Modern Societies have to consider diverse tasks strongly related to geochemistry sciences. Examples intensively discussed in the public are restoration measures for contaminated industrial fallow grounds, the safe storage of chemical-toxic and radioactive waste, carbon dioxide sequestration to reduce green-house gas emissions, the construction and operation of deep geothermal power plants, the geochemical exploration of natural resources or water and waste water treatments, including desalination efforts. Direct and urgent aspects to be dealt with are analytical and geochemical consequences of the Fukushima Daiichi nuclear disaster. All these cases have one in common – they require reliable thermodynamic data in order to forecast the fate of chemicals in the respective environment. Whereas a variety of standard methods, such as potentiometry, solubility studies, liquid-liquid extraction or electrochemical titrations, are in widespread use to generate thermodynamic data, it is far less straightforward to assign correct reaction pathways and structural patterns to the underlying chemical transformations. This especially holds for systems with strong tendencies to complexation and oligomerization. Here, it is essential to have proof of evidence for all involved species, which cannot be provided by the aforementioned methods, and is still lacking for various metal-containing systems. Spectroscopic techniques in combination with approaches from quantum chemistry can be of great benefit for such tasks. However, their application ranges are often restricted with respect to the type of element (and redox state) that can be probed. Further handicaps are imposed by detection limits or other parameters such as pH or salinity. Moreover, the spectroscopic results are often difficult to interpret in an unambiguous way. To overcome these complications at least partially, this workshop has been initiated. It shall significantly extend the application areas of spectroscopic tools important for lanthanide and actinide chemistry. Emphasis shall be placed on the development of spectroscopic methods towards more challenging environmental conditions – such as very basic pH values, elevated temperatures, pressures, or salinities – extending the range of covered elements and redox states. Furthermore, the exploration of options for lowering detection limits and increasing spatial resolution at sufficiently high signal-to-noise ratios will support future investigations on more complex systems. An approach combining the extension of spectroscopic tools with respect to elements and parameters, improvements of experimental setups, and applications of quantum chemical methods in predictive as well as interpretative ways certainly can be very beneficial. The workshop hopefully will bundle and strengthen respective research activities and ideally act as a nucleus for an international network, closely collaborating with international partners. I am confident that the workshop will deliver many exciting ideas, promote scientific discussions, stimulate new developments and in such a way be successful.

L'analyse isotopique des actinides est nécessaire pour le pilotage du cycle du combustible, le contrôle du traité de non-prolifération ou pour la surveillance environnementale. La précision de ces analyses peut être limitée par la performance du détecteur utilisé mais aussi par les incertitudes associées aux intensités d’émission disponibles dans les tables de données nucléaires. La désintégration des actinides est généralement suivie par d’intenses émissions de photons X et gamma dans la gamme d’énergie inférieure à 100 keV. Leur détection peut être intéressante pour l'analyse des actinides. Cependant, les techniques conventionnelles de mesure ne permettent pas de séparer correctement les raies des émissions concernées. Cette thèse a été consacrée à la mesure des intensités à l’aide d’un détecteur cryogénique. Ce dernier est basé sur un calorimètre métallique magnétique (MMC) qui permet de mesurer le dépôt d’énergie sous forme d’une élévation de température. Le MMC, appelé SMX3, comporte quatre pixels ; il est spécifiquement conçu pour la spectrométrie X et gamma de haute résolution dans la gamme d'énergie inférieure à 100 keV en vue de la mesure des intensités d’émission des actinides. Outre la haute résolution fournie par SMX3, due à son principe de fonctionnement, ce détecteur possède un rendement de détection constant et quasiment égal à 100% dans la gamme d'énergie inférieure à 25 keV, où les rayons XL des actinides sont émis. La courbe de rendement de SMX3 a été étalonnée par une méthode qui consiste en une mesure d’une seule source étalon d’Am-241 combinée à des simulations Monte Carlo. Les trois actinides Pu-238, Pu-239, et Cm-244 ont été mesurés afin de fournir des intensités absolues et relatives des émissions Li-Yj (avec Y=L,M,N,O,P i=1,2,3 et j=1..7). Grâce à la très haute résolution en énergie du MMC, les raies XL individuelles des actinides peuvent être séparées. Les raies satellites sont aussi détectées, leurs intensités relatives aux raies diagrammes dépendent de l’isotope en fonction des paramètres fondamentaux atomiques. Les intensités des raies XL individuelles ont pu être déterminées pour la première fois, notamment pour les transitions L₁-L₃. De plus, les intensités des régions XLi (i=1,2,3) ont été établies. Les intensités des groupes XL et XL globale sont comparées avec les calculs et les données expérimentales disponibles