[en] Ordering and self-organization are critical in determining the dynamics of reaction-diffusion systems. Here we show a unique pattern formation mechanism, dictated by the coupling of thermodynamic instability and kinetic anisotropy. Intrinsically different from the physical origin of Turing instability and patterning, the ordered patterns we obtained are caused by the interplay of the instability from uphill diffusion, the symmetry breaking from anisotropic diffusion, and the reactions. To understand the formation of the void/gas bubble superlattices in crystals under irradiation, we establish a general theoretical framework to predict the symmetry selection of superlattice structures associated with anisotropic diffusion. Through analytical study and phase field simulations, we found that the symmetry of a superlattice is determined by the coupling of diffusion anisotropy and the reaction rate, which indicates a new type of bifurcation phenomenon. Our discovery suggests a means for designing target experiments to tailor different microstructural patterns.

[en] Helium (He) presents one of the mayor concerns in the nuclear materials community as it modifies the mechanical properties of the system withstanding fast neutron spectra, promoting swelling and embrittlement. Ferritic/martensitic steels are one of the main candidates as structural materials for future nuclear applications. Experimentally the bubble distribution is observed to vary depending on irradiation conditions (temperature, dose rate and total dose). However, traditional atomistic models decouple the role of temperature in the mechanical properties from its effect on the bubble distribution. In this paper we study substitutional He segregation to screw and edge dislocations in α-Fe at different temperatures. We use an object kinetic Monte Carlo methodology to obtain general trends in bubble distribution and a canonical Monte Carlo algorithm, with full atomistic fidelity, to find the He distribution at the dislocation cores. Molecular dynamics has subsequently been applied to study the yield strength, which increases significantly in the presence of He, more remarkably for the edge dislocation. The total stress fits a Kocks relation. However, if the lattice resistance is subtracted, the relation between the critical shear stress and the temperature is non-monotonic for the screw character. To reproduce this effect, we propose to modify the Kocks relation, adding a second-order term in temperature that extends the range of applicability of the model

[en] A phase-field model for the U-Si system has been implemented in MARMOT. The free energies for the phases relevant to accident-tolerant fuel applications (U₃Si₂, USi, U₃Si, and liquid) were implemented as free energy materials within MARMOT. A new three-phase phase-field model based on the concepts of the Kim-Kim-Suzuki two-phase model was developed and implemented in the MOOSE phase-field module. Key features of this model are that two-phase interfaces are stable with respect to formation of the third phase, and that arbitrary phase free energies can be used. The model was validated using a simplified three-phase system and the U-Si system. In the U-Si system, the model correctly reproduced three-phase coexistence in a U₃Si₂-liquid-USi system at the eutectic temperature, solidification of a three-phase mixture below the eutectic temperature, and complete melting of a three-phase mixture above the eutectic temperature.

No abstract available

[en] Despite the extensive utilization of uranium dioxide (UO₂) as a fuel in commercial nuclear reactors, there is only minimal information regarding the fundamental nature of radiation damage at high temperatures, such as those experienced by the fuel under operation. In this work, molecular dynamics simulations have been performed to determine the threshold displacement energy (E_d) for oxygen and uranium in UO₂ at 1500 K. Three definitions of displacement energy were employed to fully study the nature of low energy radiation damage: 1) the probability of having the primary knock-on atom (PKA) leave its original lattice site, 2) the probability that the PKA will permanently displace atoms from their original lattice site, and 3) the probability of forming a stable Frenkel pair. Additionally, four unique interatomic potentials were utilized to investigate uncertainties associated with potential choice in high temperature radiation damage studies in UO₂. This work provides critical insight into the high temperature behavior of radiation damage in UO₂, as well as the variation in behavior between oxygen and uranium PKAs.

