Aqueous Complexation of Thorium(IV), Uranium(IV), Neptunium(IV), Plutonium(III/IV), and Cerium(III/IV) with DTPA

Aqueous complexation of Th(IV), U(IV), Np(IV), Pu(III/IV), and Ce(III/IV) with DTPA was studied by potentiometry, absorption spectrophotometry, and cyclic voltammetry at 1 M ionic strength and 25 °C. The stability constants for the 1:1 complex of each trivalent and tetravalent metal were calculated. From the potentiometric data, we report stability constant values for Ce(III)DTPA, Ce(III)HDTPA, and Th(IV)DTPA of log β₁₀₁ = 20.01 ± 0.02, log β₁₁₁ = 22.0 ± 0.2, and log β₁₀₁ = 29.6 ± 1, respectively. From the absorption spectrophotometry data, we report stability constant values for U(IV)DTPA, Np(IV)DTPA, and Pu(IV)DTPA of log β₁₀₁ = 31.8 ± 0.1, 32.3 ± 0.1, and 33.67 ± 0.02, respectively. From the cyclic voltammetry data, we report stability constant values for Ce(IV) and Pu(III) of log β₁₀₁ = 34.04 ± 0.04 and 20.58 ± 0.04, respectively. The values obtained in this work are compared and discussed with respect to the ionic radius of each cationic metal

Closing the Nuclear Fuel Cycle with a Simplified Minor Actinide Lanthanide Separation Process (ALSEP) and Additive Manufacturing

Expanded low-carbon baseload power production through the use of nuclear fission can be enabled by recycling long-lived actinide isotopes within the nuclear fuel cycle. This approach provides the benefits of (a) more completely utilizing the energy potential of mined uranium, (b) reducing the footprint of nuclear geological repositories, and (c) reducing the time required for the radiotoxicity of the disposed waste to decrease to the level of uranium ore from one hundred thousand years to a few hundred years. A key step in achieving this goal is the separation of long-lived isotopes of americium (Am) and curium (Cm) for recycle into fast reactors. To achieve this goal, a novel process was successfully demonstrated on a laboratory scale using a bank of 1.25-cm centrifugal contactors, fabricated by additive manufacturing, and a simulant containing the major fission product elements. Americium and Cm were separated from the lanthanides with over 99.9% completion. The sum of the impurities of the Am/Cm product stream using the simulated raffinate was found to be 3.2 × 10−3 g/L. The process performance was validated using a genuine high burnup used nuclear fuel raffinate in a batch regime. Separation factors of nearly 100 for 154Eu over 241Am were achieved. All these results indicate the process scalability to an engineering scale

Toward Mechanistic Understanding of Nuclear Reprocessing Chemistries by Quantifying Lanthanide Solvent Extraction Kinetics via Microfluidics with Constant Interfacial Area and Rapid Mixing

The closing of the nuclear fuel cycle is an unsolved problem of great importance. Separating radionuclides produced in a nuclear reactor is useful both for the storage of nuclear waste and for recycling of nuclear fuel. These separations can be performed by designing appropriate chelation chemistries and liquid-liquid extraction schemes, such as in the TALSPEAK process (Trivalent Actinide-Lanthanide Separation by Phosphorus reagent Extraction from Aqueous Komplexes). However, there are no approved methods for the industrial scale reprocessing of civilian nuclear fuel in the United States. One bottleneck in the design of next-generation solvent extraction-based nuclear fuel reprocessing schemes is a lack of interfacial mass transfer rate constants obtained under well-controlled conditions for lanthanide and actinide ligand complexes; such rate constants are a prerequisite for mechanistic understanding of the extraction chemistries involved and are of great assistance in the design of new chemistries. In addition, rate constants obtained under conditions of known interfacial area have immediate, practical utility in models required for the scaling-up of laboratory-scale demonstrations to industrial-scale solutions. Existing experimental techniques for determining these rate constants suffer from two key drawbacks: either slow mixing or unknown interfacial area. The volume of waste produced by traditional methods is an additional, practical concern in experiments involving radioactive elements, both from disposal cost and experimenter safety standpoints. In this paper, we test a plug-based microfluidic system that uses flowing plugs (droplets) in microfluidic channels to determine absolute interfacial mass transfer rate constants under conditions of both rapid mixing and controlled interfacial area. We utilize this system to determine, for the first time, the rate constants for interfacial transfer of all lanthanides, minus promethium, plus yttrium, under TALSPEAK process conditions, as a first step toward testing the molecular mechanism of this separation process

