PNNL

Abstract

Sub-nanometer scale spatially resolved analysis of uranium isotopes is critical for a variety of technological disciplines, including earth and planetary sciences, environmental monitoring, bioremediation, and the nuclear fuel cycle. A three dimensional characterization technique (atom probe tomography) was utilized for locally mapping uranium isotope and impurity element distribution in fine-scale microstructural features in small volumes (100 nm$^3$). Here, a metallic nuclear fuel (uranium alloyed with ten weight percent molybdenum) with different nominal enrichments of 0.20 and 19.75\% \textsuperscript{235}U was studied. Isotope and element distributions in this metallic nuclear fuel system were systematically studied and quantified in different phases and regions within the fabricated microstructure. Homogeneity of \textsuperscript{235}U distribution, and how measured uranium isotope abundances compare to material processing and specifications were determined. We demonstrate that there are small fluctuations in \textsuperscript{235}U (within 10\%) at the nanoscale, and measured enrichments are in close agreement with specification. Our results provide insight into the thermo-mechanical processing history of the metallic uranium fuel analyzed. The approach presented here could be applied to studying nanoscale variations of isotopic abundances in the broad class of actinide-bearing materials, providing key insights about origin and thermomechanical processing routes.