Computational Materials Science and Engineering of Materials in Nuclear Reactors: Defects 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

Wednesday 8:30 AM
February 26, 2020
Room: Theater A-9
Location: San Diego Convention Ctr

Session Chair: Kai Nordlund, University of Helsinki; Mike Tonks, University of Florida


8:30 AM  Invited
The Use of Molecular Dynamics Simulations for Modeling Gas - Point Defect Interaction Behavior in Nuclear Materials: Brian Wirth1; 1University of Tennessee
    The molecular dynamics (MD) simulation technique was initially developed to study defect creation during radiation damage in nuclear materials. In this presentation, I will provide an overview of recent molecular dynamics results, which highlight how the exponential increase in computational power has enabled studying much larger, more complex nuclear materials for times to address questions associated with displacement cascades in metals and fission product track interactions with fission gas bubbles in nuclear fuel. I will then address how the use of sustained duration MD simulations have simulated in excess of 1 micro-second to produce insight into the the nucleation and growth of high pressure helium bubbles in fusion plasma facing components, and the interactions between hydrogen and helium with bubbles and defects. The presentation will also discuss gaps in our ability to simulate longer time scale processes and important open questions that can be addressed by atomistic modeling.

9:10 AM  
Density Functional Theory Study of He/H Effect in W-Ni-Fe Composite for Plasma Facing Material: Wahyu Setyawan1; 1Pacific Northwest National Laboratory
    Ductile-phase-toughened tungsten composites are explored as plasma-facing materials for future fusion reactors. An example composite employs a ductile face-centered cubic random solid solution of Ni-Fe-W as a binder phase for W grains. W-Ni-Fe composites with a total Ni+Fe content of 3-10 wt.% (Ni:Fe ratio of 7:3) exhibit improved fracture toughness compared to tungsten. The toughening relies on the cohesion of the interphase boundary between W grains and the binder. In fusion, plasma-facing materials are exposed to He and H isotope irradiations. Therefore, understanding the effect of such isotopes on the interphase boundaries is important. We employ density functional theory to study the He/H effect in W-Ni-Fe composites. Segregation behavior and cohesion/decohesion effect will be explored. Furthermore, simultaneous effects of displacement damage due to neutron irradiations will be considered by introducing tungsten self-interstitial atoms or vacancies at the boundaries. The effect of He/H with and without neutron irradiation will be compared.

9:30 AM  
Ab-initio Molecular Dynamics Simulations of bcc U and U-Zr Alloys: Benjamin Beeler1; David Andersson2; Yongfeng Zhang1; 1Idaho National Laboratory; 2Los Alamos National Laboratory
    Uranium-zirconium (U-Zr) alloy fuels have a history of usage in sodium-cooled fast reactors and have recently regained interest due to the possibility of incorporating minor actinides into the fuel in order to reduce the quantity of long-lived radioisotopes generated as nuclear waste. Despite decades of irradiation experience on U-Zr alloy fuels, there still exists a relative dearth of fundamental property information. Under operation, the interior of the metallic fuel pin exists in the high-temperature body-centered cubic phase of U-Zr. This isotropic phase is particularly difficult to investigate, both experimentally and computationally, due to its low temperature mechanical instability. High performance computing resources are now allowing for the investigation of high temperature systems utilizing a density functional theory framework with minimal assumptions. In this work, the first ab initio molecular dynamics simulations are performed on body-centered cubic U and U-Zr alloys.

9:50 AM  
Diffusion and Interaction of Prismatic Dislocation Loops in Stochastic Dislocation Dynamics: Yang Li1; Max Boleininger2; Christian Robertson1; Laurent Dupuy1; Sergei Dudarev2; 1DEN-Service de Recherches Métallurgiques Appliquées; 2Culham Centre for Fusion Energy
     BCC metals exposed to irradiation form mobile prismatic loops of <111> type, that in many cases represent the dominant type of radiation defects. As the microstructure of a fusion reactor component is expected to coarsen significantly over the service lifespan, a desired outcome of fusion materials research is predicting the fundamental laws of evolution governing this process.The motion of <111> loops is driven by stochastic forces associated with thermal fluctuations. We find that the stochastic force in conjunction with internal degrees of freedom is a factor fundamentally affecting the dynamics of elastically interacting loops. The trapping reaction between loops depends critically on the internal re-orientation of their habit planes during the reaction, leading to increases in the lifetime of bound states by orders of magnitude. Inclusion of internal degrees of freedom makes the fundamental difference between loop rafts remaining bound and stable, or separating on experimentally relevant timescales.

10:10 AM Break

10:30 AM  Invited
Development and Testing of Machine Learning Interatomic Potentials for Radiation Damage Calculations: Kai Nordlund1; Ali Hamedani1; Jesper Byggmästar1; Flyura Djurabekova1; 1University of Helsinki
    Machine learning interatomic potentials have gained major attention in recent years due to their increased flexibility compared to analytical potentials. However, most potentials are trained against equilibrium properties only, and it is not clear whether they also work in radiation damage calculations. We have now taken into use the Gaussian approximation potentials (GAP) framework for radiation damage calculations in Si and W. Tests of the as-published potentials showed that they do not describe the high-energy repulsive part of the interaction in any realistic manner, and hence it is crucial to modify the GAP framework for dealing with radiation effects. In the talk, we describe our approach for doing this and show initial results on damage calculations in low-energy cascades.

11:10 AM  
Molecular Dynamics Simulations of Mixed Materials in Tungsten: Mary Alice Cusentino1; Mitchell Wood1; Aidan Thompson1; 1Sandia National Laboratories
    Tungsten and beryllium are both used in fusion reactors, as divertor and first wall materials, respectively. Beryllium transported from the first wall deposits on the tungsten divertor. Linear plasma experiments of beryllium deposition on tungsten indicate the formation of tungsten/beryllium intermetallics, which degrade the material properties and performance of the divertor. To study this phenomenon at the microscopic scale, we have used the Spectral Neighbor Analysis Potential (SNAP) approach to develop a high accuracy machine learning interatomic potential for molecular dynamics simulations of beryllium implantation in tungsten. Results indicate that both amorphous and ordered tungsten/beryllium structures form near the surface that are precursors to intermetallic formation. Beryllium is observed to mix with the tungsten substrate through an exchange mechanism. These results show the early stages of formation of tungsten/beryllium surface layers that provide unprecedented insight into experimental observations. SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525.

11:30 AM  
Plasticity of Zirconium Hydrides: an Edge and Screw Planar Discrete Dislocation Model: Luca Reali1; Daniel Balint1; Mark Wenman1; Adrian Sutton1; 1Imperial College London
     Hydrides are a major concern for the safety of Zr-alloys. Usually considered as a brittle phase embedded in a ductile matrix, micro-hydrides can withstand significant plastic deformation without cracking. Both screw and edge dislocations satisfy the criteria for slip transmission. Both cases are analysed using planar discrete dislocation plasticity (DDP). Key parameters are the strength of the interface and the internal friction. They control where dislocations accumulate, and therefore where cracks are likely to be initiated. It is argued that, with increasing hydride thickness, the transformation strain leads to forest hardening and to very high internal dislocation densities. Furthermore, interfacial dislocations at the semi-coherent interface may promote the formation of glide dislocation pile-ups in front of the hydride. The two scenarios are explored varying the hydride thickness.Novel aspects: the implementation of a screw 2D-DDP, and its combination with conventional edge 2D-DDP for a three-dimensional stress analysis.