L'étude et la compréhension de la chimie des actinides constitue un axe de recherche privilégié dans le cadre de la technologie nucléaire aussi bien en amont pour le développement de matériaux à bases d'actinides qu'en aval pour l'étude du retraitement des déchets nucléaire produits dans les centrales. Une des problématiques principales dans ces études réside dans la capacité que possèdent les actinides à subir des réactions redox et à former des assemblages de tailles et de topologies variées. Dans ce contexte, l'uranium, peu radioactif, est un bon modèle des actinides plus lourds et beaucoup plus difficiles à manipuler. Ainsi, l'objectif de ce travail réside dans la synthèse d'assemblages polymétalliques à base d'uranium en mettant à profit quelques aspects de sa réactivité redox et de sa chimie de coordination. Ainsi, la synthèse et l'étude des propriétés spectroscopiques et magnétiques de nouveaux complexes d'uranium ont été étudiées. La réactivité d'hydrolyse de complexes d'uranium trivalent en absences de ligands stériquement encombrants a permis l'accès à des assemblages oxo/hydroxo d'uranium de différentes tailles en modulant le complexe de départ et les conditions de synthèse. L'élargissement de cette stratégie de synthèse à l'oxydation contrôlée avec l'azoture a ainsi permis la synthèse d'un complexe nitrure/azoture d'uranium présentant une topologie originale. L'étude de la chimie de coordination du polymère d'uranyle pentavalent {[UO2py5][KI2py3]}n avec différents ligands a également été abordée a permis l'accès à plusieurs complexes cation-cation d'uranyle pentavalent dont la stabilité est suffisante pour que l'étude de leur dismutation ait pu être suivie par RMN du proton. La modulation des ligands a également permis l'accès à des complexes monomères stables en solution vis-à-vis de la dismutation dans les solvants organiques. Les propriétés magnétiques des complexes monomères et polymétalliques obtenus ont été étudiées et un couplage anti ferromagnétique a été mis en évidence pour le complexe dimère cation-cation d'uranyle pentavalent [UO2(dbm)2(K18C6)]2.

<p>Precision measurements of 238U(n,n'g) and 235,238U(n,2ng) partial cross sections have been performed at Triangle Universities Nuclear Laboratory (TUNL) to improve crucial data for the National Nuclear Security Administration's (NNSA) Stockpile Stewardship Program. Accurate neutron-induced reaction cross-section data are required for many practical applications, including nuclear energy and reactor technology, nuclear transmutation, and explosive nuclear devices. Due to the cessation of underground nuclear testing in the early 1990s, understanding of the performance of nuclear devices is increasingly dependent on precise model calculations which are, in turn, themselves reliant on accurate reaction data to serve as benchmarks for model codes. Direct measurement of (n,n') and (n,2n) reaction cross sections for uranium is extremely difficult due to large neutron background from fission and very close nuclear level spacing. Previous direct measurements of the cross sections are incomplete and/or discrepant over the energy range of interest. However, the (n,n'g) and (n,2ng) partial gamma-ray cross-section data obtained in the present work can be combined with model calculations to infer total (n,n') and (n,2n) reaction-channel cross sections.</p><p>A pulsed and monoenergetic neutron beam was used in combination with high-resolution gamma-ray spectroscopy to measure these partial cross sections for incident neutron energies between 5 and 14 MeV. Gamma-ray yields were measured with high-purity germanium (HPGe) clover and planar detectors. Neutron fluxes were determined from the well-measured 2+ -> 0+ transition in 56Fe to be on the order of 10^4 n/cm^2/s. Detector efficiency and attenuation of gamma rays in the target were simulated using the MCNPX Monte-Carlo radiation transport code.</p><p>Measured partial cross sections were compared with previous measurements and calculations from GNASH and TALYS Hauser-Feshbach statistical-model codes. Results are generally in good agreement with existing data and provide cross-section data for transitions in energy regions where none previously existed. Total reaction-channel cross sections are inferred from statistical-model calculations and compared with existing direct measurement data.</p> / Dissertation

