Practicum Abstracts

The National Spherical Torus Experiment Upgrade (NSTX-U) has been used to investigate the effect of wall tile surface conditioning on plasma performance during operation. Previous campaigns have demonstrated the enhanced suppression of edge-localized modes and achievement of high confinement (H-mode) conditions when reactor walls were conditioned with lithium. In addition to lithium coatings, binary alloys of lithium with other metals, such as tin, have been proposed as potential liquid metal plasma facing materials. In order to better understand the surface chemistry of lithium and its interaction with vacuum-level species, we attempted to deposit monolayer-level lithium coatings onto various materials. The systems analyzed in this study included lithium coatings on both ATJ graphite and tungsten, in addition to a liquid phase Sn-Li alloy. The Angle-Resolved Ion Energy Spectrometer (ARIES) facility at Sandia National Laboratory was used to analyze the surface interactions using low energy ion scattering spectroscopy (LEISS).

Sn-Li is a low melting-point alloy that has been identified as a material with favorable performance in plasma material interaction (PMI) studies. While lithium is a low Z material with a demonstrated ability to absorb impinging ions, pure lithium is plagued by high evaporation rates in the liquid phase. The Sn-Li alloy is a more stable alternative that provides a lower rate of evaporative flux due to the high vapor pressure of tin. In addition, in the liquid phase, the bulk segregation of lithium to the surface of the material has been observed. While the alloy is of considerable interest to the PMI community, little data has been collected on its surface chemistry in a plasma environment. In order to expand the existing body of knowledge in this area, samples of an 80% Sn—20% Li alloy were prepared and analyzed in both the solid and liquid phase in order to assess the surface composition and degree of lithium segregation in the liquid phase. The Angle-Resolved Ion Energy Spectrometer (ARIES) at Sandia National Laboratories was used to probe the surfaces of the alloy. After synthesizing an 80% Sn—20% Li alloy, I worked with Dean Buchenauer to analyze the structure of the material in the solid phase using Auger spectroscopy and X-ray fluorescence (XRF) techniques. An Auger depth profile was performed in order to characterize the change in composition from the surface to the bulk. In addition, Sn-Li samples were also analyzed using low energy ion scattering (LEIS) in the ARIES device. The surface composition of the material in the solid phase was determined. Due to unforeseen delays, the LEIS experiments are still ongoing as of the time of this writing. In the coming weeks the samples will be melted and LEIS will be used to characterize the evolution of lithium coverage at the surface of the material in the liquid phase.

Hydrogen degradation of structural materials, such as nickel-based alloys, is characterized by both enhanced dislocation processes and grain boundary decohesion leading to intergranular fracture. Nanoindentation and scanning probe microscopy (SPM) were used to characterize slip transfer across high- and low-energy grain boundaries in commercially pure nickel alloys before and after hydrogen charging. Both low-energy recrystallization twin boundaries and high-energy random boundaries and were identified for indentation using electron backscatter diffraction. Nanoindentation produced local deformation along grain boundaries, causing material pile-up and slip steps; thermal hydrogen-charging altered the observed response to local deformation. Additional indentation within specific grains indicated hydrogen charging reduces elastic modulus. Coupled nanoindentation and SPM investigations provide a unique method for analyzing local hydrogen effects as a function of grain boundary type and grain orientation, which can be used to develop grain boundary engineered materials.

When operating, a nuclear reactor emits on the order of 10^20 antineutrinos per second. The number of antineutrinos detected varies with the isotope being fissioned and linearly with the power of the reactor. Changes in a reactor’s antineutrino signature can indicate either a change in the operation of the reactor or in the fuel. Currently, groups at Sandia and Lawrence Livermore National Laboratories are working to develop antineutrino detectors for reactor monitoring. Reactor simulations are being performed to determine how significant changes in the reactor are on antineutrino signatures. By simulating several operating modes and fuel configurations, an estimate can be obtained of what changes in the reactor are noticeable with antineutrino detection.