Due to a natural occurrence, a good resistance to radiation damage and a low solubility in neutral and basic conditions, Nd-britholite of formula Ca9Nd(PO4)5(SiO4)F2 was already considered as a potential host matrix for the specific immobilization of trivalent actinides. The incorporation of tetravalent actinides like Th, U or Ce in the structure was examined through the elaboration of Ca9Nd1−xAnIVx(PO4)5−x(SiO4)1+xF2 samples. This study was the early beginning of the incorporation of 239Pu and/or 238Pu in order to evaluate the effects of α-decay on the britholite structure. The preparation of the samples was realized through dry chemistry methods using mechanical grinding in order to optimize the conditions of synthesis. The results showed that the incorporation of thorium in britholite structure was successful up to 20 wt.% leading to the formation of solid solutions. For U(IV), the incorporation was incomplete: only 5–8 wt.% of uranium was incorporated instead of 10 wt.% expected. The secondary phase, which was uranium enriched, was identified as the calcium uranate CaU2O5+y. The incorporation of Pu(IV) was studied using Ce(IV) as a surrogate. The results were quite good: major part of cerium was introduced in the structure but was partly reduced in Ce(III) during the heating treatment.

The structural evolution of UO2 during its oxidation to U3O8 at 250°C in air was studied by in-situ synchrotron X-ray diffraction on the D2AM-CRG beamline at ESRF. The aim of this study is to determine the phases that are likely to appear during the long-term storage of spent nuclear fuel. Our results are in disagreement with the literature in which the existence of an intermediate cubic phase is not reported. Instead, an α-U3O7 tetragonal phase (c/a < 1) was mentioned but not definitively observed. These previous interpretations may have been the result of poor instrumental resolution.

Recent results from inelastic neutron scattering studies on Ce0.9Th0.1 are used to demonstrate the importance of electron-phonon coupling to the physical properties of f-electron bonded metals. In the case of Ce0.9Th0.1, the phonon density of states (DOS) of α-phase shows a significant softening when heated but shows no change across the α-γ transition despite the 11% volume change. This is supported by analysis of the magnetic spectra showing that most of the transition entropy can be accounted for with the crystal field and spin fluctuations. The precursor phonon softening, the lack of any phonon change across the transition, the magnetic spectra, and the volume transition itself can all be explained in terms of the atomic displacement sensitivity of the hybridization of the local f-electrons with conduction electrons. The electron-phonon coupling resulting from these displacement-sensitive electronic states appears to be essential to understanding cerium. Some of the behavior characteristic of these states, a large volume changes and precursor phonon softening, occurs in many other f-electron bonded metals suggesting that the phenomena is not unique to cerium.

UAsSe and USb2 single crystals were studied at 15K by angle-resolved photoemission spectroscopy (ARPES) in the photon energy range between 20 eV and 110 eV. The high energy and momentum resolution (24 meV and 0.09Å−1, respectively) allows observation of very narrow and dispersive photoemission features within 100 meV of the Fermi level. The natural linewidth of the near EF feature of USb2 was found to be less than 10 meV with the dispersion of 10 meV observed in normal emission spectra. The near EF feature in UAsSe is slightly broader, situated closer to the Fermi edge, and exhibits 20 meV dispersion in the normal emission spectra. It gives evidence that neither UAsSe nor USb2 have purely 2D electronic structure. Both these compounds should be treated as quasi-2D materials.

We earlier reported the measured decrease of electrical resistivity during isochronal-annealing of ion irradiation damage that was accumulated at low-temperature (10 or 20K), and the temperature dependence of the resistance of defect-populations produced by low-temperature damage-accumulation and annealing in a stabilized δ-phase plutonium alloy, Pu(3.3 at%Ga)[1]. We noted that the temperature dependence of the resistance of defects resulting from low-temperature damage accumulation and subsequent annealing exhibits a -ln(T) temperature dependence suggestive of a Kondo impurity. A discussion of a possible “structure-property” effect, as it might relate to the nature of the δ-phase of Pu, is presented.

The fundamental research topic in actinide chemistry and materials sciences is the role that the 5f electrons play in bond formation, which provides the central focus for DOE BES-supported actinide science. Structural systematics of the actinide metals, oxides, and other compounds as a function of atomic number are well established. Magnetic measurements have shown that the light actinide metals have delocalized 5f orbitals (i.e., the 5f electrons form bands), whereas the f electrons become localized at americium. Thus, the magnetic behavior of the first part of the actinide series resembles that of the d transition metals whereas the heavier actinides exhibit behavior similar to the rare earth metals. Spectroscopic results have established electronic energy levels, crystal field splitting, and near-neighbor coordination. The 5f orbitals participate in the band structure of materials that contain the light actinide metals and some of their intermetallic compounds, and perhaps in molecular compounds. Molecular-level information on the geometry and bonding in solids, at surfaces, and in clusters can now be obtained at BES-supported facilities.