Density functional (DFT) methods are first used to study 22 of the most stable solution-phase UN4O12 isomers containing uranyl nitrate, UO2(NO3)2. Based on relative free energy calculations, 4 solution (a6, a5, a8, and a1) and 5 gas-phase isomers (a1, a2, a3, b1, and b2) are identified as the strongest candidates to exist and possibly predominate within their respective environments. DFT is then applied to a new form of binucleating Schiff–base polypyrrolic macrocycles containing actinyl ions [AnO2]n+ (An = U, Np, Pu; n = 1, 2) and 3d transition metals (TM): Mn, Fe, Co, and Zn. Formal bond order evidence is provided for 24 TM to actinyl–endo–oxygen partial bond formations. Special structural cases are discussed. Redox potentials for AnVIO21/AnVO21– couples closely follow the Np > Pu > U trend seen for AnO2(H2O)52+/1+. Predictions of –1.10, 0.25, and 0.01 eV are made for U, Np, and Pu redox potentials.

The current UK strategy for radioactive waste management is to permanently store the waste in an underground repository. Final disposal of the radwaste may then be preceded by chemical conditioning and physical encapsulation. The objective of this work was to determine the extent of actinide immobilisation in cement. Since actinides are hazardous and costly to study directly, a chemical analogue approach to studying actinide immobilisation was adopted. Th(IV), Ce(III, IV) and Eu(III) were chosen as actinide simulants and their suitability assessed by a critical review of the literature. Ca(OH)₂ and C-S-H dominate the observed chemical properties of the aqueous phase in cement. As they are of such importance, it was these cement components which were used to investigate the reaction of the simulant elements with cement. The phases found to be predicted were ThO₂, ThSiO₄, Eu(OH)₃, Ca₂Eu₈(SiO₄)₆O₂, CeO₂, CeSiO₄ and Ca₂(SiO₄)₆O₂. CeSiO₄ and Ca₂Ce₈(SiO₄)O₂ are newly reported phases, produced by hydrothermal synthesis. Rietveld refinement confirmed CeSiO₄ to have the zircon structure, with space group 14₁/amd and cell parameters a = 6.9564(3) A, c = 6.1953 (4) A. Ca₂Ce₈(SiO₄)₆O₂ exhibits the apatite structure, with space group P6₃/m and cell parameters a = 9.4343(3) a, c = 6.8885(4) A. Preliminary solubility studies were carried out on all of the solubility-limiting phases. Phase impurity, poor crystallinity and incongruent solubility of phases hindered the generation of solubility product data. Nevertheless, these phases have naturally occurring analogues which are known to be environmentally stable and have low solubilities. On the basis of the experimental results obtained, it may be concluded that cement has the potential to be a very effective immobilisation matrix for actinide elements. Recommendations for future experiments using active elements are discussed.

Density functional (DFT) methods are first used to study 22 of the most stable solution-phase UN4O12 isomers containing uranyl nitrate, UO2(NO3)2. Based on relative free energy calculations, 4 solution (a6, a5, a8, and a1) and 5 gas-phase isomers (a1, a2, a3, b1, and b2) are identified as the strongest candidates to exist and possibly predominate within their respective environments. DFT is then applied to a new form of binucleating Schiff–base polypyrrolic macrocycles containing actinyl ions [AnO2]n+ (An = U, Np, Pu; n = 1, 2) and 3d transition metals (TM): Mn, Fe, Co, and Zn. Formal bond order evidence is provided for 24 TM to actinyl–endo–oxygen partial bond formations. Special structural cases are discussed. Redox potentials for AnVIO21/AnVO21– couples closely follow the Np > Pu > U trend seen for AnO2(H2O)52+/1+. Predictions of –1.10, 0.25, and 0.01 eV are made for U, Np, and Pu redox potentials.

X-ray spectroscopic experiments, including X-ray absorption spectroscopy (XAS), X-ray emission spectroscopy (XES) and resonant inelastic X-ray scattering (RIXS), were performed in a large energy range (60-9000 eV) on a variety of systems, probing different elements and orbitals. The systems can preferably be divided into three groups: Actinides, copper systems and amorphous materials. We studied oxidation states of Pu in PuO2 using a closed-source experimental setup, developed by us, to lower the safety restrictions associated with such experiments. The presence of Pu with higher oxidation state than Pu(IV) was found in polycrystalline PuO2. This is an important finding since Pu with higher oxidation states has higher solubility affecting the safety of a proposed undeground repository of spent nuclear fuel which can eventually be infiltrated by groundwater. In the proposed underground repository the molecular oxygen will vanish at some point due to microbial activity and mineral oxidation. At this stage sulfur is the most important candidate for causing copper corrosion. We studied Cu films and foils exposed to sulfide solutions at both Cu L2,3, Cu K and S L2,3 edges and found for example that S L2,3 XES is useful for distinguishing between different copper sulfides and Cu Kβ XES can be used to identify monovalent Cu compounds. Amorphous, zirconium containing alloys are potentially useful materials for future nuclear reactors. The electronic structure of such materials was also investigated.

