NSUF - Nuclear Science User Facilities

Andrew Nelson

Profile Information
Name
Dr. Andrew Nelson
Institution
Oak Ridge National Laboratory
Position
Section Head / Distinguished Staff Scientist
h-Index
26
ORCID
0000-0002-4071-3502
Biography

Dr. Andrew T. Nelson is a distinguished staff scientist and section head of the Fuel Development Section at Oak Ridge National Laboratory. Prior to joining ORNL in 2018, Dr. Nelson served as the team leader for Ceramic Nuclear Fuels in the Materials Science and Technology Division at Los Alamos National Laboratory. He received his Ph.D. in Nuclear Engineering from the University of Wisconsin-Madison. Dr. Nelson currently holds numerous leadership roles in DOE-NE and NNSA fuel development programs. He is the National Technical Lead for Accident Tolerant Fuels within the U.S. Department of Energy’s Advanced Fuels Campaign and the Fuel Fabrication and Development Lead for the NNSA’s Advanced LEU Fuels for Nonproliferation Applications program. Dr. Nelson’s research interests are focused on the development and assessment of novel nuclear fuels, with an emphasis on advanced fuel systems for light water reactors. He is also active in development of high-density dispersion and particle fuel concepts for nonproliferation applications; this work seeks to couple advanced manufacturing and in situ diagnostics to improve performance and reduce conservatisms inherent to reference nuclear fuel fabrication methods. Dr. Nelson has authored or co-authored over one hundred peer-reviewed publications in the areas of nuclear fuel and cladding materials, ceramics, and high temperature materials.

