Identical samples of uranium coupons were prepared and each exposed to hydrogen for different times (where this time is significantly less than a classically understood ‘induction time’). Samples were prepared from rolled depleted uranium stock: as-received oxide was removed on all surfaces and two faces (~12x12 mm) were polished to a sub-micron standard. Samples were individually taken through a Vacuum Thermal Pre-Treatment cycle from room temperature to 200°C to the reaction temperature (80°C) over 40 hours and subsequently exposed to 10 mbar O2 for 24 hours. After O2 was removed, the samples were exposed to hydrogen for pre-determined times of up to 48 minutes. Examination of the samples by Scanning Electron Microscopy (SEM) has, as expected, identified small features protruding from the surface believed to have been caused by sub-surface precipitation of UH3. In general these features are circular and isolated from each other, have a diameter of less than 3μm and appear as either ‘flat-topped’ or ‘domed’ morphology. In addition, longer time exposure samples show a predominance of ‘area attack’ where coalesced sub-surface precipitation appears to be confined to particular metal grains. X-Ray Diffraction (XRD) data show an increase in the quantity of UH3 with time.

In d-wave unconventional superconductors, superconducting Cooper pairs are believed to be formed via magnetic fluctuations. In fact, the superconducting transition temperature Tc roughly correlates with the antiferromagnetic spin fluctuation energy in d-wave unconventional superconductors including high Tc cuprates. In addition to this correlation, the superconducting pairing symmetry and the magnetic anisotropy of the normal state are found empirically to be strongly correlated in f-electron unconventional superconductors having crystallographic symmetry lower than cubic. In antiferromagnetic systems, unconventional superconductivity appears with singlet (d-wave) pairing for cases of XY anisotropy. In contrast, in ferromagnetic systems, unconventional superconductivity with triplet (e.g. p-wave) pairing appears for cases of Ising anisotropy. In this report, the d-wave case is addressed, the origin of XY anisotropy is discussed in terms of the orbital character; and the angular momentum character jz for each piece of Fermi surfaces is determined.

Inelastic x-ray scattering measurements of the phonon density of states (DOS) of PuO2(+2%Ga) were made and compared to recent predictions from the literature made using three leading theoretical approaches; Density Functional Theory (DFT), DFT plus the Hubbard U (DFT+U), and Dynamical Mean-Field Theory (DMFT). The DFT prediction, which does not account for strong electronic correlations, underestimates the measured energies of most features. The DFT+U and DMFT predictions, which include approximations to strong correlation effects, more accurately reflect the low energy features but exaggerate splitting in the highest energy optic oxygen modes. The exaggeration of the splitting is worse for DFT+U than for DMFT. The transverse acoustic mode shows the least sensitivity to calculation type, and is well reproduced by all three theories. The longitudinal acoustic mode, which is thought to control the thermal conductivity, is more sensitive to calculation type, suggesting an important role for electronic correlations in making application-critical predictions.

In order to reduce the inherent uncertainty in kinetic theory models and promote their transition to become predictive methodologies, a multi-scale modeling approach is proposed and demonstrated in this work. KiValues of key materials properties such as point defect (vacancy and interstitial) migration enthalpies, as well as kinetic factors, such as dimer formation and defect recombination coefficients and self-interstitial atom – interstitial loop reaction rates, were obtained by ab initio/molecular dynamics calculations. A rate theory model was used to interpret the evolution of dislocation loops in irradiated molybdenum. Calculations of the dose dependence of average loop diameter were performed and compared to experimental measurements obtained from irradiations with high-energy electrons. The comparison demonstrates reasonable agreement between model-predicted and experiment-measured data.

Two major causes of hardening and subsequent embrittlement in ferrite steels are the spinodal decomposition of the binary Fe-Cr solid solution and the carbide formation due to the presence of carbon as foreign interstitial atoms. In the present work, simulations of the microstructure evolution due to thermal ageing are performed by means of a kinetic Monte Carlo code and using a state-of-the-art interatomic potential based on density functional theory (DFT) predictions and experimental data. The main issues concern the possibility to perform thermal ageing simulations in an acceptable computational time frame and to reproduce a realistic behavior of carbon kinetics and carbide formation. The simulations on the binary system show the microstructural evolution during thermal ageing and allowed to find an exponential trend related to the acceleration as a function of temperature. With the insertion of carbon in the model, the chromium precipitation tends to accelerate. The carbon clustering, analyzed separately, is faster with higher C concentrations and in lattices with segregated chromium.

