Symposium EN07 : Issues, Challenges and Opportunities in Actinide Materials

From potential contamination of individuals with radioactive fission products after a nuclear accident to the therapeutic use of radio-isotopes for cancer diagnostics and treatment, the biological chemistry of actinides has become increasingly relevant to a number of applied problems. Understanding the fundamental bonding interactions of selective metal assemblies presents a rich set of scientific challenges and is critical to the characterization of f-element coordination chemistry in environmentally and biologically relevant species, and to the development of highly efficient separation reagents or new therapeutic agents. Our approach to these challenges uses a combination of biochemical and spectroscopic studies on both in vitro and in vivo systems to characterize the selective binding of f-block metal ions by natural and biomimetic hard oxygen-donor architectures and the subsequent macromolecular recognition of the resulting assemblies.
Luminescence sensitization, UV-Visible, X-ray absorption, and X-ray diffraction spectroscopic techniques allow us to tune specific actinide coordination features by ligands that drive the differentiation of different metals through stabilization in specific oxidation states and provide information on their respective electronic structures. In addition, X-ray diffraction analyses using the mammalian iron transport protein siderocalin as a crystallization matrix reveals remarkable aspects of the protein’s interactions with chelated metals, establishing series of isostructural systems that can be used to derive trends in the later 5f-element sequence, when combined with theoretical predictions. These results will be discussed with a perspective on how such studies have important implications for the combined use of spectroscopic, thermodynamic, and biokinetic methods to exploit the fundamental knowledge of the role of f-electrons in actinide bonding for the development of new transport, separation, luminescence, and therapeutic applications.

During the past three years, our group in Soochow University has deeply looked into two parallel research directions, both combining two fields of radiochemistry and metal-organic frameworks (MOFs). The first one is the synthesis and characterizations of actinide MOF compounds. This system is unique not only because compared to the transition metals and lanthanides, the actinide MOF systems are substantially less explored, but also that these compounds can not be simply predicted by those analogues of transition metals and lanthanides owing to the uniqueness of actinide ions in bonding and coordination. In addition, we have found many interesting potential applications for these compounds including actinide waste form design for geological disposal, ion-exchange for remediation of radioactive contamination, and detection of extremely low-dose ionization radiations, further highlighting the bright future of adopting actinide ions in building of unique MOFs with potential applications in the nuclear industry. The other research direction is the design and build of non-radioactive MOFs for rapid, efficient, and selective removal and detection of soluble radioisotope ions including UO22+, Sr2+, Cs+, and TcO4- from aqueous solutions. Specifically, I will talk about three interesting examples within this direction: several single-crystalline zirconium phosphonate MOFs that are able to survive from fuming acids including aqua regia and can remove large amounts of uranium even from acidic solutions; a luminescent mesoporous MOF equipped with abundant Lewis basic sites, which can be used for sequestration and detection of trace amounts of uranyl ion in the natural water systems including seawater; the first experimental investigation of 99TcO4- removal by a cationic MOF material showing many promises over the traditional anion-exchange materials. These works clearly reveal that all the possible advantages for ideal radioisotope sorbent materials including high capacity, fast kinetics, excellent selectivity, and great stability and recyclability etc. can be indeed integrated in the MOF system.

The collider accelerator complex at Brookhaven National Laboratory operates a proton LINAC which provides protons for high energy nuclear physics experiments and production of medical isotopes. The Linac proton energy can be incrementally tuned from 66 to 200 MeV in steps at 90, 118, 140, 160, 180 MeV. More than 90% of the pulses are directed via beamline to Brookhaven Linac Isotope Producer (BLIP) which was built with a shielded shaft that allows targets to be inserted into and out of beam in an underwater target station utilized for target cooling. The beam can deliver both rastered and focused beam patterns at an average current as high as 165 µA. BNL is part of the DOE tri-Lab effort consisting of ORNL, LANL and BNL to evaluate high energy proton irradiations of Thorium to provide multi-Curie quantities of Ac-225 for clinical applications. This requires developing robust targetry for the power distribution resulting from the high energy and current to developing the chemistry to isolate the Ac-225 from the thorium target. Presented will be the efforts at BNL in developing a thorium target to produce curie quantities using their 200 MeV linear accelerator as well as chemistry efforts to isolate the Ac-225 from the more than 400 other isotopes produced.

In the 75+ years since plutonium was discovered there has been an enormous amount of work reported on it – physical properties, chemistry, and materials science. In the last 10 years significant progress has been made in understanding the role of the 5f electrons, in both experiment and theory.

This talk will try to cover the experimental situation, as we now know it [1]. Recent experiments include photoemission spectroscopy, where we can compare metallic Pu with PuTe, a compound that is known to have a significant fractional occupation of the 5f6 state, in addition to the 5f5. Core level spectroscopies and resonant X-ray emission spectroscopy (RXES) of Pu also provide evidence for a mixed-configuration ground state. On a finer energy level, neutron inelastic scattering has shown that the ground state of Pu metal is a Kondo singlet, with a Kondo energy of ~ 85 meV (~1000 K) [2].

On the theory side, which I will cover only briefly, LDA+U gives a magnetically ordered state as the ground state. However, in a landmark paper in 2007, Shim et al. [3] showed that the application of dynamical mean-field theory (DMFT) led to a non-magnetic solution, and they predicted within 20% the Kondo energy. A recent theoretical paper by Amadon [4] has further emphasized the importance of the DMFT approach, and calculates almost all structural and elastic properties of Pu metal.

References:
[1] J. J. Joyce & G. H. Lander, Pu-Handbook 2018 (to be published) Ch. 10: “Pu Experimental Electronic Structure.”
[2] M. Janoschek et al. Sci. Adv. 1:e1500188 (2015)
[3] J. H. Shim, K. Haule, & G. Kotliar Nature 446, 513 (2007)
[4] B. Amadon, Phys. Rev. B 94, 115148 (2016)

Based on first-principle calculations¹, we have systematically explored the nature of the elastic stability and the delta-delta'-epsilon phase transitions in pure Pu at high temperature. It is found that, both the electron-phonon coupling and the spin fluctuation effects tend to decrease the tetragonal elastic constant (C') of delta-Pu, accounting for its anomalous softening at high temperature. The lattice thermal expansion together with the electron-phonon coupling can stiffen C' of epsilon-Pu, promoting its mechanical stability at high temperature. The delta-epsilon transition is calculated to take place around 750-800 K, and is dominated by the phonon vibration. The delta' intermediate phase is realized around 750 K mainly because of the thermal spin fluctuation.
1) Li, C-M ; Johansson, B and Vitos, L SCIENTIFIC REPORTS 7, Article Number: 5632, (2017)

The last decade or so has seen a continued development of better experimental techniques to measure equation-of-state (EOS) for various materials. These improvements of both static (diamond anvil-cell) and shock compression approaches have increased the accuracy of the experimental EOS and further challenged the complimentary theoretical modeling. The conventional modeling of EOS, at least at pressure and temperature conditions that are not too extreme, is founded on density-functional theory (DFT). Naturally, there is an increased interest in the accuracy of DFT as the measurements are becoming more exact as well as the robustness of DFT for predicting the EOS for materials and regimes where experimental data are not available.
In this presentation, we focus on a broad and large set of elemental solids from low Z up to the high Z actinide metals. The intent is to compare DFT with experimental zero temperature isotherms up to 1 Mbar and draw conclusions regarding the theoretical errors and quantify a reasonable and defensible approach to define the theoretical uncertainty. We will show a subset of the results and focus more deeply on the actinide metals. This work performed under the auspices of the U.S. DOE by LLNL under Contract DE-AC52-07NA27344.

