Separations Research for Advanced Nuclear Fuel Cycles - ACS

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Chapter 2

Separations Research for Advanced Nuclear Fuel Cycles T. A. Todd* Laboratory Fellow and Director, Fuel Cycle Science and Technology, Division Idaho National Laboratory, P.O. Box 1625, Idaho Falls, ID 83415 *[email protected]

The United States Department of Energy has been conducting research into advanced separation methods for the recycle of used nuclear fuel components for the last decade. Separation of certain used fuel constituents allows for improved waste management (by developing waste forms tailored for specific long-lived radioisotopes) and transmutation of long-lived actinide elements. One incentive for processing used fuel is to reduce the time that the overall radiotoxicity of the used fuel is greater than that of natural uranium ore. Spent fuel must be managed for geologic time scales (300,000+ years) while fuel that is processed to recycle and transmute actinide elements requires management for engineering time scales (hundreds of years). Efficient separation processes are needed for treatment of current light water reactor used fuel and future fast transmutation fuel (metal or oxide). Low process losses and product purity sufficient to meet fuel specifications are needed.

Introduction The United States has recently commissioned a “blue ribbon commission” to provide recommendations for developing a safe, long-term solution to managing the Nation’s used nuclear fuel and nuclear waste (1). One strategy that will likely be considered by the commission is the treatment of used nuclear fuel to recycle actinide elements for their unused energy value, as well as to transmute higher actinides. The United States Department of Energy has been researching advanced separation methods for the recycle of used nuclear fuel components © 2010 American Chemical Society In Nuclear Energy and the Environment; Wai, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


for about the last decade (2, 3). The focus of the current research program is long-term, science-based research into advanced recycle technologies. Paramount to any transmutation scenario is the separation of transuranium actinides, primarily plutonium, neptunium, americium and curium from the rest of the used fuel constituents. A number of technologies have been recently studied and demonstrated in laboratory-scale experiments (4, 5). Additional examples are provided in the following chapters of this symposium series book. The potential benefits of separating out the actinides from used fuel for reuse include utilizing their unused energy potential and transmutation to shorten the time they are radioactive. Used nuclear fuel that is directly disposed to a repository retains a radiotoxicity (relative to natural uranium ore) that requires management of the fuel for several hundred thousand years. By separating the actinides and recycling them into reactors (either thermal or fast) the radiotoxicity decreases dramatically and for the scenario of recycle in fast spectrum reactors, is less radiotoxic than natural uranium ore in several hundred years. This equates to the responsible management of nuclear wastes for geologic time scales in the case of direct disposal or engineering time scales in the case of separation/transmutation. There are still some very long lived fission products that would require disposal in a repository, (iodine-129, technetium-99 and Cs-135), but because their half-lives are very long (15.9 million years, 213,000 years, and 2.3 million years, respectively) their specific activity (i.e. radiotoxicity) is quite low. The radiotoxicity (normalized to natural uranium ore) for used fuel directly disposed and with the uranium and transuranics recycled is shown in Figure 1. Additionally, any geologic repository will have to manage heat from the decay of used fuel or radioactive waste products. By removing plutonium and americium from the fuel, transmuting and not placing it in the repository, the peak long-term heat load in the repository is greatly reduced. This was a major benefit for the proposed Yucca Mountain repository, and would likely benefit other repository designs as well, but to a lesser extent. The U.S. separations research program has recently shifted from an applied process development/demonstration focus to a more fundamental research program. It is recognized that any successful process must have a solid understanding of the process chemistry before it could be successfully implemented. A primary focus of the research program it to develop robust, simplified methods of minor actinide separation (americium or americium and curium) from lanthanides. Research into thermodynamics, kinetics, coordination chemistry, radiation chemistry and interfacial transport mechanisms is currently underway for aqueous and electrochemical technologies. Additionally, research into innovative separation technologies, involving reactive gases and/or volatility and alternative process chemistries such as carbonate chemistry is in progress.

