Ken Nash

The role of carboxylic acids in TALSQuEAK separations

Recent reports have indicated that Trivalent Actinide–Lanthanide Separation by Phosphorus reagent Extraction from Aqueous Komplexes (TALSPEAK)-type separations chemistry can be improved through the replacement of bis-2-ethyl(hexyl) phosphoric acid (HDEHP) and diethylenetriamine-N,N,N′,N″,N″-pentaacetic acid (DTPA) with the weaker reagents 2-ethyl(hexyl) phosphonic acid mono-2-ethylhexyl ester (HEH[EHP]) and N-(2-hydroxyethyl)ethylenediamine-N,N′,N′-triacetic acid (HEDTA), respectively. This modified TALSPEAK has been provided with an adjusted acronym of TALSQuEAK (Trivalent Actinide–Lanthanide Separation using Quicker Extractants and Aqueous Komplexes). Among several benefits, TALSQuEAK chemistry provides more rapid phase transfer kinetics, is less reliant on carboxylic acids to mediate lanthanide extraction, and allows a simplified thermodynamic description of the separations process that generally requires only parameters available in the literature to describe metal transfer. This article focuses on the role of carboxylic acids in aqueous ternary (M-HEDTA-carboxylate) complexes, americium/lanthanide separations, and extraction kinetics. Spectrophotometry (UV-Vis) of the Nd3+ hypersensitive band indicates the presence of aqueous ternary Nd–Lac–HEDTA species (Lac = lactate, K 111 = 1.83 ± 0.01 at 1.0 mol L−1 ionic strength, Nd(HEDTA) + Lac− ⇄ Nd(HEDTA)Lac−). While lower levels (0.1 mol L−1 vs. 1.0 mol L−1) of carboxylic acid will still be necessary to control pH and encourage phase transfer of the heavier lanthanides, application of different carboxylic acids does not have an overwhelming impact on Ln/Am separations or extraction kinetics relative to conventional TALSPEAK separations. TALSQuEAK separations come to equilibrium in two to five minutes depending on the system pH using only 0.1 mol L−1 total lactate or citrate.

Two-Phase Calorimetry. II. Studies on the Thermodynamics of Cesium and Strontium Extraction by Mixtures of H+CCD− and PEG-400 in FS-13

Abstract Thermochemical characterization of the partitioning of cesium and strontium from nitric acid solutions into mixtures of the acid form of chlorinated cobalt dicarbollide (H+CCD−) and polyethylene glycol (PEG-400) in FS-13 diluent has been completed using isothermal titration microcalorimetry and radiotracer distribution methods. The phase transfer reaction for Cs+ is a straightforward (H+ for Cs+) cation exchange reaction. In contrast, the extraction of Sr2+ does not proceed in the absence of the co-solvent molecule PEG-400. This molecule is believed to facilitate the dehydration of the Sr2+ aquo cation to overcome its resistance to partitioning. The phase transfer reactions for both Cs+ and Sr2+ are enthalpy driven (exothermic), but partially compensated by an unfavorable entropy. The results of the calorimetry studies suggest that the PEG-400 functions as a stoichiometric phase transfer reagent rather than acting simply as a phase transfer catalyst or phase modifier. The calorimetry results also demonstrate that the extraction of Sr2+ is complex, including evidence for both the partitioning of Sr(NO3)+ and endothermic ion pairing interactions in the organic phase that contribute to the net enthalpic effect. The thermodynamics of the liquid-liquid distribution equilibria are discussed mainly considering the basic features of the ion solvation thermochemistry.

Chemistry of radioactive materials in the nuclear fuel cycle

Abstract: From the days of the Manhattan Project, the chemistry of actinides and selected fission products has shaped decisions on the handling of irradiated nuclear fuel. This chemistry is characterized by the diversity of the fission products, the rich redox chemistry of the light actinides, high radiation levels, concentrated nitric acid used to dissolve the fuel and the nuclear chemistry of both actinides and fission product lanthanides. This chapter introduces the actinide and fission product chemistry relevant to the nuclear fuel cycle, from the isolation of uranium from mined ores through reprocessing to management of the byproduct wastes. The important features of historically successful solvent extraction separations and alternative chemical processes are described. Finally, the role of nuclear energy as a source of primary power sans greenhouse gases is discussed.

Managing Zirconium Chemistry and Phase Compatibility in Combined Process Separations for Minor Actinide Partitioning

NEUP Program Supporting Fuel Cycle R&D Separations and Waste Forms call DE-FOA-0000799 requests long term R&D projects focusing on streamlining separation processes for advanced fuel cycles. An example of such a process relevant to the U.S. DOE FCR&D program would be one combining the functions of the TRUEX process for partitioning of lanthanides and minor actinides (from PUREX/UREX raffinates) with that of the TALSPEAK process for separating transplutonium actinides from fission product lanthanides. Experience teaches (and it has been demonstrated at the lab scale) that, with proper control, multiple process separation systems can be made to operate successfully. However, it is also recognized that considerable economies of scale could be achieved if multiple operations were merged into a single process based on a combined extractant solvent. Work is underway in the U.S. and Europe on developing several new options for combined processes (TRUSPEAK, ALSEP, SANEX, GANEX, ExAm are examples). There are unique challenges associated with the operation of such processes, some relating to organic phase chemistry, others arising from the variable composition of the aqueous medium. This project targets two general problematic issues in designing combined process systems: managing the chemistry of challenging aqueous species and optimizing the composition and properties of combined extractant organic phases. The primary focus areas of this research program are 1) developing improved information on the thermodynamics of problematic fission product zirconium and 2) developing a framework for an organic phase solvation model based on molecular-scale interactions between extractant molecules and organic solvent molecules. Both zirconium chemistry and “diluent effects” have been investigated in the prior literature. As each issue represents a particularly challenging obstacle to combined process development and the details of the science remain largely unresolved, this project aims to build on existing information with carefully designed and executed investigations of each subject and integration of the parallel thrusts where it is reasonable to do so. The research team includes Professor Nathalie Wall (PI), WSU who has recently reported results on the application of radiotracer-based solvent extraction studies of Tc(IV) complexes, Professor Kenneth L. Nash (WSU), Dr. Leigh R. Martin (Idaho National Lab), and Dr. Cécile Marie (CEA-Marcoule, France). The collaboration team brings expertise in actinide/lanthanide chemistry, separation science and the thermodynamics and kinetics of metal ligand interactions. Drs. Marie, Martin, and Nash have collaborated previously on similar investigations. The overarching objectives of this investigation are to improve the state of knowledge on zirconium chemistry in the nuclear fuel cycle, to add new analysis tools to support development of long term improvements in the predictability of fuel cycle separations systems, and ultimately to enable the creation of compact and efficient separations methods for advanced nuclear fuel reprocessing.

Radiochemistry Education at Washington State University: Sustaining Academic Radiochemistry for the Nation

Since 2002, Washington State University has been building radiochemistry as a component of its overall chemistry program. Using an aggressive hiring strategy and leveraged funds from the state of Washington and federal agencies, six radiochemistry faculty members have been added to give a total of seven radiochemists out of a department of twenty‐five faculty members. These faculty members contribute to a diverse curriculum in radiochemistry, and the Chemistry Department now enjoys a significant increase in the number of trainees, the quantity of research expenditures, and the volume and quality of peer‐reviewed scientific literature generated by the radiochemistry faculty and the trainees. These three factors are essential for sustaining the radiochemistry education and research program at any academic institution.