Editorial pubs.acs.org/CR
Introduction to Nuclear Chemistry radiation dose of a nuclear repository, as short-lived fission products have decayed after several hundreds of years. The earlier actinides (U, Np, Pu, Am, and Cm) display a very rich redox chemistry. They can exist in a variety of oxidation states, ranging from +3 to +7 for neptunium and plutonium, from +3 to +6 for americium, and from +3 and +4 for curium. It is noteworthy that Pu(IV), Pu(V), and Pu(VI) can coexist in oxic aqueous solutions due to their similar redox potentials. Geochemical transport models require, as input, accurate thermodynamic data of solid and solution radionuclide species that may form via chemical reactions with groundwater components and the surrounding geomedia. Altmaier and colleagues summarize the advances of aqueous actinide chemistry and thermodynamics. Results of these studies are important contributors to thermodynamic databases assessing the risk of migration of radionuclides in potential waste repositories and contaminated environments. Furthermore, data need to be available for elevated temperatures because the decay heat of nuclear waste stored in an underground repository can generate temperatures up to several hundred degrees. Knope and Soderholm discuss experimental studies of actinide hydrates and their hydrolysis and condensation products that provide direct structural information of metal− ligand coordination numbers and bonding distances in solution. Recent refinements of existing techniques, such as intensity increases and spatial resolution improvements for synchrotronbased X-ray absorption fine structure (EXAFS) studies, and the application of X-ray scattering techniques to solution species, especially high-energy X-ray scattering (HEXS), have been instrumental to significant advances of the field. Such studies provide tangible metrics for an improved interpretation of thermodynamic solution measurements and are essential for calibration and refinement of theoretical molecular modeling calculations. To continue the theme of actinide speciation in the environment, Walther and Denecke present a review of the origin, distribution, and analysis of natural and anthropogenic actinide colloids and particles generated from thorium and uranium minerals, reactor accidents in Chernobyl and Fukushima, above-ground nuclear testing, accidents during the transport of nuclear devices, and depleted uranium ammunition and shielding. These colloids are considered important species for the transport of specific radionuclides in aqueous media. Tetravalent plutonium ions can form or become associated with colloidal sized particles. This represents another vector for transporting actinides through the geosphere. Plutonium in this form will not behave like a dissolved species and may exhibit considerably different migration behavior than the dissolved species. Depending on their size and charge relative to the surrounding porous media, colloids can move more rapidly or
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uclear chemistry has its origin in the research of Marie and Pierre Curie, who isolated the first radioactive elements, polonium and radium, from several tons of uranium ore in 1898. With the increased understanding of the atom stemming from the research of Rutherford, Bohr, and many other scientists in the beginning of the last century, the phenomenon of radioactivity was recognized as a nuclear process. During the ensuing years of the twentieth century, the field has grown and matured to a highly diverse discipline. In addition to frontier research, nuclear chemistry has found essential uses and applications in a broad variety of applied disciplines, such as nuclear medicine and isotope production, radio-pharmaceutical preparation, nuclear stockpile stewardship and security, surveillance of clandestine nuclear activities, nuclear power, separation science, nuclear waste processing and minimization, waste isolation and remediation of nuclearcontaminated sites, and prediction and monitoring of behavior of actinides in the environment, to name only a few. This thematic issue of Chemical Reviews, although far from comprehensive, covers a selected variety of applications as well as fundamental nuclear research themes. Cutler and co-workers discuss the application of radioactive isotopes in medicine for diagnostics, imaging, and radiotherapy. Medical isotopes are used in more than 10,000 hospitals worldwide, and 