How Surface Chemistry Affects Spent Nuclear Fuel

The fluorite-structured actinide dioxides (AnO₂, particularly UO₂) are the most commonly used fuels in nuclear reactors. Understanding the surface chemistry of the AnO₂ series therefore is of both fundamental and technological importance. Many fundamental questions relating to AnO₂ surface chemistry need answering for the safe processing, recycling and reuse, long-term storage, and environmental effects of nuclear fuels. Examples of issues relevant to the surface properties of these materials include the atomistic description of AnO₂ surface structures, the stability of different surface planes, the effect of defects on the properties of AnO₂ surfaces, and the mechanisms of interaction between environmental molecules and AnO₂ surfaces. However, our understanding of AnO₂ surface chemistry has been so far limited by the difficulties associated with the handling of radioactive actinides in experiments. The computational approaches for the atomic simulation of surfaces have reached a state of maturity such that modeling studies can now supplement experimental efforts.

Limits of actinide dioxide knowledge

Our knowledge of AnO₂ surface chemistry is limited primarily due to two factors. First, specialized laboratory equipment is required to handle actinides experimentally because these elements are radioactive. Early actinides such as thorium and uranium are relatively abundant in nature and their most stable isotopes have long radioactive decay times, making them appropriate for laboratory work. The highly radioactive transuranic elements meanwhile have shorter lifetimes and are therefore not found in nature; they are produced artificially and are only available in small quantities. Accordingly, most experimental studies of AnO₂ surfaces have focused on UO₂, leaving the remainder of the AnO₂ series comparatively unexplored. Experimental studies of single-crystallized materials of transuranic oxides are particularly rare in the literature.

The second reason for our limited understanding of AnO₂ surface chemistry is that we lack characterization techniques with sufficient precision at the atomic scale. Most of the methods used to probe surface properties, such as X-ray photoelectron spectroscopy (XPS) and Auger Electron Spectroscopy (AES), have penetration depths varying from a few nanometers to micrometers, and the signals from these techniques typically represent an average over these depths. Directly observing the atomic surface structure and chemical properties of AnO₂ surfaces using these experimental techniques is challenging without help from atomistic simulations.

Driven by the experimental difficulties studying AnO₂ surfaces and the improvement in computational power and methodology, the last few years have seen extensive efforts to simulate AnO₂ surfaces using ab-initio electronic structure calculations. However, treatment of actinides using density functional theory (DFT) remains non-trivial due to a combination of factors not present in the lighter elements of the periodic table, e.g.: relativistic effects, strongly-correlated 5f electrons, and noncollinear magnetism. Different approaches have been proposed to circumvent this problem of standard DFT functional. One of the most popular methods is DFT+U, which involves the introduction of an empirical Hubbard U correction term to the Hamiltonian. This method has been widely used because it does not increase the computational cost and it can greatly improve the description of the 5f electrons. Here, we present two examples demonstrating the importance of atomic simulations in understanding AnO₂ surface chemistry using DFT studies. The first study focuses on controlling the surface energy of different AnO₂ planes to tune the morphology of AnO₂ nanoparticles. The second shows the critical effect of surface electronic structure on the reactivity of AnO₂ surfaces using water splitting as an example.

Ligand-induced shape transformation of ThO₂ nanocrystals

Nanocrystals with size- and shape-dependent properties are of great scientific interest. Remarkable progress has been made in the controlled synthesis of nanocrystals of stable elements in the past two decades, however, our knowledge of actinide nanocrystals has been considerably limited due the difficulties described above. Recently, a non-aqueous surfactant-assisted synthesis has been used for the preparation of actinide oxide nanocrystals (ThO₂, UO₂, and NpO₂), yielding structures with different morphologies such as branched nanocrystals, nanodots, and nanorods. The nanocrystals were synthesized in a mixture of organic molecules including oleic acid, acetylacetone, oleylamine, and trioctylphosphine oxide (Fig. 2). However, both the underlying mechanism that controls the morphologies and the role of different ligands in nanocrystal growth are unclear.

