Symposium S.CT03—Expanding the Frontiers of Actinide Materials Science Through Experiment and Theory

A cardinal obstacle to performing quantum-mechanical simulations of strongly-correlated matter is that, with the theoretical tools presently available, sufficiently-accurate computations are often too expensive to be ever feasible. Here we design a computational framework combining quantum-embedding (QE) methods with machine learning. This allows us to bypass altogether the most computationally-expensive components of QE algorithms, making their overall cost comparable to bare Density Functional Theory (DFT). We perform benchmark calculations of a series of actinide systems, where our method describes accurately the correlation effects, reducing by orders of magnitude the computational cost. We argue that, by producing a larger-scale set of training data, it will be possible to apply our method to systems with arbitrary stoichiometries and crystal structures, paving the way to virtually infinite applications in condensed matter physics, chemistry and materials science.

arXiv:2006.15227 (2020)

Due to its position at the boundary between the light and heavy actinides, Plutonium (Pu)
has liminal characteristics that make modeling its 5f-orbital electrons especially challenging.
Consequently, a complete understanding of the electronic structure of this elemental solid remains
in question. In order to explore the role of strong electronic correlations in the δ-phase
of Pu, we have performed Fermi surface calculations using the generalized gradient approximation
with dynamical mean-field theory and the Gutzwiller wavefunction approximation to
compare their differing approaches to electron correlation effects. The de Haas-van Alphen
(dHvA) frequencies and effective masses were then calculated to analyze correlation induced
renormalization effects on the Fermi surface topologies. We have predicted that the electronic
correlation gives rise to a volume expansion of 200% in both the hole and electron
pockets of the Fermi surface in addition to an intermediate mass enhancement. This is in
agreement with the band renormalization theory for strongly correlated materials.

Acknowledgments: U.S. Department of Energy (DOE) National Nuclear Security Administration, Contract No. 89233218CNA000001.
LANL LDRD Program. NNSA Advanced Simulation and Computing Program. Center for Integrated Nanotechnologies, a DOE BES user facility, in partnership with LANLInstitutional Computing Program for computational resources.

We discuss the impact of 5f electron interactions on structural properties of pure actinides and phase of plutonium[1][2]. We first use an ab initio calculation of effective Coulomb interactions and we find that the calculated interactions in plutonium and americium are much smaller than usually expected. With these interactions, we reproduce the experimental transition from low volume early actinides (uranium, neptunium, α-plutonium) to high- volume late actinides (δ-plutonium, americium, and curium) and a good agreement of volume of phases of plutonium with experiment. We also discuss phonon spectra of plutonium. Then we discuss our values of effective 5f electron interactions U. In some lanthanides and actinides [3], higher values are expected to reproduce experimental photoemission spectra and structural properties. We discuss general ways to improve the calculation to be more predictive.

[1] B. Amadon Phys. Rev. B 94, 115148 (2016), Phys. Rev. B 97, 039903 (2018)
[2] B. Amadon and B. Dorado J. Phys.: Condens. Matter 30 405603 (2018)
[3] J.B. Morée and B. Amadon Phys. Rev. B 98, 205101 (2018)

Studying the local moment and 5f-electron occupations sheds insight into the electronic behavior in actinide materials. X-ray absorption spectroscopy (XAS) has been a powerful tool to reveal the valence electronic structure when assisted with theoretical calculations. In this work, we employed the DFT+Gutzwiller rotationally-invariant slave boson method to obtain the local Hamiltonian of the single-impurity Anderson model (SIAM), and used exact diagonalization (ED) method to calculate the XAS spectra from the model. An in-house built computational code was developed for the ED method. By applying this technique to the recently discovered 5f-electron topological Kondo insulator plutonium tetraboride (PuB₄), we were able to determine the signature of 5f-electronic correlation effects in the theoretical X-ray spectra. We found that the Pu 5f-6d hybridization effect provides an extra channel to mix the j=5/2 and 7/2 orbitals in the 5f valence. As a consequence, the resultant electron occupation number and spin-orbit coupling strength deviate from the intermediate coupling regime. We have also applied this approach to the d-phase of Pu.

The electron correlation effect gives rise to an abundant variety of physical properties particularly in the d- and f-based materials. Among this class of materials, the actinide compounds share a unique position due to the interplay between their magnetic and superconducting properties. Particularly, the coexistence of a large magnetic moment and unconventional superconductivity is the most significant aspect of uranium-based compounds. These distinctive behaviors are due to the strongly correlated U 5f states which are located at the boundary between simple localized and itinerant pictures of electrons. To understand the origin of these remarkable physical properties, it is essential to unveil the U 5f electronic structures.

In this talk, I will review the electronic structure studies of strongly correlated uranium compounds by photoelectron spectroscopies with soft X-ray synchrotron radiation [1,2]. The following subjects will be discussed.