[en] Modelling and simulation of advanced nuclear reactors is a significant challenge due to the strong coupling between the various physical phenomena in such systems. As a result, the traditional operator-splitting based simulation tools must make way for a multi-scale, multi-physics simulation approach. A phenomenon of particular interest in the Molten Salt Reactor (MSR) is the corrosion of structural materials by high temperature molten salts. The Idaho National Laboratory’s Multiphysics Object Oriented Simulation Environment (MOOSE) is an open-source, finite element framework for multi-scale, multi-physics simulations of nuclear reactors and a new MOOSE-based application, namely Yellowjacket, is being developed to simulate corrosion by coupling models of electrodynamics with thermochemistry and phase field. This paper presents the design and development of a new Gibbs Energy Minimiser (GEM) within Yellowjacket, its applications in computing chemical potentials and driving forces for corrosion, and its role in enabling coupling of thermochemical equilibrium calculation in multi-scale, multi-physics simulations to model problems of interest in advanced reactors design and deployment. (author)

[en] In this work, we present a multiphysics phase field model for capturing microstructural evolution during solid-state sintering processes. The model incorporates modifications of phase field equations to include rigid-body motion, elastic deformation, and heat conduction. The model correctly predicts consolidation of powder particles during sintering because of two competing mechanisms—neck formation and grain growth. The simulations show that the material undergoes three distinctive stages during the sintering process—stage I where neck or grain boundary between two particles is formed, stage II in which neck length stabilizes and growth or shrinkage of individual particles initiates, and finally stage III with rapid grain growth leading to disappearance of one of the grains. The driving forces corresponding to different mechanisms are found to be dependent on the radius of the particles, curvature at the neck location, surface energy, and grain boundary energy. In addition, variation in temperature is found to significantly influence the microstructure evolution by affecting the diffusivity and grain boundary mobility of the sintered material. The model is also used to compare sintering simulation results in 2D and 3D. It is observed that due to higher curvature in 3D, model predicts faster microstructural evolution in 3D when compared to 2D simulations under identical boundary conditions.

[en] This report summarizes the lower-length-scale effort during FY 2016 in developing mesoscale capabilities for microstructure evolution, plasticity and fracture in reactor pressure vessel steels. During operation, reactor pressure vessels are subject to hardening and embrittlement caused by irradiation induced defect accumulation and irradiation enhanced solute precipitation. Both defect production and solute precipitation start from the atomic scale, and manifest their eventual effects as degradation in engineering scale properties. To predict the property degradation, multiscale modeling and simulation are needed to deal with the microstructure evolution, and to link the microstructure feature to material properties. In this report, the development of mesoscale capabilities for defect accumulation and solute precipitation are summarized. A crystal plasticity model to capture defect-dislocation interaction and a damage model for cleavage micro-crack propagation is also provided.

[en] The Grizzly code has been under development for the U.S. Department of Energy’s Light Water Reactor Sustainability program to provide predictive tools for the evolution of material degradation in critical light water reactor structural components due to long-term exposure to the environmental conditions of normal reactor operation, and for the effects of this degradation on the safety of these components. This development has primarily targeted reactor pressure vessels and reinforced concrete structures. Work performed during Fiscal Year 2019 added important features for both of these types of structures. For probabilistic fracture mechanics analysis of reactor pressure vessels, capabilities were added to account for the effects of warm prestressing and residual stresses, as well as to model crack initiation, growth, and arrest. For modeling microstructure evolution in reactor pressure vessels, a cluster dynamics model for evolution of Mn-Ni-Si phases has been implemented. For modeling degraded reinforced concrete structures, capabilities to consider the combined effects of damage and creep in the concrete constitutive model and model nonlinear behavior of reinforcing bars has been added. These newly-developed features fill important gaps in Grizzly’s feature set for engineering-scale analysis of degraded reactor pressure vessels and reinforced concrete structures. Grizzly development in both of these areas has reached a point where it is ready for extensive testing on relevant engineering problems.

[en] The intent of this presentation is to demonstrate the preliminary efforts made by the new INL team to the eXtreme Materials project goal of developing an engineering scale simulation of oxidation lifetime predictions.