In order to understand the role of ligands in ThO₂ growth we performed a DFT study on ThO₂ surfaces and their interactions with different surfactant ligands. The low Miller index (111), (110), and (100) lattice planes were considered because their surface energies span a wide range, thus they can represent most surfaces of ThO₂ (see the explainer of Miller indices on the opposite page). The thermodynamic stability of ThO₂ nanocrystals in different shapes was then evaluated employing a modified Wulff construction as a thermodynamic model. Ligand molecules with long tails were represented using smaller ligands with the same functional group to maintain the same chemistry with reduced computational costs; specifically, we used acetic acid (C₂H₄O₂) to represent oleic acid (C₁₈H₃₄O₂). It was found that the (111) surface of ThO₂ has the lowest surface energy among the studied models, therefore pristine ThO₂ nanocrystals will tend to grow in an octahedral geometry terminated by (111) planes (Fig. 3).

The adsorption of ligands can significantly affect the surface energy for different facets, in turn altering the morphologies of ThO₂ nanocrystals due to selective adsorption on different surfaces. In particular, molecules with carboxylate group (–COOH) display stronger affinity for the (110) surface (Fig. 4). The selective adsorption of acetic acid on the (110) surface is attributed to the differences in the surface structures—Th atoms on the (110) surface have a lower coordination number than on the (111) surface, allowing the ligands to bind more strongly. Moreover, the (110) surface has a flat exterior with Th and O atoms on the same plane, while the topmost layer of (111) and (100) surfaces show the O atoms creating stronger repulsion between the ligand/carboxylate O atoms and the surfaces.

 

The adsorption of ligands can stabilize the growth of different ThO₂ surfaces. The more stable surface will grow as the nanocrystal forms, changing the shape, for instance, from octahedral to nanorod in the case of increasing coverage of oleic acid (Fig. 3). Interestingly, other ligands such as acetylacetone, oleylamine, and trioc-tylphosphine oxide do not modify the equilibrium shape of ThO₂ nanocrystals. These calculations highlight the critical role of surface-ligand interactions in determining the nanocrystal morphology and may pave the way towards a comprehensive under-standing of the formation and growth of other AnO₂ nanocrystals with well-defined sizes and shapes for future applications.

 

Water splitting on AnO₂ surfaces with oxygen vacancies

The interaction of water with AnO₂ compounds has attracted a great deal of interest due to its importance in corrosion studies of nuclear fuels. It has been shown that the adsorption of H₂O molecules on the perfect UO₂ (111) surface is reversible at 300 K, indicating that H₂O is weakly adsorbed. In contrast, when oxygen vacancies are present, H₂O strongly interacts with the (111) surface and can split into H₂. Nonetheless, our understanding of the electronic structure and catalytic properties across the AnO₂ surfaces is still lacking, especially with respect to surfaces with oxygen vacancies, and the effects of these vacancies on actinide oxidation states and surface chemical reactions.

Motivated by the aforementioned questions, computational modeling was used to study the electronic structure and water splitting chemistry of ThO₂, UO₂, and PuO₂ (111) surfaces with oxygen vacancies. Because of the formally O^2– state of oxygen in these materials, each oxygen vacancy leaves two excess electrons on the surface (we use the term “formal” in chemistry to describe integral electron counting in a molecule). We found that these excess electrons distribute differently depending on the actinide involved (Fig. 5). On the ThO₂ surface, the electrons remain at the vacancy site and form a lone pair. At the other extreme, excess electrons localize on two Pu centers on the PuO₂ surface, and consequently reduce their oxidation state from Pu⁴⁺ to Pu³⁺ (i.e., each Pu center has formally gained an electron). In contrast, on the UO₂ surface, one of the excess electrons remains at the vacancy site and the other is located in a 5f orbital of a neighboring uranium ion.

