Molecular Dynamics Solvent Extraction Modeling for Spent Nuclear Fuel Applications

Molecular Dynamics Solvent Extraction Modeling for Spent Nuclear Fuel Applications

Presenter: Brian Gullekson, Oregon State University

Molecular Dynamics (MD) computational modeling is a powerful technique for characterizing molecular scale chemical behavior.  Individual molecular models are placed randomly into a simulation space with allotted system energy and allowed to equilibrate, displaying the chemical mechanics of chemical systems of interest, such as solvent extraction of spent nuclear fuel.  This technique is useful for identifying properties such as ion complex types, third phase development as a function of solute loading, reverse micelle size, and D-values for specific metal-ligand complexes.  The techniques for developing these models are well characterized, but are not currently widely utilized as a method of characterizing spent nuclear fuel reprocessing chemistry other than purely tri-butyl phosphate (TBP) systems or predicting chemical behavior for development of new extraction schemes.  These areas of development possess great potential for further expansion of MD simulations.

 Before performing dynamics on large, multi-component systems, individual molecular models must first be created which accurately represent physically observed macroscopic chemical properties, such as fluid density and interfacial tension.  These molecules are assigned characteristic bond angles and dihedrals specific to their molecule types.  Charge fields are also assigned through Lennard-Jones potentials for representing van der Walls interactions and partial coulombic charges for representing polar centers.  Charges are assigned by quantum characterization of electron orbital structure and subsequent corrections to bridge the gap between simplifications made in the execution of this characterization and observed physical parameters.¹  Further accuracy can be obtained by assigning classical Drude oscillator particles to individual atoms in the molecules of interest to simulate polarizability using spring dynamics.²  This technique generally increases the computational expense of a simulation, but provides a means for increasing the physical accuracy of created simulations.

Upon the creation of individual models, extraction systems can be modeled by placing bulk aqueous and bulk organic layers adjacent to each other and performing time dynamics, allowing for phase dispersion characteristic of solvent extraction processes.  Displacement of molecules is computed by Newtonian dynamics resultant from the forces local to individual atomic sites.  The overall system potential energy is defined as the deformation energies of bonds and bond angles, as well as intermolecular forces such as coulombic repulsion.  As these forces direct the system toward thermodynamic equilibrium, characteristics typical of solvent extraction processes can be observed while phases are still interacting, a useful technique to studying the evolution of these systems from interaction to equilibrium. 

The development of new extraction models, such as simulations for currently employed extraction techniques possess several advantages and potential prohibiting factors.  Of recent interest in spent nuclear fuel reprocessing are synergistic extraction techniques which effectively separate all transuranic elements from other fission products, optimizing high level waste disposal and effective fuel cycle management.³ These techniques employ a number of different extractants, such as CMPO, HDEHP, and various acid ions in the aqueous phase, each used for different purposes in the extraction process.  MD modeling can be used to further the fundamental understanding of the mechanism through which these extractants perform chemical separations.  Furthermore, optimization studies can be performed by MD simulations, as a wide variety of extraction parameters can be tested without the added burden or chemical procurement, waste disposal, and radiation and non-proliferation protection.  These systems, however, will require very large simulations in order to minimize inherent statistical errors.  This need for large simulation sizes could cause simulations to become prohibitively computationally expensive without proper means of minimizing system size while preserving accurate extraction behavior.  Furthermore, incorrect system initialization may cause dynamics to produce physically unrealistic situations.  Proper system initialization which incorporates both minimized equilibration time and accurate physical behavior is an area which must be researched prior to varying testing conditions for efficient use of computational resources.

Typical PUREX models, incorporating nitric acid, TBP, and n-dodecane have been well characterized with MD modeling techniques, but variants on this system have not yet been well documented.  A logical place to begin expanding into new extractant molecules is to develop molecules similar to TBP, but with extended alkane chains from the central phosphate group.  Extractants such as tri-hexylphosphate (THP) and tri(2-ethylhexyl) phosphate (TEHP) have been shown to possess favorable extractant characteristics, such as increased solute loading before third phase formation and less susceptibility to radiolysis.⁴  Furthermore, TEHP is an analog to HDEHP, an extractant used in conjunction with CMPO in the TRUEX process for the extraction of even trace amounts of higher trivalent actinides such as Am(III) and Cm(III).⁵  Development of these models was simplified by the fact that they are very similar to models which have already been created and well documented.  Their development however, has followed a logical sequence toward the development of more advanced extraction systems, and will serve as a backbone in the development of a comprehensive TRUEX MD simulation.

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