Density-functional electronic calculations have been used to investigate the high-pressure behavior of americium.The phase transitions calculated agree with the recent sequence obtained experimentally under pressure: double hexagonal close packed → face centered cubic → face centered orthorhombic, primitive orthorhombic. In the first three phases the 5f electrons are found localized, only in the fourth phase (Am IV) the 5f electrons are found delocalized. The localization of the 5f electrons is modelled by an anti-ferromagnetic configuration which has a lower energy than the ferromagnetic ones. In this study the complex crystal structures have been fully relaxed.

The adsorption of H2O on UO2 and PuO2 surfaces is studied using density functional theoretical approaches. Periodic slab models are employed to represent the oxide surface in which five layers of the oxide are explicitly treated. For both oxides the dihydroxyl form corresponding to dissociated H2O is more stable than the molecularly adsorbed H2O on the (111) and (110) surfaces. Adsorption of H2 on PuO2 and Pu2O3 is also investigated where stable forms corresponding to dissociated H atoms are found.

High-energy x-ray scattering (HEXS) is used to probe correlations about selected actinide ions in solution. The experiments were designed to favor homoleptic water coordination in order to compare the preferred structural environment of UO22+, NpO2+, and Cm3+ with similar ligands. In addition to comparisons between the three actinide ions in solution, the results from the HEXS studies are also compared with known solid state structures. Marked similarities are found between near- and next-near neighbor coordination in solution and in the solid state.

Time-resolved photoinduced reflectivity data were measured for the itinerant antiferromagnet UNiGa5 (TN ≈ 85 K) from room temperature down to 10 K. The relaxation time τ increases sharply near TN, which we attribute to the opening of a spin gap. In addition, at the lowest temperatures τ increases with a T1 dependence, which is similar to that shown by the heavy fermion YbAgCu4, but with no blocking of electron-phonon scattering within the DOS peak. The transient amplitude increase at TN with a temperature dependence that is consistent with the appearance of a spin gap.

The standard method to determine the band structure of a condensed phase material is to (1) obtain a single crystal with a well defined surface and (2) map the bands with angle resolved photoelectron spectroscopy (occupied or valence bands) and inverse photoelectron spectroscopy (unoccupied or conduction bands). Unfortunately, in the case of Pu, the single crystals of Pu are either nonexistent, very small and/or having poorly defined surfaces. Furthermore, effects such as electron correlation and a large spin-orbit splitting in the 5f states have further complicated the situation. Thus, we have embarked upon the utilization of unorthodox electron spectroscopies, to circumvent the problems caused by the absence of large single crystals of Pu with well-defined surfaces. Our approach includes the techniques of resonant photoelectron spectroscopy [1], x-ray absorption spectroscopy [1,2,3,4], electron energy loss spectroscopy [2,3,4], Fano Effect measurements [5], and Bremstrahlung Isochromat Spectroscopy [6], including the utilization of micro-focused beams to probe single-crystallite regions of polycrystalline Pu samples. [2,3,6]

The mechanism responsible for the stability of the delta-phase in plutonium alloys remains poorly understood despite previous extensive studies. In the present experiments, three alloys (Pu-8at%Am, Pu-15at%Am and Pu-2at%Ga) were studied under pressure by X-ray diffraction in diamond anvil cells and revealed i) a pressure-induced softening in the compressibility of the delta-phase and ii) the presence of an intermediate γ’-phase across the δ → α’ phase transition. The softening, an anomalous behaviour, is shown to be coherent with other known unusual properties found for these alloys like NTE (Negative Thermal Expansion). Furthermore, it has been observed that the cell volume for the α’-phase formed under pressure depends on the rate of the pressure increase. This behaviour may be related to the diffusion of solute atoms to specific sites within the α’-phase unit cell. Such diffusion may also explain the α’-phase unit cell’s volume shrinkage versus time in a Pu-2at%Ga sample kept at a constant pressure.

These XRD results, obtained under pressure, may be used to test first-principle calculations or other general modelling which aim at predicting the solute’s diffusion and/or self-irradiation effects in plutonium alloys.

Differential scanning calorimetry (DSC) is used as an alternative approach to determining the time-temperature-transformation (TTT) diagram for the martensitic delta to alpha-prime transformation in a Pu-2.0 at% Ga alloy. Previous work sugests that the TTT diagram for a similar alloy exhibits an unusual double-C curve for isothermal holds of less than 100 minutes. Here, we extend this diagram to 18 hours, and confirm the double-C curve behavior. When the sample is cooled prior to the isothermal holds, the delta to alpha-prime transformation is observed as several overlapping exothermic peaks. These peaks are very reproducible, and they are believed to be the result of different kinds of delta to alpha-prime martensitic transformation. This may be due to the presence of different nucleation sites and/or different morphologies.

