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Visualizing Uranium in the Environment with Superconducting Technology
Takumi Yomogida (Graduate Student, Department of Earth and Planetary Science) *Former Researcher
Yoshio Takahashi (Professor, Department of Earth and Planetary Science)
The use of nuclear power generation is one of the most important options to achieve carbon neutrality, which is an urgent issue today.
The most important issue in nuclear power generation is the disposal of radioactive waste, and the possibility of geological disposal of spent fuel without recycling (direct disposal) has recently been considered as a promising method for this purpose.
In particular, in the case of direct disposal, the major element in the radioactive waste is uranium (U), and it is important to understand the migration behavior of U in the environment in order to assess the safety of geological disposal of radioactive waste.
In order to accurately determine the migration behavior of U in the environment, it has been desired to develop a new analytical technique to detect only trace amounts of U signals without interference from elements with high concentrations in the sample.
We (Takumi Yomogida (currently Researcher, Japan Atomic Energy Agency) and Yoshio Takahashi), together with Associate Professor Shinya Yamada of the Faculty of Science, Rikkyo University, have conducted research to elucidate the true nature of trace amounts of U in the environment using the Transition Edge Sensors (TES), an advanced X-ray detector based on superconductivity technology. TES can detect X-ray fluorescence (XRF) from trace elements excited by X-rays with high energy resolution and high sensitivity, but has not been used for environmental chemistry and geochemistry due to its limited counting rate. In this study, TES with improved counting rate by using multiple elements is applied to μ-XRF and μ-XANES (X-ray absorption near edge structure) methods using high-brilliance microbeam X-rays obtained at SPring-8 for the first time in the world, and the distribution state of trace U in real environmental samples, which cannot be detected with ordinary semiconductor detectors (SDD), and the The present study was the first application of the μ-XRF and μ-XANES (X-ray absorption near edge structure) method in the world.
In this study, we paid particular attention to biotite, a mineral that has the potential to reduce and fix U(VI) to U(IV) in the environment (also important as a mineral that fixes U in U deposits), and elucidated its fixation mechanism. Conventional μ-XRF mapping using SDD could not distinguish the distribution of U from that of Rb due to interference by the XRF of rubidium (Rb), which is abundant in biotite (Fig. 1a), but the U mapping results obtained by TES-μ-XRF (Fig. 1b) showed that the XRF of Rb and trace amounts of U could be separated and measured (Fig. 1c). (Fig. 1c), the correct distribution of U was obtained. The TES-µ-XANES analysis also showed that the valence of U at this site was reduced to U(IV). These results indicate that U is reduced and concentrated at the sites where Rb and other elements are dissolved by weathering of biotite in the environment. This indicates that U is reduced and immobilized in biotite and is less likely to move in the formation. The mechanism by which biotite retains U was clarified from the analysis of actual samples using TES.
µ-XRF mapping of U and Rb in biotite using SDD (a: upper left) and TES (b: upper right) and XRF spectrum of biotite (c: lower)
This study enables us to analyze ultra-trace elements in environmental samples with µm spatial resolution and to elucidate the mechanisms of elemental migration behavior from the atomic and molecular scale (molecular geochemistry), and is expected to be applied to the study of environmental migration behavior of various elements in addition to U. TES can be used for cosmic X-ray observation, atomic and molecular The results of this study are also expected to be widely applied to Earth, environment, extraterrestrial samples and biological samples, such as nondestructive analysis of extraterrestrial samples obtained in the future small body sample return project.
The results of this research were published in T. Yomogida, et al. Analyst. 149, 2932 (2024).
(Press release on April 9, 2024)
Published in The Rigakubu News July 2024
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