Minor Actinide Separation The U.S. Fuel Cycle Research and Development (FCR&D) program is researching new methods of separating the minor actinides (Am, Cm) from 14 In Nuclear Energy and the Environment; Wai, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


Figure 1. Used nuclear fuel radiotoxicity vs. time (see color insert) lanthanides, as well as methods to partition americium from curium. Specific areas of research are focused on: • • • •

Developing a more robust one-step An/Ln separation process Developing a broad operating envelope of An/Ln separation processes through better fundamental understanding Developing a viable method to separate Am from Cm and Ln Establishing a strong scientific basis for future process selection

The primary benefit of this research is expected to be a significant simplification of fuel recycle methods, resulting in overall improvement in economics and increased acceptability of the nuclear fuel cycle. One of the underlying aspects of developing a one-step An/Ln separation process as well as developing a broad operating envelope and scientific basis, is to understand the competing factors in current separation processes such as the TALSPEAK process (6). This process, and related processes such as the SANEX process (7), rely on the strong complexation of actinides and lanthanides by an organic extractant such as di-2-ethylhexylphosphoric acid (HDEHP), and the complexation of the actinides in the aqueous phase by another complexant such as diethylenetriaminepentacetic acid (DTPA), in the presence of a buffer such as lactic acid. Investigations of the thermodynamic and kinetic effects of the TALSPEAK process have led to the development of a preliminary thermodynamic model. However, this model predicts a different pH dependence on An (III) and Ln (III) distribution ratios than is observed experimentally (8). Possible causes for this discrepancy include ternary metal complex formation involving lactate and HDEHP in the organic phase, a ternary metal complex formation involving lactate and DTPA in the aqueous phase, activity coefficient variation, possible micellization behavior, and kinetic effects. It is expected that as a greater understanding of the TALSPEAK chemistry is developed, this knowledge will 15 In Nuclear Energy and the Environment; Wai, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


translate to other An/Ln separation chemistry applications (such as alternative extractants) as well. Purposeful manipulation of ternary metal complexes in the aqueous phase in a TALSPEAK-type extraction system is another avenue of research being investigated to effect Am/Cm selectivity. Research into appropriately sized aqueous complexants has shown some promise in fitting the secondary ligand in the open coordination site, making complexation more favorable for the slightly larger Am (III) cation. Another approach to a simplified An (III) separation is the combining of the extractants of the TRUEX and TALSPEAK processes. The hypothesis being explored is that the combined extractant would extract An (III) and Ln (III) elements in nitric acid with selective stripping of An (III) occurring in a buffered, complexant solution. This approach is discussed in Chapter 10 by Lumetta et al. Molecular modeling is being employed to identify and select targets for synthesis of soft-donor ligands in the bis(dithiophosphinic) acid family. This effort is focused on identifying ligands that are designed for optimal Am(III) binding, and therefore provide some selectivity for Am over Cm and the lanthanides. The above methods seek to identify ligands or ligand systems that selectively complex Am(III) in the presence of Ln(III). In a different approach, selective extraction of Am (V) or (VI) from Cm (III) and Ln (III) has been demonstrated in organic solvents (9). The greatest challenge with this approach is being able to hold the Am in the higher oxidation state long enough to extract it. Another approach would be to extract Am (III) with Cm (III) and Ln (III) in a TRUEX-like extractant, and then selective strip Am (V) from the loaded solvent. A third approach would be to utilize an actinyl-selective complexant that would stabilize Am in the pentaor hexavalent state. Electrochemical methods for separation of Am (IV) or (VI) from Cm (III) and Ln (III) in room temperature ionic liquids is another approach being investigated. Finally, methods to separate Am (III) from Ln (III) using alkaline carbonate conditions or inorganic ion exchangers are also being investigated. It can be seen that this science-based approach is investigating multiple possible solutions to the problem of actinide separations, and is thus expected to yield an optimal solution.

Fundamental Separations Research Understanding the fundamental properties of separation processes is a major effort of the FCR&D program. A key activity that supports the minor actinide separation activities is developing an understanding of the thermodynamic and kinetic behavior of the trivalent actinides and lanthanides in the TALSPEAK process using microcalorimetry, fluorescence and conventional spectrophotometric techniques, radiochemical techniques, and stopped flow kinetics. This work builds on recently developed methodologies and will focus on a) studying the behavior of the fundamental thermodynamic parameters ΔGº, ΔHº and ΔSº in relation to changes in the extraction media; b) characterization of the solution chemistry parameters for the actinides and lanthanides within the operational envelope of the TALSPEAK process. It is envisaged that the results of 16 In Nuclear Energy and the Environment; Wai, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


these studies will be useful in improving the TALSPEAK process and/or provide a useful starting point for the development of alternative Am/Cm separation methods. Another key area of research is the area of radiation chemistry (10, 11). Radiation chemistry may adversely affect the separations process in a number of ways. In addition to direct radiation damage, the formation of free radicals in solution may also damage organic solvents indirectly. Gamma and alpha radiolysis are of particular interest, as well as understanding radiolysis mechanisms needed to develop predictive tools/models. In the past, most work involved performing solvent extraction experiments on irradiated solvents. Today, a more fundamental chemical investigation includes steady state radiolysis and identification of decomposition products as well as pulse radiolysis to understand the kinetics of free radical reactions with solutes. These data will be important in the overall development of any separation process in the presence of high radiation fields that exist in used nuclear fuel.