Resonant photoelectron spectroscopy study of heavy fermion superconductors
We have elucidated the nature of the electron correlation effect in heavy fermion superconductors UPd₂Al₃ [3] and UTe₂ [4] by imaging their partial U 5f density of states (pDOS) using U 4d-5f resonant photoelectron spectroscopy. It was found that incoherent peaks emerge on the higher binding energy side in their U 5f pDOS, suggesting that the electron correlation effect takes an essential role in their electronic structures

Three-dimensional angle-resolved photoelectron spectroscopy study of URu₂Si₂
The complete volume electronic structure of the heavy fermion superconductor URu₂Si₂ in the paramagnetic phase was revealed by the three-dimensional (3D) angle resolved photoelectron spectroscopy (ARPES). It was found that the electronic structure of URu₂Si₂ has a strong dependence along the perpendicular k_z direction, suggesting that it has a considerable 3D nature. Furthermore, the U 5f states in URu₂Si₂ form dispersive bands with their energy dispersions of sub-electron volts, and contribute to the Fermi energy. This suggests that U 5f electrons in URu₂Si₂ have an itinerant aspect.

Work performed in collaboration with I. Kawasaki, Y. Takeda, T. Okane, Y. Saitoh, A. Fujimori, H. Yamagami, Y. Haga, E. Yamamoto, Y. Onuki, A. Nakamura, Y. Homma, and D. Aoki.

References
[1] S. Fujimori, et al., J. Phys. Soc. Jpn. 85, 062001 (2016).
[2] S. Fujimori, J. Phys.: Condens. Matter 28, 153002 (2016).
[3] S. Fujimori, et al., Phys. Rev. B 99, 035109 (2019).
[4] S. Fujimori, et al., J. Phys. Soc. Jpn. 88, 103701 (2019).

The debate whether uranium 5f electrons are closer to being localized or itinerant in the intermetallic compound UGa₂ is not yet fully settled, partly due to inconclusive spectroscopic observations of the uranium 5f electrons [1,2]. The present state of the theoretical understanding of the UGa₂ electronic structure is not satisfactory either. The magnetic moments carried by the uranium atoms in the low-temperature ferromagnetic phase are severely underestimated by the band theory (LDA), and although the correlated band theory (LDA+U) makes the moments larger, the increase is still not large enough to match the experiment [3,4].

We attempt to gain more insight into the electronic structure of UGa₂ by means of LDA+DMFT calculations using exact diagonalization as the impurity solver [5]. The LDA+DMFT definitely improves the description of magnetism with respect to LDA, but obtaining magnetic moments as large as in experiments turns out to be difficult even for this advanced electronic structure method – even though the uranium 5f electrons come out as rather localized. Apart from the magnetism, we calculate and analyze also the spectral properties to be compared with experimental valence-band photoemission and M-edge x-ray absorption.

[1] S.-i. Fujimori, M. Kobata, Y. Takeda, T. Okane, Y. Saitoh, A. Fujimori et al., Phys. Rev. B 99, 035109 (2019).
[2] T. Gouder, L. Havela, M. Diviš, J. Rebizant, P. M. Oppeneer, M. Richter, J. Alloys Compd. 314, 7 (2001).
[3] M. Diviš, M. Richter, H. Eschrig, L. Steinbeck, Phys. Rev. B 53, 9658 (1996).
[4] A. V. Andreev, K. P. Belov, A. V. Deryagin, Z. A. Kazei, R. Z. Levitin et al., Sov. Phys. JETP 48, 1187 (1978).
[5] J. Kolorenč, A. B. Shick, A. I. Lichtenstein, Phys. Rev. B 92, 085125 (2015).

The actinyl cation (D_∞h point group) possesses symmetric and asymmetric stretching modes that are typically Raman and Infrared active, respectively. Lowering the symmetry of the actinyl moiety will activate both modes causing two stretching bands to be present within the spectral window. While activation of these bands suggests perturbation to the actinyl bond, there is currently a limited understanding on how intermolecular forces influence this process or the extent of the effect on the actinyl bond in solid state compounds. In this presentation, I will focus on the analysis of Raman spectra associated with neptunyl (NpO₂⁺/NpO₂²⁺) compounds and explore activation and vibrational coupling effects of these materials. Specifically, the impacts of counterions and hydrogen bonding will be explored experimentally and compared to theoretical calculations. In addition, the neptunyl system will be compared to the related uranyl complexes to further explore the subtle differences in bonding within actinyl cations.