Corium which simulates the molten core of a Boiling Water Reactor was prepared as a parameter of Zr content, and melting temperatures and thermal conductivities were measured. The melting temperatures were measured by the thermal arrest method and were 2622 oC, 2509 oC and 2540 oC, respectively, in the specimens of 24.3 at%, 49.0 at% and 73.5 at% Zr content. Thermal conductivities had low values of 2.0-3.5 W/m oC at temperatures of 400 to 1600 oC.

In order to characterize the microstructure of oxide layers formed on Zircaloy-4 tubes during the breakaway transition, oxidation tests in a flowing steam environment were performed at 1000°C with a different oxidation time. It was found that breakaway oxidation occurred after the oxidation time of 3000s, and zirconium dioxide layers existed in two mixed crystallographic forms of the tetragonal and monoclinic phase in all samples. The zirconium oxide layers showed enhanced crystallinity, increase in grain size, and fine pores at the grain boundary after breakaway oxidation. We found that the initiation of breakaway-oxidation instability originated from these microstructural changes.

Uranyl adsorption/reduction by Fe2+ on hematite and pyrite has been studied at neutral pH under anoxic and CO2-free conditions. XPS results confirm that more U3O8 precipitates on hematite than on pyrite reacted for 24 h in 160 μM uranyl nitrate and 160 μM Fe2+ solution at initial pH 7.3. These results are explained in terms of co-adsorption energy and U atom Mulliken charge transfer by quantum mechanical calculations. Moreover, in situ fluid tapping-mode AFM experiments on hematite indicate a deceleration of the U reduction rate within 24 h due to the passivation of the surface caused by the formation of orthorhombic U3O8 crystals. In addition, crystals observed using AFM show morphologies of orthorhombic schoepite appearing on hematite after 5 h.

Two LaBS glasses containing 9.5 wt.% (#1) and 5.0 wt.% PuO2 (#2) were prepared by melting in Pt ampoules at 1500 C and examined by scanning electron microscopy with energy dispersive X-ray spectroscopy. The bulk of sample #1, both as-prepared and stored for 3 yrs, was amorphous with homogeneous PuO2 distribution. Sample #2, especially after storage for 2-3 yrs, was partly devitrified primarily in the near-surface area. As followed from X-ray elemental maps, the vitreous phase was enriched with Al and Si whereas larger elongated and smaller dendrite crystals strongly enriched with rare earths (La, Nd, Gd) and Si and minor amounts of Hf may be attributed to britholite. A minor concentration of Pu was also observed in this phase. Moreover, relatively minor amounts of white regular crystals with high PuO2 and lower HfO2 contents were observed in the samples and are probably associated with PuO2 and a PuO2-HfO2 cubic solid solution phase. Nevertheless, even in devitrified areas of the samples, the majority of the Pu remained in the vitreous phase where it was homogeneously distributed.

Besides the importance of the actinide dioxide series as a nuclear fuel, the magnetic properties of these compounds at low temperatures are particularly interesting. Their surprisingly varied physical properties at low temperatures stimulate continuing interest for both theory and experiment. Recently, we have performed 17O-NMR studies for the first time on Pu and Amcontaining dioxide systems, (Pu1-xAmx)O2. For the x=0.09 sample, a temperature-dependent NMR line broadening has been observed at low temperatures. By comparing the experimental data with the results of NMR line simulations, we have estimated the effective moment of Am ions to be Peff=1.38 μB. The value suggests the 5f5 (Am4+) state of the Am ion in PuO2. For the x=1 (=AmO2) sample, on the other hand, our 17O-NMR data provide the first microscopic evidence for a phase transition at 8.5 K as a bulk property in this system. A spectrum with a triangular line shape indicates that the internal field is distributed very nearly randomly in the ordered state.