Countercurrent Actinide Lanthanide Separation Process (ALSEP) Demonstration Test with a Simulated PUREX Raffinate in Centrifugal Contactors on the Laboratory Scale

An Actinide Lanthanide Separation Process (ALSEP) for the separation of trivalent actinides (An(III)) from simulated raffinate solution was successfully demonstrated using a 32-stage 1 cm annular centrifugal contactor setup. The ALSEP solvent was composed of a mixture of 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester (HEH[EHP]) and N,N,N′,N′-tetra-(2-ethylhexyl)-diglycolamide (T2EHDGA) in n-dodecane. Flowsheet calculations and evaluation of the results were done using the Argonne’s Model for Universal Solvent Extraction (AMUSE) code using single-stage distribution data. The co-extraction of Zr(IV) and Pd(II) was prevented using CDTA (trans-1,2-diaminocyclohexane-N,N,N′,N′-tetraacetic acid) as a masking agent in the feed. For the scrubbing of co-extracted Mo; citrate-buffered acetohydroxamic acid was used. The separation of An(III) from the trivalent lanthanides (Ln(III)) was achieved using citrate-buffered diethylene-triamine-N,N,N′,N″,N″-pentaacetic acid (DTPA), and Ln(III) were efficiently back extracted using N,N,N′,N′-tetraethyl-diglycolamide (TEDGA). A clean An(III) product was obtained with a recovery of 95% americium and curium. The Ln(III) were efficiently stripped; but the Ln(III) product contained 5% of the co-stripped An(III). The carryover of Am and Cm into the Ln(III) product is attributed to too few actinide stripping stages, which was constrained by the number of centrifugal contactors available. Improved separation would be achieved by increasing the number of An strip stages. The heavier lanthanides (Pr, Nd, Sm, Eu, and Gd) and yttrium were mainly routed to the Ln product, whereas the lighter lanthanides (La and Ce) were mostly routed to the raffinate

Recovery of High Specific Activity Molybdenum-99 From Accelerator-Induced Fission on Low-Enriched Uranium for Technetium-99m Generators

A new process was developed to recover high specific activity (no carrier added) 99Mo from electron-accelerator irradiated U3O8 or uranyl sulfate targets. The process leverages a novel solvent extraction scheme to recover Mo using di(2-ethylhexyl) phosphoric acid following uranium and transuranics removal with tri-n-butyl phosphate. An anion-exchange concentration column step provides a final purification, generating pure 99Mo intended for making 99Mo/99mTc generators. The process was demonstrated with irradiated uranium targets resulting in more than 95% 99Mo recovery and without presence of fission products or actinides in the product

Aqueous Complexation of Thorium(IV), Uranium(IV), Neptunium(IV), Plutonium(III/IV), and Cerium(III/IV) with DTPA

Aqueous complexation of Th­(IV), U­(IV), Np­(IV), Pu­(III/IV), and Ce­(III/IV) with DTPA was studied by potentiometry, absorption spectrophotometry, and cyclic voltammetry at 1 M ionic strength and 25 °C. The stability constants for the 1:1 complex of each trivalent and tetravalent metal were calculated. From the potentiometric data, we report stability constant values for Ce­(III)­DTPA, Ce­(III)­HDTPA, and Th­(IV)­DTPA of log β₁₀₁ = 20.01 ± 0.02, log β₁₁₁ = 22.0 ± 0.2, and log β₁₀₁ = 29.6 ± 1, respectively. From the absorption spectrophotometry data, we report stability constant values for U­(IV)­DTPA, Np­(IV)­DTPA, and Pu­(IV)­DTPA of log β₁₀₁ = 31.8 ± 0.1, 32.3 ± 0.1, and 33.67 ± 0.02, respectively. From the cyclic voltammetry data, we report stability constant values for Ce­(IV) and Pu­(III) of log β₁₀₁ = 34.04 ± 0.04 and 20.58 ± 0.04, respectively. The values obtained in this work are compared and discussed with respect to the ionic radius of each cationic metal

Demonstration of the ALSEP Process in Centrifugal Contactors using Spiked Simulated Raffinate Solution