X-ray absorption data, obtained from Np(V) in two environmentally-related samples, show evidence of significant metal-ion reduction in the synchrotron beam. The data are presented and possible sources of reduction are discussed.

Plutonium (Pu) is well known to have complex and unique physico-chemical properties [1]. Notably, the pure metal exhibits six solid-state phase transformations with large volume expansions and contractions along the way to the liquid state: α → β → γ → δ → δ' → ε→ liquid. Unalloyed Pu melts at a relatively low temperature ∼640 °C to yield a higher density liquid than that of the solid from which it melts, (Figure 1). Detailed understanding of the properties of plutonium and plutonium-based alloys is critical for the safe handling, utilization, and long-term storage of these important, but highly toxic materials. However, both technical and and safety issues have made experimental observations extremely difficult.

The δ to α′ phase transformation in Pu-Ga alloys is intriguing for both scientific and technological reasons. On cooling, the ductile fcc δ-phase transforms martensitically to the brittle monoclinic α′-phase at approximately −120°C (depending on composition). This exothermic transformation involves a 20% volume contraction and a significant increase in resistivity. The reversion of α′ to δ involves a large temperature hysteresis and begins just above room temperature. In an attempt to better understand the underlying thermodynamics and kinetics responsible for these unusual features, we are examining the δ/α' transformations in a Pu-0.6 wt% Ga alloy using differential scanning calorimetry (DSC) and resistometry. Both techniques indicate that the martensite start temperature is −120°C and the austenite start temperature is 35°C. The heat of transformation is approximately 3 kJ/mole. During the α ′ → δ reversion, “spikes” and “steps” are observed in DSC and resistometry scans, respectively. These spikes and steps are periodic, and their periodicity with respect to temperature does not vary with heating rate. With an appropriate annealing cycle, including a “rest” at room temperature, these spikes and steps can be reproduced through many thermal cycles of a single sample.

Phosphate materials are usually considered as potential candidates to perform the immobilization of actinides coming from an advanced reprocessing of spent fuel in the field of an underground repository. Among them, Thorium Phosphate-Diphosphate (TPD) and monazites have been already extensively studied. The elaboration of TPD/monazite based materials was thus envisaged in order to immobilize simultaneously tri- and tetravalent actinides and a neutron absorber. Two chemical ways of synthesis were considered and the compounds were easily prepared in the powder and in the pellet form. A good chemical compatibility was found between TPD and monazite since the properties of both phosphates were kept in these composites. Moreover, the relative density of the pellets reached 90 – 95 % of the value calculated from XRD data. The normalized dissolution rates determined in acidic media did not exceed 5.10−4 g.m−2.day−1 which confirmed the very good durability of such materials during leaching tests.

In the field of the immobilization of high level activity and long life radwaste for a deep underground repository, several phosphate matrices were already proposed to delay the release of actinides to the biosphere. Among them, Thorium Phosphate-Diphosphate (TPD) and associated solid solutions were considered. In order to improve the homogeneity of the samples prepared, a new way of preparation based on the precipitation in closed containers, at 150°C, of the Thorium-Phosphate HydrogenPhosphate hydrate (TPHP) or associated Thorium – Uranium (IV) Phosphate HydrogenPhosphate hydrate (TUPHP) was developed. Well crystallized and single phase TUPD solid solutions of general formula Th4−xUxP6O23 were prepared after heating above 950°C. Dense TUPD pellets were also obtained through a two-steps procedure involving a uniaxial pressing of the powder at room temperature then a heating treatment at 1250°C for 10 hours. Several properties of the pellets such as their chemical durability during leaching tests were considered carefully. All the normalized dissolution rates of powdered and sintered TUPD solid solutions remained low by comparison to that reported for several other ceramics studied in the same objective.

Recent experiments on single crystals of the compounds CeRh1−xCoxIn5 and PrOs4Sb12 are briefly reviewed. The temperature-composition (T-x) phase diagram of the heavy fermion pseudoternary system CeRh1−xCoxIn5, delineating the regions in which superconductivity, antiferromagnetism, and the coexistence of these two phenomena occur, has been established. Entropy vs x isotherms and residual resistivity vs x plots exhibit peaks near the critical concentration xcr ≈ 0.8 at which the Néel temperature appears to vanish (quantum critical point). The filled skutterudite compound PrOs4Sb12 exhibits unconventional superconductivity below Tc = 1.85 K that involves heavy fermion quasiparticles with an effective mass m* ≈ 50 me, where me is the mass of the free electron. The unconventional superconducting state appears to consist of several distinct superconducting phases and to break time reversal symmetry. A high field ordered phase occurs below ∼ 1 K and between ∼ 4.5 T and ∼ 15 T that appears to be associated with quadrupolar order. The heavy fermion state and superconductivity in PrOs4Sb12 may originate from the interaction between Pr3+ electric quadrupole moments and the charges of the conduction electrons. When Ru is substituted for Os in PrOs4Sb12, a minimum in Tc occurs at Pr(Os0.4Ru0.6)4Sb12, suggesting a competition between two types of superconductivity.

