Luke Thomas Townsend

Crystalline ceramics are candidate materials for the immobilization of radionuclides, particularly transuranics (such as U, Pu, and Am), arising from the nuclear fuel cycle. Due to the α-decay of transuranics and the associated recoil of the parent nucleus, crystalline materials may eventually be rendered amorphous through changes to the crystal lattice caused by these recoil events. Previous work has shown irradiation of titanate-based ceramics to change the local cation environment significantly, particularly in the case of Ti which was shown to change from 6- to 5-fold coordination. Here, this work expands the Ti-based study to investigate the behavior in Fe-based materials, using LaFeO₃ as an example material. Irradiation was simulated by heavy ion implantation of the bulk LaFeO₃ ceramic, with the resulting amorphous layer characterized with grazing angle X-ray absorption spectroscopy (GA-XAS). Insights into the Fe speciation changes exhibited by the amorphized surface layer were provided through quantitative analysis, including pre-edge analysis, and modeling of the extended X-ray absorption fine structure (EXAFS), of the GA-XAS data.

This is the first comprehensive study to compare the long term stability of U and Sr (bio)remediation end-points following oxidative remobilisation ( via oxygen/nitrate) yielding insights into retention capabilities and removal mechanisms.

Citrate is a key decontaminant used in the nuclear industry and here we explore its biogeochemical fate in the presence of Ni²⁺ and U(VI)O₂ ²⁺ under conditions relevant to low level radioactive waste (LLW) disposal. Anaerobic microcosm experiments were performed under nitrate- and sulfate-reducing conditions at between pH 9 and 10. Citrate (1 mM) was supplied as both an electron donor and a potential metal ion complexant. Incubation experiments with citrate, inoculated with nitrate- or sulfate-reducing microbial consortia, were challenged with three different concentrations of Ni: 0.01, 0.1 or 1 mM, or U: 0.005, 0.05, or 0.5 mM. The nitrate- and sulfate-reducing inocula were enriched from well characterised alkaline sediments obtained from high pH lime-workings. A multi-technique approach was adopted to characterise the aqueous geochemistry, solid phase mineralogy, and bacterial communities in each incubation system. In the 0.01 mM Ni systems citrate underwent full biodegradation under both nitrate and sulfate-reducing conditions in less than 15 days. In the sulfate-reducing experiments, 50% of the added 0.01 mM Ni_(aq) was removed from solution and black solids formed; SEM and TEM analysis suggested that these were Ni-sulfides. For the higher Ni concentration incubations, no changes were observed in the nitrate-amended experiments. In the sulfate-amended experiments only citrate fermentation was observed, likely because elevated levels of Ni were toxic to nitrate- and sulfate-reducing bacteria in the inocula. Interestingly, although fermentative bacteria were key citrate degraders in the sulfate-amended experiments they did not dominate in the nitrate-amended experiments presumably due to competition from other microbes. In the U experiments, citrate degradation took place over 55 days in all systems except the 0.5 mM U/nitrate-amended incubations. In all U/sulfate-amended experiments, a dark-coloured precipitate formed and XAS analysis indicated that these solids contained reduced U(IV) with EXAFS suggesting that non-crystalline U(IV)–phosphate phases dominated. Microbial community analysis by 16S rRNA gene sequencing of endpoint samples identified fermenters and nitrate- and sulfate-reducing bacteria in the relevant incubations. Overall, findings suggest microbial degradation of citrate occurs under repository relevant conditions with Ni (at 0.01–0.1 mM) and U (at 0.005–0.5 mM) but with an inhibitory effect particularly at elevated Ni concentrations. Significantly, the work suggests that under anaerobic conditions relevant to LLW disposal, citrate undergoes biodegradation leading to the development of poorly soluble Ni sulfides and/or bioreduction of U(VI) to poorly soluble U(IV) phases. This suggests that both removal of citrate, and retention of Ni and U can occur in these environments and this information can be used to further inform development of safety cases for radioactive waste disposal.

Over 60 years of nuclear activity have resulted in a global legacy of contaminated land and radioactive waste. Uranium (U) is a significant component of this legacy and is present in radioactive wastes and at many contaminated sites. U-incorporated iron (oxyhydr)oxides may provide a long-term barrier to U migration in the environment. However, reductive dissolution of iron (oxyhydr)oxides can occur on reaction with aqueous sulfide (sulfidation), a common environmental species, due to the microbial reduction of sulfate. In this work, U(VI)-goethite was initially reacted with aqueous sulfide, followed by a reoxidation reaction, to further understand the long-term fate of U species under fluctuating environmental conditions. Over the first day of sulfidation, a transient release of aqueous U was observed, likely due to intermediate uranyl(VI)-persulfide species. Despite this, overall U was retained in the solid phase, with the formation of nanocrystalline U(IV)O₂ in the sulfidized system along with a persistent U(V) component. On reoxidation, U was associated with an iron (oxyhydr)oxide phase either as an adsorbed uranyl (approximately 65%) or an incorporated U (35%) species. These findings support the overarching concept of iron (oxyhydr)oxides acting as a barrier to U migration in the environment, even under fluctuating redox conditions.