A fusion-fission (hybrid) reactor is a combination of a fusion device and a subcritical fission reactor, where the fusion device acts as a neutron source and the power is mainly produced in the fission core. Hybrid reactors may be suitable for transmutation of transuranic isotopes in the spent nuclear fuel, due to the safety margin on criticality imposed by the subcritical fission core. The SFLM Hybrid project is a theoretical project that aims to point out the possibilities with steady-state mirror-based hybrid reactors. The quadrupolar magnetic mirror vacuum field is based on the Straight Field Line Mirror field and the central cell is 25 m long. A fission mantle surrounds the mirror cell. The fission to fusion power ratio is about 150 with keff = 0.97, implying that almost all the produced energy comes from fission. Beyond each mirror end magnetic expanders are located, which increase the plasma receiving “divertor” area and provide tolerable heat load on wall materials. The plasma is heated with ion cyclotron radio frequency heating and the fission mantle is cooled using a liquid lead-bismuth eutectic. The device is self-sufficient in tritium, and does not seem to suffer from severe material problems. A remaining issue may be the plasma electron temperature, which need to reach about 500 eV for efficient power production.  In this doctoral thesis, theoretical work has been done with the magnetic coil system of such a device and also with the overall concept. A new coil type, the fishbone coil, suitable for single cell quadrupolar mirrors, has been invented. Two vacuum field coil sets with satisfying properties have been found, where the most recent coil set consists of fishbone coils. Finite ß effects on the magnetic field have been investigated, showing that the flux tube ellipticity increases with ß. The ellipticity of the vacuum field increases slightly with radius, but with finite ß it decreases with radius. The maximum flux surface radial extensions decrease with ß, which is an unexpected and beneficial result. A radial invariant has also been identified, and particle simulations have been made to emphasize that quadrupolar mirrors must be symmetric or confinement may be lost.

