Microstructural Processes in Irradiated Materials: Nuclear Fuels and Ceramics
Sponsored by: TMS Structural Materials Division, TMS: Nuclear Materials Committee
Program Organizers: Thak Sang Byun, Pacific Northwest National Laboratory; Chu-Chun Fu, Commissariat à l'énergie atomique et aux énergies alternatives (CEA); Djamel Kaoumi, University of South Carolina; Dane Morgan, University of Wisconsin-Madison; Mahmood Mamivand, University of Wisconsin-Madison; Yasuyoshi Nagai, Tohoku University

Thursday 2:00 PM
March 2, 2017
Room: Del Mar
Location: Marriott Marquis Hotel

Session Chair: Mark Asta, University of California-Berkeley; William Weber, University of Tennessee

2:00 PM  Invited
Amorphization and Recrystallization in Ion-irradiated Ceramics: William Weber1; Eva Zarkadoula2; Ritesh Sachan2; Haizhou Xue1; Yanwen Zhang2; 1University of Tennessee; 2Oak Ridge National Laboratory
    During ion irradiation of ceramics, amorphization and recrystallization are often competing processes that depend on temperature. In this work, experimental and computational approaches are used to investigate the separate and combined effects of nuclear and electronic energy loss on amorphization and recrystallization in several complex oxide ceramics. Experimentally, ion mass and energy are used to control the ratio of electronic to nuclear energy loss, and large-scale atomistic simulations that include ballistic and thermal spike processes are used to model these effects. The results demonstrate that electronic energy loss, typical of MeV ions, can lead to additive or synergistic effects on amorphization; however, below a threshold in electronic energy loss, ionization-induced recrystallization dominates. These results have significant implications for interpreting and modeling the irradiation response of nuclear ceramics in accelerated testing using MeV ion irradiation. This work was supported by the U.S. DOE, BES, MSED.

2:30 PM  
Insights on Dramatic Radial Fluctuations in Track Formation by Energetic Ions: Ritesh Sachan1; Yanwen Zhang1; Eva Zarkadoula1; Matthew Chisholm1; William Weber2; 1Oak Ridge National Laboratory; 2University of Tennessee
    We discuss the unexpected dramatic radial variations originated during the recrystallization process in an ion track formation by swift heavy ion irradiation at a constant electronic energy-loss in pyrochlore structured Gd2TiZrO7. Though previous studies have shown track formation and average track diameter measurements, this work brings further clarity on how the recrystallization process in the ion track formation in Gd2TixZr(1-x)O7 systems brings more morphological complexities than the currently accepted behavior of ion tracks. This study shows the diameter variations to be as large as ~40% within an extremely short incremental track length of ~20 nm. Our molecular dynamics simulations show that these fluctuations in diameter are attributed to the partial substitution of Ti atoms by Zr atoms on the B-site in the lattice which have large difference in ionic radii. This results to the local favorability of phase formation during recrystallization and leads to radial fluctuations in the ion track.

2:50 PM  
Characterization of Radiation Effects in Complex Oxides: New Application of Neutron Total Scattering Techniques: Jacob Shamblin1; Eric O'Quinn1; Raul Palomares1; Maik Lang1; 1University of Tennessee
    Complex oxides, such as pyrochlore (A2B2O7) exhibit broad structural and chemical diversity, making them attractive candidates for inert matrix fuels and durable solids for the immobilization of radionuclide wastes. We present neutron total scattering with pair distribution function (PDF) analysis as a new approach for the characterization of radiation effects in complex oxides. Neutrons scatter strongly from low-Z elements, permitting a detailed analysis of both cation and anion defect behavior. PDF analysis elucidates the local defect structure, including changes in site occupation, coordination, and bond distance. We have investigated ion-irradiated complex oxides at the Spallation Neutron Source at Oak Ridge National Laboratory. Key to this approach is the use of GeV ions with a very high penetration depth to produce sufficiently large irradiated sample mass (~150 mg). Ion beam-induced structural modifications including defect formation, disordering, and amorphization were studied in terms of both the average long-range and local sample structures.

