Open Access

Show Abstract ▼

Abstract: Operations at uranium (U)-mining and nuclear facilities have left a global legacy of significant radionuclide contamination in groundwaters which must be managed to minimize environmental harm. Uranium groundwater contamination is present at several sites globally, including Oak Ridge National Laboratory and Hanford, USA and Sellafield nuclear site, UK. In situ phosphate biomineralisation offers a promising method for radionuclide (including 90Sr and U) remediation at these sites. Typically, phosphate-generating amendments are injected into the subsurface to sequester select radionuclides in groundwaters by precipitation of poorly soluble Ca-phosphate phases and subsequent adsorption and/or incorporation of radionuclides to these poorly soluble phases, a remediation route being explored for both U and 90Sr. In this study, we investigate the mechanisms of U-phosphate precipitation in two phosphate-generating amendments (Ca-citrate/Na-phosphate and glycerol phosphate) under conditions relevant to Sellafield, UK. Using aerobic batch sediment experiments, we show both Ca-citrate/Na-phosphate and glycerol phosphate amendments are effective at enhancing removal of U(VI) from representative groundwaters (from 94% to >97%). Aqueous geochemical data coupled to speciation modelling highlighted that precipitation of U(VI) phosphate phases was the likely mechanism of U(VI) removal from groundwaters. Further X-ray absorption spectroscopy (XAS) analysis of solids confirmed U was present as a highly insoluble uranyl orthophosphate-like phase after treatment with both Ca-citrate/Na-phosphate and glycerol phosphate amendments. These data provide underpinning information on U-phosphate remediation in Sellafield relevant conditions thus expanding the range of treatment options for radionuclide contaminated groundwaters and defining the transport and fate of U during phosphate biomineralisation.

Open Access

Show Abstract ▼

Abstract: Catastrophic failure in brittle, porous materials initiates when smaller-scale fractures localise along an emergent fault zone in a transition from stable crack growth to dynamic rupture. Due to the rapid nature of this critical transition, the precise micro-mechanisms involved are poorly understood and difficult to image directly. Here, we observe these micro-mechanisms directly by controlling the microcracking rate to slow down the transition in a unique rock deformation experiment that combines acoustic monitoring (sound) with contemporaneous in-situ x-ray imaging (vision) of the microstructure. We find seismic amplitude is not always correlated with local imaged strain; large local strain often occurs with small acoustic emissions, and vice versa. Local strain is predominantly aseismic, explained in part by grain/crack rotation along an emergent shear zone, and the shear fracture energy calculated from local dilation and shear strain on the fault is half of that inferred from the bulk deformation.

Show Abstract ▼

Abstract: Breadcrust bombs are pyroclasts displaying fractured, dense surfaces enveloping expanded interiors, and are associated with Vulcanian explosions. We document pyroclasts from the 2008–2009 CE eruption of Chaitén (Chile) that are internally as well as externally breadcrusted. The pyroclasts are cut by intersecting micrometer- to millimeter-thick tuffisites with dense glassy walls, which grade into strongly inflated pumiceous material. We find H2O diffusion gradients proximal to the breadcrusted surfaces, such that H2O is depleted from far-field magma (0.68 ± 0.04 wt%) into dense, fractured vein walls (0.2–0.3 wt%), indicating a spatial association between H2O mass transfer within the pyroclast interior and both suppressed vesiculation and breadcrusting. We experimentally confirm that diffusive H2O depletion suppresses bubble growth at shallow conduit conditions. Therefore, we interpret the breadcrust formation to be induced by H2O diffusion and the associated rise in viscosity rather than by cooling in the classical breadcrust-formation models. We posit that a “dehydration quench” is important as degassing continues to very low H2O contents in shallow-conduit magma that continues to vesiculate.

Open Access

Show Abstract ▼

Abstract: Selenium (Se) is a toxic contaminant with multiple anthropogenic sources, including 79Se from nuclear fission. Se mobility in the geosphere is generally governed by its oxidation state, therefore understanding Se speciation under variable redox conditions is important for the safe management of Se contaminated sites. Here, we investigate Se behavior in sediment groundwater column systems. Experiments were conducted with environmentally relevant Se concentrations, using a range of groundwater compositions, and the impact of electron-donor (i.e., biostimulation) and groundwater sulfate addition was examined over a period of 170 days. X-Ray Absorption Spectroscopy and standard geochemical techniques were used to track changes in sediment associated Se concentration and speciation. Electron-donor amended systems with and without added sulfate retained up to 90% of added Se(VI)(aq), with sediment associated Se speciation dominated by trigonal Se(0) and possibly trace Se(-II); no Se colloid formation was observed. The remobilization potential of the sediment associated Se species was then tested in reoxidation and seawater intrusion perturbation experiments. In all treatments, sediment associated Se (i.e., trigonal Se(0)) was largely resistant to remobilization over the timescales of the experiments (170 days). However, in the perturbation experiments, less Se was remobilized from sulfidic sediments, suggesting that previous sulfate-reducing conditions may buffer Se against remobilization and migration.

Show Abstract ▼

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.