We have studied the crystal structure of Pa metal under high pressure by means of first-principles calculations based on the density functional theory (DFT) using the generalized gradient approximation (GGA). The body centered tetragonal (bct) to orthorhombic (α-U) phase transition was calculated to take place at 29 GPa and with a volume change of 1.3%. The calculated c/a for the bct phase reaches the ideal c/a value (0.816) at around 50 GPa. A bulk modulus of 113 GPa was derived from a Murnaghan equation of state (EOS) fitting procedure. Our results are in general good agreement with recent experiment performed by Haire et al. [Phys. Rev. B 67, 134101 (2003)]. We have also calculated the phonon spectrum for fcc, bct and bcc Pa. The latter gives imaginary frequencies showing the low temperature instability of this crystallographic phase for Pa.

A combinative approach of microhardness testing, tensile testing, and TEM microstructural analysis was employed to study the microstructure and mechanical instability of a water-quenched U-6wt.% Nb (WQ-U6Nb) alloy subjected to different aging schedules including artificial aging at 200°C, 15-year natural aging at ambient temperatures, and 15-year natural aging followed by accelerative aging at 200°C. The changes in mechanical property during and after the aging processes were examined using microhardness and tensile-testing methods. During the early stages of artificial aging at 200°C, the microhardness of WQ-U6Nb alloy increased, i.e., age hardening, as a result of the development of nanoscale modulation caused by spinodal decomposition. Coarsening of the modulated structure occurred after a prolonged aging at 200°C for 16 hours, and it led to a decrease of microhardness, i.e., age softening. Phase instability was also found to occur in WQ-U6Nb alloy that was subjected to a 15-year natural aging at ambient temperatures. The formation of partially ordered domains resulting from a spinodal modulation with an atomic-scale wavelength rendered the appearance of swirl-shape antiphase domain boundaries (APBs) observed in TEM images. Although it did not cause a significant change in microhardness, 15-year natural aging has dramatically affected the aging mechanisms of the alloy isothermally aged at 200°C. Microhardness values of the NA alloy continuously increased after isothermal aging at 200°C for 96 hours as a result of the phase decomposition of partially ordered domains into Nb-depleted α phase and Nb-enriched U3Nb ordered phase in the alloy. It is concluded that the long-term natural aging changes the transformation pathway of WQ-U6Nb, and it leads to order-disorder transformation and precipitation hardening of WQ-U6Nb alloy.

Hydrogen peroxide formed by radiolysis of carbonate –bicarbonate media was followed by 13C NMR using dissolved [233UO2(13CO3)3]4- as the alpha source (Dα=12.1 Gy/hr). It is shown that the speciation of the peroxide carbonates (or other species) that are formed in the pH region 5.9 and 11.6 by titration with hydrogen peroxide are common to those formed by hydrogen peroxide generated by radiolysis. Radiolytic generation of hydrogen peroxide was followed by observing the formation of a uranyl peroxide carbonate complex. Complex formation was observed to accelerate for about 1200 hours and to terminate between 1×10-4 and 5×10-4 M. It is assumed that either a steady state H2O2 production rate was established in solution or that some limiting feature of the experiment was responsible for slowing the yield of product.

Recently uranyl peroxide nanoclusters containing 24, 28, and 32 uranyl polyhedra were chemically and structurally characterized under alkaline conditions. Such nanoclusters could conceivable form from oxidative alteration of nuclear waste in a geological repository by incorporating peroxide formed by alpha-radiolysis of water or in tanks where high-level waste is stored. The stability and persistence of uranyl peroxides in the vadose zone will be impacted by their interaction with geological media. Here we report batch experiments of solutions containing monodisperse nanoclusters in contact with crushed welded tuff. Within the first 72 hours, U concentrations in solution remained unchanged; however concentrations of Si, Al, Ca, Mg, Na, and Fe increased due to the dissolution of calcium-bearing aluminum silicate minerals in the welded tuff. Despite the presence of excess Li+ in solution, within two weeks crystals precipitated in which Ca2+ replaced Li+ in the nanocluster cage.

