Heavy Element Chemistry Program
Actinide Chemistry Research Program
The LBNL Actinide Chemistry Research Program portfolio addresses several aspects of the DOE BES Grand Challenges, Discovery Research, and Use-Inspired Research that will contribute new fundamental knowledge and understandings of actinide science necessary for a secure energy future, particularly from the underlying perspective of nuclear energy. Examples from the LBNL program relevant to these DOE research objectives include: controlling actinide synthesis bond-by-bond; developing an understanding of f-electron behavior in a diverse range of actinide materials; developing new experimental techniques for spectroscopic investigations of actinides; understanding the chemical interactions from which future advanced separations could be based; and exploring fundamental actinide molecular chemistry in the gas phase.
ACTINIDE COORDINATION AND ORGANOMETALLIC CHEMISTRY
Transuranic Coordination Chemistry
Since the invention of the atomic bomb in 1945, the energy potential of the actinides has impacted the world; actinides are used in power generation, military applications, and space exploration. These applications have generated many environmental and health concerns. The risks of environmental contamination and exposure to the population, caused by accidental or intentional release of actinides, have created a need for improved methods for the production and reprocessing of reactor fuel materials for increased efficiency and reduced waste. There is also a need for safe and effective chelating agents for decorporation and decontamination, as well as for separation technologies. However, our fundamental knowledge of the chemistry of the actinides is relatively limited. Understanding the coordination properties and solution behavior of f-element complexes will enable us to address issues central to the priority missions of the U. S. Department of Energy (DOE), including the development of ligands for in vivo decorporation, and new technologies for environmental cleanup and hazardous waste reduction in contaminated sites.
DFT-minimized structure of Eu(3,4,3-Li-1,2-HOPO) - (left) and a schematic of the
The goal of this program is to prepare organometallic and coordination compounds of the f-block elements that show the differences and similarities of the chemical and physical properties among the f-elements and between the f- and d-transition metals. The synthetic goal is to prepare new and unusual molecules and to study their chemical and physical properties in order to show the role that f-electrons play in bonding.
SOLID STATE ACTINIDE CHEMISTRY
Structural, Electronic, and Magnetic f-Electron Interactions in Intermetallic Systems
The magnetic and electronic properties of f-electron intermetallics are determined, in part, by the Kondo and Rudermann-Kasuya-Kittel-Yosida (RKKY) interactions. The competition between these effects gives rise to a wide range of exotic behavior, such as heavy fermions, intermediate valence, and possibly non-Fermi liquids and superconductivity. Lattice disorder is often a determining component, giving rise to glassy magnetic states and possibly non-Fermi liquid behavior. Another determining component is the degree of localization of the f-shell, a particularly vexing quantity in, for instance, the elemental d- form of plutonium. These issues are explored with a focus on the lanthanide and actinide 115’s, such as PuCoGa5. Local structure studies are utilized to search for lattice disorder and determine substitution sites, such as in PuCoGa5-xInx, to help determine and explain properties such as the dimensionality of the superconducting state.
(Left) RIXS scans for PuSb2 and d–Pu. Note the clearly broadened maximum in d–Pu compared to
Electronic Structure and Surface Chemistry of Actinides Using Soft X-Ray Synchrotron Radiation
Microspectroscopic and fluorescence-based techniques have been exploited for the investigations of actinide materials with soft X-ray synchrotron radiation (SR) because of the capability to perform experiments with small amounts of actinide materials. The objectives are to elucidate and understand the roles of the 5f electrons in the chemical bonding of the actinides; characterize the electronic structures of actinide materials; develop surface chemistry of actinide materials; optimize soft X-ray SR techniques for actinide investigations: and develop new solid state synthetic methods. The fundamental knowledge gained from the investigations will have impact in areas of actinide science related to f-electron bonding, separations, aging, corrosion, gas-solid interactions, actinide transport, future nuclear energy technologies, the development of nanoscale actinide materials characterization techniques, and will advance scientific needs related to improved energy utilization.
N K-edge XES spectra at 425 eV yielding the N 2p occupied
ACTINIDE CHEMISTRY IN THE GAS PHASE AND SOLUTION
Gas Phase Actinide Chemistry
A better understanding of the molecular chemistry of the 5f actinide series of elements is crucial to developing advanced technologies for their use in stewardship, energy, medical, defense and other applications. Several components of the DOE BES Heavy Element Chemistry program focus on bonding and reactivity of actinide molecules and complexes, with a particular emphasis on the prevalent, chemically complex, scientifically important, and problematically long-lived alpha-emitting actinides, thorium through curium. An overall goal of this subtask is to provide an underpinning for a better understanding of actinide molecular and solution properties. This is directly relevant to needs in actinide chemistry, particularly in environmental remediation and actinide materials stewardship.