80% of all nuclear medicine applications apply technetium-99, resulting in about 30 million uses per year. The authors review the nuclear and chemical properties of radionuclides currently used in nuclear imaging and therapy, together with their production methods and preclinical and clinical trials. Mayer et al. present an overview of nuclear forensic science. Nuclear forensics is a relatively new branch of nuclear chemistry that combines radiochemistry, nuclear physics, and material science to identify the origin of clandestine nuclear materials. The review concentrates on the nuclear forensics of uranium and plutonium, as both elements have fissionable isotopes, and their origin, history, and intended use may be of interest for national security. The next four articles review selected fundamental nuclear chemistry research areas that provide indispensable data necessary for an improved understanding of radionuclide transport in contaminated surface and subsurface environments. This information is vital for conducting performance assessment of potential geologic high-level nuclear waste repositories. The worst-case scenario for risk assessment is the intrusion of ground or surface water into a nuclear waste repository. This can corrode the waste forms and thus potentially transport radionuclides to the accessible environment. The release rate is then controlled by a variety of different processes: dissolution of the waste form, radionuclide solubility, redox and complexation interactions, as well as precipitation reactions with inorganic and natural and anthropogenic organic water components, and finally by the various geochemical and physical reactions with the host rock and its components. Actinides are the main contributors to the © 2013 American Chemical Society
Special Issue: 2013 Nuclear Chemistry Published: February 13, 2013 855
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in nuclear reactors or accelerators. These reactions transmute long-lived actinides into higher actinides. Some of these will also undergo neutron-induced fission. This separation is essential for a successful transmutation of these actinides because the lanthanides would interfere, as they are strong neutron absorbers. Important radionuclides for removal are the fission products strontium-90, technetium-99, iodine-129, and cesium-135 to reduce the heat load on the nuclear waste form, and the trivalent actinides americium and curium. In their paper titled “Complexation and Extraction of Trivalent Actinides and Lanthanides by Triazinylpyridine N Donor Ligands”, Panak and Geist summarize the newest developments for the separation of trivalent americium and curium from fission lanthanides. Lastly, Türler and Pershina report on the “Advances in the Production and Chemistry of the Heaviest Elements”. During the last three decades, eight new elements have been synthesized, independently confirmed, and named with approval from IUPAC. Since 1999, a collaboration between scientists from the Joint Institute for Nuclear Research (JINR), Russia, and the Lawrence Livermore National Laboratory (LLNL), USA, synthesized elements 112 through 118 at previously unknown high reaction cross sections. This was accomplished via a new reaction route using a high-intensity calcium-48 beam on neutron-rich actinide targets from uranium to californium. These experiments resulted in six new elements (113−118) and more than 50 previously unknown isotopes. In 2012, elements 114 and 116 were named flerovium (Fl) and livermorium (Lv), respectively. Elements 113, 115, 117, and 118 are still awaiting independent confirmation. Recently oneatom-at-a-time gas-phase volatility studies placed element 112, copernicium (Cn), as a d-metal in group 12 in the Periodic Table, which is in agreement with theoretical predictions. The first experiments for studying the chemical behavior of Fl are in progress. I would like to thank all the authors for providing the outstanding review articles for this thematic issue of Chemical Reviews. I truly appreciate their tremendous efforts to help showcase the exciting field and the vibrancy of nuclear chemistry. I also thank the editorial staff and the editor Dr. Guy Bertrand for their hard work in publishing this issue on nuclear chemistry in Chemical Reviews. I very much hope that this publication provides a welcome reference to the experts as well as interesting information about the diversity of the field to the curious reader.