Elastic moduli are fundamental thermodynamic susceptibilities that connect directly to thermodynamics, electronic structure and mechanic properties. It is important to determine the origin of changes in elastic properties in ²³⁹Pu and its Ga alloys as a function of time. The most-likely sources of these changes include a) ingrowth of radioactive decay products like He and U, b) the introduction of radiation damage, c) δ-phase instabilities towards α-Pu or to Pu₃Ga. The measurement of mechanical resonance frequencies can be made with extreme precision and used to compute the elastic moduli without corrections giving important insight in this problem. Using Resonant Ultrasound Spectroscopy, time-dependent measurements were made of the mechanical resonance frequencies of fine-grained polycrystalline δ-phase ²³⁹Pu, from room temperature up to 480K. At room temperature, both shear (G) and bulk (B) moduli increase in time with the rate of G at least a factor of 3 faster than that of B. As the temperature is increased, the rates of change increase exponentially with both G and B becoming stiffer with time. For T>420K an abrupt change in the time dependence is observed indicative of different G and B time dependence, however no changes in rate are observed when the temperature (400K) corresponding to the α-β lines is crossed. These measurements suggest that the changes in time observed in Ga-stabilized δ-²³⁹Pu are consistent with the thermally activated creation of defects, that can be (partially) annealed for T>420K. Our measurements also rule out the decomposition of δ-Pu(Ga) towards α-Pu or Pu₃Ga as the main source of changes in δ-Pu in laboratory time frame.

This presentation will describe some recent work on heat capacity studies for several Pu alloys that include low weight percent Ga alloys (Pu-0.614at.%Ga – Pu-1.425at.%Ga) as well as some thermophysical and metallurgy results on the monocarbide PuC_1-x.
Specific heat measurements have been completed on several gallium stabilized delta-Pu alloys up to 7at.%Ga. The measurements suggest that the low weight percent compositions transition to the alpha-prime Pu structure at low temperatures. Some additional trends will be discussed in relation to electronic correlations and Sommerfeld coefficients in these materials.

The radioactive decay processes at work in plutonium produce a complex set of changes in the material. In addition to ever-changing chemical makeup, defect structures are continuously created by radiation damage, and annealed by multiple processes that are activated at various temperature regimes. Our initial work in this area involved storing a sample of plutonium at liquid helium temperatures to turn off all natural annealing, and probing the local structure of Pu and Ga atoms as the sample was warmed through various stages of an isochronal annealing experiment. This demonstrated the differing damage and annealing behaviors of Pu and Ga in the lattice.

More recent investigations have repeated these measurements on additional samples. In addition, we have investigated the effects on local structure of high temperature annealing using materials of varying ages and Ga concentrations, leading to an improved understanding of the response of Pu and Ga local structure in different states of sample damage.

Aging of fcc δ-Pu is becoming a forefront challenging problem in Pu science, as we try to understand the effects of radiological decay on the phase stability. The most influential drivers of δ-Pu aging include He and U ingrowth, radiation-induced lattice defect accumulation, and phase instability, which may affect the overall integrity in mechanical and electronic properties. Due to multiple processes that occur while δ-Pu ages, computational efforts, such as DFT, may provide fundamental insight and guidance into the most prominent defects that will impact the stability of the lattice and the electronic properties. We have explored a variety of point defects in unalloyed and Ga-alloyed δ-Pu, which include vacancies, self-interstitials, and defects containing the radioactive daughter decay product, U. Formation energies and binding energies of these defects, and migration barriers of selective defects will be discussed, along with corresponding radial distribution functions (RDFs).

Topological insulators (TIs) have recently attracted intense theoretical and experimental interest for the existence of topologically-protected surface states in an otherwise insulating bulk. In the topological insulators, the spin-orbit coupling (SOC) plays an important role in locking the spin and momentum of electronic states. Since 5f-electrons maintain an intrinsically strong SOC, Kondo insulators will be an excellent candidate in the search of TIs. In this talk, we present our first-principles study of electronic structure and topological property of PuB₄ in comparison with PuB₆. We show how the SOC dramatically changes the band structure in these Pu compounds. The topological property is then analyzed by calculating the Z₂ topological invariant in the Wilson loop method, and we show the surface electronic states from the calculations of the slab structure. The effects of electronic correlations are also discussed, and the results are compared against experiment.

Uranium dioxide is an important material which is the primary fuel used in commercial nuclear reactors. It has been extensively studied, but newer and improved techniques allow us to further explore and expand our understanding of its behavior at extreme conditions, which can be applied to optimize simulation and manufacturing techniques. It has a magnetic transition 30.8 K, corresponding to unusual physical behavior, indicating strong mangeto-structural coupling. Below T_N, UO₂ orders antiferomagnetically and shows a small volume collapse due to a lattice distortion, as well as a rarely seen piezomagnetic behavior. UO₂ crystalizes in the calcium fluoride structure, but the binary uranium-oxygen system (UO_2+x) stablizes in a wide range of stoichiometries with similar crystal structure, having both deficiencies and interstitial inclusions in the oxygen site. The structural, electronic, and magnetic properties of the material can be highly dependent on x. Previous studies on powder uranium dioxide have been done over the last several decades, but the samples used in them often had mixed stoichiometries due to manufacturing techniques and oxidation. Using high quality and uniform single crystals of UO_2+x with known stoichiometries, we have systematically examined the change in thermal properties. <div id="UMS_TOOLTIP" style="position: absolute; cursor: pointer; z-index: 2147483647; background-color: transparent; top: -100000px; left: -100000px; background-position: initial initial; background-repeat: initial initial;"> </div>

Thorium, a potential nuclear fuel candidate, has been recently suggested for use in some new advanced reactors designs including molten-salt reactors, high-temperature gas reactors and some accelerator-driven reactors. In most reactor applications, the oxide form of thorium is considered as a possible fuel choice, however, metallic thorium alloys, which received some interest in the early days of reactor fuel development, could provide some distinct advantages over the oxide form, such as higher thermal conductivities and higher fuel densities.