Inspired by our recent discoveries of the remarkable and unexpected structural behaviors in SrUO₄ [1] and CaUO₄ [2], which both show reversible oxygen-vacancy ordering and symmetry lowering with increasing temperature, we have revisited the closely related CdUO₄ using synchrotron X-ray and neutron powder diffraction supported by X-ray absorption spectroscopic measurements and thermogravimetric analysis.
Depending on synthesis conditions, CdUO₄ was previously reported to form two structural types based on laboratory X-ray powder diffraction [3]. One polymorph (α-CdUO₄) was described as isostructural to α-SrUO₄ and CaUO₄, having a rhombohedral structure in space group R-3m. The other polymorph (β-CdUO₄) was described as orthorhombic in space group Cmmm. A high temperature third polymorph, denoted as γ-CdUO₄, was also suggested based on a thermogravimetric study [4]. The structure of γ-CdUO₄ was not solved but described as a form of the rhombohedral α-CdUO₄ that is sub-stoichiometric.
We have synthesized both α- and β-CdUO₄ and determined their crystal structures using high resolution synchrotron X-ray and neutron powder diffraction for the first time. While the structure of α-CdUO₄ was confirmed as previously reported rhombohedral structure in R-3m, we propose that the correct space group for the orthorhombic β-CdUO₄ is Ibmm, isostructural to β-NiUO₄ [5].
In situ synchrotron X-ray diffraction measurements were also carried out with both α- and β-CdUO₄ from room temperature to 1000°C in sealed quartz capillaries. β-CdUO₄ was found to undergo an irreversible reconstructive structural phase transformation around 900°C. Although the X-ray diffraction pattern of the high temperature phase has a passing resemblance to that of α-CdUO₄, there are clear peak splittings that cannot be accounted for by the rhombohedral structural model. Note that these splittings are too small to be resolved by laboratory X-ray diffractometers, therefore they were not observed in previous studies. Using a similar approach to that in our earlier study of SrUO₄ [1], the high temperature structure of CdUO₄, denoted as δ-CdUO_4-x, was solved for the first time as triclinic in space group P-1. When α-CdUO₄ was heated to around 750°C it also underwent a first order phase transformation to a mixture of the β and δ phases.
Similar to SrUO₄, oxygen vacancies are believed to play a key role in the phase transformation behavior of CdUO₄. The results of this study will be discussed with the other related monouranates.

References
[1] G. L. Murphy, C.-H. Wang, G. Beridze, Z. Zhang, J. A. Kimpton, M. Avdeev, P. M. Kowalski and B. J. Kennedy, “Unexpected Crystallographic Phase Transformation in Nonstoichiometric SrUO_4-x: Reversible Oxygen Defect Ordering and Symmetry Lowering with Increasing Temperature”, Inorg. Chem. 57 (2018) 5948-5958.
[2] G. L. Murphy, C.-H. Wang, Z. Zhang, P. M. Kowalski, G. Beridze, M. Avdeev, O. Muransky, H. E.A. Brand, Q.-F. Gu, and B. J. Kennedy, “Controlling Oxygen Defect Formation and its Effect on Reversible Symmetry Lowering and Disorder-to-Order Phase Transformations in Non-Stoichiometric Ternary Uranium Oxides”, Inorg. Chem.58 (2019) 6143-6154.
[3] T. Yamashita, T. Fujino, N. Masaki, and H. Tagawa, "The crystal-structures of alpha-CdUO₄ and beta-CdUO₄," J. Solid State Chem., 37(1981) 133-139.
[4] H. Tagawa and T. Fujino, "Anomalous change of the oxidation-state of uranium in the phase-transformation of cadmium monouranate," Inorganic & Nuclear Chemistry Letters, 16 (1980) 91-96.
[5] G. L. Murphy, P. Kegler, Y. Zhang, Z. Zhang, E. V. Alekseev, M. D. de Jonge, and B. J. Kennedy, "High-Pressure Synthesis, Structural, and Spectroscopic Studies of the Ni–U–O System," Inorg. Chem., 57 (2018) 13847-58.

The focus of this talk highlight on-going research efforts on laser spectroscopy. At present, there are a number of challenges that are involved with meeting the 30 ppy (pits/year) and some of them are scientific and engineering. In this presentation, an overview of hand-held laser induced breakdown spectroscopy (HH-LIBS) an initial measurements on alpha and delta Pu metal standards. There will also be collaborative research that involves the use of chemometric techniques that have been applied to Ce samples for the purpose of method development. Highlights from on-going USMA/LANL collaboration will be discussed.

Nuclear forensics requires accurate identification of distinguishing material characteristics to determine the history of interdicted nuclear materials. Key material signatures, such as morphological and chemical variations, that pinpoint recent provenance can reside within the bulk oxide scale formed over nuclear materials. This paper identifies improvements in extracting and analyzing key material signatures that exists within the oxide scale formed at the surface of Pu metals. We will describe our multiplatform approach using high-resolution microscopic techniques to identify spatially resolved morphological and chemical features of Pu oxide scales. 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 the distinguishing morphological and chemical characteristics between the original oxide scale formed from the controlled storage environment and the scale that is formed during subsequent exposure to external environments. Internal chemical composition, structure, and pore geometry of oxide scales are different, attributable to oxidative corrosion from exposure to outside the storage environment. These altered oxide scales provide distinguishing characteristics for recent environmental exposures. Furthermore, we will discuss our continuing effort to improve understanding internal morphologies and methods to generate database of morphological and chemical features including quantification applicable to nuclear forensics.
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 support does not constitute an express or implied endorsement on the part of the Government. LLNL-ABS-795544

Nuclear forensic science is the science of identifying the purpose and provenance of radiological and/or nuclear material found outside of regulatory control. Because the ability to perform a nuclear forensics examination can help to identify gaps or weaknesses in physical security or safeguards of nuclear material, (which can then be strengthened in response) it is increasingly recognized as an integral part of a robust nuclear security program.

