase using Atomistic Simulations

Actinide materials are central to the fission-based nuclear energy systems. The challenges with the nuclear energy generation are the safety of nuclear power plants, the disposal of nuclear waste, and the possible diversion of nuclear material for the production of nuclear weapons. These problems must be addressed to make nuclear energy safer and cheaper. A deep understanding and characterization of actinide materials are required to design the next-generation nuclear energy systems. This thesis focuses on actinide materials in aqueous phase using computational methods. Di-oxo actinide cations (AnO$^{n+}2$, $n$=1, 2) or better known as actinyl ions are commonly found and important species in solutions and in nuclear fuel cycle. Speciation, thermodynamic and transport properties of the actinyl ions are of the primary focus in the present work. The most appropriate way to study these properties are by using molecular dynamics (MD) simulations. Central to MD simulations are the force field parameters, which model the interactions between atoms. First, a general approach is developed to obtain force fields for actinide systems via electronic structure-- quantum-mechanical (QM) calculations which include important many body solvation effects. Effective pairwise additive force field parameters for AnO$2^{n+}$--water interactions (An = U, Np, Pu, Am and $n$ = 1, 2) were obtained using the proposed approach. The force field utilizes a simple Lennard-Jones plus electrostatics form, and can be further used to extend the force field to other systems using mixing rules. Further, intramolecular force fields for stretching and bending of the actinyl ions were developed from QM calculations. Developed force field parameters were used in MD simulations to simulate actinyl ions in water. The predicted hydration free energies and water coordination numbers for actinyl ions match well with the available experimental data. Studying the dynamic properties, actinyl ions were found to diffuse with five water molecules as an 'iceberg'', resulting in considerably slower diffusion when compared to pure water. Monocation actinyl ions diffuse only slightly faster than their dication counterparts. Mechanisms and rate constants for the water exchanges between the first and outer coordination shells of actinyl ions were studied. An associative interchange exchange mechanism was observed for +2 charged actinyl ions, whereas a dissociative mechanism was observed for +1 charged actinyl ions. Factors governing the exchange were identified: O=An=O angle flexibility for +2 actinyl ions, whereas distance between central actinide and water's oxygen atoms for +1 actinyl ions matter. Obtained results are in good agreement with the available literature. Further, the interactions between actinyl ions and several ligands commonly found in aqueous phase $viz.$ Na$^{+}$, Cl$^{-}$, OH$^{-}$, F$^{-}$, NO$_{3}^{-}$, CO$_{3}^{2-}$, and SO$_{4}^{2-}$ were studied. Potential of mean force and stability constants were obtained for the interactions. Trends and values of stability constants match reasonably well with the experimental results. Further, spatial distribution functions were studied to identify various contact ion pairs, solvent shared ion pairs and solvent separated ion pairs in the aqueous solution. In the end, we focused on actinide nano-scale clusters which are made by the self-assembly of 20-60 actinyl polyhedra. Building blocks for some of the nano-scale clusters-- uranyl peroxide and uranyl phosphite were studied in the aqueous phase by first developing force fields from QM methods and using them in MD simulations. Predicted structural properties agree well with those from the experiments. The force field parameters proposed for building blocks of actinide nanoclusters paves the way to simulate the nanoclusters. The large nanoclusters could be simply filtered out, thus have the potential to be very useful in nuclear waste separation. The study of actinide systems in this work are helpful in characterization and understanding of the physio-chemical behavior of actinides at the molecular level. Such an understanding is very fundamental and instrumental in designing new experiments, in meeting the specific design goals, and in building the scientific basis of advanced nuclear systems. Finally, the computational methodologies used and developed in this work can be extended to other systems as well.