Electrochemical Technologies Electrochemical technologies offer another approach to processing used nuclear fuel (12). This approach is particularly important for the recycle of transuranic elements from metal fast reactor fuel. The current research approach utilizes electrochemical separation in a molten KCl/LiCl salt phase. The focus of R&D for electrochemical technology will be to demonstrate key technical issues directly related to the feasibility of a sustainable electrochemical technology. Of particular importance are TRU recovery (losses and purity), understanding of the process operating envelope over a broad range of conditions, removal of TRU and fission products from salt (to enable salt recycle and improve economics) development and optimization of spent salt and metallic waste forms, and on-line near-real time process monitoring and control.

Transformational Separation Technologies A new area of research is in the area of innovative separation technologies that could result in a new approach to used fuel treatment, beyond conventional methods using nitric acid dissolution/solvent extraction or electrochemical processing in molten salts. This approach may utilize alternative media (e.g. ionic liquids, supercritical fluids, alkaline chemistry, gaseous phase, etc) to facilitate advanced separation methods. The goal of any new process would be to simplify processing, and maintain needed selectivity.

Summary Treatment of used nuclear fuel to separate actinides and recycle them for additional energy and transmutation of long-lived isotopes can provide substantial benefit in the long-term management of used fuel. For a closed fuel cycle to be cost effective, new approaches to used fuel separations are being investigated, as well 17 In Nuclear Energy and the Environment; Wai, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.

as developing a greater understanding of the fundamental chemistry of separation processes.

Acknowledgments


Work supported by the U.S. Department of Energy, Office of Nuclear Energy, under DOE Idaho Operations Contract DE-AC07-05ID14517.

References 1.

Secretary Chu Announces Blue Ribbon Commission on America’s Nuclear Future, 2010. U.S. Department of Energy. http://www.energy.gov/news/ 8584.htm. 2. Todd, T. A.; Felker, L. K.; Vienna, J. D.; Bresee, J. C.; Lesica, S. The Advanced Fuel Cycle Initiative Separations and Waste Campaign: Accomplishments and Strategy. In Proceedings of Global 2009, Paris, September 2009. 3. Todd, T. A.; Wigeland, R. A. Advanced Separation Technologies for Processing Spent Nuclear Fuel and the Potential Benefits to a Geologic Repository. In Separations for the Nuclear Fuel Cycle in the 21st Century; Lumetta, G. J., Nash, K. L., Clark, S. B., Friese, J. I., Eds.; ACS Symposium Series 933; American Chemical Society: Washington, DC, pp 41−55. 4. Periera, C.; Vandegrift, G. F.; Regalbuto, M. C.; Bakel, A. J.; Laidler, J. J. A Summary of the Lab-Scale Demonstrations of UREX+ Processes at Argonne National Laboratory. In Proceedings of Global 2007, Boise, ID, September 2007. 5. Jubin, R. T.; et al. CETE R&D at the Oak Ridge National Laboratory Supporting Management of Nuclear Waste. In Proceedings of Waste Management ’09, Phoenix, AZ, March 2009. 6. Weaver, B.; Kappelmann, F. A. J. Inorg. Nucl. Chem. 1968, 30, 263–272. 7. Hill, C.; Guillaneux, D.; Berthon, L. SANEX-BTP Process Development Studies. In Proceedings of the International Solvent Extraction Conference (ISEC), 2002; 1205−1209. 8. Nilsson, M.; Nash, K. L. Solvent Extr. Ion Exch. 2009, 27, 354–377. 9. Mincher, B. J.; Martin, L. R.; Schmitt, N. C. J. Inorg. Chem. 2008, 47, 6984–6989. 10. Mincher, B. J.; Modolo, G.; Mezyk, S. P. Solvent Extr. Ion Exch. 2009, 27, 1–25. 11. Martin, L. R.; Mexyk, S. P.; Mincher, B. J. J. Phys. Chem. A. 2009, 113, 141–145. 12. Li, S. X.; Johnson, T. A.; Westphal, B. R.; Goff, K. M.; Benedict, R. W. Experience for Pyrochemical Processing of Spent EBR-II Driver Fuel. In Proceedings of Global 2005, Tsukuba, Japan, October 2005.

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Separations Research for Advanced Nuclear Fuel Cycles - ACS

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