Nuclear forensics science matured as a discipline in the mid-1990s, coincident with an increasing number of nuclear smuggling incidents (Kristo et al., 2016). Labs were tasked with responding to incidents involving interdictions of low enriched uranium (LEU), highly enriched uranium (HEU), and even Pu in Russia, Lithuania, Germany, Bulgaria and elsewhere (IAEA, 2011; Kristo, 2012). As the casework increased throughout the mid-1990’s, so too did the sophistication of the analytical approaches to nuclear forensics examinations. Nuclear forensics practitioners seek to identify not only what materials are on the most basic level, but what might have been their intended use as well as how, when and where the materials were produced, used or stored. As the casework and policy implications of the field have evolved, so too has the science of nuclear forensics.

A combination of geochemical, materials science, and radiochemical analysis methods have traditionally formed the basis for nuclear forensics examination approaches. These methods exploit the characteristics of front end materials produced through mining, milling, and conversion in which the geologic and materials processing signatures are significant. Methods with a geochemical basis are also important for materials from the back end of the fuel cycle, including radiochronometric techniques that allow age-dating of the purification of materials (e.g. Mayer et al., 2013; Eppich et al., 2013; Kristo et al., 2016).

The search for identifying characteristics, or signatures, has been the focus of nuclear forensics research since the beginning. In the context of a nuclear forensics examination, signatures are defined as characteristics or sets of characteristics of a given sample that enables the assessment and evaluation of that sample’s provenance. The most valuable signatures are both readily measurable and also persistent through the fuel cycle. Some examples of signatures that persist up to the point of enrichment might include REE patterns, U isotopic compositions, or isotope compositions of certain trace elements including ⁸⁷Sr/⁸⁶Sr or ⁹⁸Mo/⁹⁵Mo (Varga et al., 2009; Brennecka et al., 2010; Rolison et al., 2019).

Other signatures, while not persistent, are characteristic of specific places or processes. These could include enrichment, minor U isotope composition, parameters associated with grinding and metallurgy, trace elements, or even trace organic compounds (e.g. Pajo et al, 2001; Svedkauskaite-LeGore et al., 2008; Badaut et al., 2009; Kennedy, 2013).

The future of nuclear forensics research is focused on developing a greater understanding of signatures, both those that are persistent, as well as those that are characteristics of places, production methods, and design parameters.

Recently, we have acquired and analyzed samples of uranium ore material and uranium ore concentrate for a variety of analytes in order to assess the behavior of isotopic and elemental signatures during processing. This work is focused on identifying persistent signatures that may serve as a predictive indicator of material provenance. Results of this work will be presented, as well as an overview of the current state of nuclear forensic signatures for materials from the front end of the nuclear fuel cycle. The talk will conclude with a discussion of some promising future directions for the field.

Lanthanide orthophosphates with the monazite crystalline structure are considered as potential matrices for encapsulation of nuclear materials due to their high resistance to radiation damage. Here, we will present a review done on a series of monazites having the crystalline structure La_xM_1−xPO₄ (M=RE, An, 0≤x≤1). We have studied their local structure using high resolution ³¹P solid-state NMR. The experiments were done using the dedicated NMR facility for transuranic materials which is available at JRC-Karlsruhe. In a second part, in order to understand the magnetic nature of these samples, we also performed magnetic susceptibility measurements. Analyses of the magnetic susceptibility data were carried out using the free ion model, crystal field theory and/or ab initio calculations allowing to extract the electronic structure. All of these data will be discussed and a comparison between the actinide monazites and their lanthanide surrogates will be presented.

Knowledge about the interaction of UO₂ with the environment is of high importance to assess its stability in oxidizing and in reducing condition. For this reason, understanding the UO₂ oxidation process is central to the performance assessment of nuclear fuels. Oxidizing condition can convert UO₂ into U₃O₈ leading, in some cases, to powdering of the fuel matrix or to failure of the cladding [1]. The redox behaviour of UO₂ is also relevant for the management of nuclear waste forms, as oxidation results in the production of soluble uranyl cations, UO₂²⁺, that can easily migrate in the biosphere.
This talk summarizes surface science research activities carried out at JRC Karlsruhe in support of nuclear safety and non-power application of nuclear materials. I will also describe a unique set-up developed in-house, the Labstation [2], including sample preparation modules, by DC Sputtering or evaporation, and several surface characterisation techniques. Among the latter, High-resolution X-ray Photoelectron Spectroscopy (XPS) and Bremstrahlung Isochromat Spectroscopy (BIS) are particularly useful to characterize the uranium oxidation state in single and mixed oxides. In a first example, I will show the process of oxidation and reduction of U oxides using O₂, H₂O and H₂, plasma. For the first time, U₂O₅ films [3-4] could be produced and studied, filling a gap in the knowledge about U⁵⁺ oxidation state. While U⁴⁺, U⁵⁺ and U⁶⁺ have representative satellite peaks positioned, respectively, at 7, 8 and 4 & 10 eV out of the main 4f_5/2,7/2 excitation lines, a satellite peak at 9 eV is observed when UO_2+x in contact with sodium is heated at 400 °C under oxygen. In a second example, we report the first in-situ preparation of ThF₄ [5] whose band gap of 10.2(2) eV is significantly larger than the 229mTh excitation energy, making ThF₄ a possible candidate material for a solid-state nuclear clock based on the vacuum ultraviolet γ decay.