dans la littérature. / Isotopic analysis of actinides is necessary for fuel cycle management, non-proliferation treaty control or environmental monitoring. The accuracy of these analyses may be limited by the performance of the detector used but also by the uncertainties associated with the emission intensities available in the nuclear data tables. The disintegration of actinides is generally followed by intense X-ray and gamma photon emissions in the energy range below 100 keV. Their detection may be of particular interest for actinide analysis. However, conventional measurement techniques do not allow to properly separate the lines of the considered emissions. This thesis was devoted to the measurement of intensities using a cryogenic detector. The latter is based on a magnetic metallic calorimeter (MMC) that measures energy deposition as a temperature rise. The MMC, called SMX3, has four pixels and is specifically designed for high resolution X-ray and gamma spectrometry in the energy range below 100 keV to measure actinide emission intensities. In addition to the high resolution provided by SMX3 through its operating principle, this detector has a constant detection efficiency and almost equal to 100% over the energy range below 25 keV, where actinides' XL rays are emitted.The SMX3 yield curve was calibrated by a method that consists of a single standard source measurement of Am-241 combined with Monte Carlo simulations. The three actinides Pu-238, Pu-239, and Cm-244 were measured to provide absolute and relative intensities of Li-Yj emissions (with Y=L,M,N,O,P i=1,2,3 and j=1..7). Thanks to the very high energy resolution of the MMC, the individual XL lines of actinides can be separated. Satellite lines are also detected, their intensities relative to the diagram lines depend on the isotope according to the fundamental atomic parameters. The intensities of the individual XL lines could be determined for the first time, especially for the transitions L₁-L₃. In addition, the intensities of the XLi regions (i=1,2,3) were established. The intensities of the overall XL and XL groups are compared with the calculations and experimental data available in the literature.

In 2012, The Institute of Resource Ecology at the Helmholtz-Zentrum Dresden Rossendorf organized the first international workshop of Advanced Techniques in Actinide Spectroscopy (ATAS). A very positive feedback and the wish for a continuation of the workshop were communicated from several participants to the scientific committee during the workshop and beyond. Today, the ATAS workshop has been obviously established as an international forum for the exchange of progress and new experiences on advanced spectroscopic techniques for international actinide and lanthanide research. In comparison to already established workshops and conferences on the field of radioecology, one main focus of ATAS is to generate synergistic effects and to improve the scientific discussion between spectroscopic experimentalists and theoreticians. The exchange of ideas in particular between experimental and theoretical applications in spectroscopy and the presentation of new analytical techniques are of special interest for many research institutions working on the improvement of transport models of toxic elements in the environment and the food chain as well as on reprocessing technologies of nuclear and non-nuclear waste. Spectroscopic studies in combination with theoretical modelling comprise the exploration of molecular mechanisms of complexation processes in aqueous or organic phases and of sorption reactions of the contaminants on mineral surfaces to obtain better process understanding on a molecular level. As a consequence, predictions of contaminant’s migration behaviour will become more reliable and precise. This can improve the monitoring and removal of hazardous elements from the environment and hence, will assist strategies for remediation technologies and risk assessment. Particular emphasis is placed on the results of the first inter-laboratory Round-Robin test on actinide spectroscopy (RRT). The main goal of RRT is the comprehensive molecular analysis of the actinide complex system U(VI)/acetate in aqueous solution independently investigated by different spectroscopic and quantum chemical methods applied by leading laboratories in geochemical research. Conformities as well as sources of discrepancies between the results of the different methods are to be evaluated, illuminating the potentials and limitations of cou-pling different spectroscopic and theoretical ap-proaches as tools for the comprehensive study of actinide molecule complexes. The test is understood to stimulate scientific discussions, but not as a competitive exercise between the labs of the community. Hopefully, the second ATAS workshop will continue to bundle and strengthen respective research activities and ideally act as a nucleus for an international network, closely collaborating with international partners. I am confident that the workshop will deliver many exciting ideas, promote scientific discussions, stimulate new developments and collaborations and in such a way be prosperous. This workshop would not take place without the kind support of the HZDR administration which is gratefully acknowledged. Finally, the or-ganizers cordially thank all public and private sponsors for generous funding which makes this meeting come true for scientists working on the heavy metal research field.

