(514e) 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.

1. Allen, M.P., Tildesley, D.J. Computer Simulation
of Liquids. Oxford Science Publications, Oxford, NY, 1987

2. Anisimov, et.al.; J. of Chem.
Theory and Computation 2005, 1, 153-168

3. Tkac, P. et.al.; J. Chem. Eng. Data. 2009, 54, 1967-1974

4. Crouse, D. J.; Arnold, W. D.; Hurst, F. J.; Alternate
Extractants to Tributyl Phosphate for Reactor Fuel

Reprocessing. Oak Ridge National Laboratory, Oak Ridge, TN, 1983

5. Horwitz, E. P., Schulz, W.
W. The Truex Process: A Vital Tool for Disposal of U.S. Defense Related
Nuclear

Waste. New Separation
Chemistry Techniques for Radioactive Waste and Other Specific Applications. pp

21-29. Argonne National Laboratory, Argonne, IL.
1991