References
[1] NEA (2005), The Safety of the Nuclear Fuel Cycle, 3^rd Edition, OECD, Paris.
[2] T. Gouder, F. Huber, R. Eloirdi, R. Caciuffo, R. J. Vis. Exp. 144, e59017b, (2019)
[3] T. Gouder, R. Eloirdi, R. Caciuffo, Scientific Reports 8, 8306 (2018)
[4] N. A. Brincat, S. C. Parker, M. Molinari, G. C. Allen and M. T. Storr, Dalton Trans. 44, 2613–2622, (2015) Molinari, M., Brincat, N. A., Allen, G. C. & Parker, S. C. Structure and properties of some layered U₂O₅ phases: A density functional theory study. Inorg. Chem. 56, 4468 (2017). Molinari, M., Brincat, N. A., Allen, G. C. & Parker, S. C. Structure and properties of some layered U₂O₅ phases: A density functional theory study. Inorg. Chem. 56, 4468 (2017). Molinari, M., Brincat, N. A., Allen, G. C. & Parker, S. C. Structure and properties of some layered U₂O₅ phases: A density functional theory study. Inorg. Chem. 56, 4468 (2017). Molinari, M., Brincat, N. A., Allen, G. C. & Parker, S. C. Structure and properties of some layered U₂O₅ phases: A density functional theory study. Inorg. Chem. 56, 4468 (2017).
[5] T. Gouder, R. Eloirdi, R. L. Martin, M. Osipenko, M. Giovannini, and R. Caciuffo, Phys. Rev. Research 1, 033005, (2019)

Magnetic order in 5f materials offers interesting benefits, based in principle on very strong spin-orbit interaction, bringing large orbital moments into systems with itinerant, i.e. bonding, states. This yields very strong magnetocrystalline anisotropy, determined not by CEF effects as in lanthanides, but by bonding anisotropy. Hybridization of the 5f states with other states having low effective mass means that a re-orientation of 5f magnetic moments can have a dramatic effect of electrical conduction, bringing a giant magnetoresistance effect. Other related prominent characteristics include a giant Kerr rotation as on of important magnetooptical phenomena.
Although highly tunable by composition and external variable, the 5f magnetism remained restricted to low temperature range, discarding e.g. inexpensive uranium from the pool of prospective magnetic materials. For example, Curie temperatures of U-based 5f-ferromagnets only exceptionally exceeds the limit of 200 K. The trick of mixing the f magnetism with 3d magnetism of transition metals, so successful in lanthanide-based permanent magnets, does not work in the case of light actinides. Both 3d and 5f states being band-like, they strongly hybridize, which degrades both the 3d and 5f desirable characteristics. Due to the same reason U-compounds very rich in Fe or Co simply lose any distinct 5f contribution. Heavy actinides (from Am onwards), in principle analogous to heavy lanthanides, undergo a fast radioactive decay.
We have been trying to understand which type of tuning can support the 5f magnetism besides the traditional game with inter-actinide spacing. The case of uranium hydrides showed that using a ionic-covalent bonding can lead to a suppression of the 5f-6d hybridization, which is an undesirable mechanism, contributing the washout of U moments. The 6d hybridization with ligand states can efficiently liberate the 5f states, although they can still maintain their band character. A general hint is to look at U compounds with a certain charge transfer and to use external pressure for fine tuning of crystal structure.
We have focused on U pnictides, and chose UCu₂P₂ as the material with record high T_C = 216 K [1]. Applying hdrostatic pressure the T_C value increases, reaching a maximum of 290 K in p = 7-8 GPa, i.e. first time obtaining a room temperature ferromagnetism with pure 5f ferromagnet. Details of electronic structure and their pressure variations have been provided by ab initio calculations.
[1] D. Kaczorowski and R. Troc, J.Phys.:Cond.Mat. 2, 4185 (1990).
This work was supported by the Czech Science Foundation under the grant No. 18-02344S.

Molten fluoride salts are used as solvents for uranium and thorium fuel for molten salt nuclear reactors. The relatively low volatility and high solubility of actinides and fission products in fluoride salt mixtures at temperatures in the range of 500 to 800^oC render fluorides highly attractive solvents for liquid nuclear fuel. While a molten salt reactor experimental reactor operated in the 1960s and was supported by an extensive experimental campaign on the chemistry and physical chemistry of actinide-bearing molten fluoride salts, important data gaps still remain, and important fundamental understanding questions remain to be explored. This talk will provide a review of recent advances in on the physical chemistry of liquid fluoride salt fuel, in the context of development and licensing of molten salt reactors.

