Using Fission-Track Radiography Coupled with Scanning Electron Microscopy for Efficient Identification of Solid-Phase Uranium Mineralogy at a Former Uranium Pilot Mill (Grand Junction, Colorado)

Abstract

At a former uranium pilot mill in Grand Junction, Colorado, mine tailings and some subpile sediments were excavated to various depths to meet surface radiological standards, but residual solid-phase uranium below these excavation depths still occurs at concentrations above background. The combination of fission-track radiography and scanning electron microscope energy-dispersive X-ray spectroscopy (SEM-EDS) provides a uniquely efficient and quantitative way of determining mineralogic associations of uranium that can influence uranium mobility. After the creation of sample thin sections, a mica sheet is placed on those thin sections and irradiated in a nuclear research reactor. Decay of the irradiated uranium creates fission tracks that can be viewed with a microscope. The fission-track radiography images indicate thin section sample areas with elevated uranium that are focus areas for SEM-EDS work. EDS spectra provide quantitative elemental data that indicate the mineralogy of individual grains or grain coatings associated with the fission-track identification of elevated uranium. For the site in this study, the results indicated that uranium occurred (1) with coatings of aluminum–silicon (Al/Si) gel and gypsum, (2) dispersed in the unsaturated zone associated with evaporite-type salts, and (3) sorbed onto organic carbon. The Al/Si gel likely formed when low-pH waters were precipitated during calcite buffering, which in turn retained or precipitated trace amounts of Fe, As, U, V, Ca, and S. Understanding these mechanisms can help guide future laboratory and field-scale efforts in determining long-term uranium release rates to groundwater.

Combining Fission-Track Radiography and Scanning Electron Microscopy to Elucidate Uranium Mobility Controls

“…4) and a consistent color (Online Resource Fig. S3) indicate that the precipitate is Al-rich rather than clay that has a molted appearance (Johnson et al 2021). The stronger signal of the Al elemental map than the Si map (Fig.…”

“…In brief, this method exposes a thin section to radioactivity, and the resulting radioactive decay of uranium creates " ssion tracks" in an overlying mica sheet. The prepared thin section slides were placed in polyethylene tubes and irradiated in a research reactor at the U.S. Geological Survey in Denver, Colorado after a mica was placed over the thin sections following the methods of Johnson et al (2021). Samples were irradiated at a neutron ux of 2 × 10 12 neutrons per square centimeter per second for 8 hours for a total maximum neutron dose of ~ 11.6 × 10 16 neutrons (less time or a lower neutron ux [or both] is required for higher concentrations of uranium).…”

“…A variety of sophisticated spectroscopic and diffraction techniques have been used to analyze uranium associations with complex mineral assemblages in sediments from some of the U.S. Department of Energy's legacy sites, e.g., the Hanford site (e.g., Arai et al 2007;Catalano et al 2004;Singer et al 2009; Stubbs et al 2009), Naturita site (e.g., Jové Colón et al 2006), and Grand Junction site (e.g., Johnson et al 2021). Speci cally, X-ray absorption ne structure spectroscopy (XAFS) and X-ray absorption near edge structure spectroscopy (XANES) have been commonly used to determine uranium speciation onto solid phases.…”

“…However, despite its potential, it has not been extensively used to elucidate uranium associations in natural sediments to inform conceptual site models. Johnson et al (2021) applied ssion-track technology on trace uranium characterization at Grand Junction, Colorado site; however, much remains unknown in direct observation of uranium hosts at a larger site scale.…”

CT Scanning of Structural Characteristics of Glacial Till in Moxi River Basin, Sichuan Province

“…The electron beam is focused on the surface of the sample. Under the action of the deflection coil between the second condenser and the objective lens, the electron beam scans the sample in a raster shape, and simultaneously detects the electrons and photons scattered from the surface of the sample after the incident electrons interact with the research object to obtain the surface morphology and composition analysis of the corresponding materials [25]. The energy of the secondary electrons scattered from the surface of the material is generally less than 50 eV, and most of their energy is about 2 to 3 eV.…”