[en] We present research relating to iodine-induced stress corrosion cracking (ISCC) and draw insights relevant to the initiation process. The means by which this corrosion initiates is currently unknown. Our previous work has highlighted some of the chemical processes and properties that must be considered in ISCC, and enable us to make some possible connections in the overall corrosion mechanism. A series of calculations has been performed to better characterize the iodine interaction with zirconium, following a hybrid approach that integrates both molecular and solid-state calculations for incorporation into large scale simulations

[en] Highlights: • A cluster expansion representation of direct DFT calculations is produced. • The adsorption energy at 0 K is found to increase with coverage. • The most stable configuration at 0 K occurs around a coverage of 0.6 ML. • The adsorption isotherms were recorded from grand canonical MC simulations. • A plateau region was observed for the 300 K but not the 773 K isotherm. - Abstract: The design of corrosion resistant zircalloys is important for a variety of technological applications ranging from medicine to the nuclear industry. Since corrosion resistance is mainly attributed to the formation of a surface oxide layer, developing a detailed understanding of this process may assist in future corrosion resistance design. In this work, we conduct a systematic multi-scale investigation of the early stages of oxide formation. This was accomplished by first using a database of fully relaxed DFT calculations to build a cluster-expansion description of the potential function. The developed potential was reasonably good at predicting DFT energies as evidenced by the cross-validation score of 4.4 meV/site. The effective cluster expansion parameters were indicative of repulsive adsorbate interactions in the adlayer in agreement with the literature. The potential then allowed for a systematic investigation of the oxygen configurations on the Zr(0001) surface via Monte Carlo simulations. The adsorption energy was recorded as a function of coverage and an increasing trend was observed in agreement with DFT predictions and the repulsive nature of interactions in the adlayer. The convex hull diagram was recorded indicating the most stable configuration to occur around a coverage of 0.6 ML. The adsorption isotherm was also recorded and contrasted for two temperatures relevant for different applications.

[en] Electrocatalysis is controlled by the interplay between the active catalytic sites and the influence of their complex environment at the electrified aqueous/metal interface. The most active electrocatalytic materials exquisitely integrate the atomic assembly of the active metal sites responsible for the elementary bond making and breaking steps, together with the carbon support to carry out efficient electron transfer, and polymer electrolyte and water to facilitate proton transfer, thus establishing an optimal three-phase interface. Understanding the elementary catalytic processes along with the atomic scale features that control them, however, is obscured by the complexity of this three-phase interface and the dynamic changes that occur to it under operating conditions.

[en] Surface properties of Tc-rich and Fe-rich portions of the Tc-Fe binary alloy phase diagram were computed in this work on the basis of density functional theory. Tc and Fe were found to have minimal degrees of mixing in the parent phases, consistent with the experimentally derived phase diagram. The influence of oxygen on surface phase stability was also studied, with no significant impact on surface segregation or degree of surface mixing. Oxygen adsorption was shown to change the ordering of surface facets in Tc, such that the pyramidal phase becomes lower in energy than the prismatic phase, even with low coverage of oxygen. No evidence for increased surface segregation upon oxidation was found for the solid-solution phases. A potential-pH surface Pourbaix diagram was derived for Tc and H, OH and O adsorbed sub-monolayers were shown to be precursors to oxide formation. While Tc and Fe have similar reactivities and properties in their parent phases, and hence, also in solid-solution, the properties of the intermetallic are expected to be significantly different due to the size-mismatch between the elements.

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

[en] Highlights: • The SQS method was used to generate Ni-Cr alloy supercells and surfaces. • Mo atoms were substituted and 0.25 ML O adsorption investigated. • Cr and Mo helped stabilize O adsorption. • The local environment and the net charge transfer are for adsorption is important. • O adsorption increases workfunction for all cases. - Abstract: Nickel-based alloys are extensively utilized in high-temperature applications where corrosion resistance and mechanical strength are important. The formation of an oxide-layer on these surfaces is associated with corrosion resistance but the early stages of this process are not well understood and difficult to model. The studies in the available literature are currently limited to introducing an element such as Cr to the top layer of the nickel surface slab. While this approach has yielded some useful insights, it is not entirely accurate. In this work, we used the special quasi-random structure (SQS) method to first build reliable representations of the Ni-Cr alloy. Next, several surface slabs were cut from these alloys and oxygen adsorption at 0.25 ML coverage was investigated. It was generally observed that Cr made the oxygen adsorption process more thermodynamically favorable and the nature of the interaction was partially electrostatic. We then introduced Mo substitutions to the top layer of the surface to investigate the effect of low levels of Mo on the oxygen adsorption. It was revealed that Mo helped further stabilize the adsorbed configuration. A correlation was observed between the net charge transferred and the thermodynamic stability of the metal-oxygen system and the stability of the adsorbate on the alloy surface was mainly dependent on the local environment of the adsorbate. The subsurface atoms for the cases considered in this study provided only minor contribution to the stability of the alloy-oxygen system and hence utilizing the doping approach on the top layer may be appropriate for studying oxygen adsorption on this particular alloy surface to a good approximation but this issue requires further investigation. Finally, the workfunctions of the surfaces and the effects of oxygen adsorption on the workfunctions were also characterized and the implications of the results for corrosion resistance were discussed.