Auch mehr als 80 Jahre nach der Entdeckung des ersten Transuranelements sind die fundamentalen Eigenschaften der Actiniden noch nicht hinreichend untersucht. Dies zeigt sich zum Beispiel in der relativ geringen Anzahl von strukturell charakterisierten Actinid- (8.790 Treffer) und insbesondere Transuran-Komplexen (537 Treffer) in der Cambridge Structural Database (Stand 29.05.20). Die Motivation dieser Doktorarbeit ist es daher, die bestehende Wissenslücke über die grundlegenden Eigenschaften der frühen Actinide, d.h. der Elemente Thorium bis Plutonium, durch die Untersuchung ihrer Koordinationschemie mit organischen Ligandmolekülen zu verringern. Zu diesem Zweck wurden während dieser Arbeit 36 neue Actinidverbindungen synthetisiert und charakterisiert, darunter auch die ersten Transuran-Amidinat-Komplexe und der erste metallorganische Neptuniumkomplex, welcher eine koordinative Np‒Br Bindung aufweist. Die Charakterisierung dieser Verbindungen erfolgte nicht nur im festen Zustand, sondern auch in Lösung. Zusätzlich wurden quantenchemische Berechnungen durchgeführt, sodass ein umfassendes Bild der Komplexstrukturen und ihrer Bindungssituation erstellt werden konnte. Diese Arbeit beinhaltet Komplexverbindungen der frühen Actiniden in deren wichtigsten Oxidationsstufen (+III bis +VI), wobei der Hauptschwerpunkt auf der drei- und vierwertigen Oxidationsstufe aufgrund der einfachen Zugänglichkeit und Vergleichbarkeit mit Lanthaniden und Übergangsmetallen liegt. Das Ziel dieser Arbeit ist es zudem, die potentielle Beteiligung der Valenzorbitale der Actiniden an der Bindung zu weniger harten Donoratomen, wie Stickstoff, aufzuklären. Dazu werden die Eigenschaften der Actinid–Stickstoff-Bindungen innerhalb von Serien von Actinid-Amidinat-Komplexen mit ihren Lanthanid- und Übergangsmetall-Analoga verglichen. Diese Untersuchungen werden durch quantenchemische Berechnungen unterstützt, um eine detaillierte Analyse der elektronischen Struktur der Komplexverbindungen zu ermöglichen. Im Rahmen dieser Arbeit wurden zwei verschiedene Arten von Amidinaten, N,Nʹ- Bis(isopropyl)-benzamidinat (iPr2BA) und (S,S)-N,Nʹ-Bis(1-phenylethyl)-benzamidinat ((S)-PEBA), zur Synthese von isostrukturellen Bis- und Tris(amidinat)-Komplexen eingesetzt. Es konnte dabei gezeigt werden, dass die maximal zugängliche Stöchiometrie, d.h. das Metall-zu-Ligand Verhältnis, durch den Ionenradius der jeweiligen Metallkationen und den sterischen Bedarf des Liganden bestimmt wird. So bilden die relativ kleinen vierwertigen Übergangsmetalle Titan und Hafnium ausschließlich Bis(amidinat)-Komplexe mit den verwendeten Benzamidinaten, während Tris(amidinat)-Komplexe für die größeren vierwertigen Kationen Zirkonium und Cer sowie die Actiniden Thorium, Uran, und Neptunium synthetisiert werden konnten. Der Unterschied zwischen der Koordinationschemie der Übergangsmetalle Zirkonium und Hafnium mit den verwendeten Amidinatliganden ist unerwartet und könnte hinweisgebend für zukünftige Trenntechnologien der beiden Elemente sein. Es wurden zudem zwei Serien von Tris(amidinat)-Komplexen ([MIVCl(iPr2BA)3] und [MIVCl((S)-PEBA)3]) synthetisiert und hinsichtlich ihrer koordinativen Bindungslängen analysiert. Einkristall-Röntgenbeugungsdaten (SC-XRD) weisen auf eine vorwiegend ionische Bindung zwischen den Metallen und den koordinierenden Atomen (Stickstoff und Chlor) hin. Eine bemerkenswerte Ausnahme bildet dabei der vierwertige Cerkomplexes [CeCl((S)- PEBA)3], welcher deutlich längere Bindungslängen als die isostrukturelle Actinidenkomplexserie aufweist. Dies deutet auf ein unterschiedliches Bindungsverhalten zwischen den 4f- und 5f-Elementen hin, welches mit quantenchemischen Berechnungen untersucht wurde. Die Analyse mittels Quantum Theory of Atoms in Molecules (QTAIM) und Natural Bond Orbitals (NBO) weist auf einen kovalenteren Charakter der Ce‒N-Bindungen im Vergleich zu den Th‒N-Bindungen hin. Dieses unerwartete Ergebnis unterstreicht die Notwendigkeit quantenchemischer Berechnungen und zeigt den starken Einfluss verschiedener Kristallpackungseffekte auf die Aufklärung der