Both experimental and theoretical calculations of metallic thorium and its alloys are rare, resulting in a sizeable gap between theory and applicability. In this work, we seek to lessen that gap by probing the nature and formation of point defects in the two allotropes of metallic thorium (an FCC α-phase and a high temperature BCC β-phase). We use the Density Functional Theory (DFT) ) implemented in the Vienna Ab-Initio Simulation Package (VASP) to calculate the electronic groundstate of both phases, first determining lattice constants, elastic properties, and cohesive energies. We use a revised version of the Perdew-Burke-Ernzerhoff (PBE) generalized gradient approximation (GGA) of the exchange-correlation functional known as the RPBE. Comparing it to the PBE we find the RPBE to produce the better lattice constants and slightly better elastic constants, while the PBE produced the most accurate cohesive energies. The elastic constants of the BCC phase show it to be mechanically unstable at 0K while also having a lower cohesive energy than the FCC phase. From the elastic constants, the elastic moduli of both the FCC and BCC allotropes and the Debye temperature of the FCC allotrope are calculated, for which the RPBE and PBE results are nearly equivalent. Using the RPBE, we then introduce vacancy and interstitial point defects into the lattice and calculate defect formation energies based on the relative supercell total energies. For the FCC phase, the vacancy defect had the lowest formation energy at 2.110 eV while the octahedral position had the lowest formation energy of the interstitials at 4.502 eV. The BCC defect formation energies were lower than the FCC energies across the board with the vacancy being lowest overall at 1.232 eV and the <110> DB being the most favorable interstitial at 3.140 eV. We also investigate formation energies associated with uranium atoms in defect positions in the FCC Th lattice finding the substitutional position to be most favorable with a formation energy of 1.478 eV and the U-interstitial energies to be, in general, slightly lower than their Th self-interstitial counterparts.

An atomistic study of threshold displacement energy(TDE) has been conducted on two primary phases of metallic uranium by employing molecular dynamic simulations. The directional dependency of the TDE is investigated through the uniform sampling of the minimal symmetry area with respect to each phase lattice. A generalized cubic-subdivision sampling strategy is introduced for applications in orthorhombic lattices. The temperature dependent defect recover mechanisms are examined by testing the ground-state phase at three temperatures and observing the shifts in TDE. Effective TDE for each phase/temperature is calculated by averaging over all sampling directions and extrapolating the PKA energy that corresponds to displacement probability of 50%.

Understanding the source term produced in accidents or terroristic actions involving nuclear and radioactive materials is essential, whether in terms of remediation, forensic analysis or risk prevention. Due to the availability of industrial radioactive sources and disused thermoelectric generators in countries with little or no safeguard control, the risk of dirty bombs explosion cannot be excluded. In order to assess the aerosol formation during such event, full scale experiments, in particular for radioactive dispersion devices (dirty bombs), have already been performed. However given the difficulty of their implementation, systematic studies for varying elemental, chemical, and environmental conditions at true or even reduced scale, parameters are almost impossible. JRC Karlsruhe has developed a laboratory scale instrumentation with the aim to perform systematic experimental studies on the formation of aerosols in conditions that have close thermal characteristics to the ones obtain in full scale experiments. To this end a facility has been designed that includes an atmosphere controlled containment vessel, a high power laser serving as heat and energy source, an impactor or filters to collect aerosols. Aerosol analysis is carried out with the help of Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), Raman spectroscopy and/or Inductively couple plasma mass spectrometry (ICP-MS). With these techniques it is possible study the partitioning of the relevant elements over the aerosol size fractions, and assess whether concentration in the small size, inhalable fractions takes place.

Numerous experiments have been performed on relevant materials such as fresh nuclear fuel, simulated irradiated nuclear fuel, as well as on materials used in radioisotope thermal generators and industrial radioactive sources. Furthermore, the effect of atmospheres, and the presence of containment material was investigated.

In this lecture some of the main results will be presented:
-The enrichment of radiotoxic elements in the smallest aerosol size fractions, presenting the highest probability of inhalation in experiments with simulated irradiated nuclear fuel as source material.
-Formation of chemical compounds chemical characteristics different from the source material as a result of gas-gas or gas-aerosol reactions. This is important for source term predictions as well as remediation following an event.
-The effect of the metallic source containment material on the aerosols characteristics.
-The partitioning of mother and daughter nuclides with size fraction and the consequences for age determination of the source material for nuclear forensic analysis.

These experimental results are an important source of information for the dispersion modelling of accidents or terroristic actions involving nuclear material.

When exposed to humidity in an oxidizing atmosphere hydrated uranium oxide grows as a secondary mineral on aged UO₂ and U₃O₈, and incorporates the oxygen isotope signature of the water vapor into the secondary uranium oxide, as well as hydrogen and oxygen into any hydration water. Because geospatial variation in δ²H and δ¹⁸O values of atmospheric humidity and precipitation is well understood, the H and O stable isotope composition of mineral hydration waters can give information on the environment of mineral formation. We present stable H and O isotope results of mineral hydration waters in uranium oxide materials analyzed by a new methodology wherein precise heating by thermogravimetric analysis is used to liberate water vapor for subsequent isotope analysis via a laser-based isotope ratio infrared spectroscopy instrument (Picarro L-2130i), which we abbreviate as TGA-IRIS. XRD analysis of U₃O₈ before and after heating to 350°C indicates that the U₃O₈ had a measurable metaschoepite phase ((UO₃) nH₂O, n < 2; where n is the number of hydration water molecules) along with alpha-U₃O₈. After heating, the metaschoepite is eliminated but the alpha-U₃O₈ is retained, meaning that the heating successfully extracted the water in the metaschoepite but did not disturb the U₃O₈. Precision for stable isotope values yielded by the TGA-IRIS method on U₃O₈ is ± 1.3 ‰ for δ²H and ± 0.60 ‰ for δ¹⁸O, which may be higher than that made on liquid water samples (± 1.2 ‰ for δ²H and ± 0.17 ‰ for δ¹⁸O), but is adequate given the large range of δ²H and δ¹⁸O values in natural waters. TGA-IRIS analysis of UO₂ is more difficult because heating caused oxidation with the evolved water, creating oxygen isotope exchange with the water, and subsequently a perturbation in the stable isotope abundance. This does not appear to be as much of an issue for hydrogen in the evolved water. Further work is needed to optimize TGA-IRIS for analysis of UO₂. The ability of TGA-IRIS to generate detailed water yield data and δ²H and δ¹⁸O values of water at varying temperatures allows for the differentiation of water in varying states of binding on mineral surfaces and within the mineral matrix. Thus, TGA-IRIS presents the possibility to isotopically differentiate the various oxygen reservoirs in hydrated minerals (including uranium oxides), which may allow TGA-IRIS to open new avenues and possibilities for research on hydrated mineral phases.

Nuclear forensics requires accurate identification of distinguishing characteristics and provenance of interdicted nuclear materials. It has been known that various production processes and interactions with changing environments can affect the morphology and associated chemistry of nuclear materials. In this presentation, we describe our multiplatform approach using high-resolution microscopic techniques to identify spatially resolved morphological and chemical features within the bulk plutonium (Pu) and uranium (U) materials. The approach involves electron microanalysis with scanning electron microscopy (SEM), focused ion-beam (FIB) tomography with SEM, three-dimensional (3D) spatial modeling, chemical analysis with Auger and X-ray photoemission spectroscopies. Our work shows that the three-dimensional (3D) characterization and spatial modeling of the interior of the bulk nuclear material provide higher-fidelity morphological features than conventional two-dimensional (2D) morphological characterization methods. Both Pu and U metals have microscopic inclusions distributed extensively with complex morphology. Inclusions from impurities such as carbon and iron reveal the production history tied to the chemical and metallurgical processes. Both Pu and U metals have morphological and chemical features entrapped within their bulk oxide scales that form from exposure to changes in atmospheres. These altered oxide scales provide distinguishing characteristics for recent provenance.