[en] Density functional theory was applied to the initial steps of uranium hydriding: surface phenomena, absorption, bulk transport and trapping. H adsorbs exothermically to the (0 0 1) surface, yet H absorption into the bulk is endothermic, with off-center octahedral absorption having the lowest absorption energy of 0.39 eV, relative to molecular H₂. H absorption in interstitial sites causes a local softening of the bulk modulus. Diffusion of H in unstrained α-U has a barrier of 0.6 eV. The energy of H absorption adjacent to the chemical impurities C, S, Si was lowered by an amount proportional to the size of the impurity atom, and the resulting lattice strain Si > S > C. Thus, impurities may promote hydriding by providing surfaces or prestrained zones for H uptake.

[en] We present the first-principle equations of state of several zirconium iodides, ZrI₂, ZrI₃, and ZrI₄, computed using density functional theory methods that apply various methods for introducing the dispersion correction. Iodides formed due to reaction of molecular or atomic iodine with zirconium and zircaloys are of particular interest due to their application to the cladding material used in the fabrication of nuclear fuel rods. Stress corrosion cracking (SCC), associated with fission product chemistry with the clad material, is a major concern in the life cycle of nuclear fuels, as many of the observed rod failures have occurred due to pellet–cladding chemical interactions (PCCI) [A. Atrens, G. Dannhäuser, G. Bäro, Stress-corrosion-cracking of zircaloy-4 cladding tubes, Journal of Nuclear Materials 126 (1984) 91–102; P. Rudling, R. Adamson, B. Cox, F. Garzarolli, A. Strasser, High burn-up fuel issues, Nuclear Engineering and Technology 40 (2008) 1–8]. A proper understanding of the physical properties of the corrosion products is, therefore, required for the development of a comprehensive SCC model. In this particular work, we emphasize that, while existing modeling techniques include methods to compute crystal structures and associated properties, it is important to capture intermolecular forces not traditionally included, such as van der Waals (dispersion) correction. Furthermore, crystal structures with stoichiometries favoring a high I:Zr ratio are found to be particularly sensitive, such that traditional density functional theory approaches that do not incorporate dispersion incorrectly predict significantly larger volumes of the lattice. This latter point is related to the diffuse nature of the iodide electron cloud

[en] In this paper we develop and apply an atomistic framework for predicting the corrosion tendencies of metallic waste forms that are based on the iron–technetium (Fe–Tc) binary system. These elements were selected due to their importance for the development of metal alloy waste forms for fission product disposition. A kinetic Monte Carlo model based on an off-lattice, modified embedded atom method (MEAM) representation of the Fe–Tc binary system was applied to understand and predict the corrosion behavior of Fe–Tc alloys, as a function of structure (phase and surface-orientation) and composition. During active dissolution, metal atoms are in the free-corrosion state, in which there is a bare metal surface exposed to the environment. The Brønsted–Evans–Polanyi relationship was applied to link atomic cohesive energies, as evaluated using the parameterized MEAM potential, to activation barriers for dissolution. The active dissolution scenario may occur in situations where the passive film has either not formed, is electrochemically unstable, or has been damaged due to the application of stress or pitting attack. Our simulations of the active dissolution process suggest that the corrosion of candidate alloy waste forms will be highly sensitive to Tc loading, as well as phase selection. Hexagonally close-packed alloys are predicted to have lower corrosion rates compared to body-centered cubic. Similarly, ordered structures appear to have a stronger corrosion resistance than randomly dispersed alloys. Finally, our results indicate an optimal loading of Tc in the alloy, which is consistent with electrochemical corrosion experiments

[en] Density functional theory calculations have been performed to provide details of the structural and charge-transfer details related to the solid solution of hydrogen in (δ)-plutonium. We follow the Flanagan model that outlines the process by which hydrogen interacts with a metal to produce hydride phases, via a sequence of surface, interstitial and defect-bound (trapped) states. Due to the complexities of the electronic structure in plutonium solid-state systems, we take the pragmatic approach of adopting the ‘special quasirandom structure’ to disperse the atomic magnetic moments. We find that this approach produces sound structural and thermodynamic properties in agreement with the available experimental data. In δ-Pu, hydrogen has an exothermic binding energy to all of the states relevant in the Flanagan model, and, furthermore, is anionic in all these states. The charge transfer is maximized (i.e. most negative for hydrogen) in the hydride phase. The pathway from surface to hydride is sequentially exothermic, in the order surface < interstitial < grain boundary < vacancy < hydride (hydride being the most exothermic state). Thus, we find that there is no intermediate state that involves an endothermic increase in energy, consistent with the general experimental observations that the hydriding reaction in plutonium metal can proceed with zero apparent activation barrier. (paper)