Structural materials in the new Generation IV reactors will operate in harsh radiation conditions coupled with high levels of hydrogen and helium production and will experience severe degradation of mechanical properties. Therefore, understanding of the physical mechanisms responsible for the microstructural evolution and corresponding mechanical property changes is critical. As the involved phenomena are very complex and span in several length scales, a multiscale approach is necessary in order to fully understand the degradation of materials in irradiated environments. In previous work, we used molecular dynamics simulations to develop critical rules for the mobility of dislocations in various iron alloys and their interaction with several types of defects that include, among others, helium bubbles and grain boundaries. In this work, Dislocation Dynamics simulations of iron alloys are used to study the mechanical behavior and the degradation under irradiation of large systems with high dislocation and defect densities.

Sample of zirconate ceramic with a composition corresponding to formula Gd1.7241Am0.3Zr2O7 was synthesized by heat-treatment of mechanically activated and compacted in pellet oxide mixture at 1500 °C for 30 min. The d values on XRD pattern of the sample soon after synthesis (D = 7.9×1015 α-decays/g or 0.001 dpa) demonstrated fluorite structure with the most intensive peak with d111 =3.042 Å (a = 5.269 Å) and very weak diffuse reflections due to d-pyrochlore. At a dose of 7.9×1017 α-decays/g or 0.11 dpa the reflections were broadened by approximately 20% and their relative intensity slightly reduced. At higher doses all the weak superstructure reflections disappeared and the growth in intensity and narrowing of the main reflection occurred. Lattice parameter a increased with the dose and reached 5.343 Å (d111 = 3.085 Å) at a dose of 4.6×1018 α-decays/g or 0.42 dpa. At a dose of 5.5×1018 α-decays/g or 0.78 dpa positions of reflections were shifted to lower d-spaces (d111 value reduced to 3.071 Å) and the half-width of the major reflection was 67% of initial. For the 241Am-doped Gd-zirconate the structure recovery rate exceeds disordering rate and no amorphization occurred at doses higher than ∼0.2-0.3 dpa.

In the performance assessment (PA) for the Waste Isolation Pilot Plant (WIPP), the solubility of uranium (VI) was conservatively set at 10-3 M for all expected WIPP conditions, including the potential and likely effects of carbonate complexation [1]. Under WIPP-relevant conditions, long-term experiments were performed to establish the uranium (VI) solubility limits in WIPP-simulated brine over a broad range of pCH+ values [7.5-12.5] and to evaluate the contribution of carbonate complexation and hydrolysis to uranium (VI) speciation. Data obtained in carbonate-free ERDA-6 brine, a simulated WIPP brine, were reported earlier [2]. In the absence of carbonate, uranium solubility approached 10-7 M at the expected pCH+ in the WIPP (~ 9.5). In the presence of a significant amount of carbonate (millimole levels), recent experimental results showed that uranium (VI) concentrations will not exceed 10-4M. This measured solubility limit is an order of magnitude lower than the uranium solubility value currently used in the WIPP PA [3]. A small effect of borate complexation was found in the pCH+ range [7.5-10]. At pCH+ ≥ 10, hydrolysis overwhelmed carbonate effects, and no amphoteric effect was observed.

We report a theoretical investigation of changes in the electronic structure of americium metal due to applied pressure. We employ a variant of the LDA+DMFT method that takes into account not only the correlations among the 5f electrons, but also the feedback of these correlations on the rest of the system by means of an appropriate adjustment of the electronic charge density. We observe only minor modification of the electronic structure in the compressed lattice, which is in accord with recent resonant x-ray spectroscopy experiments.

There is a growing body of evidence that a number of mixed-valent and heavy-fermion materials show renormalized hybridization gaps either at the Fermi-energy or close to the Fermi-energy. In the former case, a heavy-fermion semiconducting state occurs and in the later case, the system remains metallic at low temperatures. The magnitudes of the hy-bridization gaps are observed to decrease with increasing temperature. The existence of a low-energy electronic energy scale creates a possibility that the Born-Oppenheimer ap-proximation may fail and that there may be a resonant coupling between the phonons and the electronic excitations. Here we argue that such a mechanism may be the cause of the phonon anomalies observed in neutron scattering experiments on the high-temperature phase of alpha-uranium.

