Chemical state mapping of simulant Chernobyl lava-like fuel containing material using micro-focused synchrotron X-ray spectroscopy

Abstract

Uranium speciation and redox behaviour is of critical importance in the nuclear fuel cycle. X-ray absorption near-edge spectroscopy (XANES) is commonly used to probe the oxidation state and speciation of uranium, and other elements, at the macroscopic and microscopic scale, within nuclear materials. Two-dimensional (2D) speciation maps, derived from microfocus X-ray fluorescence and XANES data, provide essential information on the spatial variation and gradients of the oxidation state of redox active elements such as uranium. In the present work, we elaborate and evaluate approaches to the construction of 2D speciation maps, in an effort to maximize sensitivity to the U oxidation state at the U L 3-edge, applied to a suite of synthetic Chernobyl lava specimens. Our analysis shows that calibration of speciation maps can be improved by determination of the normalized X-ray absorption at excitation energies selected to maximize oxidation state contrast. The maps are calibrated to the normalized absorption of U L 3 XANES spectra of relevant reference compounds, modelled using a combination of arctangent and pseudo-Voigt functions (to represent the photoelectric absorption and multiple-scattering contributions). We validate this approach by microfocus X-ray diffraction and XANES analysis of points of interest, which afford average U oxidation states in excellent agreement with those estimated from the chemical state maps. This simple and easy-to-implement approach is general and transferrable, and will assist in the future analysis of real lava-like fuel-containing materials to understand their environmental degradation, which is a source of radioactive dust production within the Chernobyl shelter.

Deep anoxic aquifers could act as sinks for uranium through microbial-assisted mineral trapping

“…U EXAFS measurements were performed at the National Synchrotron Light Source II (NSLS-II), Brookhaven National Laboratory (USA) at beamline 4-BM (XFM). The XFM beamline utilizes an air-bearing fixed-exit Si <111> double-crystal monochromator 87 . Calcite crystal was packed into a double layer of 50 µm Kapton tape and mounted on a motorized three-axis sample stage, positioned at 45°to the incident beam.…”

A new Kirkpatrick–Baez-based scanning microscope for the Submicron Resolution X-ray Spectroscopy (SRX) beamline at NSLS-II

“…The National Synchrotron Light Source II (NSLS-II) has a broad portfolio of imaging beamlines with multimodal and multiscale capabilities that comprise the Imaging and Microscopy Program. In particular, the Submicron Resolution X-ray Spectroscopy (SRX) beamline (Allwood et al, 2018;Yadav et al, 2017) plays a vital role in filling the resolution gaps between the microprobe beamlines such as the X-ray Fluorescence Microprobe (XFM) (Ding et al, 2021), Tender Energy X-ray Absorption Spectroscopy (TES) (Northrup, 2019), and nanoscale imaging beamlines that include the Hard X-ray Nanoprobe (HXN) (Yan et al, 2018;Nazaretski et al, 2015Nazaretski et al, , 2017, the Full-field X-ray Imaging (FXI) (Ge et al, 2018;Coburn et al, 2019), and the Coherent Diffraction Imaging (CDI) beamlines (Williams et al, 2017). To effectively bridge the micro-imaging capabilities with the nanoscale imaging capabilities at the NSLS-II, a new scanning X-ray microscope with state-of-the-art capabilities has been designed, constructed, and is under commissioning at the SRX beamline.…”

Exploring spectroscopic X-ray nano-imaging with Zernike phase contrast enhancement

“…Traditional XANES spectroscopy measures the intensity of X-rays before and after the sample with gas ionization chambers, which are mainly developed for bulk analysis. Although, micro-XANES is available with a synchrotron source 2 – 5 , the microscale resolution is insufficient for complex systems that require nanoscale chemical state changes or probing local information. Recently, spectroscopic full-field transmission X-ray microscopy (TXM-XANES) has emerged as a new technique for chemical imaging at the nanoscale with two-dimensional (2D) information 6 – 9 .…”