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Abstract: Uranium dioxide (UO2) and metaschoepite (UO3•nH2O) particles have been identified as contaminants at nuclear sites. Understanding their behavior and impact is crucial for safe management of radioactively contaminated land and to fully understand U biogeochemistry. The Savannah River Site (SRS) (South Carolina, USA), is one such contaminated site, following historical releases of U-containing wastes to the vadose zone. Here, we present an insight into the behavior of these two particle types under dynamic conditions representative of the SRS, using field lysimeters (15 cm D x 72 cm L). Discrete horizons containing the different particle types were placed at two depths in each lysimeter (25 cm and 50 cm) and exposed to ambient rainfall for 1 year, with an aim of understanding the impact of dynamic, shallow subsurface conditions on U particle behavior and U migration. The dissolution and migration of U from the particle sources and the speciation of U throughout the lysimeters was assessed after 1 year using a combination of sediment digests, sequential extractions, and bulk and μ-focus X-ray spectroscopy. In the UO2 lysimeter, oxidative dissolution of UO2 and subsequent migration of U was observed over 1–2 cm in the direction of waterflow and against it. Sequential extractions of the UO2 sources suggest they were significantly altered over 1 year. The metaschoepite particles also showed significant dissolution with marginally enhanced U migration (several cm) from the sources. However, in both particle systems the released U was quantitively retained in sediment as a range of different U(IV) and U(VI) phases, and no detectable U was measured in the lysimeter effluent. The study provides a useful insight into U particle behavior in representative, real-world conditions relevant to the SRS, and highlights limited U migration from particle sources due to secondary reactions with vadose zone sediments over 1 year.

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Abstract: Radioactive ‘hot’ particles can be deposited in the environment as a result of illicit activities, nuclear accidents (e.g., Chernobyl, Fukushima), weapons use, mining, and/or nuclear waste disposal. Understanding the long-term behaviour of such materials in the environment is important for understanding risk and environmental impact, and for designing remediation strategies. However, mechanistic knowledge of hot particle alteration processes, reaction products, and radionuclide speciation are limited, especially at finely resolved spatial scales. In this talk, we provide two case-studies that detail how micro- to nano-focus synchrotron X-ray techniques can be used as part of an analytical “tool kit” to fully characterise nuclear industry born hot particles. In turn, this data can inform safety assessments and clean-up / decommissioning efforts at radioactively contaminated sites. In both case-studies, we examine highly radioactive micro-particles that were found in soil samples taken from nuclear exclusion zone that surrounds the Fukushima Daiichi Nuclear Power Plant (FDNPP). These particles were emitted from the damaged FDNPP reactors during the 2011 accident. Recent work by our group [1, 2] has shown that these particles are common forms of contamination in the nuclear exclusion zone, but the possible environmental and human-health impacts of the particles are not yet known. Recent work [3, 4] on Diamond Light Source Beamlines I18 (micro-focus X-ray spectroscopy) and I14 (Hard X-ray nanoprobe), and the Swiss Light Source micro-XAS Beamline, has permitted detailed chemical characterisation of these challenging materials. In case study 1, we will present micro-focus data that describes the speciation of actinide elements in whole FDNPP hot particles [3]. The data includes the first speciation information for plutonium released from the damaged FDNPP reactors. In case study 2, we present nano-probe characterisation of recently discovered hot particles derived from FDNPP reactor Unit 1 [4]. These particles have the highest ever recorded 134+137Cs radioactivities for particles released from the FDNPP. In our work, FIB sectioning of the particles permitted detailed SIMS, electron microscopy, and hard X-ray nano-probe analysis of the particles. In particular, combined electron-microscopy and synchrotron-based nano-focus XRF and XRD analyses were used to characterise the particles (e.g., Figure 1). For both case studies we will provide an overview of sample preparation, analysis considerations, and discuss how the results inform management of the FDNPP legacy.

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Abstract: Traces of Pu have been detected in material released from the Fukushima Daiichi Nuclear Power Plant (FDNPP) in March of 2011; however, to date the physical and chemical form of the Pu have remained unknown. Here we report the discovery of particulate Pu associated with cesium-rich microparticles (CsMPs) that formed in and were released from the reactors during the FDNPP meltdowns. The Cs-pollucite-based CsMP contained discrete U(IV)O2 nanoparticles, <~10 nm, one of which is enriched in Pu adjacent to fragments of Zr-cladding. The isotope ratios, 235U/238U, 240Pu/239Pu, and 242Pu/239Pu, of the CsMPs were determined to be ~0.0193, ~0.347, and ~0.065, respectively, which are consistent with the calculated isotopic ratios of irradiated-fuel fragments. Thus, considering the regional distribution of CsMPs, the long-distance dispersion of Pu from FNDPP is attributed to the transport by CsMPs that have incorporated nanoscale fuel fragments prior to their dispersion up to 230 km away from the Fukushima Daiichi reactor site.

Open Access

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Abstract: Metaschoepite is commonly found in U contaminated environments and metaschoepite-bearing wastes may be managed via shallow or deep disposal. Understanding metaschoepite dissolution and tracking the fate of any liberated U is thus important. Here, discrete horizons of metaschoepite (UO3●nH2O) particles were emplaced in flowing sediment/groundwater columns representative of the UK Sellafield site. The column systems either remained oxic or became anoxic due to electron donor additions, and the columns were sacrificed after 6- and 12-months for analysis. Solution chemistry, extractions, and bulk and micro-/nano-focus X-ray spectroscopies were used to track changes in U distribution and behavior. In the oxic columns, U migration was extensive, with UO22+ identified in effluents after 6-months of reaction using fluorescence spectroscopy. Unusually, in the electron-donor amended columns, during microbially-mediated sulfate reduction, significant amounts of UO2-like colloids (>60% of the added U) were found in the effluents using TEM. XAS analysis of the U remaining associated with the reduced sediments confirmed the presence of trace U(VI), non-crystalline U(IV), and biogenic UO2, with UO2 becoming more dominant with time. This study highlights the potential for U(IV) colloid production from U(VI) solids under reducing conditions and the complexity of U biogeochemistry in dynamic systems.

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Abstract: Stainless steel coupons have been exposed to uranium-containing nitric acid solutions, in conditions similar to those found in various uranium handling nuclear facilities across the nuclear fuel cycle. Solid state analysis of the stainless steel samples and solution composition analysis were undertaken to gain a better understanding of the contamination process mechanisms. Stainless steel coupons were immersed in 12 M HNO3 containing uranium (1 g/L), in the form of uranyl, for periods of up to 255 days. Uranium contamination was observed across all time lengths of exposure. Solution analysis indicated that the levels of contamination reached an equilibrium state after ~14 days. Investigations using Raman microscopy, synchrotron microfocus X-ray fluorescence and X-ray absorption spectroscopy showed inhomogeneous localization of uranyl species within the passive layer of the stainless steel surface. Over longer time lengths of exposure these contaminant species were predominantly found to locate within intergranular regions of the stainless steel. This finding should be taken into consideration when developing decontamination protocols for corroded stainless steel that has been exposed to uranium, to facilitate metal reuse/recycle and minimize hazardous waste volumes.