Prepared by LLNL under Contract DE-AC52-07NA27344. This work was supported by the Office of Defense Nuclear Nonproliferation Research and Development within the U.S. Department of Energy’s National Nuclear Security Administration. This work has been supported by the U.S. Department of Homeland Security (DHS), Domestic Nuclear Detection Office (DNDO), under a competitively awarded IAA No. HSHQDN-16-X-00044. This support does not constitute an express or implied endorsement on the part of the Government. LLNL-ABS-740142

Nuclear forensics involves determination of the origin and history of interdicted nuclear materials based on the detection of signatures associated with their production and trafficking. The surface oxidation undergone by UO₂ when exposed to air is a potential signature of its atmospheric exposure during handling and transport. To assess the sensitivity of this oxidation to atmospheric parameters, surface sensitive grazing-incidence x-ray diffraction (GIXRD) measurements were performed on UO₂ samples exposed to air of varying relative humidity (34%, 56%, and 95% RH) and temperature (room temperature, 50 °C, and 100 °C). Near-surface unit cell contraction was observed following exposure, indicating oxidation of the surface and accompanying reduction of the uranium cation ionic radii. The extent of unit cell contraction provides a measure of the extent of oxidation, allowing for comparison of the effects of various exposure conditions. No clear influence of relative humidity on the extent of oxidation was observed, with samples exhibiting similar degrees of unit cell contraction at all relative humidities investigated. In contrast, the thickness of the oxidized layers increased substantially with increasing temperature, such that differences on the order of 10 °C yielded readily observable crystallographic signatures of the exposure conditions.

In case of a nuclear reactor accident, one of the biggest problems in term of safety is the release of radioactive elements in the environment. It is thus important to be able to evaluate the nature, quantity and release kinetics of these elements from the nuclear fuel UO₂. Our work is focused on two fission products, caesium and molybdenum, which are abundant and whose behaviour is suspected to be interconnected to each other and to the fuel chemistry. Mo is chemically reactive and is present in the UO₂ fuel under several chemical forms, metallic, oxides or as compounds with other FP like Cs. Moreover, it has a direct influence on the UO₂ oxidation and it is often described as a buffer to the fuel oxidation. Cs is a volatile species whose release rate may be influence by the presence of Mo in the fuel.
The aim of our work is to study the effect of high temperature and irradiation on the release of Cs and Mo from uranium dioxide. In particular, we investigate the effect of the fuel stoichiometry on the migration of Mo and Cs and also, on the behaviour of the matrix under these extreme conditions. To do so, UO_2+x pellets of different stoichiometry are prepared by wet oxidation of UO₂. Then, Cs and Mo are introduced, either separately or together, by ion implantation in UO₂ or UO_2+x samples at a mean concentration ranging from 0.1 to 1 at%. High temperature treatments at 1600°C or irradiations are then led. Different irradiation regimes are studied, involving the production of different kind of defects in the material: ballistic defects, electronic defects or a combination of ballistic and electronic defects. The latter case is representative of what occurs in the fuel during the FP production. After treatments, Mo and Cs concentration profiles in samples are obtained by SIMS (Secondary Ions Mass Spectrometry) in order to measure the element release and if possible, their diffusion coefficients. X-ray absorption spectroscopy is used to characterize the Mo chemical form in the matrix. In parallel, an investigation of the microstructure evolution of UO₂ and UO_2+x is followed by combination of µ-Raman spectroscopy and X-ray diffraction.
On the FP behaviour, we confirm the strong correlation between Mo and Cs migration when these two elements are present at the same time. We also show that the combined effect of irradiation and temperature leads to a species migration, especially in high electronic stopping regime. This can be relied to the defects created in the matrix by the irradiation and can be explained by the thermal spike model. The effect of the UO₂ hyper-stoichiometry will be discussed, in term of FP release and matrix micro-structure resistance to irradiation.

Uranyl fluoride (UO₂F₂), the hydrolysis product of uranium hexafluoride (UF₆), is a byproduct of the nuclear fuel cycle. Understanding the chemical behavior of uranyl fluoride in various environmental conditions is important for nuclear forensics applications. Uranyl fluoride exists in two known crystal structures. The hexagonally-coordinated anhydrous crystal can be converted to a pentagonally-coordinated hydrate of the form ([(UO₂F₂)(H₂O)]₇●(H₂O)₄) in the presence of gas-phase water. We have probed this phase transition using in-situ Raman spectroscopy with temperature and relative humidity control.

At elevated water vapor pressure, the uranyl fluoride hydrate undergoes further transformations to uranyl hydroxide and peroxide species. These transitions were studied using micro-Raman spectroscopy and scanning electron microscopy with energy-dispersive X-ray spectroscopy to analyze particles equilibrated at different water vapor pressures. The formation of uranyl peroxide species is especially noteworthy because of the absence of hydrogen peroxide in the system, which has generally been assumed to be necessary for the formation of these species.

The incorporation of transition metal nanoparticles into porous organic substrates has demonstrated to be advantageous over free nanoparticle counterparts with respect to applications in heterogeneous catalysis and hydrogen storage. It may therefore be hypothesized that a similar methodology could further exploit the useful properties of actinide nanoparticles, including their characteristic radioactivity and application in the nuclear fuel cycle. The effects of incorporation of 5f nanoparticles into substrates as a new set of functional metamaterials, however, has yet to be fully explored.

Recently, the synthesis of 5f nanoparticle-substrate systems has been empirically developed through use of vapor impregnation of volatile precursors into porous organic substrates serving as host templates. The structural attributes of these constructs, however, still remains elusive, in part due to the complexity of sub 5-nm nanoparticles and the nature of the inorganic-organic interface. In order to gain insight into the structure of these actinide nanoparticle constructs, as well as the mechanism through which they are able to incorporate and bind to their porous hosts, X-ray absorption fine structure (XAFS) measurements at the Th and U L3 absorption edges were collected at beamline 11-2 of the Stanford Synchrotron Radiation Lightsource. XAFS proves advantageous in resolving nanoparticle structure, since the finite size of nanoparticles causes broadening of features using conventional diffraction methods. In addition, XAFS provides useful information concerning amorphous-like nanoparticle surface layers and distinguishes them from characteristic bulk material structure. Our key findings suggest that the nanoparticle surface structure is altered upon incorporation into the substrates. Namely, while the first coordination shell exhibits bulk-like characteristics, deviation in both the first coordination shell and in particular the higher order coordination shells is observed in comparison to bulk analogues. This degree of structural specificity informs the mechanism behind the binding motif between the particles and host substrate and also provides important implications on the resulting nanoparticle properties, which are intimately connected to their surface structure.

