Computational Materials Science and Engineering of Materials in Nuclear Reactors: Thermomechanical Properties 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 2:00 PM
February 26, 2020
Room: Theater A-9
Location: San Diego Convention Ctr
Session Chair: Mahmood Mamivand, Boise State; Kelvin Xe, TAMU; Patrick Burr, UNSW
2:00 PM Invited
Thermochemical and Phase Equilibria (CALPHAD) Modeling of Nuclear Fuel Materials: A Constant in Reactor Development: Theodore Besmann1; 1University of South Carolina
The earliest development of nuclear energy and weapons required understanding the thermodynamics and phase equilibria of actinide and related systems, with references dating as early as 1943. Standard fluorite-structured UO2 fuel is itself complex, exhibiting substantial hyper- and hypo-stoichiometry and has been the subject of modeling efforts for well over 50 years. That coupled with the generation of fission product elements, high temperatures and a steep thermal gradient, and radiation make it exceptionally challenging to thermochemically understand oxide fuel behavior. Similar comments can be made regarding the various alternative fuel systems explored over the decades, and some of which are being considered today in accident tolerant fuel options such as silicide, borides, nitrides, and carbides, metallic alloys, and molten salts. This paper will review thermochemical modeling of oxide fuels and remaining challenges, and explore the state of understanding for alternatives such as actinide silicides and molten salts.
2:40 PM
Recent Development of Thermochimica for Simulations of Nuclear Materials: Max Poschmann1; Bernard Fitzpatrick1; Srdjan Simunovic2; Markus Piro1; 1University of Ontario Institute of Technology; 2Oak Ridge National Laboratory
The open-source equilibrium thermochemistry library Thermochimica has previously been employed to study uranium dioxide nuclear fuel for light-water reactor applications. Recently, significant improvements to the efficiency and range of applications of Thermochimica have been made. We will discuss these advances and demonstrate applications of Thermochimica for next-generation nuclear technologies, such as MSRs and TRISO fuels. Calculations on popular molten salt fuel materials, such as FliNaK, FliBe and fission product containing salts, have been enabled through the implementation of the quadruplet approximation to the modified quasichemical model in Thermochimica, which takes into account first and second-nearest-neighbor short-range ordering contributions to the Gibbs energies of liquid solution phases. Coupling of Thermochimica to various other software packages, such as the Multiphysics Object Oriented Simulation Environment (MOOSE), Coolant-Boiling in Rod Arrays - Two Fluids(CTF), Virtual Environment for Reactor Applications (VERA), and Oak Ridge Isotope GENeration (ORIGEN) for nuclear fuel applications will also be demonstrated.
3:00 PM
Thermodynamic Properties at the Rim in High Burnup UO2 Fuels: Dillon Frost1; Jessica Veliscek-Carolan2; Conor Galvin1; Edward Obbard1; Michael Cooper3; Patrick Burr1; 1UNSW; 2ANSTO; 3Los Alamos National Laboratory
Permanent bonding between UO2 and zirconium alloy cladding occurs at burnups exceeding 47 GWd/t. A solid solution of (U,Zr)O2 is produced. A combinatorial approach of sol-gel synthesis and MD simulations was used to examine the thermodynamic properties of (U,Zr)O2. Increasing the concentration of Zr in the (U,Zr)O2 mixtures resulted in a reduction of thermal conductivity at room temperature which became negligible at reactor operating temperatures. Tetragonal ZrO2 is found at the rim at moderate (20-30 GWd/t) burnups which results in a significantly reduced thermal conductivity at the rim compared to that of the (U,Zr)O2 mixtures. The (U,Zr)O2 layer at the pellet-clad interface is somewhat beneficial as it provides a thermal conductivity which is an order of magnitude higher than that of He and fission gases.
3:20 PM
Atypical Melting Behaviour of (Th,U)O2, (Th,Pu)O2 and (Pu,U)O2 Mixed Oxides: Conor Galvin1; Patrick Burr2; Michael Cooper3; Paul Fossati4; Robin Grimes5; 1UNSW + Imperial; 2UNSW Sydney; 3Los Alamos National Laboratory; 4CEA Gif-sur-Yvette; 5Imperial College London
Numerous discrepancies and anomalies have been reported in the melting behaviour of the ternary UO2–ThO2–PuO2 system, sometimes attributed to limitations of the experimental set-up. Here we use molecular dynamics to shed light on the peculiar melting behaviour of the mixed oxides. We show that minima in solidus and liquidus are indeed observed at ∼5% additions of the oxide with higher melting point, as proposed in some experiments, and that this is an inherent property of the solid solutions, not an artefact of experimental procedures. We propose that the root cause for this reduction in melting point is the enhanced formation of Frenkel pairs. The energy of Frenkel pairs is observed to reduced at low solute concentrations due to the presence of small amounts of cations of larger ionic radius, causing local distortion in the lattice. These defects may act as nucleation sites for early-onset melting.
3:40 PM Break
4:00 PM Invited
Developing Capabilities to Investigate the Effect of Curvature on the Radiation Response of Solid-state Interfaces: Sisi Xiang1; Thien Duong1; Emmeline Sheu1; Michael Demkowicz1; Kelvin Xie1; 1Texas A&M University
We describe an ongoing effort to elucidate the effect of curvature on the radiation response of solid-state interfaces. This work integrates modeling and experiments to discover how curvature-induced behavior changes with interface structure. We begin by selecting a set of model heterophase interfaces that pair Cu with another FCC metal or with a BCC metal. We then develop a set of atomic potentials to describe all these binary systems. Transmission electron microscopy is used to investigate samples with highly curved interfaces after implantation with helium (He), focusing on the effect of composition and interface structure on the resulting He bubble microstructure. Work towards the convergence of experimental and modeling investigations will be discussed.
4:40 PM
Analyzing U-Zr Experimental Data Using Quantitative Phase-field Simulation and Sensitivity Analysis: Michael Tonks1; Jacob Hirschhorn1; Assel Aitkaliyeva1; Cynthia Adkins2; 1University of Florida; 2Idaho National Laboratory
Uranium-zirconium (U-Zr) is a candidate fuel for nuclear fast reactors. An improved mechanistic understanding of the diffusion and phase behavior of the alloy will allow us to design safer and more efficient fuel. In this work, we apply quantitative phase field simulations based on thermodynamic data to analyze existing diffusion couple data of unirradiated U-Zr and constituent redistribution in irradiated U-Zr fuel rods. We fit various parameters in the models by comparing to the available data. We then use sensitivity analysis to determine which parameters have the largest impact on the simulation predictions, in order to direct future experiments.
5:00 PM
Mesoscale Modeling and Experiments for Predicting the Thermal Conductivity of UZr Fuels: Karim Ahmed1; Sean Mcdeavitt1; Mitchell Meyer2; 1Texas A&M University; 2Idaho National Laboratory
The effective thermal conductivity of nuclear fuels strongly depends on the underlying microstructure. We conducted a combined experimental and computational work to investigate this relationship in UZr fuel pellets. A combined phase-field and finite-element model was developed to simulate effective thermal conductivity of these pellets. The model accounts for the thermal resistance of the interfaces and able to predict the effective conductivity of the fuel for different fuel compositions, microstructures, and temperatures. The model has been implemented in the MOOSE framework. A companion experimental work was also conducted to correlate the effective thermal conductivity of U-10Zr pellets measured using the laser flash method and the underlying microstructure quantified via 3D X-ray tomography. The model was validated against the experimental data. The model results agree well with the experimental data obtained in this work and from literature.