Bindungseigenschaften. Darüber hinaus zeigen QTAIM- und NBO-Analysen eine Zunahme des kovalenten Charakters innerhalb der Actinidenserie von Thorium zu Uran, gefolgt von einem Plateau ähnlicher Kovalenz von Uran zu Plutonium. Die Ursache der kovalenten Wechselwirkung unterscheidet sich jedoch. Bei Uran ergibt sich der Hauptbeitrag zum kovalenten Charakter aus der Beteiligung von 6d-Orbitalen, während bei Plutonium der 5f-Beitrag dominant ist. Die bestimmende Rolle der 5f-Orbitale bei der Bindung zu Stickstoff-Donor-Liganden zeigt sich daher nur für vierwertiges Neptunium und Plutonium und nicht für die leichteren Actiniden. Im Gegensatz zu den Untersuchungen der [MIVCl((S)-PEBA)3]-Komplexreihe bestätigt eine detaillierte Analyse der Bindungseigenschaften der dreiwertigen Actinidkomplexe vom Typ [MIII((S)-PEBA)3] eindeutig einen höheren kovalenten Anteil der AnIII‒N-Bindungen im Vergleich zu den Lanthanid-Analoga sowohl durch experimentell bestimmte MIII‒ N- Bindungslängen als auch durch quantenchemische Berechnungen mittels QTAIM- und NBO-Analysen. Dieser Unterschied im kovalenten Charakter von drei- und vierwertigen Actinidkomplexen kann auf die höhere LEWIS-Acidität der Letzteren zurückgeführt werden. Dies wurde zum ersten Mal unter Verwendung des gleichen Typs von Stickstoff-Donor-Liganden für beide Oxidationszustände im Rahmen dieser Arbeit bestätigt. Die Ergebnisse deuten weiterhin auf eine Beteiligung der Valenzelektronen der Actiniden an der Bindung und damit auf einen erhöhten Überlapp mit den Ligandorbitalen, insbesondere für die dreiwertigen Actiniden, hin. Die vierwertigen Actinid-Chloro-Komplexe wurden außerdem hinsichtlich ihrer Reaktivität untersucht, wobei (Pseudo-)Halogenid-Austauschreaktionen durchgeführt wurden. Diese ergaben die entsprechenden Fluoro-, Bromo- und Azidokomplexe. Diese Substitutionschemie wurde dabei erstmals für ein Transuranelement angewandt, wodurch unbekannte Neptunium-(Pseudo)halogenid-Komplexe hergestellt werden konnten. Somit wurde die bekannte Koordinations- und Substitutionschemie von Urankomplexen erfolgreich auf Neptunium übertragen, was neue Möglichkeiten eröffnet, die grundlegenden Eigenschaften der frühen Actiniden mit einem breiteren Spektrum zugänglicher Verbindungen zu untersuchen. Im Rahmen dieser Arbeit wurden außerdem einige dieser grundlegenden Eigenschaften der Actinid-Amidinat-Komplexe mit Hilfe der kernmagnetischen Resonanzspektroskopie (NMR) untersucht. Zum ersten Mal wurde dabei die Abhängigkeit der paramagnetischen Hyperfeinverschiebung von der Anzahl der f-Elektronen für vierwertige Actinidkomplexe systematisch untersucht. Generell konnte dabei eine Zunahme der Pseudokontaktverschiebung mit zunehmender Anzahl von f-Elektronen nachgewiesen werden, mit der bemerkenswerten Ausnahme des [UF((S)-PEBA)3]-Komplexes. In diesem Komplex verändert das stark koordinierende Fluorid das Ligandenfeld des vierwertigen Urans, um ein umgekehrtes Verhalten der Pseudokontaktverschiebung zu induzieren. Dabei ist besonders hervorzuheben, dass dieses Verhalten nicht für den isostrukturellen Neptuniumkomplex [NpF((S)-PEBA)3] beobachtet wird, was auch durch quantenchemische Berechnungen bestätigt werden konnte. Dieses unerwartete Verhalten eines relativ einfachen vierwertigen Urankomplexes unterstreicht die Notwendigkeit einer tiefgreifenden Analyse der paramagnetischen Eigenschaften der Actiniden, um letztendlich ihre NMR-Spektren detaillierter interpretieren zu können. Zusammenfassend konnte während dieser Doktorarbeit gezeigt werden, dass Actiniden in allen untersuchten Oxidationszuständen starke Wechselwirkungen mit Stickstoff-Donor-Liganden eingehen, was zur Synthese einer Vielzahl unbekannter Actinidkomplexe führte. Die Bindungsanalyse der An–N-Bindung zeigte dabei unterschiedliche Kovalenzanteile, mit einem allgemeinen Trend zu höheren Anteilen für die weicheren Actinid bzw. Actinyl-Kationen (+III und +V). Mit Hilfe von Stickstoff-Donor-Liganden als weniger harte Donoren konnten somit die grundlegenden Eigenschaften und insbesondere die Koordinationschemie der Actiniden