Silicate species are known to be highly abundant in the environment and even more so in the nuclear waste repository as components of the structural materials and the storage matrix. Therefore the silicate ions need to be considered as potential reactants which may interact with radionuclides and, among others, with actinides. Thorite, ThSiO₄, and coffinite, USiO₄, are two naturally occurring phases which have been extensively studied because of their abundance in the environment [1,2]. The supposed natural formation mechanism of coffinite, which implies the alteration of oxide phases in reductive and silica rich media, rose important questions about the radionuclides’ behaviour in spent nuclear fuel under geologic repository conditions [3]. Moreover, the formation of oxyhydroxy actinide silicate colloids has been observed for thorium, uranium and neptunium in weakly basic carbonate media at room temperature and dramatically increased the mobility of actinides in the environment [4]. In the case of plutonium (IV), silicate compounds’ formation has been suspected for Pu-containing precipitates observed in basic media [5] and for plutonium borosilicate glasses altered by vapour hydration [6]. Therefore, the synthesis and the determination of the thermodynamic properties of PuSiO₄ could be a crucial issue in the assessment of the behaviour of plutonium in the environment.

However, even if PuSiO₄ has already been hydrothermally synthesized once [7], the favourable conditions for the formation of this phase and its stability domain are not well constrained. To provide more data on the Pu system and to determine suitable synthesis conditions three surrogates, CeSiO₄, ThSiO₄ and USiO₄, which crystallize in the same zircon type-structure (space group I4₁/amd) [7,8] as PuSiO₄, were investigated.

Optimized conditions of synthesis were determined for these three silicate-based systems as a function of several experimental parameters (pH, ligands -including carbonates-, concentration, temperature, reactants, redox state, atmosphere…) and then transposed to the synthesis of the plutonium-silicate system. This comparative study has highlighted the differences between plutonium and its most common surrogates in interaction with silicate ions and allowed to assess in which conditions PuSiO₄ may be formed in the environment.

[1] A. Mesbah et al., Inorganic Chemistry, 54, 6687-6696, 2015
[2] S. Szenknect et al., Inorganic Chemistry, 52, 6957-6968, 2013
[3] J. Janeczek, R. Ewing, Materials Research Society, Symposium Proceedings, 257, 497-504, 1992
[4] H. Zänker et al., Chemistry Open Reviews, 5, 174-182, 2016
[5] N. Krot et al., PNNL-11901, UC-2030, 1998
[6] J. Fortner et al., Materials Research Society, Symposium Proceedings, 608, 739-744, 1999
[7] C. Keller, Nukleonik, 5, 41-48, 1963
[8] J. Skakle et al., Powder Diffraction, 15, 234-238, 2000

We performed the material characterizations of electron microscopies (SEM and TEM), X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy to reveal structural evolution of the crystalline α-U₃O₈ powders during a long-term (>30 years) storage in the ambient humidity and temperature. A rim of amorphous uranyl-hydrate phase was observed forming around the α-U₃O₈ crystalline core. The nano-scale fcc UO₂ phase was also observed, which occurred either in the amorphous uranyl hydrate layer or at the boundary between the crystalline α-U₃O₈ core and surface amorphous phase. A preferential crystallographic orientation for epitaxial UO₂ growth on the U₃O₈ was verified. Different from the UO₂ oxidation process that involves the transition phase of U₄O₉ or U₃O₇ before transforming into U₃O₈, the occurrence of the middle phases during the reducing progress has not been identified. The stability and oxidation mechanism of U₃O₈ exposed to humidity will be discussed.

The work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344 and funded by the Office of Defense Nuclear Nonproliferation Research and Development within the U.S. Department of Energy’s National Nuclear Security Administration under Project Number LL15-U_Surface_Oxidation-NDD3B.

Actinide oxalates are a family of materials important for nuclear industry. In the field of nuclear waste management, actinides as well as oxalate are present in nuclear waste in geological repositories. Therefore, the interactions between actinides and oxalate are important for performance assessment (PA) for geological repositories for nuclear wastes. As an example, in the Waste Isolation Pilot Plant (WIPP), a U.S. DOE geological repository for defense-related transuranic (TRU) waste in the bedded salt formations in New Mexico, USA, the inventory of oxalate in waste was 1.99 × 10⁴ kg for the WIPP Compliance Application Re-Certification in 2014 (CRA-2014). The oxalate concentrations were 1.18 × 10^–2 M. As actinide oxalates have low solubilities, actinide oxalates could form in geological repositories to become solubility-controlling phases for actinides.
In the nuclear power fuel cycle, long-lived and alpha-emitting actinides with high radiotoxicity are usually separated from other radioactive elements, as oxalates for immobilization for disposal or for transmutation, or purification for reuse. For instance, ²⁴¹Am can be separated, and be used as radioisotope thermoelectric generators (RTGs) for space programs. RTGs are a key technology for data acquisition and communications in space missions. Pu in used nuclear fuel can also be co-separated with U as oxalates, which then can serve as a feeder for manufacturing mixed oxide (MOX) fuel. In addition, the uranyl oxalate system has been used as a chemical actinometer.
As actinide oxalates are important to numerous fields, the knowledge of their solubility constants is a key to the aqueous processes in which they are precipitated. In this work, we present our evaluation of the solubility constants of actinide oxalates including americium, curium, plutonium and uranium oxalates. In our evaluation, we use the Pitzer model to describe activity coefficients of aqueous species. The computer code, EQ3/6 Version 8.0a, is used as a modeling platform. The solubility data that are selected for evaluation are in oxalic and nitric acids, and in the mixtures of these two acids. The media are characterized with high ionic strengths, up to ~ 8 m.

The solubility constants of actinide oxalates evaluated by this study are expected to find applications in many fields, including the description of the stability of actinide oxalates in the near field of geological repositories.

^A This research is funded by the WIPP programs administered by the Office of Environmental Management (EM) of the U.S. Department of Energy.
^B Sandia National Laboratories is a multimission laboratory operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA-0003525. SAND2017-11366 A

During the packaging of PuO₂ for storage the conditions are closely controlled to limit water uptake by the oxide as radiolytic decomposition of any adsorbed water will lead to the formation of both a potentially flammable atmosphere containing molecular hydrogen as well as reactive oxygen species which may be incorporated into the oxide phase. To underpin the safety case for the long term storage of PuO₂, a better understanding of the fundamental physical, chemical and materials degradation processes occurring at the water-oxide interface inside the storage canisters is desirable.
This study investigates the effect of the radiolysis of adsorbed water on PuO₂ and various surrogate oxides. Samples of baked oxide powder were equilibrated in humidity chambers to attain a wide range of masses of water adsorbed onto the powder surface. In the case of PuO₂, once the surface water on the oxide had equilibrated with the humid atmosphere, the samples were sealed into a glass vessel held at constant humidity with either an argon or a nitrogen over-gas and left for up to 3 months. Periodic sampling of the headspace was undertaken and the atmosphere above the oxide analysed using gas chromatography. In the case of the surrogate oxides, once the oxides had adsorbed the maximum quantity of water, the samples were flame sealed under argon into a glass tube and Co-60 gamma irradiated. Following gamma irradiation, the headspace was analysed using gas chromatography. Prior to and after gamma irradiation, the oxide powders were characterised using a variety of different microscopy and spectroscopy methods.
In nearly all of the experiments a linear production of molecular hydrogen as a function of radiation dos was observed. For PuO₂, the rate at which H₂ was produced increased with increasing water loading. This result contrasts with the experimental data in this study and from similar experiments in the literature utilising UO₂, CeO₂ and ZrO₂ which showed a higher rate of production for lower water coverages. There was no evidence of a steady state hydrogen concentration being reached in the PuO₂ experiments, an outcome opposite of that observed in storage canisters and in experiments performed by other groups in which the PuO₂ was not held under constant humidity. Radiolytic production of molecular oxygen was not observed. Comparison of the properties of the surrogate oxide (ZrO₂) powder prior to and post irradiation did not reveal any differences in stoichiometry, surface functionalization or the formation of oxygen-centered free radicals, so the location of the sibling oxidant to the evolved molecular hydrogen currently remains unanswered.

