Computational Materials Science and Engineering of Materials in Nuclear Reactors: Chemical Interactions and Modeling
Sponsored by: TMS Structural Materials Division, TMS: Nuclear Materials Committee
Program Organizers: Dilpuneet Aidhy, Clemson University; Michael Tonks, University of Florida; Mahmood Mamivand; Giovanni Bonny, Belgian Nuclear Research Center
Tuesday 8:30 AM
February 25, 2020
Room: Theater A-9
Location: San Diego Convention Ctr
Session Chair: Jaime Marian, UCLA; Mike Tonks, University of Florida
8:30 AM Invited
Corrosion of Silicon Carbide in Nuclear Environments: Izabela Szlufarska1; Jianqi Xi1; Cheng Liu1; Dane Morgan1; 1University of Wisconsin-Madison
Due to its excellent physical and chemical properties, SiC has been known for its potential as a structural material in nuclear reactors. One of the remaining issues that can limit usability of this material is corrosion. In this talk, I will discuss our theoretical studies of SiC corrosion exposed to molten salt and hydrothermal water. For molten salt, we show that Si dissolves into the salt much easier than C, thus leaving a C-rich layer during corrosion. A strategy to suppress SiC corrosion is discussed based on Be doping of the coolant. For hydrothermal water, we identify mechanisms of water attack on SiC surface. Specifically, we discover that attack of water on SiC occurs by a hydrogen scission reaction in which H breaks a Si-C backbond and leads to disordering of the surface. Hydrogen scission occurs originates from a new metastable species (bridging hydroxyl group) also discovered in our study.
9:10 AM
Modeling of Interface Evolution during Zirconium Alloy Corrosion: Natalia Tymiak Carlson1; Richard Smith1; Bruce Kammenzind1; 1Naval Nuclear Laboratory, Bettis Site
A fundamental description of the evolution of the metal/oxide interface is central to the development of a mechanistic model of zirconium alloy corrosion. Despite continuing advances in the characterization of oxide films produced under various oxidation conditions, an understanding of whether observed cracks, grain boundary porosity, and chemical segregation either result from or facilitate the growth process is still lacking. Corrosion models focus almost exclusively on phenomena affecting charge carrier transport. Attention has to be paid to the structural rearrangements and chemical reactions at the moving interface if a self-consistent picture of the evolving structure/phase/stress/stoichiometrty, and oxidation rate is the ultimate goal. A synergy of atomistic and continuum scale models is utilized to evaluate the impact of interlinked stress, stoichiometry, phase content, impurities, and structural defects of zirconium oxide at the metal/oxide interface on the local oxidation rates and evolving interface topography.
9:30 AM
Influence of Coordination Numbers on Representing Molten Salts for Nuclear Reactor Applications Using the Modified Quasi-Chemical Model (MQM): Matthew Christian1; Theodore Besmann1; 1University of South Carolina
Accurately modeling the thermochemical properties of fuel/coolant mixtures in molten salt reactors (MSRs) is critical to simulating operational conditions and accidental scenarios. The modified quasi-chemical model (MQM) is frequently used to represent the salt’s molten state because it accounts for short-range ordering that arises from strong ionic interactions via atomic coordination numbers. Coordination numbers for each atomic species are typically approximated from representative crystal solids and may not be reflective of the molten state. Therefore, the short-range ordering imposed by the coordination numbers may produce inaccurate phase relations. This presentation will show how coordination numbers, and hence short-range ordering, influences modeled phase diagrams using the MQM and how a of realistic understanding can be implemented.
9:50 AM
Amorphous Zirconia: a Host for Excess Oxygen in Cladding Barrier Oxides?: Simon Middleburgh1; Michael Rushton1; Iuliia Ipatova1; Lee Evitts1; William Lee1; 1Nuclear Futures Institute
Amorphous zirconia (a-ZrO2) has been simulated using a synergistic combination of state-of-the-art methods: employing reverse Monte-Carlo, molecular dynamics and density functional theory together. This combination has enabled the complex chemistry of the amorphous system to be efficiently investigated. Notably, the a-ZrO2 system was observed to accommodate excess oxygen readily – through the formation of neutral peroxide defects – a result that has implications not only in the a-ZrO2 system, but also in other systems employing network formers, intermediates and modifiers. The structure of the a-ZrO2 system was also determined to have edge-sharing characteristics similar to structures reported in the amorphous TeO2 system and other chalcogenide-containing glasses. Additionally, yttria stabilized zirconia has been investigated to shed light on the impact of dopants in the zirconia system. Here, modelling results are corroborated by targeted experimental observations from X-ray diffraction and Raman spectroscopy.
10:10 AM Break
10:30 AM Invited
A Physical Model of Zircaloy Corrosion in Water for Simulating Nuclear Reactor Clad Response: Jaime Marian1; Qianran Yu1; Peng Wang2; Michael Reyes1; Asghar Aryanfar3; Gary Was2; 1UCLA; 2University of Michigan; 3Bahçeşehir University
The long-term integrity of the fuel elements in the reactor core is highly dependent on the response of Zirconium cladding to corrosion, and it is therefore imperative that reliable models of oxide scale buildup be devised. In this work, we present a model of the evolution of the Zr oxide layer accounting for thermo-migration and electro-migration of oxygen transport in the clad, in addition to standard thermally-activated diffusion. As well, the model is guided by quantum mechanical calculations of the stability of zirconium-oxygen compounds. Irradiation is captured in the form of radiation enhanced diffusion, with model predictions informed by well-controlled proton irradiation experiments. Finally, we also provide results of the formation and growth of hydride platelets below the oxide layer using kinetic rate theory models fitted with atomistic data.
11:10 AM
Atomistic Studies of Nuclear Materials with Temperature: Uranium Nitride and Thermocouples: Ember Sikorski1; Lan Li1; 1Boise State University
Understanding the mechanistic atomic behavior of nuclear materials requires a consideration of temperature in a reactor. To predict the behavior of uranium nitride in the event of cladding failure, we have modeled uranium nitride surfaces in the presence of water at a high temperature. Additionally, studying the behavior of new nuclear materials in research reactors requires robust instrumentation. High Temperature Irradiation Resistant thermocouples (HTIR-TCs) outlast traditional TCs in the reactor environment, but they can still exhibit signal drift. We have studied the interaction of Nb HTIR leg with Al2O3 insulation and molecular oxygen. Our results advance an understanding of mechanism of oxygen diffusion into Nb, which ultimately leads to signal drift.
11:30 AM
Stabilizing Gamma Hydrides in Zr through Mechanical Stress: Jacob Bair1; Nicole Overman1; Shawn Riechers1; Ewa Ronnebro1; David Collins1; David Abrecht1; 1Pacific Northwest National Laboratory
Recent studies have indicated that certain mechanical stresses may stabilize otherwise metastable hydride phases in Zr. The delta phase is extremely brittle and leads to issues with the mechanical integrity of nuclear fuel rod claddings. The metastable zeta and gamma phases are much higher in ductility, making them preferable to the brittle delta phase. In this study, experimental and computational approaches are used to determine stress and heat treatments which could stabilize gamma hydrides. A phase field model tests the stability of gamma hydrides in several common reactor temperature conditions with a variety of applied stresses. Samples from rolled Zr sheets charged to varying H concentrations are treated according to the modeling parameters and studied with EBSD to determine the fraction of each hydride phase. Fiducial marks are used to study the same local areas with AFM to obtain a microstructural map of local hardness correlated with each phase.