umfassend untersucht werden. Diese Untersuchungen sollten auch für weitere Studien über die Komplexierung von Actiniden mit Liganden in Betracht gezogen werden, welche naturstoffnahe funktionelle Gruppen tragen, um ihren Einfluss auf das Verhalten von Actiniden in der Umwelt detaillierter bewerten zu können. / The intriguing chemistry of the actinide elements lacks a fundamental understanding of their inherent properties even more than 80 years after the discovery of the first transuranium element, neptunium. This is for instance reflected in the relatively small number of structurally characterized actinide (8,790 hits) and especially transuranium complexes (537 hits) in the Cambridge Structural Database (as of 05/29/2020). The motivation behind this PhD work is thus the investigation of the coordination chemistry of the early actinides (i.e. thorium to plutonium) with organic ligand molecules to narrow this knowledge gap and to deduce their fundamental properties. To this end, this work has synthesized and characterized 36 new compounds, among those the first transuranium amidinate complexes and the first metal-organic neptunium complex possessing a Np–Br bond. These compounds have been characterized not only in the solid state, but wherever possible solution structures have been determined and high-level quantum chemical calculations have been performed to obtain a comprehensive picture of their structures and bonding situation. The thesis covers the most important oxidation states (+III to +VI) of the early actinides but mainly focuses on the tri- and tetravalent oxidation state owing to their accessibility for the early actinides and also for lanthanide and transition metals to compare their properties with. This work aims in particular to elucidate the potential participation of the actinides’ valence orbitals in the bonding to medium hard donor atoms like nitrogen in order to compare the actinide–nitrogen bond properties with their lanthanide and transition metal analogues. The degree of covalent interaction in actinide complexes plays an important role in the prediction of their behavior in naturally relevant systems and for separation processes in nuclear industry. The properties of the An–N bonds are studied by the synthesis of series of actinide complexes using amidinate ligands and are then compared to isostructural lanthanide and transition metal analogues. The analysis of the coordinative bond lengths together with an elucidation of the structures in solution is the key to understand the behavior of the actinide complexes. These investigations are supported by quantum chemical calculations for a detailed analysis of the complexes’ electronic structure. Two different types of amidinate ligands, N,Nʹ-Bis(isopropyl)-benzamidinate (iPr2BA) and (S,S)-N,Nʹ-Bis(1-phenylethyl)-benzamidinate ((S)-PEBA) are used to synthesize series of isostructural bis- and tris(amidinate) complexes. It is shown that the maximum accessible stoichiometry, i.e. the metal-to-ligand ratio, is determined by the ionic radius of the respective metal cation and the steric demand of the ligand itself. Hence, the relatively small tetravalent transition metal analogues titanium and hafnium exclusively form bis(amidinate) complexes with the used benzamidinates whereas tris(amidinate) complexes could be synthesized for the bigger tetravalent cations zirconium and cerium, as well as the actinides thorium, uranium, and neptunium. The difference between the coordination chemistry of the transition metals zirconium and hafnium with the used amidinate ligands is highly unexpected and could hold implications for future separation technologies. Two series of tris(amidinate) complexes [MIVCl(iPr2BA)3] and [MIVCl((S) - PEBA)3] have been synthesized and analyzed regarding their coordinative bond lengths. Single crystal X-ray diffraction (SC‑XRD) data indicates a predominantly ionic bonding interaction between the metals and the coordinating atoms, i.e. N and Cl, with the notable exception of the tetravalent cerium complex [CeCl((S)-PEBA)3] showing