Intermetallic compounds containing actinide ions exhibit a broad spectrum of different physical phenomena at low temperatures. The latter include heavy quasiparticles, unconventional superconductivity and various forms of magnetic ordering. The complex and sometimes enigmatic properties of these compounds derive from the strong correlations among the 5f electrons. Previous model calculations suggested that the intra-atomic Hund’s rule-type correlations may lead to partial localization which is reflected e. g. in the co-existence of itinerant 5f-derived heavy quasiparticles and local magnetic excitations. The conjectured "dual nature" of 5f electrons which is closely related to the question of the 5f valence of the actinide ions is not directly probed by ground state properties and the low-energy excitations. Here we present microscopic calculations for core-level spectroscopy emphasizing the consequences of strong intra-atomic correlations of the 5f shell.

A new Mo potential, developed recently by using an ab initio quantum mechanics theory, was used to study formation and time evolution of radiation defects, such as self-interstitial atoms (SIAs), vacancies, and small clusters of SIAs, using molecular dynamics (MD). MD models were developed for calculation of the diffusion coefficients of vacancies, self-interstitials, and small dislocation loops containing 2 to 37 SIAs; and the rate constants were calculated. Interactions of small SIA loops with SIAs were simulated. The results show that rotation of SIA from one <111> to another equivalent direction is an important mechanism that significantly contributes to kinetic coefficients.

Density-functional formalism is applied to study the ground state properties of γ-U-Zr and γ-U-Mo solid solutions. Calculated heats of formation are compared with CALPHAD assessments. We discuss how the heat of formation in both alloys correlates with the charge transfer between the alloy components. The decomposition curves for γ-based U-Zr and U-Mo solid solutions are derived from Ising-type Monte Carlo simulations. We explore the idea of stabilization of the δ-UZr2 compound against the α-Zr (hcp) structure due to increase of Zr d-band occupancy by the addition of U to Zr. We discuss how the specific behavior of the electronic density of states in the vicinity of the Fermi level promotes the stabilization of the U2Mo compound. The mechanism of possible Am redistribution in the U-Zr and U-Mo fuels is also discussed.

Enhanced thermal conductivity oxide fuels offer increases in both safety and efficiency of commercial light water reactors. Low-temperature oxidative sintering and Spark Plasma Sintering (SPS) techniques have been used to produce UO2-SiC composite pellets. Oxidative sintering performed for 4 hours at 1200∼1600oC and SPS was employed only for 5 mins at the same temperature. While oxidative sintering failed to achieve enhanced thermal conductivity, the SPS sintered pellet obtained promising features such as higher density, better interfacial contact, and reduced chemical reaction. Thermal conductivity measurement at 100oC, 500oC, and 900oC revealed maximum 62% higher thermal conductivity value, when compared to UO2 pellets, in SPS sintered UO2-10vol% SiC composite pellet. The result shows that the SPS technique is required to sinter UO2-SiC nuclear fuel pellets with a high value of thermal conductivity.

Plutonium and Pu-Ga alloys have been observed to have anomalous hydrogen solubility behavior, including a significant concentration dependence of hydrogen diffusivity in the dilute regime, a sharp drop off in the hydrogen solubility constant in the dilute regime, and a near complete absence of change in the Sieverts’ constant as the alloys are heated across phase transformation boundaries. We are investigating the possibility that a vacancy mechanism is responsible for this behavior. X-ray diffraction measurements show a 0.14% lattice contraction in Pu-2 at. % Ga alloys when they are charged with ~2 at. % hydrogen. The lattice re-expands when the hydrogen is removed. Density functional calculations show that increasing the number of hydrogen atoms associated with a vacant lattice site in Pu lowers the energy of the hydrogen-vacancy complex. These observations support the idea that vacancies are stabilized by hydrogen in the Pu lattice well beyond their thermal equilibrium concentration and could be responsible for the anomalous hydrogen response of Pu.