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The chemistry and physical properties of actinide complexes has become increasingly significant and relevant since the dawn of the nuclear age. In addition to increasing the potency of nuclear power and the safety and disposal of its subsequent waste products, exploration of the chemistry of actinide complexes provides a fascinating insight into the increased complexity and divergence of reactivity of these complexes when compared to transition metal complexes. Chapter One provides a brief introduction to the chemistry of actinides and in particular, the major focus of this work, of thorium. This is followed by a survey of examples of rare examples of thorium complexes with a formal oxidation state other than Th (IV). Following this is a review of selected examples of thorium (IV) complexes exhibiting unusual reactivity surveying thorium hydride and alkyl complexes initially. This progresses into reviewing the chemistry of thorium complexes containing multiple bonds to non-metal atoms, beginning with carbon atoms and then progressing to atoms in the chalcogen and pnictogen groups. The introduction finishes with an investigation into the properties of the terphenolate ligands used in this study, including examples of unusual complexes that they have been shown to stabilise. In Chapter Two, an exploration into the catalytic activity of fairly simple actinide amide catalysts, N”2Th (IV) {k2-N(SiMe3)SiMe2CH2, N”2U (IV) {k2-N(SiMe3)SiMe2CH2} and UN”3, upon terminal acetylenes is presented. The chapter begins with a brief introduction summarising the previous reactivity observed in the catalysis of terminal acetylenes, with particular focus on actinide-based catalyst mediated reactions. The catalytic results on a variety of terminal acetylenes with different steric and electronic properties is then reported upon. It is found that high conversions and selectivities can be achieved upon optimisation of the catalytic process. It was also found that the different catalysts and substrates favoured different products, with selective oligomerisation and cyclotrimerisation reactions observed. The differing reactivities lend support to the role of f-electrons upon the catalytic route of the reaction. Conclusions are discussed at the end of the chapter. In Chapter Three, the synthesis and characterisation of heteroleptic terphenolate thorium chloride complexes and their subsequent reactivity was investigated. The synthesis and characterisation of ThCl2(OTerMes)2DME and ThCl2(OTerMes)2(H2O)3 are initially described. The reactivity of these complexes favoured transmetallation of the terphenolate ligands, with the complexes; [Li(OTerMes)THF]2, [Li(OTerMes)]2THF, μ3- (TerMesO)μ3-(CH2SiMe3)3Li4, LiAlH2(OTerMes)2, [(THF)K(OTerMes)]2, MgCl(OTerMes)(THF)2, MgBr(OTerMes)(THF)2 and Fe(OTerMes)2(py)2 synthesised and characterised from reactions attempting to transform the ancillary chlorido-ligands. The reactivity of ThCl2(OTerMes)2DME was found to not be solely transmetallation of the terphenolate ligands as elucidated by the synthesis and characterisation of [Th(OTerMes)2(Cl)2(4,4’- bipyridyl)1.5]∞ and [MgTh2μ2-Cl2μ3-Cl(OTerMes)2(C4H7)2μ-η3:η3(C4H7)H]. The synthesis of [MgTh2μ2-Cl2μ3-Cl(OTerMes)2(C4H7)2μ-η3:η3(C4H7)H] was found to proceed via a reductive elimination route with concomitant formation of a terphenolate transmetallation product Mg(OTerMes)2(THF)2. The formation of[Th(OTerMes)2(Cl)2(4,4’- bipyridyl)1.5]∞ was achieved via reaction with the Lewis base 4-4’ bipyridine. Reactions attempting to form heteroleptic uranium terphenolate complexes were also detailed. Conclusions are discussed at the end of the chapter. In Chapter Four, the synthesis and characterisation of heteroleptic terphenolate thorium borohydride complexes and their subsequent reactivity was investigated. It was found that the conversion of ThCl2(OTerMes)2DME to Th(BH4)2(OTerMes)2DME proceeded smoothly using a precedented reaction route. In contrast to ThCl2(OTerMes)2DME, reaction with a Lewis acid was found to result in abstraction of the solvating DME molecule, resulting in the synthesis and characterisation of Th(BH4)2(OTerMes)2. In similarity to ThCl2(OTerMes)2DME, Th(BH4)2(OTerMes)2DME was found to react with a Lewis base (4-4’ bipyridine) to form Th(BH4)2(OTerMes)2(4,4’ bipyridine)∞. However, despite the increased robustness and versatility of the borohydride complexes, transmetallation of the terphenolate complexes remained an issue as shown by the synthesis and characterisation of Mg(OTerMes)((μ-H)3BH)THF)2. Th(BH4)2(OTerMes)2 was found to be able to facilitate small molecule activation in a variety of substrates, encompassing CO, CO2 and CS2 amongst others. In most cases this small molecule activation favoured the formation of BMe3, with the concomitant formation of HB(OTerMes)2 in the case of CO2 and CS2. Attempts at catalysis of isonitriles and terminal acetylenes by Th(BH4)2(OTerMes)2 are presented with mixed results. Conclusions are discussed at the end of the chapter. In Chapter Five, investigations into the effects of changing the donor atom of the terphenyl moiety were probed. The chapter began by examining the differing properties of a phosphorous atom acting as a ligating atom, as opposed to the oxygen atom seen in Chapters Three and Four. The chapter continued by detailing the result of reactions attempting to synthesise and characterise terphenyl phosphino-actinide complexes. It was found that in the case of actinides with easily accessible lower oxidation states, i.e. U (IV), that reductive elimination was favoured, culminating in the isolation of (TerMesPH)2. Following this result attempts were made to modify the ligand system in an attempt to divert the reaction away from this product, in the hope of isolating a phosphino-actinide complex. Reactions attempting to ligate the terphenyl moiety via the aryl α-carbon to thorium were also detailed, resulting in radicular degeneration and the isolation of nBuTerTrip and ClTerTrip. Conclusions are discussed at the end of the chapter. Experimental and characterising data are provided in Chapter Six.

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