Kenneth L. Nash

ACTINIDE PARTITIONING—A REVIEW

Reagents and methods that have been developed during the past 20 years for hydrometallurgical partitioning of actinides from different types of transuranium (TRU) wastes and dissolved fuels are reviewed. Emphasis is placed on the extraction performance of the fully-optimized reagents rather than on the structural iterations that were undertaken (and in some cases are still being conducted) to identify the optimum species. Particular attention is paid to separation processes that have been demonstrated in batch and counter-current solvent extraction, and batch and column mode extraction chromatography. The salient features of the various techniques and reagents for actinide recycle are compared. Sections of the review focus on neptunium behavior in hydrometallurgy and on characterization of those reagents best suited to the separation of trivalent actinides from fission product lanthanides. Selected flowsheets that have been reported for the separation and recovery of actinides from TRU wastes are presented.

A REVIEW OF THE BASIC CHEMISTRY AND RECENT DEVELOPMENTS IN TRIVALENT f-ELEMENTS SEPARATIONS

ABSTRACT Among the most difficult of separations of metal ions are the intra- and intergroup separation of Ianthanides and trivalent actinides. Yet environmental concerns related to radioactivity, and new high-tech developments which have increased the demand for pure lanthanides have combined to foster a greater need for effective procedures to attain these separations. The most recent review which concentrated on this problem is now nearly 20 years old. While the methods developed over the preceeding 50-60 years of research are still in use, new understanding and some promising new methods are currently under development. The present manuscript is partly a tutorial of the basic chemical principles leading to successful separations, and partly a review of the results of the past 10-12 years on the separation of trivalent lanthanides and actinides, both intra-, and intergroup.

Separations Chemistry for Actinide Elements: Recent Developments and Historical Perspective

Abstract With the end of the cold war, the principal mission in actinide separations has changed from production of plutonium to cleanup of the immense volume of moderately radioactive mixed wastes which resulted from fifty years of processing activities. In order to approach the cleanup task from a proper perspective, it is necessary to understand how the wastes were generated. Most of the key separations techniques central to actinide production were developed in the 40′s and 50′s for the identification and production of actinide elements. Total actinide recovery, lanthanide/actinide separations, and selective partitioning of actinides from inert constitutents are currently of primary concern. To respond to the modern world of actinide separations, new techniques are being developed for separations ranging from analytical methods to detect ultra-trace concentrations (for bioassay and environmental monitoring) to large-scale waste treatment procedures. In this report, the history of actinide separations,...

f-element complexation by diphosphonate ligands.

To develop a fundamental understanding of the nature of lanthanide and actinide coordination complexes with ligands bearing phosphonate groups, we have examined the kinetics, thermodynamics and spectroscopy of lanthanide and actinide complexes with phosphonate ligands in aqueous solutions and resolved crystal structures for selected lanthanide complexes. The combined results of two-phase distribution experiments, titration calorimetry, laser-induced fluorescence decay, and molecular mechanics modeling have been used to infer the. structure of 1:2 complexes of Eu(III) with gem-diphosphonic acids in aqueous solution. A complementary crystallographic and FTIR investigation of the structure of lanthanide complexes in the solid state indicates a general similarity between amorphous and crystalline complexes, differing in the relative hydration of the solid. There are no inner sphere water molecules in the solid state. Parallel studies of the kinetics of formation and dissociation of f-element diphosphonate complexes demonstrate that the relative stability of these complexes is governed primarily by differences in the rate of complex formation. A related investigation has established the existence of a correlation between complex formation rate and cationic radius which is independent of the charge on the cation.

Hydrolytic and Radiolytic Degradation of Oφd(iB)Cmpo: Continuing Studies

Abstract The hydrolytic and radiolytic degradation of octyl(phenyl)-N, N-diisobutylcarbamoyl-methylphosphine oxide (CMPO) in tetrachloro-ethylene (TCE) and in tributylphosphate-TCE mixtures (TRUEX process solvent) in contact with HNO3 has been investigated. Gas chroma-tographic analysis of the degraded solutions has allowed identification and quantitation of a variety of organophosphorus degradation compounds derived from CMPO. The effect of degradation of the extractant solutions on the extraction and stripping of americium has been addressed, as has the effectiveness of Na2CO3 scrub procedures for restoring extractant performance. Extrapolation of these results to normal process application conditions indicates that neither hydrolysis nor radiolysis should compromise the americium extraction performance of TRUEX-TCE.

