Green Separation Techniques for Nuclear Waste Management - ACS

Chapter 5

Downloaded by EMORY UNIV on March 8, 2016 | http://pubs.acs.org Publication Date (Web): October 15, 2010 | doi: 10.1021/bk-2010-1046.ch005

Green Separation Techniques for Nuclear Waste Management Chien M. Wai* Department of Chemistry, University of Idaho, Moscow, ID 83844 *[email protected]

Recent developments utilizing green solvents, ionic liquids and supercritical fluid carbon dioxide (sc-CO2), for managing nuclear wastes are summarized in this chapter. Direct dissolution of uranium dioxide and lanthanide oxides in sc-CO2 can be accomplished with a CO2-soluble tri-n-butylphosphate-nitric acid complex. On-line separation of uranium and lanthanides dissolved in the sc-CO2 phase can be achieved by a countercurrent stripping technique. Using room temperature ionic liquid coupled with sc-CO2 for extraction of uranium from acidic solutions and for dissolution of solid uranium dioxide are also described. Potential applications of these green separation techniques for decontamination of nuclear wastes and for reprocessing spent fuels are discussed.

Introduction Replacing traditional fossil fuels with environmentally sustainable energy sources is necessary for curbing the threat of global warming. Nuclear power is one of the known energy sources which is free of carbon emission and has a power production cost comparable to that of gas (1). Currently nuclear power contributes to about 20% of the electricity generated in the USA whereas in France over 80% of the electricity is generated by nuclear energy. One public concern for expanding use of nuclear energy in the USA is the economic and environmental issues associated with the wastes produced by nuclear power generation. Traditional methods of treating nuclear wastes and reprocessing spent fuel require acid solutions and organic solvents for dissolution and separation of radioactive elements with an unavoidable consequence of generating large © 2010 American Chemical Society Wai and Mincher; Nuclear Energy and the Environment ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


quantities of liquid wastes. Minimizing waste generation in the nuclear fuel cycle is an important factor to make nuclear energy environmentally sustainable and acceptable to the public. Utilizing green solvents for treating nuclear wastes and for reprocessing spent fuel is one approach of minimizing waste generation in the nuclear fuel cycle (2). Supercritical fluid carbon dioxide (sc-CO2) and ionic liquids (ILs) are considered green solvents for chemical reactions and separations (3, 4). Research in utilizing sc-CO2 as a solvent for dissolution and extraction of metal species started nearly two decades ago (5, 6). The successful demonstration of extracting lanthanides and uranium from aqueous solutions using CO2-soluble ligands in the early 1990s led to the speculation that sc-CO2 extraction could be an attractive green technology for nuclear waste management because it does not require conventional liquid solvents. Later reports showed that direct dissolution of lanthanide oxides and uranium dioxide in sc-CO2 could be achieved with a high efficiency using a CO2-soluble TBP-nitric acid complex (7–9). These reports established the value of the supercritical fluid technology for management of nuclear wastes. Current industrial interest in supercritical fluid technology for nuclear waste management is demonstrated by a project undertaken by AREVA for recovering enriched uranium from the incinerator ash generated by the light-water reactor fuel fabrication process (10). The process is discussed in detail by S. Koegler in Chapter 6 of this book. Possibilities of utilizing sc-CO2 technology for reprocessing spent nuclear fuels have also been investigated by other research projects (11, 12). Room temperature ionic liquids (RTILs) have unique properties including non-flammable nature, near zero vapor pressure and high solubility for a variety of ionic and neutral compounds (1, 13, 14). These properties make them attractive for replacing volatile organic solvents traditionally used in various liquid-liquid extractions. However, separation of dissolved metal species from RTILs by conventional methods such as acid stripping, distillation, or organic solvent back-extraction is often inefficient and results in production of secondary wastes. It is known that sc-CO2 can dissolve in RTILs effectively whereas latter have negligible solubilities in the fluid phase (15). This property allows easy recovery of dissolved metal species in RTIL by sc-CO2 without contamination or loss of the IL. A combination of IL dissolution and sc-CO2 extraction may provide another green approach for managing nuclear wastes (16). This chapter summarizes current status of sc-CO2 extraction technology for nuclear waste management with an emphasis on industrial applications. Recent developments in IL dissolution of lanthanide and actinide oxides and their subsequent extraction and separation by sc-CO2 are discussed. One unique property of RTILs is their unusual solvation character, which is reflected in their ability to dissolve both ionic species and neutral compounds. Little is known at the present time regarding the chemical environments of uranyl and other actinide species dissolved in RTILs. In this chapter, some information based on spectroscopic studies of uranyl species dissolved in a RTIL are also given to illustrate the coordination environments of uranyl in the ionic liquid phase.

