Abstract
Spatially resolved analysis of uranium (U) isotopes in small volumes of actinide-bearing materials is critical for a variety of technical disciplines, including earth and planetary sciences, environmental monitoring, bioremediation, and the nuclear fuel cycle. However, achieving subnanometer-scale spatial resolution for such isotopic analysis is currently a challenge. By using atom probe tomography—a three-dimensional nanoscale characterisation technique—we demonstrate unprecedented nanoscale mapping of U isotopic enrichment with high sensitivity across various microstructural interfaces within small volumes (~100 nm3) of depleted and low-enriched U alloyed with 10 wt% molybdenum that has different nominal enrichments of 0.20 and 19.75% 235U, respectively. We map enrichment in various morphologies of a U carbide phase, the adjacent γ-UMo matrix, and across interfaces (e.g., carbide/matrix, grain boundary). Results indicate the U carbides were formed during casting, rather than retained from either highly enriched or depleted U feedstock materials. The approach presented here can be applied to study nanoscale variations of isotopic abundances in the broad class of actinide-bearing materials, providing unique insights into their origins and thermomechanical processing routes.
Highlights
Uranium (U) is the heaviest element naturally occurring in the Earth’s crust in significant amounts, and is used in its natural and anthropogenic forms
3D elemental distribution across precipitate-matrix interfaces. In both depleted U-10Mo (DU-10Mo) and low-enriched uranium (LEU)-10Mo alloys, the main microstructural features are uranium carbide (UC) inclusions and the surrounding γ-UMo matrix[44]. Representative images of both DU-10Mo and LEU-10Mo microstructures are shown in Fig. 1. (Example micrographs of other UC morphologies are provided in Fig. S1 of the Supplementary Information)
Characterisation of UC and γ-UMo matrix phases in DU and LEU alloyed with 10 wt% Mo was performed to demonstrate the capability of Atom probe tomography (APT) for quantitative measurement of U isotopes in actinide-bearing materials with varying nominal enrichments
Summary
Uranium (U) is the heaviest element naturally occurring in the Earth’s crust in significant amounts, and is used in its natural and anthropogenic forms. Fuel swelling is directly related to geometric stability (e.g., bowing, deflection) and robustness of fuel plates under irradiation, contributing to phenomena such as coolant channel closure, or release of fission gas products to the coolant[25] These phenomena make it critical to analyse the distribution of 235U in a nuclear fuel at a high spatial resolution to account for 235U enrichment variation across all possible nanoscale heterogeneities in the microstructure. Current capabilities for measurement of U isotopic abundance typically involve mass spectrometry or spectroscopy methods, including the following: inductively coupled plasma mass spectrometry (ICP-MS)[35], time-of-flight and nanoscale secondary ion mass spectrometry (SIMS)[36,37], thermal ionisation mass spectrometry (TIMS)[35], and gamma spectroscopy[38] These techniques each have their own merits, and all vary in terms of accuracy, resolution (e.g., spatial, depth, mass), cost, sample preparation requirements, analysis time, and complexity of operation[39].
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