The behaviour of Ce-containing zirconolites in hot isostatically pressed (HIPed) materials is complex, characterised by redox interactions between the metallic HIP canister that result in reduction of Ce⁴⁺ to Ce³⁺. In this work, a glass–ceramic of composition 70 wt.% CaZr_0.9Ce_0.1Ti₂O₇ ceramic in 30 wt.% Na₂Al₂Si₆O₁₆ glass was produced by HIP (approx. 170 cm³ canister) to examine the extent of the material–canister interaction. A complex material with six distinct regions was produced, with the extent of Ce reduction varying depending on the distance from the canister. Notably, the innermost bulk regions (those approximately 7 mm from the canister) contained only Ce⁴⁺, demonstrating that a production-scale HIPed glass–ceramic would indeed have a bulk region unaffected by the reducing environment induced by a ferrous HIP canister despite the flow of glass at the HIP temperature. Each of the six regions was characterised by XRD (including Rietveld method refinements), SEM/EDX and linear combination fitting of Ce L₃-edge XANES spectra. Regions in the lower part of the canister were found to contain a significantly higher fraction of Ce⁴⁺ compared to the upper regions. Though zirconolite-2M was the major crystalline phase observed in all regions, the relative abundances of minor phases (including sphene, baddeleyite, rutile and perovskite) were higher in the outermost regions, which comprised a significantly reduced Ce inventory.

Zirconolite is considered to be a suitable wasteform material for the immobilization of Pu and other minor actinide species produced through advanced nuclear separations. Here, we present a comprehensive investigation of Dy³⁺ incorporation within the self-charge balancing zirconolite Ca_1–xZr_1–xDy_2xTi₂O₇ solid solution, with the view to simulate trivalent minor actinide immobilization. Compositions in the substitution range 0.10 ≤ x ≤ 1.00 (Δx = 0.10) were fabricated by a conventional mixed oxide synthesis, with a two-step sintering regime at 1400 °C in air for 48 h. Three distinct coexisting phase fields were identified, with single-phase zirconolite-2M identified only for x = 0.10. A structural transformation from zirconolite-2M to zirconolite-4M occurred in the range 0.20 ≤ x ≤ 0.30, while a mixed-phase assemblage of zirconolite-4M and cubic pyrochlore was evident at Dy concentrations 0.40 ≤ x ≤ 0.50. Compositions for which x ≥ 0.60 were consistent with single-phase pyrochlore. The formation of zirconolite-4M and pyrochlore polytype phases, with increasing Dy content, was confirmed by high-resolution transmission electron microscopy, coupled with selected area electron diffraction. Analysis of the Dy L₃-edge XANES region confirmed that Dy was present uniformly as Dy³⁺, remaining analogous to Am³⁺. Fitting of the EXAFS region was consistent with Dy³⁺ cations distributed across both Ca²⁺ and Zr⁴⁺ sites in both zirconolite-2M and 4M, in agreement with the targeted self-compensating substitution scheme, whereas Dy³⁺ was 8-fold coordinated in the pyrochlore structure. The observed phase fields were contextualized within the existing literature, demonstrating that phase transitions in CaZrTi₂O₇–REE³⁺Ti₂O₇ binary solid solutions are fundamentally controlled by the ratio of ionic radius of REE³⁺ cations.

Neptunium (²³⁷Np) is an important radionuclide in the nuclear fuel cycle in areas such as effluent treatment and the geodisposal of radioactive waste. Due to neptunium’s redox sensitivity and its tendency to adsorb strongly to mineral phases, such as iron oxides/sulfides, the environmental mobility of Np can be altered significantly by a wide variety of chemical processes. Here, Np interactions with key iron minerals, ferrihydrite (Fe₅O₈H·4H₂O), goethite (α-FeOOH), and mackinawite (FeS), are investigated using X-ray Absorption Spectroscopy (XAS) in order to explore the mobility of neptunyl(V) (Np(V)O₂⁺) moiety in environmental (radioactive waste disposal) and industrial (effluent treatment plant) scenarios. Analysis of the Np L_III-edge X-ray Absorption Near-Edge Structure (XANES) and Extended X-ray Absorption Fine Structure (EXAFS) showed that upon exposure to goethite and ferrihydrite, Np(V) adsorbed to the surface, likely as an inner-sphere complex. Interestingly, analysis showed that only the first two shells (O_ax and O_eq) of the EXAFS could be modelled with a high degree of confidence, and there was no clear indication of Fe or carbonate in the fits. When Np(V)O₂⁺ was added to a mackinawite-containing system, Np(V) was reduced to Np(IV) and formed a nanocrystalline Np(IV)O₂ solid. An analogous experiment was also performed with U(VI)O₂²⁺, and a similar reduction was observed, with U(VI) being reduced to nanocrystalline uraninite (U(IV)O₂). These results highlight that Np(V) may undergo a variety of speciation changes in environmental and engineered systems whilst also highlighting the need for multi-technique approaches to speciation determination for actinyl (for example, Np(V)O₂⁺) species.

Geological disposal is the globally preferred long-term solution for higher activity radioactive wastes (HAW) including intermediate level waste (ILW). In a cementitious disposal system, cellulosic waste items present in ILW may undergo alkaline hydrolysis, producing significant quantities of isosaccharinic acid (ISA), a chelating agent for radionuclides. Although microbial degradation of ISA has been demonstrated, its impact upon the fate of radionuclides in a geological disposal facility (GDF) is a topic of ongoing research. This study investigates the fate of U(VI) in pH-neutral, anoxic, microbial enrichment cultures, approaching conditions similar to the far field of a GDF, containing ISA as the sole carbon source, and elevated phosphate concentrations, incubated both (i) under fermentation and (ii) Fe(III)-reducing conditions. In the ISA-fermentation experiment, U(VI) was precipitated as insoluble U(VI)-phosphates, whereas under Fe(III)-reducing conditions, the majority of the uranium was precipitated as reduced U(IV)-phosphates, presumably formed via enzymatic reduction mediated by metal-reducing bacteria, including Geobacter species. Overall, this suggests the establishment of a microbially mediated “bio-barrier” extending into the far field geosphere surrounding a GDF is possible and this biobarrier has the potential to evolve in response to GDF evolution and can have a controlling impact on the fate of radionuclides.