Stability constants for europium(III) complexes with substituted methane diphosphonic acids in acid solutions

Abstract The stability constants of europium complexes with methane diphosphonic (MDPA), ethane-1,1- diphosphonic (EDPA), 1-hydroxyethane-1,1-diphosphonic (HEDPA), vinylidene-1,1-diphosphonic (VDPA) and 1,2-dihydroxyethane-1,1-diphosphonic (DHEDPA) acids were determined by solvent extraction distribution experiments in 2.0 M Na/HNO 3 media at 25 °C. The protonation constants for these tetrabasic acids were also determined by potentiometric titration. The latter two diphosphonic acids have not been previously reported in the open literature. Stability constants for a variety of protonated complexes with M:L stoichiometries varying from 1:1 to 1:3 are indicated in the least-squares analysis of the distribution data. Calculations based on the thermodynamic data indicate that diphosphonic acids are superior complexing agents for europium compared to carboxylic acids and aminopolycarboxylic acids in solutions of acidity greater than 0.01 M. The implication of these complexes for possible applications in separation processes is discussed.

ANALYTICAL-SCALE SEPARATIONS OF THE LANTHANIDES: A REVIEW OF TECHNIQUES AND FUNDAMENTALS

Separations chemistry is at the heart of most analytical procedures to determine the rare earth content of both man-made and naturally occurring materials. Such procedures are widely used in mineral exploration, fundamental geology and geochemistry, material science, and in the nuclear industry. Chromatographic methods that rely on aqueous solutions containing complexing agents sensitive to the lanthanide cationic radius and cation-exchange phase transfer reactions (using a variety of different solid media) have enjoyed the greatest success for these procedures. In this report, we will briefly summarize the most important methods for completing such analyses. We will discuss in some detail the basic aqueous (and two-phase) solution chemistry that accounts for separations that work well and offer explanations for why others are less successful.

FUNDAMENTAL INVESTIGATIONS OF SEPARATIONS SCIENCE FOR RADIOACTIVE MATERIALS

ABSTRACT Fundamental investigations of solvent extraction and ion exchange separations of radioactive materials have been conducted within the National Laboratory system of the U. S. Department of Energy (and its predecessor agencies) for the past 50 years. Basic research conducted at Oak Ridge and Argonne National Laboratories has produced both high quality new science and important applications in nuclear technology. The present contribution is an attempt to summarize the most important scientific results arising from this research during the past 10 years, a time of great change in the nuclear separations field, and to suggest possible directions for the next stage of research and development in this field.

The role of triaxiality in the ground states of even-even neutron-rich Ru isotopes

Abstract Partial decay schemes for the very neutron-rich nuclei 108,110,112 Ru and 114 Ru, about which nothing was previously known, have been determined from the study of prompt, triple-γ coincidences in 248 Cm fission fragments. Levels with probable spin up to 10ħ have been observed and γ-ray branching ratios obtained. Ratios of reduced electric quadrupole transition probabilities obtained from the branching ratios are in good agreement with predictions of a simple model of a rigid triaxial rotor.

Actinide immobilization in the subsurface environment by in-situ treatment with a hydrolytically unstable organophosphorus complexant: Uranyl uptake by calcium phytate

An attractive approach to decreasing the probability of actinide migration in the subsurface is to transform the ions into a less mobile form by remote treatment. We have under development a process that relies on a polyfunctional organophosphorus complexant to sequester the mobile metal ions by complexation/cation exchange. The insoluble complex subsequently decomposes, transforming the actinides into phosphate mineral forms as a thermodynamically stable isolation medium. In this report, we briefly describe the process with focus on the cation exchange behavior of the calcium salt of the organophosphorus sequestrant.