Acknowledgement: This work was supported by the Dalton Cumbrian Facility Project, a joint initiative of the University of Manchester and the Nuclear Decommissioning Authority, by the UK Engineering and Physical Sciences Research Council and by the US Department of Energy, Office of Nuclear Energy.

Soft X-ray synchrotron radiation methodologies are being employed at the Advanced Light Source (ALS) of Lawrence Berkeley National Laboratory (LBNL) to characterize radioactive and actinide materials using spectromicroscopy approaches developed for the nanoscience investigations. Results from these studies have begun to provide improved fundamental knowledge that can be used as a scientific basis for the enhanced design of actinide materials, complexes, ligands, and the overall understanding of actinide materials. The experimental developments at the ALS have centered on studies of radioactive materials with the Molecular Environmental Science (MES) soft X-ray scanning transmission X-ray microscope (STXM) at Beamline 11.0.2 for spatially-resolved near-edge X-ray absorption spectroscopy (XAS). The spectromicroscopy capabilities of the STXM provide the means to determine the speciation and composition in a range of actinide materials, particularly those of technological and environmental interest with spatial resolution that can reach to the true nanoscale. A particular emphasis has been on the use of light atom (B, C, N, O, F, Na, Mg, Al, Si) ligand K–edge XAS technique to determine the electronic structure characteristics in an array of unique and relevant materials. Furthermore, there are a host of additional electron energy level thresholds (such as the L–edges of the transition metals, the M–edges of the lanthanides, and others) that can be probed by near-edge XAS in the soft X-rays. A key component of the aforementioned STXM spectromicroscopy investigations has been the partnership with theory that has provided the fundamental basis from which to interpret and more fully understand the information contained within the near-edge XAS spectra. Contemporary STXM spectromicroscopy studies have focused on fundamental studies of the electronic structure in actinide complexes, speciation of radioactive elements in environment, forensic science, and most recently to fast ion beam (FIB) prepared radioactive materials specimens in collaboration with Idaho National Laboratory. Future scientific developments and applications of soft X-ray STXM spectromicroscopy investigations utilizing ptychography methodologies will be discussed.

Properties of Uranium hydride, the oldest known 5f ferromagnet, haven't been yet really well established, despite decades of research. The reason is primarily the form of the material, which is normally obtained as a fine pyrophoric powder. Hence it is rather difficult to obtain a reliable information on basic features of electronic structure by means of electron spectroscopies. Another principially possible route is in-situ synthesis of the hydride films by means of reactive sputtering in H₂ containing atmosphere, although interaction of hydrogen with UHV systems may lead to a disappointing oxidation. This route proved feasible, yielding basic valence-band and 4f core-level spectra for UH₃ [1]. Here we present results of an extended spectroscopic study of U, U-Mo, and (U,Mo)H₃ films studied by XPS (using monochromatized Al-Kα radiation), UPS (using He I and He II UV) and by Bremsstrahlung Isochromat Spectroscopy (BIS), the last providing information on empty electronic states above the Fermi level. The study si complemented by ex-situ XRD study, proving that the sputter deposition leads to β-UH₃ type of hydride, which is, however, highly textured and exhibits large compressive residual strain. The results are confronted with fully relativistic ab-initio calculations, providing an overall agreement of energies of large scale spectral features. The situation around the Fermi level is, however, showing some discrepancy between the theory and experiment, which may indicate many body effects. Those would be rather unexpected in a band 5f-system, assumed at least as a good starting point for description of uranium compound with short U-U spacing of 330 pm. The data will be discussed from the point of view of possible charge transfer between the U-6d and 7s states and the H-1s states, its impact on the U-5f states, which can be one of reasons of the relatively very high Curie temperature of 165 K. The study also revealed interesting structure variations of the hydrides depending on the substrate material and temperature.

The work at the Charles University was supported by the Czech Science Foundation under the grant No. 15-01100S.

[1] T. Gouder et al., Phys.Rev. B 70, 235108 (2004).

Recently, at SLAC National Accelerator Laboratory we have expanded our high-resolution x-ray spectroscopy capabilities in the tender x-ray regime (1.6-5.0 keV). We have built an emission spectrometer developed on a Rowland geometry (500mm of radius) via cylindrically bent Johansson analyzers that operates in a wide range of diffraction angles (~30^o-65^o) while having a subnatural line-width energy resolution (~0.35eV @ 2400eV) at a dispersive geometry. Although the spectrometer is enclosed in a vacuum chamber, an independent sample chamber, separated with a thin window from the main chamber, has been incorporated to enable a flexible sample environment (e.g. solid/gas/liquid samples, in-situ cells, radioactive materials, etc.). This has provided as the opportunity to built a multidisciplinary science program in the tender x-ray regime related to catalysis, energy materials and actinides. Here we will summarize the developed capabilities and present the study of a series of Uranium intermetallics using resonant and non-resonant X-ray emission. Future plans and directions will be discussed.

I discuss a theoretical description of the resonant x-ray emission spectroscopy (RXES) that is based on the Anderson impurity model. The parameters of the model are determined from material-specific LDA+DMFT calculations. Recently, this approach was shown [1] to accurately reproduce the L-edge RXES measured in lanthanides [2]. Here, the method is extended to the M-edge spectra in actinide compounds. The same theoretical approach applies also to the x-ray absorption spectra measured in the high-energy-resolution fluorescence-detection mode (HERFD-XAS) [3]. As an example, I investigate the origin of large variations observed among the M-edge HERFD-XAS spectra of uranium compounds where uranium atoms are in the same oxidation state. For instance, both UO₃ and torbentite have U atoms in the U(VI) state, but the shape of their x-ray absorption spectra differs quite substantially [4,5].
[1] J. Kolorenč, Physica B (2017), DOI: 10.1016/j.physb.2017.08.069
[2] J. A. Bradley et al., Phys. Rev. B 85, 100102 (2012).
[3] K. Hämäläinen, D. P. Siddons, J. B. Hastings, and L. E. Berman, Phys. Rev. Lett. 67, 2850 (1991).
[4] Y. Podkovyrina et al., J. Phys.: Conf. Ser. 712, 012092 (2016).
[5] K. O. Kvashnina, Y. O. Kvashnin, S. M. Butorin, J. Electron. Spectrosc. Relat. Phenom. 194, 27 (2014).