The differences in the distribution of excess electrons can be explained by the contraction and energy decrease of the 5f orbitals from Th to Pu: the 5f orbitals of Pu are lower in energy than the vacancy states and therefore favor reduction of the metal centers, whereas in Th, the 5f orbitals are much higher in energy, hindering the reduction. The change in electronic structure from ThO₂ to PuO₂ surfaces with oxygen vacancies leads to different chemical properties and reactivity of the AnO₂surfaces. We have demonstrated this in our studies of the splitting of H₂O catalyzed by ThO₂, UO₂, and PuO₂ (111) surfaces. On stoichiometric surfaces, the formation of H₂ from catalytic splitting of H₂O is endothermic (i.e., energy-absorbing) for all three AnO₂ (111) surfaces evaluated. In contrast, the H₂O splitting reaction and H₂ production is spontaneous in the presence of oxygen vacancies. The ThO₂ surface shows the most exothermic (energy-releasing) reaction process with small energy barriers. This is due to a lone electron pair at the vacancy site on ThO₂ (Fig. 5a) that can readily participate in chemical reactions with H₂O. Meanwhile, on the PuO₂surface, there is a larger energy barrier for the H₂ production, although the H₂O adsorption and H₂ production is energetically preferred.

Summary

Understanding actinide dioxide surface chemistry is relevant to many stages in the nuclear industry and significant work has been performed in the research community to explore the properties and reactivity of AnO₂ surfaces. In combination with experimental results, atomic simulations can provide insights into electronic structure and chemical reactions on the AnO₂ surfaces. Our recent work has demonstrated the significant role of ligand-surface interactions in the determination of ThO₂ nanocrystal morphology, in addition to vacancy formation and its impacts on catalyzing the production of H₂ on AnO₂ surfaces. Oxygen vacancies are expected to be present on the surfaces of spent nuclear fuels due to radiation damage. If spent fuels are exposed to water in a geological storage facility it will therefore lead to the production of H₂. The accumulation of H₂ can cause the pressurization of the fuel containers, which is a serious safety issue to consider for long-term storage of spent nuclear fuels.

Acknowledgments

The authors gratefully acknowledge funding from the Laboratory Directed Research and Development program of Los Alamos National Laboratory (LANL) under project 20160604ECR 20180007DR, and US DOE office of Basic Energy Science under the Heavy Element Chemistry program at LANL. The calculations were performed using EMSL (grid.436923.9), a DOE Office of Science User Facility sponsored by the Office of Biological and Environmental Research. G.W. also acknowledges a Director’s Postdoc Fellowship from LANL.

Further reading

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2. G. Jomard, et al., “Structural, thermodynamic, and electronic properties of plutonium oxides from first principles,” Phys. Rev. B, 2008, 78, 075125.
3. B. Himmetoglu, et al., “Hubbard-corrected DFT energy functionals: The LDA+ U description of correlated systems,” Int. J. Quantum Chem., 2014, 114, 14–49.
4. G. Wang, E.R. Batista, P. Yang, “Ligand induced shape transformation of thorium dioxide nanocrystals,”Phys. Chem. Chem. Phys., 2018, 20, 26, 17563–17573.
5. X.-Y. Liu, D. Andersson, B. Uberuaga, “First-principles DFT modeling of nuclear fuel materials,” J. Mater. Sci., 2012, 47, 7367–7384.
6. G. Wang, E.R. Batista, P. Yang, “Excess electrons on reduced AnO₂ (111) surfaces (An = Th, U, Pu) and their impacts on catalytic water splitting,” J. Phys. Chem. C, 2019, 123, 50, 30245–30251.
7. D. Hudry, et al., “Non-aqueous synthesis of isotropic and anisotropic actinide oxide nanocrystals,” Chem. Eur. J., 2012, 18, 8283–8287.
8. D. Hudry, et al., “Controlled synthesis of thorium and uranium oxide nanocrystals,” Chem. Eur. J., 2013, 19, 5297–5305.
9. A.S. Barnard, P. Zapol, “A model for the phase stability of arbitrary nanoparticles as a function of size and shape,” J. Chem. Phys., 2004, 121, 4276–4283.
10. S. Senanayake, H. Idriss, “Water reactions over stoichiometric and reduced UO₂ (111) single crystal surfaces,” Surf. Sci., 2004, 563, 135–144