noticeably longer bond lengths than the isostructural actinide complex series. This points to a different binding behavior between the 4f and 5f elements which is investigated using quantum chemical calculations. Quantum theory of atoms in molecules (QTAIM) and natural bond orbital (NBO) analysis of the [MIVCl((S) - PEBA)3] complex series indicates a more covalent character of the Ce–N bonds compared to the actinide analogue thorium despite the longer than expected bond lengths. This unforeseen result emphasizes the power of quantum chemical calculations and reveals the strong impact of various crystal packing effects on the elucidation of bond properties. In addition, QTAIM and NBO analyses further reveal an increase of the covalent character when traversing the actinide series from thorium to uranium followed by a plateau from uranium to plutonium, but a different origin. For uranium the major contribution to the covalent character stems from the participation of 6d orbitals, whereas for plutonium the 5f contribution is dominant. Thus, unless expected otherwise, the prominent role of the 5f orbitals in the bonding to nitrogen donor ligands manifests itself only for tetravalent neptunium and plutonium and not for the lighter actinides. In contrast to the investigations of the [MIVCl((S)-PEBA)3] complex series, a detailed analysis of the binding properties of the trivalent actinide complexes of [MIII((S)-PEBA)3] type unequivocally confirms a higher degree of covalent character of the AnIII– N bonds compared to the lanthanide analogues by both, experimentally determined MIII – N bond lengths and quantum chemical calculations using QTAIM and NBO analyses. This difference in covalent character of tri- and tetravalent actinide complexes is expected to stem from the higher LEWIS acidity of the latter, but has been corroborated for the first time within this thesis using the same type of nitrogen donor ligand for both oxidation states. The results indicate a participation of the actinides’ valence electrons in the bonding and hence an increased overlap with the ligand orbitals especially for the trivalent actinides. The tetravalent actinide chloro tris(amidinate) complexes have been further investigated regarding their reactivity using (pseudo)halide exchange reactions yielding the corresponding fluoro, bromo, and azido complexes. It has to be pointed out, that this substitution chemistry is applied for the first time for a transuranium element, yielding unprecedented neptunium (pseudo)halide complexes. Thus, the well-known coordination and substitution chemistry of uranium complexes has been successfully expanded to neptunium which opens new opportunities to study the fundamental properties of the early actinides with a broader range of accessible compounds. Within this thesis some of these fundamental properties have been studied for the actinide amidinate complexes by means of nuclear magnetic resonance (NMR) spectroscopy. For the first time the dependence of the paramagnetic hyperfine shift on the number of f electrons has been investigated systematically for tetravalent actinide complexes. Generally, an increase of the hyperfine shift with increasing number of f electrons could be revealed with the remarkable exception of the [UF((S)-PEBA)3] complex. The strongly coordinating fluoride is altering the ligand field of the tetravalent uranium to induce an inverted behavior of the pseudocontact shift. Most remarkably, this behavior is not observed for the isostructural neptunium complex [NpF((S)-PEBA)3], which could be confirmed by multi-configurational quantum chemical calculations. This unexpected behavior of a relatively simple tetravalent uranium complex further underlines the need for a profound analysis of actinides’ paramagnetic properties to finally interpret their NMR spectra in more detail. In summary, during this PhD work it could be shown that nitrogen donor ligands show strong interactions with actinides in all investigated oxidation states. The binding shows varying degrees of covalency, evident in experimental