|"Bubble Character, Kr Distribution and Chemical Equilibrium in UO2" Todd Allen, Anter EL-AZAB, Jian Gan, Mahima Gupta, Lingfeng He, Hunter Henderson, Michele Manuel, Andrew Nelson, Janne Pakarinen, Billy Valderrama, Journal of Nuclear Materials Vol. 2015 Link|
"Bubble formation and Kr distribution in Kr-irradiated UO2"
Todd Allen, Anter EL-AZAB, Jian Gan, Mahima Gupta, Andrew Nelson, Janne Pakarinen, Billy Valderrama, Lingfeng He, Abdel-Rahman Hassan, Hunter Henderson, Marquis Kirk, Michele Manuel,
Journal of Nuclear Materials
In situ and ex situ transmission electron microscopy observation of small Kr bubbles in both single-crystal and polycrystalline UO2 were conducted to understand the inert gas bubble behavior in oxide nuclear fuel. The bubble size and volume swelling are shown as weak functions of ion dose but strongly depend on the temperature. The Kr bubble formation at room temperature was observed for the first time. The depth profiles of implanted Kr determined by atom probe tomography are in good agreement with the calculated profiles by SRIM, but the measured concentration of Kr is about 1/4 of the calculated concentration. This difference is mainly due to low solubility of Kr in UO2 matrix and high release of Kr from sample surface under irradiation.
"Effect of Grain Boundaries on Krypton Segregation Behavior in Irradiated Uranium Dioxide"
Todd Allen, Darryl Butt, Jian Gan, Lingfeng He, Hunter Henderson, Brian Jaques, Michele Manuel, Janne Pakarinen, Billy Valderrama,
Journal of Metals
Fission products, such as krypton (Kr), are known to be insoluble within UO2, segregating toward grain boundaries and eventually leading to a lowering in thermal conductivity and fuel swelling. Recent computational studies have identi?ed that differences in grain boundary structure have a signi?cant effect on the segregation behavior of fission products. However, experimental work supporting these simulations is lacking. Atom probe tomography was used to measure the Kr distribution across grain boundaries in UO2. Polycrystalline depleted UO2 samples were irradiated with 0.7 MeV and 1.8 MeV Kr-ions and annealed to 1000C, 1300C, and 1600C for 1 h to produce a Kr-bubble dominated microstructure. The results of this work indicate a strong dependence of Kr concentration as a function of grain boundary structure. Temperature also influences grain boundary chemistry with greater Kr concentration evident at higher temperatures, resulting in a reduced Kr concentration in the bulk. Although Kr segregation takes place at elevated temperatures, no change in grain size or texture was observed in the irradiated UO2 samples.
"Influence of instrument conditions on the evaporation behavior of uranium dioxide with UV laser-assisted atom probe tomography"
Jian Gan, Billy Valderrama, Hunter Henderson, Michele Manuel,
Journal of Nuclear Materials
Atom probe tomography (APT) provides the ability to detect subnanometer chemical variations spatially, with high accuracy. However, it is known that compositional accuracy can be affected by experimental conditions. A study of the effect of laser energy, specimen base temperature, and detection rate is performed on the evaporation behavior of uranium dioxide (UO2). In laser-assisted mode, tip geometry and standing voltage also contribute to the evaporation behavior. In this investigation, it was determined that modifying the detection rate and temperature did not affect the evaporation behavior as significantly as laser energy. It was also determined that three laser evaporation regimes are present in UO2. Very low laser energy produces a behavior similar to DC-field evaporation, moderate laser energy produces the desired laser-assisted field evaporation characteristic and high laser energy induces thermal effects, negatively altering the evaporation behavior. The need for UO2 to be analyzed under moderate laser energies to produce accurate stoichiometry distinguishes it from other oxides. The following experimental conditions providing the best combination of mass resolving power, accurate stoichiometry, and uniform evaporation behavior: 50 K, 10 pJ laser energy, a detection rate of 0.003 atoms per pulse, and a 100 kHz repetition rate.
"Investigation of material property influenced stoichiometric deviations as evidenced during UV laser-assisted atom probe tomography in fluorite oxides"
Todd Allen, Jian Gan, Hunter Henderson, Michele Manuel, Billy Valderrama, Clarissa Yablinsky,
Nuclear Instruments and Methods in Physics Research B: Beam Interactions with Materials and Atoms
Oxide materials are used in numerous applications such as thermal barrier coatings, nuclear fuels, and electrical conductors and sensors, all applications where nanometer-scale stoichiometric changes can affect functional properties. Atom probe tomography can be used to characterize the precise chemical distribution of individual species and spatially quantify the oxygen to metal ratio at the nanometer scale. However, atom probe analysis of oxides can be accompanied by measurement artifacts caused by laser-material interactions. In this investigation, two technologically relevant oxide materials with the same crystal structure and an anion to cation ratio of 2.00, pure cerium oxide (CeO2) and uranium oxide (UO2) are studied. It was determined that electronic structure, optical properties, heat transfer properties, and oxide stability strongly affect their evaporation behavior, thus altering their measured stoichiometry, with thermal conductance and thermodynamic stability being strong factors.