The specific nature of uranium oxides in spent nuclear fuel is a primary concern in the storage of nuclear fuels resulting from both the volumetric expansion of uranium oxides and change in solubility with an increase in oxygen content. In particular, the growth of hyperstochiometric uranium dioxide has been linked to fracturing of spent fuel, resulting in further oxidation and the release of short-lived radionuclides trapped within the UO₂ matrix. Oxygen K-edge x-ray absorption spectroscopy is an excellent method to distinguish between different uranium oxide stoichiometries, but samples are limited to 1-2 µm thick as a result of the short penetration length of soft x-rays. To address this, low burnup pressurized water reactor (PWR) spent nuclear fuel samples were processed at Idaho National Laboratories utilizing focused ion beam (FIB) sectioning, resulting in samples ideal for soft x-ray spectroscopy. Here we present the first measurements of oxygen K-edge near-edge x-ray absorption spectroscopy, using the Beamline 11.0.2 scanning transmission x-ray microscope (STXM) at the Advanced Light Source, on FIB sections of spent nuclear fuel. The FIB sections present unique challenges in normalization and data analysis which will be discussed in this presentation. The sensitivity of the oxygen K-edge x-ray absorption spectrum between the different uranium oxides has allowed for species-specific mapping of the spent nuclear fuel at the sub-micron scale, showing UO₂ as the bulk of the fuel, with U₃O₇ localized near a defected area in the spent nuclear fuel. In addition, spectra at the uranium N_4,5 edges and the cerium M_4,5 edges are analyzed and discussed, showing preliminarily a Ce³⁺ oxidation state. This is all presented in the context of comparisons with transmission electron microscopy measurements of the same FIB sections.

The principal sources of radiation damage in actinide compounds are spontaneous fission and alpha-decay, and self-radiation damage from these processes affect microstructural evolution, phase stability, and thermodynamic properties. In general, alpha-decay is the dominant process for most actinides, and because of the long half-lives of the actinides and their daughter products in used nuclear fuel and actinide waste forms, alpha-decay damage is a concern over very long timescales. Alpha decay produces energetic alpha particles (4.5 to 5.5 MeV) and recoil nuclei (70 to 100 keV), which result in both radiation damage and the accumulation of helium. The evolution of radiation damage due to alpha-decay has been studied using short-lived actinides, such as ²³⁸Pu and ²⁴⁴Cm, in several candidate glasses and ceramics for the immobilization of actinides, and fission tracks from the spontaneous fission of ²⁴⁴Cm have been observed in crystalline ceramics. Ion beam irradiations have been more frequently employed to study radiation damage in relevant materials due to decreased costs, shorter irradiation timescales and the non-radioactive nature of the irradiated materials. Furthermore, ion irradiations can be performed over a wider range of irradiation conditions to implant helium, study radiation damage kinetics, understand the separate and combined effects of alpha particles and alpha recoils, and benchmark against alpha-decay damage due to short-lived actinides. At ion energies typically used to study alpha-decay damage in actinide compounds, the electronic and nuclear energy losses are both important, and local ionization along the ion path can affect damage production and evolution, and atomic-level simulations have provided insights into these processes. While alpha particles are much less damaging than alpha recoils, the electronic energy loss from the alpha particles can also cause athermal annealing of heavy ion damage (i.e., alpha-recoil damage) that can impact the evolution of radiation damage. Helium implantation studies have shown that the accumulation of helium and heavy ion damage leads to the formation of helium bubbles above a threshold helium concentration. These results have significant implications for interpreting and modeling the radiation response of actinide compounds.

Self-irradiation in plutonium metal at room temperature introduces defect accumulation into the lattice, such as He bubbles and other daughter product impurities, interstitials and vacancies. X-ray diffraction and dilatometry studies have shown that the room temperature self-irradiation of δ-phase Pu-Ga alloys results in swelling of the lattice that saturates after 0.1-0.2 dpa of accumulated lattice damage. For a more complete picture of the poorly understood “aging” processes of defect accumulation and damage evolution δ-phase Pu-Ga alloys, dose, dose rate, thermal history and alloy composition must be considered. This work investigates with time-of-flight neutron diffraction the influence on long-range order of room temperature self-irradiation defects in δ-phase ²³⁹Pu-Ga alloys as they influence the lattice throughout a varied history of damage accumulation and thermal cycling.

In-situ time-of-flight neutron diffraction measurements were performed on the High-pressure Preferred Orientation (HIPPO) diffractometer at Lujan Center, Los Alamos Neutron Science Center. Data were collected between room temperature and 35K in an investigation of ambient aging impact on the lattice effects detectable during thermal cycling on the lattice of a δ-phase ²³⁹PuGa alloys. Repeated thermal cycling of PuGa alloys immediately after annealing at 450°C exhibits lattice swelling similar to ambient temperature isothermal lattice swelling, but at much shorter time scales. Reported here, the amount of lattice swelling for each cycle decreased in magnitude relative to the time before indicative of a saturating strain due to stress of local defect evolution to damage accumulation and impact on long-range order. Longer idling times, as observed during the FY16 and FY17 measurements, resulted in incomplete lattice recovery, and a contracted lattice. This study is an attempt at unravelling the complex history dependence of induced stress states and corresponding lattice strain on the kinetic processes of self-irradiation damage and thermal cycling.