An ALSEP process demonstration test for the separation of trivalent actinides from spikedsimulated high active raffinate solution was run in a 32-stage 1 cm annular centrifugalcontactor setup. The solvent consisted of 0.5 mol L−1 HEH[EHP] and 0.05 mol L−1 T2EHDGAin n-dodecane. A flow sheet was calculated with the AMUSE code using single stage distributiondata, taking into account the constraint of the available number of 16 centrifugalcontactors. The co-extraction of Zr(IV) and Pd(II) was efficiently prevented using 0.05 molL−1 CDTA as a masking agent. Co-extracted Mo was scrubbed using 0.75 mol L−1 AHA and0.175 mol L−1 ammonium citrate at pH 3. The separation of An(III) from Ln(III) was achievedusing 0.015 mol L−1 DTPA and 0.2 mol L−1 ammonium citrate at pH 2. Finally, Ln(III) wereefficiently back extracted using 0.2 mol L−1 TEDGA in 0.5 mol L−1 HNO3.A fairly clean An(III) product was obtained with a recovery of 95% Am(III) + Cm (III). TheLn(III) were efficiently stripped by the hydrophilic TEDGA complexant, but the Ln(III)product contained 5% of co-stripped An(III). The carryover of An(III) into the Ln(III)product is attributed to too few actinide stripping stages, which was constrained by theavailable number of contactors.The results of the demonstration test will be presented and discussed

Demonstration of the ALSEP Process in Centrifugal Contactors using Spiked Simulated Raffinate Solution

An ALSEP process demonstration test for the separation of trivalent actinides from spikedsimulated high active raffinate solution was run in a 32-stage 1 cm annular centrifugalcontactor setup. The solvent consisted of 0.5 mol L−1 HEH[EHP] and 0.05 mol L−1 T2EHDGAin n-dodecane. A flow sheet was calculated with the AMUSE code using single stage distributiondata, taking into account the constraint of the available number of 16 centrifugalcontactors. The co-extraction of Zr(IV) and Pd(II) was efficiently prevented using 0.05 molL−1 CDTA as a masking agent. Co-extracted Mo was scrubbed using 0.75 mol L−1 AHA and0.175 mol L−1 ammonium citrate at pH 3. The separation of An(III) from Ln(III) was achievedusing 0.015 mol L−1 DTPA and 0.2 mol L−1 ammonium citrate at pH 2. Finally, Ln(III) wereefficiently back extracted using 0.2 mol L−1 TEDGA in 0.5 mol L−1 HNO3.A fairly clean An(III) product was obtained with a recovery of 95% Am(III) + Cm (III). TheLn(III) were efficiently stripped by the hydrophilic TEDGA complexant, but the Ln(III)product contained 5% of co-stripped An(III). The carryover of An(III) into the Ln(III)product is attributed to too few actinide stripping stages, which was constrained by theavailable number of contactors.The results of the demonstration test will be presented and discussed

X-ray switch for rare earth element adsorption to a liquid interface

Ions at liquid surfaces and interfaces influence many scientific and technological areas, including molecular and nanoparticle assembly for energy and separations processes. Controlled transport of ions between interfacial and bulk liquids can lead to triggering ion-induced interfacial phenomena. Here, we show that X-ray exposure alters the competitive equilibrium of rare earth elements bound to chelating ligands in bulk water and to insoluble monolayers at the water surface. Controlling the X-ray exposure leads to reversible adsorption of rare earth trivalent ions to the liquid surface. Evidence for the exposure-induced temporal variations in the ion surface density is provided by synchrotron X-ray fluorescence near total reflection (XFNTR) measurements. Varying the X-ray penetration depth from 10 nm to 2.8 µm leads to a controlled exposure of either the surface region alone or the surface monolayer plus dissolved chelating ligands and bulk water. This separation of surface and bulk processes helped identify the role of aqueous radiolysis in the adsorption cycle. Comparison of different chelates identified amine binding sites as a contributor to the cycling mechanism. The primary molecules utilized for these studies – chelating ligand DTPA and organophosphoric acid extractant DHDP – are like those used in the separation of rare earth elements from ores and in the reprocessing of nuclear fuel. The observed reversible cycling of ion adsorption may provide an opportunity for further control over these processes and enhanced separation