# Water on Actinide Dioxide Surfaces: A Review of Recent Progress
## 1 Introduction  
The fluorite structured actinide dioxides (AnO\({}_{2}\), particularly UO\({}_{2}\)) are the most common nuclear fuels currently used in nuclear reactors worldwide. Understanding the surface chemistry of these materials is of both fundamental and technological importance owing to its relevance to many aspects in the nuclear industry from the corrosion of nuclear fuels [1; 2], hydrogen gas production, and release from nuclear fuels [3], to the handling and long-term storage of spent fuels. As the interaction of nuclear fuels with environmental species (e.g., water and oxygen) starts at their surfaces, there is a pressing need to study the chemical reactions on AnO\({}_{2}\) surfaces to evaluate the environmental impact of spent nuclear wastes [4]. However, the understanding of the surface chemistry of AnO\({}_{2}\) has been severely limited by the difficulties associated with the handling of radioactive actinides in experiments and the complexity of studying actinide systems using theoretical means. Specialized laboratory equipment is required to handle actinides in experiments, because all the actinide elements are radioactive and very toxic [5; 6]. 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, for example, the most stable isotopes \({}^{232}\)Th and \({}^{238}\)U have half-lives of \(1.4\times 10^{10}\) and \(4.5\times 10^{9}\) years, respectively. The transuranics, on the other hand, are not found in nature and have been artificially made; therefore, they are available in small quantities and are highly radioactive. Therefore, most experimental studies of AnO\({}_{2}\) surface chemistry have focused on UO\({}_{2}\), leaving the rest of the AnO\({}_{2}\) essentially unexplored. Theoretical calculations can supplement experimental efforts to understand the surface chemistry of the AnO\({}_{2}\) series and some studies have tried to fill this gap. However, the treatment of actinides using electronic structure calculations remains challenging, in comparison with the lighter elements of the periodic table,owing to a combination of factors: relativistic effects, strongly correlated 5\(f\) electrons, and noncollinear magnetism. For example, standard exchange-correlation functional of density functional theory (DFT), such as Local Density Approximation (LDA) and Generalized Gradient Approximation (GGA), yields over-delocalized solutions for 5\(f\) electrons, failing to correctly describe their localization behavior [7; 8]. Different approaches have been proposed to circumvent this problem. One of the most popular methods is DFT + \(U\), which involves the introduction of an empirical Hubbard \(U\) correction term to the Hamiltonian [9; 10]. This method has found high appeal because it does not increase the computational cost and can greatly improve the description of the 5\(f\) electrons; therefore, it has been widely used to calculate the bulk and surface properties of AnO\({}_{2}\)[7; 8; 11; 12; 13; 14; 15; 16]. However, searching of ground state electronic structure in DFT + \(U\) calculations presents additional difficulties [17; 18; 19; 20]. Significant experimental and computational efforts have been made to understand the surface chemistry of AnO\({}_{2}\) in the last few years. Herein, we present a brief review of recent progress in the interaction of H\({}_{2}\)O with AnO\({}_{2}\) surfaces. We start from the atomic structures of stoichiometric AnO\({}_{2}\) surfaces and their surface energies. Then, the formation of oxygen vacancies and their influence on the oxidation states of actinide atoms are discussed. Finally, we discuss the adsorption and reactions of water on both the stoichiometric and reduced AnO\({}_{2}\) surfaces with oxygen vacancies.