Ceramic Materials for Nuclear Energy Research and Applications: Microstructural Modeling of Oxide Fuels
Sponsored by: TMS Structural Materials Division, TMS: Nuclear Materials Committee, TMS: Mechanical Behavior of Materials Committee, TMS: Energy Committee
Program Organizers: Walter Luscher, Pacific Northwest National Laboratory; Xian-Ming Bai, Virginia Polytechnic Institute and State University; Lingfeng He, North Carolina State University; Sudipta Biswas, Idaho National Laboratory; Simon Middleburgh, Bangor University

Thursday 8:30 AM
March 23, 2023
Room: 28B
Location: SDCC

Session Chair: Sudipta Biswas, Idaho National Laboratory


8:30 AM  Invited
Diffusion Properties in Uranium-plutonium Mixed Oxides: Atomic Scale Investigation of the Effect of Composition and Chemical Disorder: Marjorie Bertolus1; Maria Chiara Notarangelo1; Didier Bathellier1; Michel Freyss1; Emeric Bourasseau1; Luca Messina1; 1CEA
     Mixed uranium-plutonium oxides are used as fuel in pressurized water nuclear reactors and are the reference fuel for future Generation IV sodium-cooled fast reactors. Atom transport properties are at the origin of important phenomena taking place in (U,Pu)O2 fuels during irradiation, in particular the redistribution of Pu in the fuel and the oxygen diffusion, which governs the local oxide/metal ratio. Therefore, the precise knowledge of diffusion mechanisms and coefficients, starting from the atomic scale, is crucial to model and predict the microstructure evolution of nuclear fuels in reactor.In the (U,Pu)O2 solid solution, uranium and plutonium ions are distributed randomly on the cationic sublattice, giving rise to an extremely large amount of possible configurations. We will present the results of a systematic analysis using atomic scale methods of the effect of plutonium concentration and local disorder on the formation and migration energy of radiation-induced defects, in particular bound Schottky defects.

9:00 AM  Invited
Atomic Scale Simulation of Amorphous Intergranular Films in Nuclear Fuel Materials: Michael Rushton1; Simon Middleburgh1; William Lee1; 1Bangor University
     The structure of the grain boundaries in uranium dioxide have a significant effect on nuclear fuel performance. These boundaries are often complex and highly disordered and it has been posited that, in some cases, amorphous intergranular films may form. Atomic scale simulations have been conducted to model amorphous structures for actinide oxide fuel materials. A combination of classical molecular dynamics, reverse Monte-Carlo and quantum mechanical methods were used to produce amorphous structures. Building on this, the material's propensity to deviate from stoichiometry, the magnetic structure and amorphous surface energies were computed. Non-stoichiometry was accommodated more readily in the amorphous system than the crystalline form. This indicates that stoichiometry deviations may be more readily accommodated in amorphous phases, if present, to leave a more stoichiometric crystalline phase – impacting processes including fission gas mobility, melting points and a number of other safety relevant properties.

9:30 AM  
Simulation of Irradiation-induced Bubble Over-pressurization and Application in Fuel Performance: Michael Cooper1; Christopher Matthews1; Larry Aagesen2; Chris Stanek1; David Andersson1; 1Los Alamos National Laboratory; 2Idaho National Laboratory
    It is thought that over-pressurized bubbles (in excess of the equilibrium pressure) play an important role in the susceptibility of UO2 high burnup structure (HBS) to pulverization. Cluster dynamics show that under irradiation, the dominant diffusion mechanism for U and Xe changes at low temperature compared to at high temperatures. The results indicate that irradiation-induced interstitial diffusion plays a role in bubble over-pressurization in the cooler periphery of the pellet where HBS forms. The bubble pressure is relieved by the emission of dislocation loops or by the driving force for the interstitial-bubble reaction tending to zero at high pressures. Here, we present the results of molecular and cluster dynamics simulations exploring diffusion mechanisms at low temperatures and their impact on bubble pressure. The introduction of such diffusion mechanisms into the physics-based fission gas model in BISON has also been carried out to improve the predicted bubble pressure and morphology evolution.

9:50 AM  
Multiscale Modeling for High-burnup Structure Formation in UO2: Sudipta Biswas1; Larry Aagesen1; Sophie Blondel2; Wen Jiang1; Jia-Hong Ke1; 1Idaho National Laboratory; 2University of Tennessee
    In nuclear fuel, regions exposed to extended burnup exhibit a refined grain structure with large micron-sized bubbles, known as high-burnup structures (HBS). A multiscale multiphysics model is developed to evaluate the HBS formation and associated fission gas bubble evolution at the mesoscale. Here, the intra-granular gas evolution, including gas production, diffusion, clustering, and re-solution, is tracked by the spatially resolved cluster-dynamics code Xolotl. Whereas, the microstructural evolution, including grain subdivision and inter-granular fission gas bubble evolution, is handled by the MOOSE-based phase-field models. The cluster-dynamics model obtains the interface locations from the phase-field model and transfers the xenon monomers arriving at those interfaces. The coupled model captures the formation of HBS and associated bubble evolution reasonably well. The model demonstrates that at lower temperature higher re-solution rate releases Xe monomers that, due to accelerated GB diffusion, leads to the growth of existing bubbles and depletion of Xe concentration within grains.