54 Wai and Mincher; Nuclear Energy and the Environment ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


Dissolution and Separation of Lanthanides and Actinides in Sc-CO2 Direct dissolution of uranium dioxide (UO2) in sc-CO2 using a CO2soluble tri-n-butylphosphate (TBP)-nitric acid complex of the general form TBP(HNO3)x(H2O)y is well-established in the literature. The complex is prepared by shaking TBP with concentrated nitric acid (15.5 M) leading to the formation of a Lewis acid-Lewis base complex in the TBP phase (17). The composition of the complex is controlled by the relative amounts of TBP and the acid used in the preparation. With an equal volume of TBP and concentrated nitric acid, the complex formed has a chemical composition of TBP(HNO3)1.8(H2O)0.6 (Figure 1). Because TBP is highly soluble in sc-CO2, it serves as a carrier for introducing HNO3 into the supercritical fluid phase with a minimum amount of water. The HNO3 in the complex oxidizes UO2 from +4 oxidation state to the +6 oxidation state uranyl ion (UO2)2+ followed by the formation of UO2(NO3)2(TBP)2, which is soluble in the supercritical fluid phase. According to one report, UO2(NO3)2(TBP)2 has a solublility of 0.45 moles per liter in sc-CO2 at 40 °C and 200 atm (18). This is close to the uranium concentration utilized in the PUREX (plutonium uranium extraction) process. The supercritical fluid dissolution method, which combines dissolution and extraction of uranium dioxide in sc-CO2 in one step without formation of an aqueous phase, is attractive for treating uranium containing wastes. In conventional solvent-based extraction processes, uranium dioxide is first dissolved in an acid solution (typically 3-6 M nitric acid) followed by extracting the dissolved uranyl ions from the acid solution into an organic solvent (such as dodecane or kerosene) containing TBP. The sc-CO2 dissolution/extraction process produces far less liquid wastes compared with conventional solvent-based extraction methods. The sc-CO2 dissolution process is particularly effective for removing uranium from densely packed solid materials because sc-CO2 is capable of penetrating into small pores in solid matrixes. One example is the extraction of enriched uranium from incinerator ash generated by the light-water nuclear reactor fuel fabrication process (9, 19). The fuel fabrication process involves conversion of enriched UF6 gas (about 3.5% 235U) to solid UO2 for nuclear power production. Disposable solid waste generated by the process is reduced by a factor of about 1/20 through a carefully controlled incineration process. The incinerator ash contains approximately 8-10% by weight of enriched uranium, a valuable material for the nuclear industry. Some gadolinium (Gd), which is added to the fuel as a neutron absorber, is also present in the ash. AREVA NP in Richland, Washington has recently demonstrated that sc-CO2 containing TBP(HNO3)1.8(H2O)0.6 is very effective for removing uranium from the incinerator ash. Gadolinium and some alkaline earth metals are also dissolved in the sc-CO2 phase. Recovery of uranium from the sc-CO2 phase is achieved by a countercurrent column stripping process illustrated in Figure 2. In this process, the sc-CO2 carrying the dissolved gadolinium and uranium is fed into a stripping column from the bottom while nitric acid is introduced into the column through a nozzle from the top. The highly acidic aqueous phase strips the Gd3+ into the aqueous phase. The aqueous phase containing Gd3+ is collected in a reservoir at 55 Wai and Mincher; Nuclear Energy and the Environment ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


Figure 1. Preparation of TBP(HNO3)1.8(H2O)0.6 extractant by shaking an equal volume (5 mL each) of TBP and concentrated nitric acid (15.5 M); left: before shaking; right: after shaking. the bottom of the column. The exiting sc-CO2 from the first column is fed into a second column from the bottom and water is introduced into the column from the top. If the acidity of the aqueous phase in contact with the sc-CO2 is low, (UO2)2+ can be stripped into the aqueous phase. TBP and some uranium remaining in the sc-CO2 phase are recycled for repeated extraction of the ash. Separation of gadolinium and uranium in this process is based on the fact that UO2(NO3)2(TBP)2 and Gd(NO3)3(TBP)x exist in the sc-CO2 phase at different HNO3 concentrations. Typically, Gd is stripped around 3-4 M nitric acid whereas UO2 is stripped at

Green Separation Techniques for Nuclear Waste Management - ACS

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