Expertise
ATF, Ceramics, Nuclear Fuel, Thermophysical Properties
Publications:
"2.6 MeV proton irradiation effects on the surface integrity of depleted UO2" Todd Allen, Anter EL-AZAB, Jian Gan, Mahima Gupta, Andrew Nelson, Janne Pakarinen, Nuclear Instruments and Methods B Vol. 319 2014 100-106 Link
The effect of low temperature proton irradiation in depleted uranium dioxide was examined as a function of fluence. With 2.6 MeV protons, the fluence limit for preserving a good surface quality was found to be relatively low, about 1.4 and 7.0 × 1017 protons/cm2 for single and poly crystalline samples, respectively. Upon increasing the fluence above this threshold, severe surface flaking and disintegration of samples was observed. Based on scanning electron microscopy (SEM) and X-ray diffraction (XRD) observations the causes of surface failure were associated to high H atomic percent at the peak damage region due to low solubility of H in UO2. The resulting lattice stress is believed to exceed the fracture stress of the crystal at the observed fluencies. The oxygen point defects from the displacement damage may hinder the H diffusion and further increase the lattice stress, especially at the peak damage region.
"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 evolution in Kr-irradiated UO2 during annealing" Lingfeng He, Xianming Bai, Janne Pakarinen, Brian Jaques, Jian Gan, Andrew Nelson, Anter EL-AZAB, Todd Allen, Journal of Nuclear Materials Vol. 496 2017 242-250 Link
Transmission electron microscopy observation of Kr bubble evolution in polycrystalline UO2 annealed at high temperature was conducted in order to understand the inert gas behavior in oxide nuclear fuel. The average diameter of intragranular bubbles increased gradually from 0.8 nm in as-irradiated sample at room temperature to 2.6 nm at 1600 °C and the bubble size distribution changed from a uniform distribution to a bimodal distribution above 1300 °C. The size of intergranular bubbles increased more rapidly than intragranular ones and bubble denuded zones near grain boundaries formed in all the annealed samples. It was found that high-angle grain boundaries held bigger bubbles than low-angle grain boundaries. Complementary atomistic modeling was conducted to interpret the effects of grain boundary character on the Kr segregation. The area density of strong segregation sites in the high-angle grain boundaries is much higher than that in the low angle grain boundaries.
"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 Vol. 456 2015 125-132 Link
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.
"Bubble, stoichiometry, and chemical equilibrium of krypton-irradiated UO2" Todd Allen, Anter EL-AZAB, Jian Gan, Mahima Gupta, Lingfeng He, Michele Manuel, Janne Pakarinen, Billy Valderrama, Abdel-Rahman Hassan, Marquis Kirk, Andrew Nelson, Journal of Nuclear Materials Vol. 456 2015 125-132 Link
In situ and ex situ transmission electron microscopy observation of small Kr bubbles in bothsingle-crystal and polycrystalline UO2 were conducted to understand the inert gas bubblebehavior in oxide nuclear fuel. The bubble size and volume swelling are shown as a weakfunction of ion dose but strongly depend on the temperature. The Kr bubble formation at roomtemperature was observed for the first time. The depth profiles of implanted Kr determined byatom probe tomography are in good agreement with the calculated profiles by SRIM, but themeasured concentration of Kr is about 1/3 of calculated one. This difference is mainly due to lowsolubility of Kr in UO2 matrix, which has been confirmed by both density-functional theorycalculations and chemical equilibrium analysis.
"Microstructure changes and thermal conductivity reduction in UO2 following 3.9 MeV He2+ ion irradiation" Anter EL-AZAB, Jian Gan, Marat Khafizov, Andrew Nelson, Janne Pakarinen, Chris Wetteland, Lingfeng He, David Hurley, Todd Allen, Journal of Nuclear Materials Vol. 454 2014 283-289 Link
The microstructural changes and associated effects on thermal conductivity were examined in UO2 after irradiation using 3.9 MeV He2+ ions. Lattice expansion of UO2 was observed in X-ray diffraction after ion irradiation up to 5 × 1016 He2+/cm2 at low-temperature (<200 °C). Transmission electron microscopy (TEM) showed homogenous irradiation damage across an 8 μm thick plateau region, which consisted of small dislocation loops accompanied by dislocation segments. Dome-shaped blisters were observed at the peak damage region (depth around 8.5 μm) in the sample subjected to 5 × 1016 He2+/cm2, the highest fluence reached, while similar features were not detected at 9 × 1015 He2+/cm2. Laser-based thermo-reflectance measurements showed that the thermal conductivity for the irradiated layer decreased about 55% for the high fluence sample and 35% for the low fluence sample as compared to an un-irradiated reference sample. Detailed analysis for the thermal conductivity indicated that the conductivity reduction was caused by the irradiation induced point defects.
"Microstructure evolution in Xe-irradiated UO2 at room temperature" Todd Allen, Anter EL-AZAB, Jian Gan, Lingfeng He, Janne Pakarinen, Marquis Kirk, Andrew Nelson, Xianming Bai, Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms Vol. 330 2014 55-60 Link
In situ Transmission Electron Microscopy was conducted for single crystal UO2 to understand the microstructure evolution during 300 keV Xe irradiation at room temperature. The dislocation microstructure evolution was shown to occur as nucleation and growth of dislocation loops at low irradiation doses, followed by transformation to extended dislocation segments and tangles at higher doses. Xe bubbles with dimensions of 1-2 nm were observed after room-temperature irradiation. Electron Energy Loss Spectroscopy indicated that UO2 remained stoichiometric under room temperature Xe irradiation.
"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, Acta Materialia Vol. 112 2016 1476-1477 Link
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.
Presentations:
"Damage Structure Evolution in Ion Irradiated UO2" Todd Allen, Jian Gan, Mahima Gupta, Andrew Nelson, Jeff Terry, TMS 2014 February 16-20, (2014)
"Radiation Effects in UO2" Todd Allen, Jian Gan, Mahima Gupta, Michele Manuel, Andrew Nelson, Janne Pakarinen, Billy Valderrama, TMS 2014 February 16-20, (2014)

About Us

The Nuclear Science User Facilities (NSUF) is the U.S. Department of Energy Office of Nuclear Energy's only designated nuclear energy user facility. Through peer-reviewed proposal processes, the NSUF provides researchers access to neutron, ion, and gamma irradiations, post-irradiation examination and beamline capabilities at Idaho National Laboratory and a diverse mix of university, national laboratory and industry partner institutions.

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