"Near Surface Stoichiometry in UO2: A Density Functional Theory Study"
Todd Allen, Hunter Henderson, Michele Manuel, Billy Valderrama,
Journal of Chemistry
The mechanisms of oxygen stoichiometry variation in UO2 at different temperature and oxygen partial pressure are important for understanding the dynamics of microstructure in these crystals. However, very limited experimental studies have been performed to understand the atomic structure of UO2 near surface and defect effects of near surface on stoichiometry in which the system can exchange atoms with the external reservoir. In this study, the near (110) surface relaxation and stoichiometry in UO2 have been studied with density functional theory (DFT) calculations. On the basis of the point-defect model (PDM), a general expression for the near surface stoichiometric variation is derived by using DFT total-energy calculations and atomistic thermodynamics, in an attempt to pin down the mechanisms of oxygen exchange between the gas environment and defected UO2. By using the derived expression, it is observed that, under poor oxygen conditions, the stoichiometry of near surface is switched from hyperstoichiometric at 300K with a depth around 3 nm to near-stoichiometric at 1000K and hypostoichiometric at 2000 K. Furthermore, at very poor oxygen concentrations and high temperatures, our results also suggest that the bulk of the UO2 prefers to be hypostoichiometric, although the surface is near-stoichiometric.
"Subsurface imaging of grain microstructure using picosecond ultrasonics"
Darryl Butt, Hunter Henderson, David Hurley, Brian Jaques, Marat Khafizov, Andrew Nelson, Janne Pakarinen, Michele Manuel, Lingfeng He,
We report on imaging subsurface grain microstructure using picosecond ultrasonics. This approach relies on elastic anisotropy of crystalline materials where ultrasonic velocity depends on propagation direction relative to the crystal axes. Picosecond duration ultrasonic pulses are generated and detected using ultrashort light pulses. In materials that are transparent or semitransparent to the probe wavelength, the probe monitors gigahertz frequency Brillouin oscillations. The frequency of these oscillations is related to the ultrasonic velocity and the optical index of refraction. Ultrasonic waves propagating across a grain boundary experience a change in velocity due to a change in crystallographic orientation relative to the ultrasonic propagation direction. This change in velocity is manifested as a change in the Brillouin oscillation frequency. Using the ultrasonic propagation velocity, the depth of the interface can be determined from the location in time of the transition in oscillation frequency. A subsurface image of the grain boundary is obtained by scanning the beam along the surface. We demonstrate this subsurface imaging capability using a polycrystalline UO2 sample. Cross section liftout analysis of the grain boundary using electron microscopy was used to verify our imaging results.
|"Characterization of Oxide Fuel Surface Chemistry with Atom Probe Tomography" Anter EL-AZAB, Jian Gan, Hunter Henderson, Michele Manuel, Billy Valderrama, 2013 ANS Winter Meeting November 10-13, (2013)|
|"Comparison of Computationally Simulated Fission Product Distribution with Correlative Characterization Techniques in Surrogate Nuclear Fuel Materials" Todd Allen, Jian Gan, Lingfeng He, Hunter Henderson, Michele Manuel, Billy Valderrama, 2013 SACNAS National Conference October 2-6, (2013)|
|"Fission Products in Nuclear Fuel: Comparison of Simulated Distribution with Correlative Characterization Techniques" Todd Allen, Anter EL-AZAB, Jian Gan, Lingfeng He, Hunter Henderson, Michele Manuel, Billy Valderrama, Clarissa Yablinsky, Microscopy and Microanalysis August 4-8, (2012)|
|"Microstructural Investigations of Kr and Xe Irradiated UO2" Todd Allen, Anter EL-AZAB, Jian Gan, Mahima Gupta, Lingfeng He, Hunter Henderson, Michele Manuel, Janne Pakarinen, Billy Valderrama, Energy Frontier Research Centers Principal Investigators Meeting July 18-19, (2013)|
|"Nano-scale Irradiation Induced Chemistry Changes in Oxide" Todd Allen, Jian Gan, Lingfeng He, Hunter Henderson, Michele Manuel, Janne Pakarinen, Billy Valderrama, 2014 TMS Annual Meeting February 16-20, (2014)|
|"Nano-scale Irradiation Induced Chemistry Changes in Oxide Fuel Materials" Todd Allen, Jian Gan, Hunter Henderson, Janne Pakarinen, Billy Valderrama, TMS 2014 February 16-20, (2014)|
Atom Probe Tomography to Study Annealing Temperature Dependence on Krypton Bubble Clustering in Uranium Dioxide - FY 2013 RTE Solicitation, #397
Atom Probe Tomography to Study Fission Product Damage in Model Nuclear Fuel - FY 2011 RTE Solicitation, #325
Atom Probe Tomography to Study Helium Segregation and Bubble Formation in Uranium Dioxide - FY 2013 RTE Solicitation, #395
Atom Probe Tomography to Study the Effect of Surfaces on the Chemistry of Depleted UO2 - FY 2012 RTE Solicitation, #382
Electron Backscatter Diffraction and Atom Probe Tomography to Study Grain Boundary Chemistry Variation in Off Stoichiometric Uranium Dioxide Thin Films - FY 2013 RTE Solicitation, #429
Electron Backscatter Diffraction and Atom Probe Tomography to Study Krypton Segregation Behavior in Uranium Dioxide - FY 2013 RTE Solicitation, #409