Ceramic Materials for Nuclear Energy Research and Applications: Fundamental Defect Science in Ceramics and Thermal Transport
Sponsored by: TMS Structural Materials Division, TMS: Advanced Characterization, Testing, and Simulation Committee, TMS: Energy Committee, TMS: Nuclear Materials Committee
Program Organizers: Xian-Ming Bai, Virginia Tech; Yongfeng Zhang, Idaho National Laboratory; Maria Okuniewski, Purdue University; Donna Guillen, Idaho National Laboratory; Marat Khafizov, Ohio State University; Thierry Wiss, European Commission- JRC -Institute of Transuranium Elements – Germany

Tuesday 8:30 AM
February 28, 2017
Room: Palomar
Location: Marriott Marquis Hotel

Session Chair: Blas Uberuaga, Los Alamos National Laboratory; Marat Khafizov, Ohio State University

8:30 AM  Invited
Radiation Damage on UO2 and UN: Lingfeng He1; Jian Gan1; Marquis Kirk2; Beata Tyburska-Pueschel3; Brian Jaques4; 1Idaho National Laboratory; 2Argonne National Laboratory; 3University of Wisconsin-Madison; 4Boise State University
    Uranium dioxide (UO2) has a fcc structure of CaF2 type and it is the most widely used nuclear fuel in commercial light water reactors. Uranium mononitride (UN) has a fcc structure of NaCl type and it shows higher fissional density, thermal conductivity, melting temperature but lower oxidation resistance than UO2. The cumulative radiation damage during the fission process causes severe degradation in the thermophysical properties of UO2 and UN fuels. In this work, transmission electron microscopy (TEM) observation of defect nucleation and evolution under ion irradiation was conducted to understand the radiation damage mechanisms in UO2 and UN. Irradiation induced microstructure, including dislocation loops, inert gas bubbles, and lunar crater features are characterized using high resolution transmission electron microscopy (HRTEM), scanning transmission electron microscopy (STEM) equipped with electron dispersive X-ray spectroscopy (EDS) and electron energy loss spectroscopy (EELS) as well as atom probe tomography (APT).

9:00 AM  
Five-dimensional Representation of Grain Boundary Energies in UO2: Yongfeng Zhang1; Timothy Harbison2; Jarin French2; Joseph Carmack3; Evan Hansen2; 1Idaho National Lab; 2Brigham Young University-Idaho; 3University of Arkansas
    Following a recent theory proposed for face-centered-cubic metals, in this work a five dimensional grain boundary energy model has been established for uranium dioxide. The parameters used in the model have been fitted to the data computed with molecular dynamics simulations, using which the energies of over 170 grain boundaries are calculated. To search for the minimum energy structure, finite temperature annealing is used with the gamma surface mapping in the molecular dynamics simulations. The model fits well to the computed data, and it reproduces well the energies of a separate set of grain boundaries that are not included in the fitting procedure. The model has been directly usable in continuum scale modeling such as phase field simulations after further derivation. This five dimensional grain boundary energy model will be useful for modeling the anisotropic effects of grain boundaries on the microstructure evolution and the thermal-mechanical properties of nuclear fuels.

9:20 AM  
Study of Point and Extended Defects in Fluorite UO2 with Variable Charges Empirical Potentials: Aurélien Soulié1; Jean-Paul Crocombette1; Emmanuel Clouet1; Frederico Garrido1; 1Comissariat à l'Energie Atomique
    Point and extended defects in fluorite uranium dioxide have been analyzed using two variable charges empirical potentials: COMB and SMTBQ. These potential allow us to study charge variations in extended defects. Neither classic empirical potentials (because of their fixed charges on ions) nor density functional theory (because of too high computational costs) can perform such simulations. Point defects have been first analyzed, and the ability of such potentials to simulate different charge states is discussed. Secondly, extended defects are studied, starting with a computation of generalized stacking faults energies (GSFEs) for different planes. Preferential glide planes for dislocations and the atomic arrangements of stable stacking faults are presented. Core structures for the possible dislocations are then studied with both potentials. We conclude on the ability of these variable charge potentials to simulate the plasticity of UO2.

9:40 AM  
The Roles of Surfaces, Chemical Interfaces, and Disorder on Plutonium Incorporation in Pyrochlores: Romain Perriot1; Pratik Dholabhai1; Blas Uberuaga1; 1Los Alamos National Laboratory
    Pyrochlores, with formula A2B2O7, are one of the candidates for nuclear waste encapsulation, due to the natural occurrence of actinide-bearing pyrochlore minerals and laboratory observations of high radiation tolerance. Here, we use atomistic simulations to determine the role of surfaces, chemical interfaces, and cation disorder on the plutonium immobilization properties of pyrochlores. We find that both Pu3+ and Pu4+ segregate to the four low-index pyrochlore surfaces considered. We also find that pyrochlore/pyrochlore bicrystals A2B2O7/A’2B’2O7 can be used to immobilize Pu3+ and Pu4+ either in the same or separate phases of the compound, depending on the chemistry of the material. Finally, we find that Pu4+ segregates to the disordered phase of an order/disorder bicrystal, while Pu3+ only slightly favors the disordered phase. Together, these results provide new insight into the ability of pyrochlore compounds to encapsulate Pu and suggest new considerations in the development of waste forms based on pyrochlores.