3:10 PM  Invited
Energetics of Trivalent Substitutional Elements in Uranium Dioxide: Combined Computational and Experimental Investigations : Jonathan Solomon1; Lei Zhang2; Alexandra Navrotsky2; Mark Asta1; 1University of California, Berkeley; 2University of California, Davis
    Energetics of rare earth (RE) substituted UO2 systems are investigated employing calorimetric measurements and density functional theory calculations. Systematic trends in energetics are investigated based on the present results and previous studies for other trivalent substituted fluorite oxides. A trend towards increased energetic stability with larger size mismatch between the tetravalent and trivalent cations is found for both actinide and non-actinide systems where charge is compensated by oxygen vacancies. However, the large exothermic oxidation enthalpy in UO2 favors oxygen rich compositions where charge compensation occurs through formation of uranium cations with higher oxidation states. Calculations are undertaken to understand the impact of these energetic trends on oxygen ion mobilities, motivated by experimental observations concerning the effect of trivalent fission products on the oxidative corrosion rates of spent nuclear fuel. This research was supported by DOE-BES as part of the Materials Science of Actinides Energy Frontier Research Center (DE-SC0001089).

3:40 PM Break

3:55 PM  
Raman Characterization of Electron Irradiated UO2 to Determine U Displacement Threshold: lionel Desgranges1; ritesh mohun1; patrick Simon2; aurélien canizares2; florian duval2; pierre desgardin2; marie-France Barthe2; christophe jegou1; sandrine miro1; 1CEA; 2CNRS
    Modeling irradiation effects in uranium dioxide, the most used nuclear fuel, requires basic data such as atomic displacement thresholds. The displacement threshold for uranium atom in UO2 was previously determined using TEM characterization of dislocations created by electron irradiation. In this study we used Raman spectroscopy to characterize irradiation effect in UO2. We have demonstrated that three peaks, named U1, U2 and U3, appear in UO2 Raman spectrum when the latter is submitted to helium irradiation. These three peaks are associated to a still unknown defect that is annealed at a temperature below 525°C, and that has an exponential creation kinetic. Here we show that these three peaks are also observed in UO2 irradiated by electrons provided the energy of the impinging electron is higher than around 2 MeV. This result is compared to PAS data on the same samples and discussed in connection with U displacement threshold.

4:15 PM  
Quantification of Irradiation Defects in Silicon Carbide Using Raman Spectroscopy: Takaaki Koyanagi1; Michael Lance1; Yutai Katoh1; 1Oak Ridge National Laboratory
    The characterization of atomistic defects including Frenkel pairs, antisites, and small defect clusters is recognized as a key to understand swelling and irradiation-creep of SiC. This study, which utilizes Raman spectroscopy, provides preliminary validation of a new experimental approach for quantifying such atomistic defects. The shift and peak broadening of the longitudinal optical phonon line in the Raman spectra of SiC caused by neutron irradiation is well interpreted as a combination of lattice strain and phonon confinement. The phonon confinement model enables the estimation of the defect distance in highly disordered SiC. These results are consistent with the results of previous experimental and simulation studies. This work is supported by the U.S. DOE, Office of Fusion Energy Sciences and Office of Nuclear Energy under contact DE-AC05-00OR22725 with ORNL managed by UT-Battelle, LLC. Research was conducted using HFIR, which is sponsored by the Office of Basic Energy Sciences, U.S. DOE.

4:35 PM  
Mesoscale Modelling of Radiation-induced Recrystallization and Fission Gas Bubble Formation in Metallic U-Mo Fuel: Linyun Liang1; Zhi-Gang Mei1; Abdellatif Yacout1; 1Argonne National Laboratory
    Radiation-induced swelling is one of the safety concerns in developing U-Mo metallic fuels for high performance research reactors. The formation of intergranular fission gas bubbles is a major cause of fuel swelling in U-Mo, which takes place simultaneously with radiation-induced recrystallization. The increased grain boundaries during recrystallization can expedite the formation of gas bubbles and therefore further increase the fuel swelling. To better control the fuel swelling, we developed a mesoscale phase-field model to investigate the microstructural changes in irradiated U-Mo fuel. The current model can successfully simulate the formation of intergranular gas bubbles and recrystallization processes observed in irradiated U-Mo. The predicted recrystallization kinetics and gas bubble-induced swelling compare favorably with experimental measurements of in-pile tests of U-Mo fuels. The effects of initial fuel grain size and grain morphology on gas bubble evolution are systematically studied, which provide a potential route towards controlled fuel swelling through microstructure engineering.

4:55 PM Concluding Comments