Materials in Nuclear Energy Systems (MiNES) 2021: Fuels and Actinide Materials- HTGR Fuels
Program Organizers: Todd Allen, University of Michigan; Clarissa Yablinsky, Los Alamos National Laboratory; Anne Campbell, Oak Ridge National Laboratory

Tuesday 1:30 PM
November 9, 2021
Room: Urban
Location: Omni William Penn Hotel

Session Chair: Elizabeth Sooby, University Of Texas At San Antonio

1:30 PM
Cluster Dynamics Simulations of Fission Gas and Product (Xe, Ag) Diffusivities in TRISO UCO Fuel Kernels : Xiang-Yang Liu¹; Christopher Matthews¹; Wen Jiang²; Michael Cooper¹; Jason Hales²; David Andersson¹; ¹Los Alamos National Laboratory; ²Idaho National Laboratory
    The UCO fuel kernels used in TRISO particles consist of a uranium dioxide (UO₂) and uranium carbide (UC₂) mixture. UC₂ is added to suppress the formation of carbon monoxide gas, however it also alters the chemistry (non-stoichiometry) of the UO₂ matrix by imposing reducing conditions, which is known to influence diffusion parameters governing gas and fission product release. In the present study, density functional theory (DFT) and empirical potential calculations are used to determine the diffusion mechanisms of Xe (fission gas) and Ag (fission product) in UO₂ under the reducing conditions generated by the presence of UC₂. The calculated diffusion parameters are first used in thermodynamic and kinetic models to predict diffusion for intrinsic conditions, after which the same information is utilized in cluster dynamics simulations to estimate the impact of irradiation on defect transport. The application of the resulting diffusion model in Bison fuel performance simulations is demonstrated.

1:50 PM
Microstructural Analysis of Oxidized Tristructural Isotropic Particles (TRISO) in Mixed Gas Atmospheres: Katherine Montoya¹; Brian Brigham¹; Tyler Gerczak²; Elizabeth Sooby¹; ¹University of Texas at San Antonio; ²Oak Ridge National Laboratory
    Certain, proposed off-normal scenarios lead to the exposure of HTGR fuel elements to steam. Subsequently, graphite matrix degradation and OPyC evolution will produce volatile gaseous oxidation products and potentially expose the TRISO particle’s SiC layer to a mixed gas atmosphere. The SiC layer acts as the structural component and primary fission product barrier; oxidation induced damage to the SiC can lead to particle failure. This study investigates the impact of steam ingress to the SiC layer by characterizing microstructure evolution of the oxidized surface. Surrogate, SiC exposed, TRISO particle oxidation testing atmospheres in the thermogravimetric analyzer included low partial pressures of steam (<0.2 atm H2O) and carbon monoxide (<1000 ppm) at high temperatures (1300慢 < T <1600 慢). Microstructural analysis of the oxidized SiO2-SiC interface was performed utilizing focused ion beam milling, scanning electron microscopy and limited transmission electron microscopy in addition to surface chemistry analysis via x-ray photoelectron spectroscopy.

2:10 PM
Oxidation Performance of High Uranium Density Fuels for Light Water Reactors: Joshua White¹; Jaime Stull¹; Scarlett Paisner¹; Tim Coons¹; Erofili Kardoulaki¹; Kenneth McClellan¹; ¹Los Alamos National Laboratory
    High uranium density fuels are currently being considered for drop in replacement in the nuclear reactor fleet. This class of fuels, such as UN, provides higher thermal conductivity and improved plant economics relative to native UO2, while also overcoming the neutronic penalties of accident tolerant based alloys. Many of the fundamental mechanisms underlying the oxidation behavior of UN has not been properly investigated in the literature to date. To this end, this work investigates the fundamental corrosion mechanisms in high temperature steam and hydrogen environments to elucidate the governing corrosion mechanisms. Application of inert coatings on UN using electroless deposition will be discussed as a potential method to mitigate washout in these fuels with steam oxidation and microstructural evaluations. Discussion will assess the practical application of coatings in a commercial setting as well as evaluating this as a method to minimizing or preventing oxidation in this class of nuclear fuels.

2:30 PM
Fabrication and Properties of Uranium Dioxide-uranium Boride Composites: Erofili Kardoulaki¹; ¹Los Alamos National Laboratory
    UO₂ composites with UB₂ and UB₄ have been proposed as advanced fuel candidates due to their high thermal conductivity, high melting point, high fissile density and their ability to incorporate a built-in burnable poison by tailoring the targeted ¹⁰B/¹¹B ratio. In this work, UO₂-UB₂ and UO₂-UB₄ composites were fabricated via spark plasma sintering. The thermal diffusivities of the samples were measured (299 to 1273 K) and were found to increase as a function of boride weight fraction. Our results confirm that UO₂ composites with either UB₂ or UB₄ have increased thermal diffusivity compared to UO₂ across the studied temperature range. Assessment of these results also indicated that in-situ reactions between the UO₂ and boride phases occur that suppress the diffusivity above 800 K. Oxidation of the boride phase was proposed as the underlying reaction and was confirmed through microstructural and crystallographic characterization performed on these samples.

2:50 PM
A Review of Current Understanding of Fluff Formation in Metallic Fuel via EBR-II Data and Modelling and Simulations.: Jake Fay¹; Fidelma Di Lemma²; Luca Capriotti²; Pavel Medvedev²; Jie Lian¹; Andrei Gribok²; Douglas Porter²; ¹Rensselaer Polytechnic Institute; ²Idaho National Laboratory
    As metallic fuel rods undergo axial swelling in a reactor setting the tops of the fuel rods begin to form a porous structure currently labeled “fluff”, which impacts source term release and core reactivity. The mechanisms and environmental factors that control fluff formation are currently unknown and need to understood and modeled for the advancement of reactor designs that utilize metallic fuels. This project is a first step in this process and primarily involves examining past experimental data from the experimental breeder reactor II experiment (EBR II) in order to build correlations between fluff formation and key factors such as operating temperature, fuel composition, and burnup. The end goal of this project will be to create accurate fluff models and to incorporate them into the BISON fuel code in order to improve simulation of metallic fuels.

3:10 PM Break