Atkinson, Bill

Supercritical water (SCW) is the intended heat transfer fluid and potential neutron moderator in the proposed GEN-IV Supercritical Water Cooled Reactor (SCWR). The oxidative environment poses challenges in choosing appropriate design materials, and the

behaviour of SCW within crevices of the passivation layer is needed for developing a corrosion control strategy to minimize corrosion. Molecular Dynamics simulations have been employed to obtain diffusion coefficients, coordination number and surface density

characteristics, of water and chloride in nanometer-spaced iron hydroxide surfaces. Diffusion models for hydrazine are evaluated along with hydration data. Results demonstrate that water is more likely to accumulate on the surface at low density conditions. The effect of confinement on the water structure diminishes as the gap size increases. The diffusion coefficient of chloride decreases with larger surface spacing. Clustering of water at the surface implies that the SCWR will be most susceptible to pitting corrosion and stress corrosion cracking.

Author Keywords: Confinement, Diffusion, Hydration, MD Simulations, Supercritcal Water

Supercritical water (SCW) exhibits unique properties that differentiates it from its low temperature behaviour. Hydrogen bonding is dramatically reduced, there is no phase boundary between liquid and gaseous states, heat capacity increases, and there is a drastic reduction of the dielectric constant. Efforts are underway for researchers to harness these properties in the applications of power generation and hazardous waste destruction. However, the extreme environment created by the high temperatures, pressures and oxidizing capabilities pose unique challenges in terms of corrosion not present in subcritical water systems. Molecular Dynamics (MD) simulations have been used to obtain mass transport, hydration numbers and the influence on water structure of molecular oxygen, chloride, ammonia and iron (II) cations in corrosion crevices in an iron (II) hydroxide passivation layer. Solvation regimes marking the transitions of solvation based versus charge meditated processes were explored by locating the percolation thresholds of both physically and hydrogen bonded water clusters. A SCW flow through reactor was used to study hydrogen evolution rates over metal oxide surfaces, metal release rates and the kinetics for the oxidation of hydrogen gas by oxygen in SCW. Insights into corrosion phenomena are provided from the MD results as well as the experimental determination of flow reactor water and hydrogen chemistry.

Author Keywords: Flow Studies, Molecular Dynamics, Supercritical Water

We build a theoretical model for exploring the electronic properties of the two-dimensional (2D) electron gas that forms at the interface between insulating SrTiO3 (STO) and a number of perovskite materials including LaTiO3, LaAlO3, and GdTiO3. The model treats conduction electrons within a tight-binding approximation, and the dielectric polarization via a Landau-Devonshire free energy that incorporates STO's strongly nonlinear, nonlocal, field-, and temperature-dependent dielectric response. We consider three models for the dielectric polarization at the interface: an ideal-interface model in which the interface has the same permittivity as the bulk, a dielectric dead-layer model in which the interface has permittivity lower that the bulk, and an interfacial-strain model in which the strain effects are included.

The ideal-interface model band structure comprises a mix of quantum 2D states that are tightly bound to the interface, and quasi-three-dimensional (3D) states that extend hundreds of unit cells into the STO substrate. We find that there is a substantial shift of electrons away from the interface into the 3D tails as temperature is lowered from 300 K to 10 K. We speculate that the quasi-3D tails form the low- density high-mobility component of the interfacial electron gas that is widely inferred from magnetoresistance measurements.

Multiple experiments have observed a sharp Lifshitz transition in the band structure of STO interfaces as a function of applied gate voltage. To understand this transition, we first propose a dielectric dead-layer model. It successfully predicts the Lifshitz transition at a critical charge density close to the measured one, but does not give a complete description for the transition. Second, we use an interfacial-strain model in which we consider the electrostrictive and flexoelectric coupling between the strain and polarization. This coupling generates a thin polarized layer whose direction reverses at a critical density. The transition occurs concomitantly with the polarization reversal. In addition, we find that the model captures the two main features of the transition: the transition from one occupied band to multiple occupied bands, and the abrupt change in the slope of lowest energy band with doping.

Johri and Bhatt found singular behavior near the band edge in the density of states as well as in the inverse participation ratio of the Anderson model. These singularities mark a transition to an energy range dominated by resonant states. We study the interacting case using an ensemble of two-site Anderson-Hubbard systems. We find the ensemble-averaged density of states and generalized inverse participation ratio have more structure than in the non-interacting case because there are more transitions and in particular the transitions depend on the ground state. Nonetheless, there are regions of sharp decline in the generalized inverse participation ratio associated with specific density of state features. Moreover these features move closer to the Fermi level with the addition of interactions making them more experimentally accessible. Unfortunately resonances unique to interacting systems cannot be specifically identified.

Author Keywords: Correlated electrons, Disorder, Localization