Advanced Characterization and Modeling of Nuclear Fuels: Microstructure, Thermo-physical Properties: Thermo-physical and Microstructure Properties of TRISO and ThO2
Sponsored by: TMS Structural Materials Division, TMS: Advanced Characterization, Testing, and Simulation Committee, TMS: Energy Committee, TMS: Nanomechanical Materials Behavior Committee, TMS: Nuclear Materials Committee
Program Organizers: David Frazer, Idaho National Laboratory; Fabiola Cappia, Idaho National Laboratory; Tsvetoslav Pavlov, Idaho National Laboratory; Peter Hosemann

Wednesday 2:00 PM
March 2, 2022
Room: 202B
Location: Anaheim Convention Center

Session Chair: David Frazer, Idaho National Laboratory

2:00 PM  
Advanced Characterization of Oxidation Behavior of TRISO Fuel SiC Coating: Haiming Wen1; Adam Bratten1; Visharad Jalan1; 1Missouri University of Science and Technology
    While high-temperature gas reactors use helium as a coolant, in some accident scenarios significant amounts of air or water vapor can be introduced into the coolant and reactor core. It is important to understand the oxidation and degradation processes of TRISO particles (especially the SiC coating layer) under these conditions. In this study, surrogate TRISO particles were subjected to oxidation in oxygen or water vapor containing environments, using thermogravimetric analysis – differential scanning calorimetry, custom-built tube furnace setup, or in-situ transmission electron microscope. Kinetic parameters of oxidation were measured through the oxidation studies at different temperatures with different partial pressures of oxidants. The microstructures of the SiC coating and the oxide layer after oxidation were carefully characterized via different techniques including scanning electron microscopy, focused ion beam, transmission electron microscopy, and atom probe tomography. The oxidation mechanisms were ascertained in relation to the oxidation conditions and microstructures of the materials.

2:20 PM  
Microstructural Characterization of the Porous Pyrocarbon Buffer Layer in TRISO Fuel Particles: Claire Griesbach1; Tyler Gerczak2; Kumar Sridharan1; Yongfeng Zhang1; Ramathasan Thevamaran1; 1University of Wisconsin-Madison; 2Oak Ridge National Laboratory
    Tristructural isotropic (TRISO) nuclear fuel particles are a robust high temperature fuel architecture; however, failure occurs in rare cases—commonly beginning in the porous pyrocarbon buffer layer nearest the uranium kernel. Understanding the buffer-initiated failure mechanisms requires characterization of the buffer porosity and mechanical properties, which have been relatively unexplored to date. We perform slice-and-view experiments using a dual-beam FIB-SEM to produce 3D reconstructions of the buffer microstructure. Volumes of ~153 μm3 are analyzed at several regions within the buffer layer and on multiple near-representative surrogate particles to investigate the spatial and particle-to-particle variability of the pore structure. We find that the average porosity is significantly lower than the commonly used 50% theoretical density estimate, and the porosity varies along the radial direction. Characterization of the buffer microstructure, along with mechanical properties obtained by nanoindentation testing, will inform multiscale models designed to predict the failure of TRISO particles under irradiation.

2:40 PM  
Correlating Atomic Scale Microstructure with Mechanical Properties in Low-density Pyrocarbon Used in TRISO Particle Fuel Buffer Layer: Yongfeng Zhang1; Claire Griesbach1; Ramathasan Thevamaran1; Kumar Sridharan1; Tyler Gerczak2; Wen Jiang3; Karim Ahmed4; 1University of Wisconsin; 2Oak Ridge National Laboratory ; 3Idaho National Laboratory ; 4Texas A&M University
    TRISO fuel is a leading fuel design for advanced high temperature reactors. A critical component to its irradiation performance is the buffer layer consisting of low-density porous pyrocarbon, whose actual matrix density and atomic-scale structure are unknown. Guided by 3D experimental characterization, atomic-scale models were constructed using molecular dynamics simulations for low-density pyrocarbon. The constructed pyrocarbon contained nano-sized, graphite-like crystallites of different orientations, connected by disordered regions with atoms bonded differently than graphite. The atomic-scale structure was quantified by fractions of sp2 and sp3 bonded atoms. A new descriptor named atomic orientation was introduced for graphite-like atoms, and its spatial correlation and 3D distribution were used to determine the crystallite size and the degree of anisotropy. The atomic-scale structure was further correlated with the elastic moduli also from molecular dynamics. This work is a part of a larger multiscale effort for quantifying and correlating buffer layer microstructure with mechanical properties.

3:00 PM  
An Atomistically-informed Cluster Dynamics Approach for Defect Evolution in ThO2 under Irradiation: Sanjoy Mazumder1; Maniesha Singh1; Tomohisa Kumagai1; Anter El-Azab1; 1Purdue University
    Irradiation induced microstructural defects significantly reduce the thermal conductivity in oxide fuels like ThO2 and UO2 via phonon interactions. Thus, accurately predicting the concentration of point defects and clusters i.e. loops and voids, is paramount in studying their effect on the phonon transport mechanisms. Using an atomistically-informed cluster dynamics (CD) model for ThO2, we have systematically studied the agglomeration of point defects having formal charge i.e. Oi-2, Thi+4, VO+2 and VTh-4 into clusters over a wide range of stoichiometry. The energetic and kinetic parameters governing the CD model are the binding energy of individual point defect to clusters and its mobility in the ThO2 matrix, respectively. Using MD, with the interatomic potential developed by Cooper et al., we have built the binding energy landscape in the composition space of defect clusters. Temperature accelerated dynamics (TAD) of point defects in ThO2 have been performed to compute their diffusivities.