Thesis (M. Phil.)--University of Hong Kong, 1993.

The As- and Mo- bearing secondary mineral phases formed during the neutralization of uranium mill wastes were studied for a variety of ore blends including current and future ore sources at the Key Lake milling operation, northern Saskatchewan, Canada. A lab-scale plant model was employed to characterize secondary precipitates obtained during the mill waste neutralization process. Three scenarios of ore blends were processed through the lab-scale plant to produce mill waste solutions for neutralization before combination into final tailings. Slurry samples (n = 12) were collected from the secondary precipitates formed during the neutralization of mill wastes (raffinate) by precipitation with Ca(OH)2 (slaked lime) from pH 1.5 to 10.5. Synchrotron based X-ray absorption spectroscopy of mill and lab-scale plant precipitates showed arsenate adsorbed to ferrihydrite was the dominant As mineral phase regardless of pH or sample blend (53-77%), with fractional contributions from ferric arsenates, and adsorption to aluminum phases (AlOHSO4, As(OH)3 and hydrotalcite). Molybdate adsorbed to ferrihydrite was the dominant Mo mineral phase, regardless of pH or sample blend, with fractional contribution decreasing with increasing pH, and minor contributions from calcium molybdate, ferric molybdate and nickel molybdate. These results were used in geochemical modelling to predict the source terms for these mineral phases in tailings facilities. Sequestration of As and Mo in the model showed solubility was controlled by adsorption to both Fe and Al oxide surfaces as well as by direct precipitation with other dissolved constituents (Ni, Ca and SO4).The models developed pH profiles of mineral phase precipitation to explain the solubility of As, Mo, Fe, Al, Mg and Ni during sequestration from pH 1.5 to 10.5 that were consistent regardless of ore blend used in simulations. Since adsorption of anions to the surface of ferrihydrite has been shown to slow conversion to crystalline forms of Fe oxides (goethite and hematite) and sequestration of arsenate effectively controls As solubility at high pH (pH >10), As-bearing mineral phases are expected to be stable for thousands of years. With adsorption as well as direct precipitation considered, Mo phases though effectively sequestering below pH 8, became unstable and released Mo back into the tailings porewater (pH >10), as predicted by the thermodynamic model. Historical data obtained from as-discharged tailings as well as previously published U mill tailings studies agree with these findings.

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The safety case for a proposed geological disposal facility (GDF) for radioactive wastes relies upon a series of engineered and natural barrier systems to limit the migration of harmful radionuclides into the geosphere over geological timescales. Natural minerals, dominantly phyllosilicates, are expected to be the most reactive components of both the host rock and the clay-based backfill surrounding the highly radioactive waste canisters for as long as 100,000 years. Upon eventual canister degradation, alpha-emitting radionuclides will leach into the backfill material (and eventually beyond) and the constituent mineral systems will accumulate radiation damage upon radionuclide uptake and/or surface precipitation. The following study is an assessment of the structural and chemical effects caused by alpha-particle bombardment of silicate minerals, as proxies for the radiation stability of natural materials present in the near and far field of a GDF.Microscopy and spectroscopy studies from naturally occurring radiation damage accumulated in silicates over geological timescales (forming distinct 'radiohaloes') have shown that both alpha-particle and alpha-recoil bombardment results in altered unit cell dimensions caused by the accumulation of point (Frenkel) defects. In the example of highly damaged biotite, structural breakdown through the reorientation of discrete lattice crystallites was observed; the variability of the interlayer spacing within these regions reveal the potential for damaged mica to adopt the structure of phyllosilicate breakdown products over geological time. Controlled alpha-particle irradiation using the Dalton Cumbrian Facility's 5 MV tandem pelletron ion accelerator, combined with microfocus spectroscopy analysis has revealed the mechanisms of high fluence alpha-radiation damage across 2:1 phyllosilicate minerals (biotite and chlorite); reducing the layered structures into a series of loosely connected domains of alternating lattice expansion and collapse. Radiation induced Fe redox changes have been revealed, with Fe reduction apparent at relatively low alpha-particle doses, giving way to Fe oxidation at high doses. A 'redox gradient', based on alpha-particle energy deposition through a silicate structure has therefore been proposed. In addition, the increase in 'edge' sites generated by structural deformation has been shown to be favourable for the adsorption of the Se(IV) oxyanion to the mica surface. Comprising a body of additional work, a core sample has been extracted from a spent nuclear fuel pond wall at the decommissioned Hunterston A nuclear power station and the radioactive contamination on the painted core surface has been analysed by microfocus spectroscopy. The contaminant radiostrontium has been shown to be associated with the Ti rich pigment in the surface paint, resulting in a 'patchy' accumulation of radioactivity at the core surface. In addition, inert Cs reactivity experiments using the underlying concrete have shown that Cs is preferentially uptaken by phyllosilicates within the altered mafic clasts used in the concrete aggregate.

The current proposed method for the long-term management of intermediate and high level radioactive waste in the UK is via geological disposal. It is known that redox sensitive elements such as uranium and neptunium will significantly contribute to the total waste inventory. Recently, studies have indicated that both U and Np can be stabilised by interaction with minerals. Over long periods of time (1000’s -10,000’s years) steel canisters that encase radioactive waste in geodisposal systems will undergo anaerobic corrosion, potentially leading to the release of radionuclides, including U and Np. Anaerobic corrosion will also result in the formation of a number of oxide phases, including iron (oxyhydr)oxides e.g. magnetite and green rust. The interaction of U and Np with such forming iron (oxyhydr)oxides may lead to the sequestering of radionuclides in the environment through a range of processes such as adsorption to a mineral surface and incorporation into a mineral structure. Therefore the interactions between iron (oxyhydr)oxides and radionuclides are important to determine their fate if potentially released within the wider environment. In this study, the fate of U(VI) and Np(V) when in contact with a range of iron (oxyhydr)oxides was considered. These systems were selected to help understand the detailed mechanisms that may occur between radionuclides and iron (oxyhydr)oxides. XRD and TEM were used to characterise mineralogy, whilst acid digestions determined the distribution of U within the mineral phase. Synchrotron based XAS was used to determine oxidation state, site geometry and local bonding environment of the radionuclides associated with the mineral phases. The data suggests that: U(V) is stabilised and incorporated in octahedral coordination into both the magnetite and green rust structure in a uranate-like coordination; with increasing U concentration mineral formation favours uraninite and Fe(III) (oxyhydr)oxides; the limit of U incorporation into magnetite is 0.45 mol % U ± 0.23; Np(V) is reduced to Np(IV) on the iron (oxyhydr)oxide surface forming a bidentate binuclear complex; and that upon reoxidation, Np(IV) is partially reoxidised back to Np(V) but not released back into solution. These results highlight the significance in understanding the mechanisms when both Np and U are in contact with iron (oxyhydr)oxides which can contribute towards site environmental clean-up and waste management in the nuclear industry.

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