Trishelle Copeland-Johnson

Three-dimensional (3D) tomography using focused-ion beam (FIB) microscopy has become a powerful technique within the past few years given its ability to generate sub-micron resolution volumes that can expand upon the information acquired two-dimensionally (2D) with scanning electron microscopy (SEM). This is especially critical for highly contextualized phenomena that need to be captured 3D to get a full perspective on how the material in question has evolved, such as in the case of materials degradation mechanisms (i.e. corrosion, irradiation, etc.). When employed in conjunction with 2D cross-sectional images of the specimen, 3D tomography could expand initial observations, providing highly contextualized information at specific sites of interest identified from 2D analysis, enhancing our contextual information about degradation mechanisms at the mesoscale. Additionally, this 3D information can also provide direct observable data for validating 3D mesoscale modelling for predicting the evolution of microstructure and composition and structural materials. Computational capabilities like these would be especially useful for expediting the qualification of nuclear materials for advanced nuclear reactors [5]. FIB tomography requires correlating and compiling data acquired from multiple detectors simultaneously, including energy dispersive x-ray spectroscopy (EDS), electron backscatter diffraction (EBSD), and secondary electron imaging, would be critical to generating correlative results. The Idaho National Laboratory (INL) has published work pertaining to the use of 3D FIB tomography techniques to analyze selective diffusion of alloying element, Cr, from Alloy 617 after corrosion in molten salt and to measure the size and concentration of fission gas pores in U-Mo fuel. Both investigations attempted to overlay both elemental composition and microstructural data acquired from EDS, EBSD, secondary electron imaging to correlate trends observed between alloy composition and microstructure due to the respective degradation mechanism of study. However, the data processing proved to be a challenge due to file incompatibilities between data acquisition software packages and the need of multiple image processing software packages like DREAM.3D, ParaView, and MATLAB. Ideally, optimization of sample preparation and data acquisition practices need to be implemented to ensure high-quality reproducible datasets, in conjunction with a simplified user-interface for processing and visualizing data that would enhance accessibility of FIB tomography to users. The purpose of this investigation is to optimize a data collection, processing, and visualization workflow for 3D volumes of elemental and microstructural evolution of 3D FIB tomography at INL, utilizing the FEI Helios G3 and G4 UX dual beam FIB microscopes located in the INL Irradiated Materials and Characterization Laboratory (IMCL). Some highlights from this study include, 1) development of a standard operating procedure specifically designed for the FEI Helios G3 and G4, taking into consideration limitations on loading/unloading the instrument due to IMCL nuclear facility work controls and characterizing atmospherically sensitive materials, 2) establishing a procedure to review and upload FIB tomography datasets to the INL Nuclear Research Data System, funded by the Nuclear Science User Facilities (NSUF) program for improve data accessibility to users, and 3) developing a simplified visualization tool for combining EDS and EBSD datasets using open-sourced software packages, such as Image J and the Segment Anything Model developed by Meta, that can be readily distributed to NSUF users to process their data. Overall, the potential for FIB tomography to enhance our understanding of nuclear materials degradation mechanisms, particularly for irradiation, would be a valuable asset to invest to enhance computational prediction tools for strategic and expedited design of nuclear materials, particularly for advanced nuclear reactor technologies.

Extended dry storage of aluminum-clad spent nuclear fuel (ASNF) requires an assessment of the radiolytic generation of molecular hydrogen gas (H2) from the ASNF’s corrosion layers. H2 accumulation can potentially lead to storage canister embrittlement/rupture and the accumulation of flammable gas mixtures in accident scenarios. To date, computational models have been developed for the prediction of radiolytic H2 generation from samples exposed to lower absorbed dose regimes (up to ~3 MGy). However, greater accuracy is needed for higher absorbed doses (> 25 MGy) where the concentration of H2 ultimately reaches a steady-state. Data in this area is limited due to the amount of time taken (months) to accumulate such high gamma doses. Furthermore, a recent study observed radiation-induced damage to the surface of pre-corroded aluminum alloy coupons at high absorbed gamma doses. Despite this observation and its implications, there is currently no computational connection between the radiolytic formation of H2 and changes in the composition and morphology of the cladding’s corrosion layers with absorbed dose. To develop a better understanding of the processes and effects described above, the present study aimed to evaluate the feasibility of leveraging synchrotron x-ray capabilities for coupled accelerated irradiation and in situ surface characterization of ASNF alloys. To that end, National Synchrotron Light Source II (NSLS-II) beamtime was secured and a series of aluminum alloy 1100 (AA1100) wires, prepared under a variety of conditions, were interrogated using ex situ x-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques at Idaho National Laboratory (INL) and ex situ and in situ XRD capabilities at the NSLS-II x-ray powder diffraction (XPD) beamline. Results indicated that performing synchrotron XRD using the available XPD beamline configuration can provide sufficient radiation dose but cannot discern changes in the composition and morphology of sample corrosion layers. Additionally, ex situ XRD and SEM data from Idaho National Laboratory (INL) indicated that the pre-corrosion of AA1100 wires did not generate an appreciably thick corrosion layer, as compared with previous aluminum coupon studies. These thinner surface corrosion layers were not sufficient for detection by both ex situ and in situ synchrotron XRD, specifically under consideration of the NSLS-II XPD beamline’s configuration primarily capturing information from the sample’s bulk material, i.e., aluminum metal. In summary, the use of the NSLS-II XPD beamline for promoting accelerated radiolysis and in situ surface characterization is not appropriate for this program’s goals.