findings and confirmed by computational results, with a general trend towards higher degrees of covalency for the softer actinide/actinyl cations (+III and +V). Nitrogen donor ligands as medium hard donors have shown the potential to investigate the fundamental properties and especially the coordination chemistry of the actinides comprehensively. These investigations should also be taken into account for further studies regarding the complexation of low-valent actinides with ligands bearing nature-derived functional groups to evaluate their influence on the behavior of actinide elements in the environment.

Many technical hurdles remain to be overcome before an advanced fuel cycle in which minor actinides from spent nuclear fuel are used to generate power. Three such issues were addressed: criticality safety of minor actinides as compared to currently used fissile isotopes; accuracy of evaluated nuclear data for selected minor actinide high energy fission cross-sections; and the preliminary design optimization of a minor actinide burning/breeding fission blanket in a fission fusion hybrid reactor concept. For minor actinide compositions found in spent fuel, current safety measures for actinide solutions were found to be adequate, though concerns may remain for unmoderated transuranic materials. Additionally, computational results indicated a 5-10% error in the fission cross-section of some minor actinides above the fast fission threshold. Finally, a relatively tall annular fission blanket was found to be the most ideal configuration for the UT fission- fusion hybrid reactor concept, satisfying criticality and power output criteria. / text

The Enhanced Actinide Removal Plant (EARP) located at Sellafield, is a key facility for processing nuclear effluents in the UK. The EARP process decontaminates radioactive waste effluent by inducing the coprecipitation of Fe(III) along with any radionuclides in solution. The resulting radioactive solid phase is then separated from the decontaminated aqueous phase by ultrafiltration processes over several weeks. In the future the EARP facility's role will be expanded to treat effluents produced from nuclear decommissioning activities. However there remains a limited understanding around the mechanisms of radionuclide removal from solution and subsequent sequestration within the solid phase under conditions relevant to such industrial effluent treatment facilities. In this work, the fate of U(VI) and Th(IV) were investigated during the EARP process. XRD and TEM analyses revealed that the solid product from the EARP coprecipitation process was 2-line ferrihydrite which transformed over time to form hematite, with some goethite formation observed in the Th containing system. During this coprecipitation process U was initially removed from solution by adsorption to ferrihydrite as a bidentate, edge-sharing surface complex associated with ternary carbonate complexes. As the U containing system was aged, a maximum range of 61 - 75% U became consistently incorporated within the newly formed hematite phase for a wide range of systems containing 6 orders of magnitude total U:Fe molar ratio. Such a constant proportion of incorporated U suggests that this incorporation occurs during a particle mediated mechanism of hematite growth, such as oriented attachment. Interestingly, Th was removed from solution during the EARP coprecipitation process by a combination of both adsorption and occlusion mechanisms. EXAFS analyses revealed that the local coordination environment of Th associated with the solid phase altered considerably with increased aging time, and correspondingly Th became increasingly recalcitrant to remobilisation over time suggesting the formation of further occluded or even incorporated Th species within the transforming iron (oxyhydr)oxide phases. This thesis progresses the fundamental understanding of radionuclide interactions with iron (oxyhydr)oxide phases during coprecipitation and aging processes under conditions relevant to industrial waste treatment processes such as EARP.