more slowly than the average groundwater velocity. During the last fifty years, Pu(IV) colloid, often also called Pu(IV) polymer, as well as Pu(IV) real or eigen colloids, was considered an amorphous noncrystalline solid that is closely related to tetravalent Pu(IV) amorphous hydrous oxide, PuO2(am, hyd). Recent studies suggest that Pu(IV) colloid exists in solution in the form of nanocrystalline PuO2 that can grow in size due to cluster formation. Amorphous, hydrous plutonium dioxide, PuO2 (am, hyd), may be X-ray diffractionamorphous material because the correlation distances between the nanocrystals are too small to be sensitive to X-ray diffraction. PuO2 (am, hyd) may indeed be nanocrystalline material. This dichotomy is illustrated in the papers by Knope and Soderholm, and Walther and Denecke. Future studies of this subject will hopefully contribute to an even more advanced understanding of the formation, nature, and stability of Pu(IV) colloid. In the fourth and last paper related to the understanding of actinides in the environment, Geckeis and coauthors summarize the current status of actinide reactions at the interface between aqueous solution and minerals. Different types of interaction mechanisms, outer-sphere and inner-sphere actinide surface coordination, and actinide incorporation mechanisms are discussed together with theoretical and geochemical actinide sorption modeling. Wen et al. survey theoretical methods and their computational approximations to predict the electronic structure and physical properties of actinide oxides ranging from Th to Cm. Their paper, titled “Density Functional Theory Studies of the Electronic Structure of Solid State Actinide Oxides”, focuses on theoretical predictions of lattice constants, electronic densities of states, band gaps, and magnetic and optical properties. Qiu and Burns report on “Clusters of Actinides with Oxide, Peroxide, or Hydroxide Bridges”. Whereas there are an abundance of studies on thorium and uranium clusters reported in the literature, only a few corresponding studies on neptunium and plutonium exist. This may be due to increased experimental challenges when working with α-radiation, including the production of toxic materials and their limited availability in only small quantities at an ever increasing expense. A better understanding of actinide clusters on a fundamental level at the nanoscale may improve processing of nuclear materials as well as spent nuclear fuel separation methods. The paper by Andrews and Cahill, titled “Uranyl Bearing Hybrid Materials: Synthesis, Speciation, and Solid State Structures”, focuses on the underlying design principles of the synthesis of hexavalent uranyl hybrid materials. The authors discuss the influence of hydrolysis, pH variation, building unit selectivity, and efforts to incorporate supramolecular tools. Fundamental actinide coordination chemistry in nonaqueous solutions is a rapidly growing field. Jones and Gaunt discuss the “Recent Developments in Synthesis and Structural Chemistry of Nonaqueous Actinide Complexes” to establish an increased understanding of bonding principles and electronic structure changes of actinides with a wide range of ligands relevant to industrial nuclear processes and environmental management. New fuel cycles for fourth generation fast neutron reactors (GEN IV) require a new approach to reprocessing. New strategies for high-level spent nuclear fuel management propose sophisticated separation schemes that separate long-lived waste radionuclides from short-lived ones. The separated radionuclides can be either conditioned for underground disposal or transmuted to shorter-lived radionuclides via neutron capture
Heino Nitsche
University of California, Berkeley
AUTHOR INFORMATION Notes
Views expressed in this editorial are those of the author and not necessarily the views of the ACS. 856
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Biography
Heino Nitsche was born in Munich, Germany, in 1949. He received his B.S. (Dipl. Chem.) in 1976 and his Ph.D. (Dr. rer. nat.) in 1980 from the Freie Universität Berlin (West). He joined Lawrence Berkeley National Laboratory (LBNL) as a staff scientist in September 1980 and became principal investigator and leader of the radionuclide group in 1984. In 1993, he returned to Germany to become a chaired Professor of Radiochemistry at the Technische Universität Dresden, Germany. Additionally, he was appointed director of the Institute of Radiochemistry in the newly founded Forschungszentrum Rossendorf in Dresden. Nitsche is a professor in the Department of Chemistry at the University of California, Berkeley, and a Faculty Senior Scientist at LBNL since 1998. He is the founding director of the Glenn T. Seaborg Center at LBNL and served in this position until 2002. He has served on the advisory boards of the National Academy of Sciences, the International Atomic Energy Agency, the OECD Nuclear Energy Agency, and the Joint Research Centre of the European Union. He was Chair of the Nuclear Chemistry Section of the German Chemical Society and 2007 Chair of the Division of Nuclear Chemistry and Technology (DNCT) of the American Chemical Society (ACS). He was selected as a member of the 2011 class of ACS Fellows. He leads the Heavy Element Nuclear and Radiochemistry Group. Nitsche’s research interests include nuclear chemistry and physics of the heaviest elements, fundamental molecular-level understanding of actinides at metal-oxide and biological interfaces, the thermodynamics and kinetics of actinides in solution, and their relationship to nuclear waste disposal, environmental contamination, and separation science. He is also a member of an international team that first studied the chemistry of the heaviest elements, bohrium (107) and hassium (108), and he independently verified the discovery of element 114 in 2009 with his Berkeley team. Recently, his group discovered six new isotopes, reaching in an unbroken α decay chain from element 114 down to 104, rutherfordium. This is a major step toward better understanding how to explore the region of enhanced stability thought to lie in the vicinity of element 114and possibly beyond.
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