10:10 AM Break

10:30 AM  
Multiphysics Modeling of High Burnup UO2 at Mesoscale: Abdurrahman Ozturk1; Merve Gencturk1; David Andersson2; Wen Jiang3; Michael W.D. Cooper2; Larry Aagesen3; Mohammed Abdoelatef1; Jason Harp4; Karim Ahmed1; 1Texas A&M University; 2Los Alamos National Laboratory; 3Idaho National Laboratory; 4Oak Ridge National Laboratory
    There is a growing interest from the U.S. nuclear industry to increase the fuel peak burnup (BU). However, it is well-established that for LWR fuels, the fission gas release rate and probability of fuel fragmentation rapidly increase at HBU, particularly during thermal transients associated with DBAs. While the underlying mechanisms of this behavior are still unclear, there is a consensus that the drastic change of microstructure across the fuel pellet during normal operation through the transient holds the key for understanding these mechanisms. By combining multi-physics modeling and quantitative characterization and measurements, we shed light on the role of microstructure heterogeneity on UO2 degradation at HBU. Particularly, we couple rate-theory, phase-field, and finite-element modeling methods to fully investigate the co-evolution of microstructure and thermo-mechanical properties of HBU UO2 pellets. The coupled approach can successfully explain the difference in the response of the structured and unstructured regions of the fuels.

10:50 AM  
Quantifying the Impact of Fast Interface Diffusion and Free Surface Evolution on Fission Gas Release in UO2 Using a Phase-field Model: Md Ali Muntaha1; Michael Tonks1; Larry Aagesen2; Anders David Ragnar Andersson3; Michael William Donald Cooper3; 1University of Florida; 2Idaho National Laboratory; 3Los Alamos National Laboratory
    The transport and release of fission gas from UO2 reactor fuel has a significant impact on light water reactor safety and efficiency. This study aims to quantify the importance of interface diffusion and free surface evolution on fission gas release in UO2. We have modified a fission gas phase-field model in MOOSE to include a free surface to observe gas release and to have fast diffusion along grain boundaries and bubble surfaces. Our model predicts that incorporating fast interface diffusion changes the microstructure evolution and the fission gas release rate. Moreover, incorporating an interface thickness correction is necessary for the phase-field model to predict the behavior accurately.

11:10 AM  
Predicting Mechanical Behavior of Uranium Oxide Fuel Pellets Using Full-field Defect Diffusion Modeling in a Crystal Plasticity Framework: Aritra Chakraborty1; Conor Oscar Galvin1; Michael W.D. Cooper1; Laurent Capolungo1; 1Los Alamos National Laboratory
    Uranium-oxide (UO2) presents a great technological interest as a nuclear fuel for pressurized water reactors. Under the operating conditions of high temperatures and low-to-moderate stresses, several microstructural changes (formation of hydrogen, noble gases, etc.) can occur leading to diffusion of multiple species in the fuel pellets. This work aims to quantify the contribution from these diffusion mediated processes on the overall mechanical behavior of these pellets. As a first step, through a coupled chemo-mechanical model in a crystal plasticity framework, we predict creep response for UO2 fuel pellets considering the local defect concentration, grain size, and stoichiometry. The underlying crystal plasticity framework also accounts for the plasticity due to dislocation glide and climb— affected by the local dislocation density and defect concentration. With such full-field models local hot spots of vacancy supersaturation can be identified, acting as potential sites for damage nucleation, thus capturing failure in these systems.

11:30 AM  
Atomistic-scale Simulations used to Simulate Creep in Oxide Fuel: Conor Galvin1; Aritra Chakraborty1; Laurent Capolungo1; David Andersson1; Michael Cooper1; 1Los Alamos National Laboratory
     Doped UO2, which has been doped to produce a larger grain size than conventional UO2, has been proposed as an advanced fuel candidate. The larger grain sizes provide improved operational fuel behavior for fission gas retention and pellet-cladding interactions due to improved mechanical properties. One such property is creep.Using molecular dynamics, we predict information at the atomistic scale that can be used to develop a mechanistic UO2 creep model for use in longer time/length-scale codes. The ultimate objective of the model is to better describe the grain size dependence and impact of doping. Previously, we found that Nabarro-Herring creep was too low to capture the experimentally observed creep rates. Therefore, other mechanisms which have distinct grain size dependences have been explored. In this work, we investigate the concentration, segregation and diffusivity of various defects at grain boundaries in UO2 to explore their impact on Coble creep.

11:50 AM  
Revealing The Microstructure and Irradiation Effects on UO2 Fracture via Coupled Phase-field and MD Simulations Approach: Merve Gencturk1; Abdurrahman Ozturk1; David Andersson2; Mohammed Abdoelatef1; Larry Aagesen3; Wen Jiang3; Michael William Donald Cooper2; Karim Ahmed1; 1Texas A&M University; 2Los Alamos National Laboratory; 3Idaho National Laboratory
    Understanding the initiation and evolution of cracks is essential to enhancing the reliability of nuclear materials. Since the continuum approach does not explicitly consider the material's microstructure, this model type is not able to account directly for either microstructure or irradiation effects. Therefore, we utilize a coupled molecular dynamics and phase-field simulations to investigate the effects of point and extended defects on the fracture properties in UO2, bridging the scales from atoms to continuum. MD simulations are utilized to analyze Young's modulus, Poisson ratio, and the critical energy release rate as a function of point and extended defect densities. Informed from atomistic MD simulations, phase-field simulations are conducted to investigate the irradiation and microstructure effects at the mesoscale. It was demonstrated that dislocations and bubbles have the most pronounced effect on fracture properties. The coupled simulations also reveal the size effect in UO2 fracture.