A cardinal obstacle to performing quantum-mechanical simulations of strongly-correlated matter is that, with the theoretical tools presently available, sufficiently-accurate computations are often too expensive to be ever feasible. Here we design a computational framework combining quantum-embedding (QE) methods with machine learning. This allows us to bypass altogether the most computationally-expensive components of QE algorithms, making their overall cost comparable to bare Density Functional Theory (DFT). We perform benchmark calculations of a series of actinide systems, where our method describes accurately the correlation effects, reducing by orders of magnitude the computational cost. We argue that, by producing a larger-scale set of training data, it will be possible to apply our method to systems with arbitrary stoichiometries and crystal structures, paving the way to virtually infinite applications in condensed matter physics, chemistry and materials science.

arXiv:2006.15227 (2020)

Knowledge about the interaction of UO₂ with the environment is of high importance to assess its stability in oxidizing and in reducing condition. For this reason, understanding the UO₂ oxidation process is central to the performance assessment of nuclear fuels. Oxidizing condition can convert UO₂ into U₃O₈ leading, in some cases, to powdering of the fuel matrix or to failure of the cladding [1]. The redox behaviour of UO₂ is also relevant for the management of nuclear waste forms, as oxidation results in the production of soluble uranyl cations, UO₂²⁺, that can easily migrate in the biosphere.
This talk summarizes surface science research activities carried out at JRC Karlsruhe in support of nuclear safety and non-power application of nuclear materials. I will also describe a unique set-up developed in-house, the Labstation [2], including sample preparation modules, by DC Sputtering or evaporation, and several surface characterisation techniques. Among the latter, High-resolution X-ray Photoelectron Spectroscopy (XPS) and Bremstrahlung Isochromat Spectroscopy (BIS) are particularly useful to characterize the uranium oxidation state in single and mixed oxides. In a first example, I will show the process of oxidation and reduction of U oxides using O₂, H₂O and H₂, plasma. For the first time, U₂O₅ films [3-4] could be produced and studied, filling a gap in the knowledge about U⁵⁺ oxidation state. While U⁴⁺, U⁵⁺ and U⁶⁺ have representative satellite peaks positioned, respectively, at 7, 8 and 4 & 10 eV out of the main 4f_5/2,7/2 excitation lines, a satellite peak at 9 eV is observed when UO_2+x in contact with sodium is heated at 400 °C under oxygen. In a second example, we report the first in-situ preparation of ThF₄ [5] whose band gap of 10.2(2) eV is significantly larger than the 229mTh excitation energy, making ThF₄ a possible candidate material for a solid-state nuclear clock based on the vacuum ultraviolet γ decay.

References
[1] NEA (2005), The Safety of the Nuclear Fuel Cycle, 3^rd Edition, OECD, Paris.
[2] T. Gouder, F. Huber, R. Eloirdi, R. Caciuffo, R. J. Vis. Exp. 144, e59017b, (2019)
[3] T. Gouder, R. Eloirdi, R. Caciuffo, Scientific Reports 8, 8306 (2018)
[4] N. A. Brincat, S. C. Parker, M. Molinari, G. C. Allen and M. T. Storr, Dalton Trans. 44, 2613–2622, (2015) Molinari, M., Brincat, N. A., Allen, G. C. & Parker, S. C. Structure and properties of some layered U₂O₅ phases: A density functional theory study. Inorg. Chem. 56, 4468 (2017). Molinari, M., Brincat, N. A., Allen, G. C. & Parker, S. C. Structure and properties of some layered U₂O₅ phases: A density functional theory study. Inorg. Chem. 56, 4468 (2017). Molinari, M., Brincat, N. A., Allen, G. C. & Parker, S. C. Structure and properties of some layered U₂O₅ phases: A density functional theory study. Inorg. Chem. 56, 4468 (2017). Molinari, M., Brincat, N. A., Allen, G. C. & Parker, S. C. Structure and properties of some layered U₂O₅ phases: A density functional theory study. Inorg. Chem. 56, 4468 (2017).
[5] T. Gouder, R. Eloirdi, R. L. Martin, M. Osipenko, M. Giovannini, and R. Caciuffo, Phys. Rev. Research 1, 033005, (2019)

Studying the local moment and 5f-electron occupations sheds insight into the electronic behavior in actinide materials. X-ray absorption spectroscopy (XAS) has been a powerful tool to reveal the valence electronic structure when assisted with theoretical calculations. In this work, we employed the DFT+Gutzwiller rotationally-invariant slave boson method to obtain the local Hamiltonian of the single-impurity Anderson model (SIAM), and used exact diagonalization (ED) method to calculate the XAS spectra from the model. An in-house built computational code was developed for the ED method. By applying this technique to the recently discovered 5f-electron topological Kondo insulator plutonium tetraboride (PuB₄), we were able to determine the signature of 5f-electronic correlation effects in the theoretical X-ray spectra. We found that the Pu 5f-6d hybridization effect provides an extra channel to mix the j=5/2 and 7/2 orbitals in the 5f valence. As a consequence, the resultant electron occupation number and spin-orbit coupling strength deviate from the intermediate coupling regime. We have also applied this approach to the d-phase of Pu.