PEP for reaction 1b: UO3H2- + CH3OH; in the
Actinide Solution Chemistry: Chemical Thermodynamics and Structure of Actinide Complexes
This subtask is focused on the chemical thermodynamics and structure of actinide complexes. Thermodynamic parameters of the complexation as well as structural information on actinide complexes are obtained. This research program has two focus areas: 1) chemical thermodynamics of actinide complexation (including hydrolysis) at elevated temperatures; and 2) thermodynamics and structure of actinide complexes with ligands of interest in actinide separations and environmental transport. Results from this research provide insight into the fundamental aspects of actinide coordination in solution and support the development of advanced actinide separation processes and safe management of nuclear wastes.
Hydrolysis constants of Pu(VI),10 U(VI)2 and Np(V)3 at variable temperatures. βn,m is the equilibrium constant for the hydrolysis reaction: mAn + nH2O = Anm(OH)n + nH, where An denotes the actinides and the electric charges on the species are not shown for clarity.
Heavy Element/Nuclear Radiochemistry Program
One of the most fundamental goals in chemistry is the study of the chemical properties of the elements. We use the Berkeley Gas-filled Separator (BGS) combined with the 88-Inch Cyclotron for synthesis, identification, and chemistry studies of new heavy element isotopes, studies that are unique within the U.S. With the combination of high-intensity heavy-ion beams from the 88-Inch Cyclotron and the high efficiency and selectivity of the BGS, the Lawrence Berkeley National Laboratory (LBNL) Heavy Element and Nuclear Radiochemistry Group will continue to make important contributions in the study of the production, nuclear decay properties, and chemistry of the heaviest elements.
Schematic of the BGS with the RF gas-stopper cell, RFQ trap, trochoid mass separator, and low-background detector system.
Meeting the Challenges Posed by Advanced Nuclear Energy Systems: Understanding and Controlling Actinide and Fission Product Chemistry and Radiation Effects in Actinide Materials (SISGR)
Understanding f-Orbital Bonding
The role that 5f orbitals play in structure and bonding remains poorly understood; consequently, one Scientific Grand Challenge stated in the 2006 report Basic Research Needs for Advanced Nuclear Energy Systems (ANES) is “Resolving the f-electron challenge to master the chemistry and physics of actinides and actinide-bearing materials.” Meeting this challenge requires fundamental advances in understanding the bonding of actinides in systems of increasing complexity. The central theme of this subtask is to explore the role of f-electrons in bonding, and to determine chemical and physical properties. The nature of f-electrons in bonding is far less well understood than the role that d-electrons play in d-block compounds, but modern techniques, including synchrotron radiation methods and mass spectrometry coupled to theory, show promise for elucidating how f-orbitals affect the chemical and physical properties of f-block compounds and materials.
|Structures of the 5LIO-backbone ligands with different sensitizing moieties.|
Exploiting f-Orbital Bonding in Separations
This research task addresses the fundamental coordination chemistry of actinides and lanthanides, which has a significant impact and direct bearing on actinide separations processes, including specific sequestration of actinides and selective extraction and separation of actinides from lanthanides and other fission products in the advanced nuclear fuel cycle. This need is reflected by the ANES Scientific Grand Challenge, “Understanding and designing new molecular systems to gain unprecedented control of chemical selectivity during processing.” Certain soft-donor ligands are highly selective in solvent extraction of trivalent actinides over trivalent lanthanides. However, the thermodynamic principles and structural factors that govern the selective extraction remain unclear. In addition, while there have been extensive in vivo studies on sequestration of actinides,fundamental studies are still needed to obtain the thermodynamic parameters for actinide complexation under physiological conditions and to identify the structures of actinide complexes with the sequestering agents, so that more efficient and more specific sequestering agents can be designed and developed.
Structures of (a) eight S, (b) eight S and one O, and (c) seven S and one O in the first coordination shell. The small light blue atom in the center is the metal (Ln or An). S is in yellow, O in red, C in green, and H in white.15
Understanding and Controlling the Redox Chemistry of Tc
The redox chemistry of Tc creates problems for the safe expansion of nuclear energy. 99Tc is an abundant (~5% yield from 235U or 239Pu fission), long-lived (t1/2 = 213,000 yr) fission product, and presents challenges for the incorporation of Tc from reprocessing waste streams into durable waste forms. The research in this subtask is aimed at understanding the redox chemistry during denitration of a Tc-bearing nitric acid waste stream, during incorporation of Tc into iron oxides using aqueous routes, during heat treatment of Tc in oxides, and during the corrosion of ceramic waste forms. Ultimately, this information can be used to control the oxidation state of Tc so that the desired oxidation state may be incorporated into ceramic waste forms.
Left: combined Pourbaix diagrams for Tc and Fe. Note that the Tc(IV) regime has been expanded to lower Eh in the combined figure reflecting the fact that Tc(IV) is the kinetic product of TcO4- reduction although Tc-metal is the thermodynamic product. Diagram is from ref. 30 Center: structure of Fe3O4. Right: structure of TiFe2O4.