Martensitic transformations can occur via two modes: thermoelastic and burst. In thermoelastic martensites, deformation can be accommodated elastically and transformations occur smoothly with changes in temperature or stress. Burst martensitic transformations require both elastic and plastic deformation to accommodate strain; individual martensite particles form at the speed of sound, and the overall accumulation of martensite may increase in discrete, incremental steps. Here, we examine a unique martensitic transformation and reversion in a Pu-2.0 at% Ga alloy and show evidence that they proceed via the burst mode. Upon cooling from ambient conditions, the metastable delta phase partially transforms martensitically to the alpha-prime phase with a volume contraction of 20%. This large volume change suggests a burst transformation. Furthermore, using differential scanning calorimetry (DSC), we observed that the alpha-prime to delta reversion proceeds in discrete increments, which appear as sharp peaks in DSC data. The DSC data is compared to similar results obtained using dilatometry and resistometry. This incremental progression is believed to be the result of autocatalytic cascades of many alpha-prime particles reverting nearly-simultaneously to the delta phase. Finite-element modeling suggests that residual stresses in the regions of reverted alpha-prime particles may catalyze (or retard) additional transformation. These stresses could initiate cascades of alpha-prime particles that revert nearly-simultaneously. The cascades are likely quenched by stress and/or temperature changes resulting from the transformation itself. During the forward delta to alpha-prime transformation, burst events are not observed with the above techniques. The transformation, however, is still expected to proceed via the burst martensite mode because of the large volume changes required. Because alpha-prime must be nucleated in the delta matrix before it can grow as an individual burst, the transformation may not occur cooperatively. These individual bursts may be too small to be resolved by the above techniques, and the signal observed corresponds to a cumulative total of all the events.

The 5f electronic states in elemental Pu and Pu compounds exhibit elements of both itinerant and localized behavior. Several first-principles calculations have been presented to describe this balance, differing in the manner in which electron correlation is included in the calculation. This paper describes a calculations performed with the Mixed Level Model (MLM), presenting calculated results for the two Pu compounds, PuRhGa5 and PuCoGa5. The MLM results are compared with other calculations and the differences discussed.

As applications for extreme ultraviolet (EUV) radiation have been identified, the demand for better optics has also increased. Thorium and thorium oxide thin films (19 to 61 nm thick) were RF-sputtered and characterized using atomic force microscopy (AFM), spectroscopic ellipsometry, low-angle x-ray diffraction (LAXRD), x-ray photoelectron spectroscopy (XPS), and x-ray absorption near edge structure (XANES) in order to assess their capability as EUV reflectors. Their reflectance and absorption at different energies were also measured and analyzed at the Advanced Light Source in Berkeley. The reflectance of oxidized thorium is reported between 2 and 32 nm at 5, 10, and 15 degrees from grazing. The imaginary component of the complex index of refraction, β, is also reported between 12.5 and 18 nm. Thin films of thorium were found to reflect better between 6.5 and 9.4 nm at 5 degrees from grazing than all other known materials, including iridium, gold, nickel, uranium dioxide, and uranium nitride. The measured reflectance does not coincide with reflectance curves calculated from the Center for X-Ray Optics (CXRO) atomic scattering factor data. We observe large energy shifts of up to 20 eV, suggesting the need for better film characterization and possibly an update of the tabulated optical constants.

Complexation of NpO2+ with oxalic acid (OX), 2,2'-oxydiacetic acid (ODA), 2,2'-iminodiacetic acid (IDA) and 2,2'-thiodiacetic acid (TDA), has been studied using spectrophotometry in 1 M NaClO4. Both the position and the intensity of the absorption band of NpO2+ at 980 nm are affected by the formation of NpO2+/dicarboxylate complexes, providing useful information on the complexation strength, the coordination mode and the structure of the complexes.

Np5+ sulfate compounds were synthesized using hydrothermal methods and their structures were determined using single crystal X-ray diffraction. Several compounds were found to contain cation-cation interactions between the Np polyhedra. Magnetic susceptibility measurements of two Np5+ sulfate compounds containing cation-cation interactions exhibit ferromagnetic ordering. The presence of the cation-cation interactions may provide a superexchange pathway through which magnetic interactions can be enhanced.

The local atomic environment and vibrational properties of monoclinic α-Pu, orthorhombic α-U, and the α-U charge-density-wave (CDW) modulation are examined by extended x-ray absorption fine structure spectroscopy (EXAFS). Debye-Waller factors from temperature-dependent EXAFS measurements are fit to a correlated Debye model. The observed Pu-Pu bond Debye temperature is θcD(α-Pu) = 162 ± 5 K for the pure α-Pu phase. The temperature dependence of the U-U nearest neighbor and second nearest neighbor Debye-Waller factors exhibits a sharp discontinuity in slope near TCDW = 43 K, the transition temperature at which the charge-density wave (CDW) in α-U occurs. Our measurement of the CDW using EXAFS is the first observation of the structure of the CDW in polycrystalline α-U.