We discuss the impact of 5f electron interactions on structural properties of pure actinides and phase of plutonium[1][2]. We first use an ab initio calculation of effective Coulomb interactions and we find that the calculated interactions in plutonium and americium are much smaller than usually expected. With these interactions, we reproduce the experimental transition from low volume early actinides (uranium, neptunium, α-plutonium) to high- volume late actinides (δ-plutonium, americium, and curium) and a good agreement of volume of phases of plutonium with experiment. We also discuss phonon spectra of plutonium. Then we discuss our values of effective 5f electron interactions U. In some lanthanides and actinides [3], higher values are expected to reproduce experimental photoemission spectra and structural properties. We discuss general ways to improve the calculation to be more predictive.

[1] B. Amadon Phys. Rev. B 94, 115148 (2016), Phys. Rev. B 97, 039903 (2018)
[2] B. Amadon and B. Dorado J. Phys.: Condens. Matter 30 405603 (2018)
[3] J.B. Morée and B. Amadon Phys. Rev. B 98, 205101 (2018)

Due to its position at the boundary between the light and heavy actinides, Plutonium (Pu)
has liminal characteristics that make modeling its 5f-orbital electrons especially challenging.
Consequently, a complete understanding of the electronic structure of this elemental solid remains
in question. In order to explore the role of strong electronic correlations in the δ-phase
of Pu, we have performed Fermi surface calculations using the generalized gradient approximation
with dynamical mean-field theory and the Gutzwiller wavefunction approximation to
compare their differing approaches to electron correlation effects. The de Haas-van Alphen
(dHvA) frequencies and effective masses were then calculated to analyze correlation induced
renormalization effects on the Fermi surface topologies. We have predicted that the electronic
correlation gives rise to a volume expansion of 200% in both the hole and electron
pockets of the Fermi surface in addition to an intermediate mass enhancement. This is in
agreement with the band renormalization theory for strongly correlated materials.

Acknowledgments: U.S. Department of Energy (DOE) National Nuclear Security Administration, Contract No. 89233218CNA000001.
LANL LDRD Program. NNSA Advanced Simulation and Computing Program. Center for Integrated Nanotechnologies, a DOE BES user facility, in partnership with LANLInstitutional Computing Program for computational resources.

Magnetic order in 5f materials offers interesting benefits, based in principle on very strong spin-orbit interaction, bringing large orbital moments into systems with itinerant, i.e. bonding, states. This yields very strong magnetocrystalline anisotropy, determined not by CEF effects as in lanthanides, but by bonding anisotropy. Hybridization of the 5f states with other states having low effective mass means that a re-orientation of 5f magnetic moments can have a dramatic effect of electrical conduction, bringing a giant magnetoresistance effect. Other related prominent characteristics include a giant Kerr rotation as on of important magnetooptical phenomena.
Although highly tunable by composition and external variable, the 5f magnetism remained restricted to low temperature range, discarding e.g. inexpensive uranium from the pool of prospective magnetic materials. For example, Curie temperatures of U-based 5f-ferromagnets only exceptionally exceeds the limit of 200 K. The trick of mixing the f magnetism with 3d magnetism of transition metals, so successful in lanthanide-based permanent magnets, does not work in the case of light actinides. Both 3d and 5f states being band-like, they strongly hybridize, which degrades both the 3d and 5f desirable characteristics. Due to the same reason U-compounds very rich in Fe or Co simply lose any distinct 5f contribution. Heavy actinides (from Am onwards), in principle analogous to heavy lanthanides, undergo a fast radioactive decay.
We have been trying to understand which type of tuning can support the 5f magnetism besides the traditional game with inter-actinide spacing. The case of uranium hydrides showed that using a ionic-covalent bonding can lead to a suppression of the 5f-6d hybridization, which is an undesirable mechanism, contributing the washout of U moments. The 6d hybridization with ligand states can efficiently liberate the 5f states, although they can still maintain their band character. A general hint is to look at U compounds with a certain charge transfer and to use external pressure for fine tuning of crystal structure.
We have focused on U pnictides, and chose UCu₂P₂ as the material with record high T_C = 216 K [1]. Applying hdrostatic pressure the T_C value increases, reaching a maximum of 290 K in p = 7-8 GPa, i.e. first time obtaining a room temperature ferromagnetism with pure 5f ferromagnet. Details of electronic structure and their pressure variations have been provided by ab initio calculations.
[1] D. Kaczorowski and R. Troc, J.Phys.:Cond.Mat. 2, 4185 (1990).
This work was supported by the Czech Science Foundation under the grant No. 18-02344S.

The actinyl cation (D_∞h point group) possesses symmetric and asymmetric stretching modes that are typically Raman and Infrared active, respectively. Lowering the symmetry of the actinyl moiety will activate both modes causing two stretching bands to be present within the spectral window. While activation of these bands suggests perturbation to the actinyl bond, there is currently a limited understanding on how intermolecular forces influence this process or the extent of the effect on the actinyl bond in solid state compounds. In this presentation, I will focus on the analysis of Raman spectra associated with neptunyl (NpO₂⁺/NpO₂²⁺) compounds and explore activation and vibrational coupling effects of these materials. Specifically, the impacts of counterions and hydrogen bonding will be explored experimentally and compared to theoretical calculations. In addition, the neptunyl system will be compared to the related uranyl complexes to further explore the subtle differences in bonding within actinyl cations.