10:00 AM Break

10:20 AM  Invited
Effect of Burn-up on the Thermal Conductivity of Fast Reactor MOX Fuel: Dragos Staicu1; Thierry Wiss1; Rudy Konings1; 1European Commission, Joint Research Centre, Nuclear Safety and Security Directorate
    The knowledge on the thermal conductivity of irradiated FBR MOX is currently very limited. Only one publication is available providing an experimental result which is surprising: no degradation of thermal conductivity with burn-up was observed. The objective of this work is to review the data and models available in the literature and obtain new experimental results in order to develop an updated recommendation. The impact of them main mechanisms was investigated in depth: radiation damage concentration as a function of the irradiation parameters, effect of the plutonium content, of microstructure, of fission gas atoms, of fission products and O/M. A new correlation for the conductivity was developed on the basis of the phenomena specific to FBR fuel: high irradiation temperature, restructuring, extensive fission gas release, diffusion of plutonium and fission products.

10:50 AM  Invited
Thermal Transport Properties of Uranium Dioxide from Atomistic Simulations: Aleksandr Chernatynskiy1; Simon Phillpot2; 1Missouri Science and Technology University; 2University of Florida
    Physics-based approaches for modeling of the nuclear fuels are currently being developed to replace the previous generation of the performance codes based on burnup. These approaches replace the state variable of "burnup" with a set of variables that include the concentration of the irradiation-produced and as-manufactured defects. Such description illuminates fuel microstructure in extensive details and permits prediction of the fuel behavior outside of the normal conditions of operation. To inform these models one requires detailed input from the atomistic level, either experimental or from simulations. Here, we will discuss how ab initio and molecular dynamics methods provide an input to the mesoscopic models on the example of the Uranium Dioxide with regards to thermal conductivity. In particular, we will highlight fundamental aspects of the phonon thermal transport from ab-initio calculations and elucidate the effect of pores, gas bubbles, dislocations and grain boundaries on the thermal transport by molecular dynamics.

11:20 AM  
Molecular Dynamics Simulations of Thermal Transport in Uranium Dioxide with Intrinsic Defects and Fission Products: Xiang-Yang Liu1; M.W.D. Cooper1; K.J. McClellan1; J.C. Lashley1; D.D. Byler1; B.D.C. Bell2; R.W. Grimes2; C.R. Stanek1; D.A. Andersson1; 1Los Alamos National Lab; 2Imperial College London
    Uranium dioxide (UO2) is the most commonly used fuel in light water nuclear reactors and thermal conductivity controls the removal of heat produced by fission, therefore, governing fuel temperature during normal and accident conditions. Molecular dynamics (MD) simulations of UO2 thermal conductivity including representative intrinsic defects and fission products are carried out. These calculations employ a standard Buckingham type interatomic potential and a potential that combines the many-body embedded atom potential with Morse-Buckingham pair potentials. Physical insights from the resonant phonon-spin scattering mechanism due to spins on the magnetic uranium ions have been introduced into the treatment of the MD results, with the corresponding relaxation time derived from existing experimental data. For each defect and fission product, scattering parameters are derived for application in both a Callaway model and the corresponding high-temperature model typically used in fuel performance codes.

11:40 AM  
Anisotropic Thermal Conductivity and Interface Resistance in Pyrolytic Carbon Coated Zirconia Particles: Yuzhou Wang1; David Hurley2; Erik Luther3; Miles Beaux3; Venkateswara Rao3; Igor Usov3; Marat Khafizov1; 1The Ohio State University; 2Idaho National Laboratory; 3Los Alamos National Laboratory
    Pyrolytic carbon (PyC) is used in composite materials. Its primary function is to act as an interphase layer that arrests crack propagation offering greater mechanical integrity to the composite. Its anisotropic properties due to layered structure of graphene sheets can be a limiting factor in nuclear applications. Here we investigate thermal transport in a composite consisting of zirconia (ZrO2) spherical kernels coated with a thin PyC layer. We implement laser-based thermal wave microscopy to measure anisotropic thermal conductivity in PyC and PyC/ZrO2 interface thermal resistance. The conductivity in circumferential direction is about 15 W/m·K whereas radial conductivity is less than 1 W/m·K. The interface resistance is on the order of 10-8 m2·K/W. The results of this work have implications for development of accident tolerant fuels where precise knowledge of conductivity is a critical design parameter and measurement of anisotropy in PyC layers of TRISO fuels to assess their mechanical integrity.