An extensive investigation of oxidation in uranium has been pursued. [1] This includes the utilization of soft x-ray absorption spectroscopy, hard x-ray absorption near-edge structure, resonant (hard) x-ray emission spectroscopy, cluster calculations, and a branching ratio analysis founded on atomic theory. The samples utilized were uranium dioxide (UO₂), uranium trioxide (UO₃), and uranium tetrafluoride (UF₄). A discussion of the role of nonspherical perturbations, i.e., crystal or ligand field effects, will be presented. The conclusions are as follows. (1) The hypothesis of the potential importance of CF effects in the XAS branching ratio (BR) analysis of 5f states was incorrect. (2) Both UO₂ and UF₄ are n_5f = 2 materials. The combination of the 4d XAS BR and RXES analyses is particularly powerful. (3) CF broadening in the L₃ RXES spectroscopy does not preclude a successful analysis. (4) The prior experimental result that n_5f (UO₂) = 3 and the proposed causation by covalent bonding was incorrect. UO₂ is an n5f = 2 material and analysis within a simple, ionically localized picture provides the correct result. (5) UO₃ appears to be an n_5f = 1 material. (6) While the 4d XAS BR analysis is blind to CF effects, crystal field and covalence remain important. (7) For localized actinide systems, the 4d XAS BR analysis founded upon the utilization of the intermediate coupling scheme remains a powerful tool. (8) For delocalized actinide systems, the BR analysis is problematic.

High quality epitaxial single-phase UO_x thin films could enable new research possibilities. The recently developed actinide synthesis capability at Center for Integrated Nanotechnologies at Los Alamos National Laboratory allows the growth of different actinide thin films with excellent controllability. In this talk, we will focus on the growth of epitaxial and textured UOx thin films by pulsed laser deposition. X-ray diffraction and transmission electron microscopy results confirm that different single-phase materials including UO₂, U₃O₈, UO₃ has been stabilized on different substrates. Optical properties such as optical bandgap and Raman spectra have been also investigated. The magnetic and electronic properties have also been systematically studied in different temperatures. This research is a comprehensive study on intrinsic physical properties of single-phase UO₂, U₃O₈, and UO₃ thin films, which provides the foundation for advanced characterization.

Over the last decades, computational chemistry has seen continuous and rapid development that is driven both by the sustained development of computer technology (exemplified by Moore’s Law) and by significant advances in theory and methodology. Computational chemistry has reached a point were it can be used as “just another spectrometer” in a “black-box” fashion for certain areas and applications, while it continues its fast development in other areas.

Theoretical and computational actinide chemistry, application of the tools of computational chemistry to the early actinides (typically Th and U to Pu, but increasingly also beyond), is a research topic that is still a frontier area, despite having seen impressive advances over the last several years. This is due to challenges arising from the size of typical systems, the need to include relativistic effects, and technical difficulties such as the large number of closely spaced electronic states, amongst others. This, combined with the experimental challenges of actinide (and particularly trans-uranium) chemistry, makes the actinides a particularly fruitful area for collaborations between theoretical and experimental research.

We will begin this presentation by discussing some aspects of the computational methodology as applied to actinides (and thus, we will briefly “take a look inside the black box”). We will then focus on some applications from our recent work. In this manner, we hope to illustrate the scope of questions that can be addressed, and the kind of unique insight that computational chemistry might provide. Specifically, we will discuss representative results from the following areas:
(i) Macrocycle complexes: Polypyrrolic macrocycles (including a ligand system colloquially known as ‘pacman’ due to its shape) provide access to unique bonding schemes if complexed with one or more actinide atoms;
(ii) Mineral surface interactions: Adsorbtion of uranyl species onto TiO₂ surfaces;
(iii) 2D Materials: Surface interactions of uranyl with silicene.

In each case, we will attempt to draw specific as well as general conclusions regarding the methodology employed and the chemistry involved.

Results to characterize the extent and the energetic importance of the covalent mixing of the frontier metal orbitals with frontier ligand orbitals of d and f element compounds; in particular oxides are presented. The results for representative transition metal, lanthanide, and actinide compounds are obtained using novel theoretical methods that permit reliable assignments of the covalent character and properties.[1] It is demonstrated that, for lanthanides and actinides, the covalent mixing with frontier f and d orbitals must both be taken in to account and that they are often of comparable magnitude. Furthermore the importance of different contributions to the covalency depends strongly on the nominal oxidation state of the metal and on the nominal occupations of the frontier metal orbitals. The consequences of the covalent mixing for core-level spectroscopies is examined and it is shown the energy splittings and intensities are strongly affected by the covalent mixing of the frontier orbitals.

Support for this work by the Geosciences Research Program, Office of Basic Energy Sciences, U.S. DOE, is acknowledged.

1. P. S. Bagus, C. J. Nelin, D. A. Hrovat, and E. S. Ilton, Journal of Chemical Physics 146, 134706 (2017).

Plutonium is one of the most complex elements in the periodic table. Even after several decades of in-depth studies of plutonium compounds and surrogates, there are still questions about how its phase, microstructure and composition influence the physicochemical behavior of this material, which is important for nuclear energy, non-proliferation, homeland security, and nuclear forensics applications. Although several methodologies have been established to accurately obtain physical and chemical information of a bulk plutonium analyte, gaining analogous knowledge from the near surface layers of plutonium is crucial to better understand how these layers evolve when subjected to different conditions. The surface of plutonium has been experimentally studied through diverse techniques (for example, scanning electron microscopy, energy dispersive spectroscopy, X-ray photoelectron spectroscopy and secondary ion mass spectroscopy), which have provided valuable information in terms of morphology and composition. However, surface probe microscopy methodologies such as atomic force microscopy (AFM) provide a direct characterization route that enables the acquisition of topographical information while simultaneously mapping almost any probe-sample interaction imaginable, allowing them to be correlated to each other. Here, we report the exploration of the surface of gallium stabilized δ phase plutonium coupons via AFM. Initial results reveal that the plutonium surface is considerably rough and heterogeneous. This is further evidenced via quantitative nanomechanical mapping (QNM), which was done to explore the mechanical properties of the δ phase plutonium surface. QNM images exhibit distinct patterns that are not observed in the topography images. The ability of AFM to achieve nanometer resolution in combination with QNM allows one to collect mechanical information (i.e., elastic modulus, adhesion, deformation, dissipation, etc.) with the same resolution, which denotes a considerable advantage to identify the presence of contaminants, inclusions, surface defects on the surface of plutonium and their effect in its properties in a direct, non-destructive and straight forward manner.


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Approved for public release LA-UR-17-29888

Electronic structure calculations of actinide based materials have long been challenging due to the complex properties of strongly correlated 5f electrons. The density functional theory (DFT) fails to capture these strong 5f electron correlations which govern, in particular in actinide oxides, various features such as magnetic properties, cation mixed valence and defect charge states. (U,Pu)O₂ mixed actinide oxides are currently used in pressurized water reactors and are reference fuels for GEN IV reactors. Therefore, an accurate description of actinide oxide basic properties is essential for nuclear energy applications.
We use here the DFT+U approach and the occupation matrix control scheme (OMC) to model bulk properties of uranium-plutonium mixed oxides as a function of the Pu content using a static approach. Using ab initio molecular dynamics combined with DFT+U and the OMC scheme, we investigate UO₂, PuO₂ and (U,Pu)O₂ bulk properties as a function of temperature in particular thermal expansion and variations of enthalpy of these compounds. This study aims at feeding thermodynamic database and will be extended to complex materials for which only few thermodynamic data are available, such as (U,Pu,Am)O₂.