The present study is focused on evaluating higher actinides beyond uranium that are capable of supporting power and propulsion requirements in robotic deep space and interstellar exploration. The central technology in this thesis is based on utilizing advanced actinides for direct fission fragment energy conversion coupled with magnetic collimation. Critical fission configurations are explored which are based on fission fragment energy conversion utilizing a nano-scale layer of the metastable isotope 242mAm coated on carbon fibers. A 3-D computational model of the reactor core is developed and neutron properties are presented. Fission neutron yield, exceptionally high thermal fission cross sections, high fission fragment kinetic energy and relatively low radiological emission properties are identified as promising features of 242mAm as a fission fragment source. The isotopes 249Cf and 251Cf are found to be promising candidates for future studies. Conceptual system integration, deep space mission applicability and recommendations for future experimental development are introduced.

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The necessity of reprocessing spent nuclear fuel has arisen from increasing awareness and concern for the environment, in addition to the potential of minimising proliferation. A number of different reprocessing techniques are currently being developed around the world to allow useful spent nuclear fuel (SNF) to be recycled and reused and the remaining waste to be treated. One such technique, currently being developed in the USA is the TALSPEAK process, an advanced reprocessing method for the separation of trivalent lanthanide (Ln3+) and minor actinide (MA3+) components. This process, developed in the 1960s at Oak Ridge National Laboratory, uses DTPA to act as a holdback reagent for MA3+, in a lactate buffered aqueous phase at pH 3.6, allowing Ln3+ to be selectively extracted by organophosphate HDEHP into an organic phase of DIPB or dodecane. TALSPEAK is one of the most promising techniques being researched due to its numerous advantages, particularly its relative resistance to radiolysis and its ability to be carried out without the need for high reagent concentrations. Additionally it gives high separation factors, in the region of ~50-100, comparable to other advanced reprocessing methods under development. The chemistry of the process is very complex and not particularly well understood so it would be advantageous to simplify the process by removing the need for a separate holdback reagent and buffer. In collaboration with colleagues at the Idaho National Lab, the use of amino acids as a potential combined buffer and soft donor was investigated. Although it was found that amino acids do not act as holdback reagents in their own right, optimisation of an L-alanine buffered TALSPEAK system with DTPA was found to allow the process to be carried out effectively at a lower pH of 2, which is more preferable for industrial application. As an extension of this, separation studies were carried out using the tripeptide L-glutathione (GSH) to determine its potential for use as a combined buffer and soft-donor. As with the studies with amino acids, it was found that GSH also does not act as a holdback reagent in its own right, however it does interact with Ln-DTPA complexes at pH 4. When optimised at this pH, separation factors of up to 1200 were achieved for Eu3+/Am3+, whilst still maintaining low MA3+ partitioning. However, further studies by ICP-MS and luminescence spectroscopy showed that a GSH buffered system was not effective for extraction of heavier lanthanides, although the results show the potential for further investigation into other short and longer chain peptide buffered systems and possibly lanthanide-lanthanide separations. Further studies were carried on amino acid appended DTPA ligands which were synthesised in a one step reaction in order to create a combined buffer and soft donor. The ligands were found to self-buffer at around pH 2 and allow successful separation of Eu3+/Am3+ (SF ~ 100). The results from initial investigations by luminescence spectroscopy and solvent extraction are promising and are presented here. Further work is needed on these systems in order to optimise their extraction capability and minimise Am3+ partitioning. In the future this work could promote studies for better understanding of TALSPEAK chemistry that could be used in industrial partitioning processes.