The correlated band theory picture (LSDA+U) has been applied to UGe2, in which superconductivity has been found to coexist with robust ferromagnetism. Over a range of volumes (i.e. pressures), two nearly degenerate states are obtained, which differ most strikingly in their orbital character (on uranium). The calculated moment, and its separation into spin and orbital parts, is consistent with recent polarized neutron scattering data. These two states are strong candidates for the two ferromagnetic phases, one low-temperature -- low-pressure, the other higher-temperature -- higher pressure. Magnetic waves built from fluctuations between these uranium configurations provide a possible new mechanism of pairing in UGe2.

A series of glass samples were prepared analogously to high level waste glass using either glass frit or glass precursors combined with a high level waste surrogate containing NaTcO4. Three different technetium species were observed in these glasses depending upon the synthesis conditions. If the glasses were prepared by reducing NaTcO4 to TcO2•2H2O with hydrazine or if a large amount of organic material was present, inclusions of TcO2 were observed. If no organic material was present, technetium was incorporated as TcO4−. If only a small amount of organic material was present, isolated Tc(IV) sites were observed in the glass. The relative technetium retention of these glasses was estimated from the Tc K-edge height, and had no correlation with the oxidation state of the technetium. Pertechnetate was well retained in these glasses.

Synchrotron radiation (SR) methods have been utilized with increasing frequency over the past several years to study topics in actinide science, ranging from those of a fundamental nature to those that address a specifically-targeted technical need. In particular, the emergence of microspectroscopic and fluorescence-based techniques have permitted investigations of actinide materials at sources of soft x-ray SR. Spectroscopic techniques with fluorescence-based detection are useful for actinide investigations since they are sensitive to small amounts of material and the information sampling depth may be varied. These characteristics also serve to simplify both sample preparation and safety considerations. Examples of investigations using these fluorescence techniques will be described along with their results, as well as the prospects for future investigations utilizing these methodologies.

We address the electronic properties of uranyl ions in solids and solutions with an emphasis in theoretical understanding of charge transfer vibronic transitions and luminescence dynamics the O-U-O species. A general theory of ion-phonon interaction has been modified for modeling and simulating multi-phonon vibronic spectra. Spectroscopic data for uranyl ions in crystals and solutions have been analyzed to achieve a predictive understanding of the uranyl-ligand vibronic interactions. By adjusting the Huang-Rhys ion-phonon interaction parameters, an excellent agreement between theory and experiment has been accomplished for uranyl ions in the ligand environments we studied. Our modeling and simulation provide insights into the physical nature of uranyl vibronic interaction and its influence on spectroscopic properties, which are commonly utilized in characterizing photochemical properties of uranyl in complexes.

High resolution photoelectron spectroscopy (PES) studies were conducted on a δ-phase Plutonium sample cleaned by laser ablation and gas dosed with O2 and H2. The measurements were made with an instrument resolution of 60 meV and with the sample at 77 K. The PES data strongly support a model with Pu2O3 growth on the metal and then PuO2 growth on the Pu2O3 layer at this temperature. In vacuum, the PuO2 reduces to Pu2O3 at room temperature with a pressure of 6×10−11 Torr. In the case of H2 dosing the hydrogen appears to penetrate the surface and disrupt the valence band as evidenced by a drop in intensity of the peak at EF which is not accompanied by a drop in the main 5f manifold at ∼2eV.

Chemical shifts (ChSh) of nine emission lines of the uranium L-series in uranium oxides UO2+x (x=0−1) with respect to UO2 were studied by using a precise crystal-diffraction X-ray spectrometer and the changes in energy of spin-orbital splitting (SOS) − Δδnl± of inner nl-orbitals of the uranium atom were calculated from the data of ChSh of spin-doublet lines. For UO2+x oxides, a linear decrease in Δδnl± values with increasing degree of uranium oxidation was found.

On the basis of the comparison of experimental Δδnl± values with Dirac-Hartree-Fock atomic calculations, it was concluded that the observed variations in Δδnl± values are due to the redistribution of electron and spin density between the 5f7/2- and 5f5/2-levels of the fine structure of the uranium atom without changes in atomic charge state.