Uranium dioxide exists in hyperstoichiometric form, UO_2+x. Structure and dynamics of the excess oxygen defects and their correlation with 5f electrons are fundamental to the understanding of mechanical, thermal, and electrical properties. Those excess oxygen atoms are not random but rather partially ordered. The widely-accepted model, the Willis cluster based on neutron diffraction, cannot be reconciled with the first-principles molecular dynamics simulations present here. We demonstrate that the Willis cluster is a fair representation of the numerical ratio of different interstitial O atoms; however, the model does not represent the actual local configuration. The simulations show that the average structure of UO_2+x involves a combination of defect structures including split di-interstitial, di-interstitial, mono-interstitial, and the Willis cluster, and the latter is a transition state that provides for the rapid diffusion of the defect cluster. The 5f electrons are partially delocalized and the U⁵⁺ atoms are not spatially linked to the defect cluster. The mobility of the U⁵⁺ is thermally activated but decoupled temporally with the lattice defect. The observation can be explained by a collective, dynamical, charge transfer-coupled lattice distortion involving the U(V) ↔ U(IV) excitation, occurring coherently over an entire domain combining charge, spin, and crystal lattice. The results provide new insights in differentiating the average structure from the local configuration of defects in a solid, 5f electron – lattice defect coupling, and the transport properties of UO_2+x.

Aging in Pu is a complex phenomenon, occurring both “outside in” (corrosion in different environments) and “inside out” (self-irradiation). Self-irradiation effects lead to defects such as lattice distortions, void swelling, or formation of He bubbles that affect properties of the material, and over time, the safety and reliability of the material.
The δ-phase ²³⁹PuGa alloys lattice behavior was analyzed by neutron diffraction using the high-pressure preferred orientation neutron time-of-flight powder diffractometer, at the Lujan Neutron Scattering Center, Los Alamos National Laboratory. Data were collected at ambient pressure and over a range of temperatures (12-300K). Lattice evolution is presented as a function of time, temperature, and alloy content. Results will be compared to literature data and similar measurements.

A systematic investigation was performed on morphology change versus oxidation of stoichiometric UO₂ powders following aging for periods up to one year under controlled conditions with relative humidity ranging from 34% to 98%. The stoichiometric UO₂ powders were prepared or converted from the initial uranium oxides that are different in speciation, isotope and/or configuration, as well as origination and history, respectively. The materials were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), high-resolution X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR) spectroscopy. For the cases of low and middle relative humidity, morphology change manifested to be surface texture modification and reconstruction due to oxidation and/or hydroxylation. Oxidized amorphous rim occurred in the sample aged for a long period. For the case of high relative humidity, however, uranyl hydrate phase formed as well. The changes in morphology and structure provide useful evidences for investigation of nuclear forensics.

The work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344 and funded by the Office of Defense Nuclear Nonproliferation Research and Development within the U.S. Department of Energy’s National Nuclear Security Administration under Project Number LL15-U_Surface_Oxidation-NDD3B.

The Isotope Separator On-Line DEvice ISOLDE is a facility dedicated to the production of radioactive ion beams located at CERN, the European Organization for Nuclear Research. With over 50 years of experience, the ISOLDE facility is able to deliver more than 1000 different isotopes of 74 chemical elements for different applications in the field of nuclear and atomic physics, material science and nuclear medicine.
Radioactive nuclides are produced by irradiating thick targets of 20 cm length and 2 cm diameter, with a 1.4 GeV proton beam. In more than 65% of the beam time, the targets are made of refractory materials such as highly porous depleted uranium or thorium carbide with excess graphite (UC_x or ThC_x). In this contribution, the life cycle of actinide target materials at the ISOLDE facility will be discussed, from the raw materials and carbide production, to operation and waste disposal.
Actinide targets are used as oxides or as carbides by mixing an oxide powder with graphite, pressed into thin pellets and heat treated to produce carbides. Oxides are normally in the form of micrometric powders and occasionally of fibers in the ThO₂ case. The targets are operated for 2 weeks while they are kept under vacuum at temperatures higher than 2000 °C to promote isotope release. During irradiation the material undergoes structural changes and trap some of the produced isotopes, while others are released, which makes the material activated and the use of a hot cell required.
To increase isotope release efficiency, a new uranium based material was engineered. By making green compacts of carbon nanotubes with nanometric uranium oxide, and subsequent carbothermal reduction, a novel porous nanometric uranium carbide was produced. This target material had a stable microstructure, however the material was extremely pyrophoric due to its large surface area and required extreme care in all handling procedures. This target was successfully operated at ISOLDE to bring increased isotope beam intensities.
While both UC_x and ThC_x are pyrophoric and cannot be kept in this form for long-term storage, a safe process for the conversion into oxide is investigated. However, UC_x oxidation in air is a highly exothermic reaction that is associated with the risk of thermal runaway, which in turns depends highly on the starting microstructure. Systematic investigations are underway in order to develop a safe and controlled stabilization process. The work developed could be transferred to other facilities worldwide, as a new waste disposal channel.
N.-T. Vuong and T. Stora acknowledge that the UC_x oxidation research project has been supported by a Marie Sklodowska-Curie Innovative Training Network Fellowship of the European Commission's Horizon 2020 Programme under contract number 642889 MEDICIS-PROMED.

Mixed actinide oxides (MOX) have been used as nuclear fuel material, furthermore UO₂ fuel effectively becomes a mixed oxide during reactor operation due to transmutation and decay. As a result, understanding the behaviour of MOX is of considerable importance. In particular, the phase diagram of these MOX systems is an important tool as it defines the operational limits for nuclear fuel. Molecular dynamics (MD) is a useful tool used to predict the melting points of these system. However, these systems can be solid, a mixture of solid and liquid or liquid and predicting the solidus and liquidus lines can be challenging. A new method has been developed to predict points on these solidus and liquidus lines using MD simulations. Calculations are carried out on (U,Pu)O₂, (U,Th)O₂ and (Th,Pu)O₂ MOX systems and compared to experimental data. These simulations can help to provide important insight into how these systems behave at high temperatures.

It is of interest in nuclear forensic science to understand the relationship between a sample’s history and the resulting chemical and physical characteristics. It may be possible to glean information on the processing and storage of uranium dioxide (i.e. UO₂) from variations in the speciation of uranium. The reaction of water and molecular oxygen with high purity, stoichiometric UO₂ powder at elevated temperature was studied by high-resolution x-ray photoelectron spectroscopy (XPS), infrared (IR) spectroscopy, powder x-ray diffraction (XRD), and scanning electron microscopy (SEM). Oxidation resulting from the dissociative chemisorption of the adsorbing molecules and subsequent incorporation into the oxide lattice was observed and quantified. A shift to higher uranium oxidation states was found to be directly correlated to increased temperature for samples exposed to 20% O₂ but not for those exposed to 98% RH. Additionally, molecular oxygen was found to be a stronger oxidation agent than water at elevated temperatures but not at ambient.

The work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344 and funded by the Office of Defense Nuclear Nonproliferation Research and Development within the U.S. Department of Energy’s National Nuclear Security Administration under Project Number LL15-U_Surface_Oxidation-NDD3B.