While the majority of studies of alteration of UO2 and commercial spent nuclear fuel under simulated geological repository conditions have emphasized the importance of uranyl oxide hydrates and uranyl silicates, the influence of peroxide on repository performance has been largely overlooked. There is considerable evidence that uranyl peroxides will be important alteration phases of nuclear waste, and that these phases may impact the long-term performance of a geologic repository such as Yucca Mountain. Here we report the thermodynamics and kinetics of becquerelite, Ca[(UO2)6O4(OH)6](H2O)8, in the presence of solutions containing hydrogen peroxide. Thermodynamic calculations reveal that in solutions containing 3.5 × 10-6 M hydrogen peroxide, studtite is thermodynamically favorable over becquerelite at 298 K. To access the kinetics of this reaction, batch experiments were conducted by the reaction of becquerelite and solutions containing hydrogen peroxide. In the presence of 0.1 M hydrogen peroxide, becquerelite altered to studtite within eight hours.

The electronic structure of the Pu-based superconductor PuCoGa5 and the Pauli paramagnet UCoGa5 is investigated using photoemission spectroscopy. The photoemission data of PuCoGa5 reveal features at the Fermi energy EF and about 1-1.5 eV below EF indicative of itinerant and localized f-electrons, respectively. Angle-resolved spectra of UCoGa5 show two peaks at similar energies that are highly dispersive, providing evidence for itinerant character of the f-electrons in this material. A comparison of the PuCoGa5 and UCoGa5 data to the spectra of α-Pu and δ-Pu serves to place PuCoGa5 within the context of the more general electronic structure problem in elemental Pu.

Actinides contained in the used nuclear fuel need to be managed in the future fuel cycles for the sustainability of this source of energy. The major ones such as uranium or plutonium are very valuable for energy production within a new fuel. The minor ones such as neptunium, americium or curium are responsible for the long-term radiotoxicity of the ultimate waste if not separated and transmuted within new fuels or dedicated targets. Whatever the choice of management in the present or future, innovative synthesis methods are studied in many research institutions to elaborate new actinides based materials.

Innovative concepts for future fuels or transmutation targets focus on mixed actinides or mixed actinide-inert element materials. For their synthesis, wet methods fulfill very useful requirements such as flexibility, compatibility with a hydrometallurgical fuel processing, less dissemination of radioactive dusts during processing, and above all a better accessibility to very homogeneous compounds and interesting nanostructures. When dealing with plutonium or minor actinides, this last characteristic is of great importance in order to avoid the so-called “hot spots” and to limit macroscopic defects in the fuel material.

In this communication, experimental results are given to illustrate interesting achievements to control the composition or the structure of mixed actinides compounds at a molecular or at a nanometric scale using co-precipitating techniques or sol-gel methods.

The first illustration describes the flexibility of the oxalate ligand to modulate the nanostructure of actinides-based solid precursors and obtain mixed actinides oxide following a thermal treatment of the oxalate precursor. New mixed oxalate structures which present original features such as accepting in the same crystallographic site either a tetravalent actinide or a trivalent one are noticeably detailed. Monocharged cations equilibrate the charge in the 3D structure depending on the molar ratio of trivalent to tetravalent actinides. These oxalate compounds are particularly suitable precursors of oxide solid solutions for various actinides systems.

The second illustration deals with the control of inorganic condensation reactions of tri- and tetravalent cations in solution by using suitable ligands with a view to obtaining homogeneous oxy-hydroxyde mixtures. The results obtained using Zr(IV), Y(III) and Am(III) or Nd(III) are quite original: a very stable colloidal sol is obtained at pH 5-6 and a nanostructured mixed oxy-hydroxide phase is formed by adapting the sol-gel transition conditions. The initial interactions between the oxy-hydroxide Zr nanoparticles, the ligand and the trivalent cations at a nanometric scale in the sol give access, after gel formation and thermal treatment, to a crystallized phase (Am-bearing cubic Y-stabilized Zirconia) at comparatively low temperatures.

In both cases, the simultaneous co-precipitation or co-gelation of the involved actinides remains a challenge because of the specific properties of each actinide, properties which moreover differ according to various possible oxidation states.