The significant abundance of uranium in radioactive waste inventories worldwide necessitates a thorough understanding of its behavior. In this work, the speciation of uranyl(VI), (UO22+) in a gibbsite system under ambient conditions has been determined as a function of pH by deconvolution and analysis of luminescence spectroscopic data. Uniquely, a combined experimental and statistical approach utilizing time-resolved luminescence spectroscopy and parallel factor analysis (PARAFAC) of excitation emission matrices has been successfully utilized to identify four separate luminescent U(VI) species in the uranyl-gibbsite system for the first time. The speciation of all luminescent U(VI) species in an environmentally relevant system over a pH range of 6-11 is discerned through the analysis of emission fingerprints at low temperature (20 K). Comparison of the deconvoluted luminescence spectra with mineral standards and geochemical models of the system allows the assignment of the luminescent chemical species as metaschoepite, Na-compreignacite, surface adsorbed equivalent to AlO2-UO2(OH) and equivalent to AlO2-UO2(CO3)(2)(4-) complexes, with assignments supported by fitting of extended X-ray absorption fine structure data. The combined spectroscopic techniques in this study show that assignment and quantification of uranyl(VI) species in a sorption system over a large pH range can be accurately achieved using PARAFAC to deconvolute a three way emission spectroscopic data set.

Strontium-90 (90Sr) is a major contaminant at nuclear legacy sites. The mobility of 90Sr is primarily governed by sorption reactions with sediments controlled by high surface area phases such as clay and iron oxides. Sr2+ adsorption was investigated in heterogeneous unconsolidated aquifer sediments, analogous to those underlying the UK Sellafield nuclear site, with grainsizes ranging from gravels to clays. Batch sorption tests showed that a linear Kd adsorption model was applicable to all grainsize fractions up to equilibrium [Sr] of 0.28 mmol L-1. Sr2+ sorption values (Kd; Langmuir qmax) correlated well with bulk sediment properties such as cation exchange capacity and surface area. Electron microscopy showed that heterogeneous sediments contained porous sandstone clasts with clay minerals (i.e. chlorite) providing an additional adsorption capacity. Therefore, gravel corrections that assumed that the > 2 mm fractions are inert were not appropriate and underestimated Kd(bulk) adsorption coefficients. However, Kd (<2 mm) values measured from sieved sediment fractions, were effectively adjusted to within error of Kd (bulk) using a surface area dependant gravel correction based on particle size distribution data. Amphoteric pH dependent Sr2+ sorption behaviour observed in batch experiments was consistent with cation exchange modelling between pH 2-7 derived from the measured cation exchange capacities. Above pH 7 model fits were improved by invoking a coupled cation exchange/surface complexation which allowed for addition sorption to iron oxide phases. The overall trends in Sr2+ sorption (at pH 6.5-7) produced by increasing solution ionic strength was also reproduced in cation exchange models. Overall, the results showed that Sr2+ sorption to heterogeneous sediment units could be estimated from Kd (<2 mm) data using appropriate gravel corrections, and effectively modelled using coupled cation exchange and surface complexation processes.

Microbial ureolysis offers the potential to remove metals including Sr2+ as carbonate minerals via the generation of alkalinity coupled to NH4+ and HCO3- production. Here, we investigated the potential for bacteria, indigenous to sediments representative of the U.K. Sellafield nuclear site where Sr-90 is present as a groundwater contaminant, to utilize urea in order to target Sr2+-associated (Ca)CO3 formation in sediment microcosm studies. Strontium removal was enhanced in most sediments in the presence of urea only, coinciding with a significant pH increase. Adding the biostimulation agents acetate/lactate, Fe(III), and yeast extract to further enhance microbial metabolism, including ureolysis, enhanced ureolysis and increased Sr and Ca removal. Environmental scanning electron microscopy analyses suggested that coprecipitation of Ca and Sr occurred, with evidence of Sr associated with calcium carbonate polymorphs. Sr K-edge X-ray absorption spectroscopy analysis was conducted on authentic Sellafield sediments stimulated with Fe(III) and quarry outcrop sediments amended with yeast extract. Spectra from the treated Sellafield and quarry sediments showed Sr2+ local coordination environments indicative of incorporation into calcite and vaterite crystal structures, respectively. 16S rRNA gene analysis identified ureolytic bacteria of the genus Sporosarcina in these incubations, suggesting they have a key role in enhancing strontium removal. The onset of ureolysis also appeared to enhance the microbial reduction of Fe(III), potentially via a tight coupling between Fe(III) and NH4+ as an electron donor for metal reduction. This suggests ureolysis may support the immobilization of Sr-90 via coprecipitation with insoluble calcium carbonate and cofacilitate reductive precipitation of certain redox active radionuclides, e.g., uranium.

Sb- and Nb-doped Zr and Sn-umbites have been prepared using hydrothermal synthesis with both high purity and yield. All four materials display excellent removal of Cs+ cations from acidic, neutral, and basic solutions, abating at least 80% of the Cs present. This performance is retained in the presence of competing Na+ cations as well as across the pH range. The most sustained selectivity is observed in acidic media, with evidence of a minor reduction in selectivity under basic conditions. The umbites have successfully been shaped into pellets, introducing macroporosity and retaining the selective uptake of Cs in the presence of excess Na. Through thermal treatment, samples of partially Cs-exchanged umbite can be converted into dense silicate phases where radioactive Cs can be immobilized in a potential wasteform for long term storage. These findings present doped umbites as prospective materials for industrial use with selective abatement properties and capabilities for deployment followed by end of life geological disposal.

Iron (oxyhydr)oxide minerals are ubiquitous in the sub-surface environment (e.g. soils and sediments) and have the potential to act as a long-term barrier to the migration of radionuclides. The long-lived actinides (e.g. 237Np, 2.14 million year half-life) are particularly challenging, and pose multigenerational technical and policy issues for the remediation of contaminated land and the disposal of radioactive wastes. Over the last decade, increasing evidence indicates that actinides may substitute for Fe in the structure of iron (oxyhydr)oxides. Given the thermodynamic stability of iron (oxyhydr)oxides over the crustal continuum, incorporation may provide long-term immobilisation pathways for anthropogenic actinide contaminants. Here, the speciation and coordination adopted by key actinides (i.e. U, Np, Pu and Am) on incorporation into iron (oxyhydr)oxides is reviewed. The stability of these phases under fluctuating environmental conditions is also discussed, to provide constraints on the long-term fate of actinides incorporated into iron (oxyhydr)oxides.

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.

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 LIII-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 (Oax and Oeq) 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.

A wide range of organic-containing wastes exists throughout Europe, the majority of which can be classed as intermediate-level or long-lived low-level waste. In some countries, organic-containing wastes have already been disposed within near-surface or geological disposal facilities, whilst in others, the wastes are stored pending the availability of a suitable disposal facility. The organic materials can be summarized as follows: bitumen, organic ion-exchange resins, halogenated and nonhalogenated polymers, cellulose materials, and other polymers such as rubber, polyurethane, polyamides, etc. The amounts and proportions of these organic materials vary by country and this reflects their different nuclear industry and power generation activities. In many countries, ion-exchange resins dominate the inventory of organic ILW, where it results from nuclear power generation using light-water reactors. Higher inventories of halogenated plastics and other related plastic and cellulose wastes result from countries that undertake fuel reprocessing and have a larger inventory from nuclear research. In addition to inorganic cement encapsulants, bitumen has been commonly used to condition a range of wastes. Once disposed of, these organic materials will provide a major source of organic carbon that has the potential, following degradation, to fuel anaerobic microbial processes.

Modeling of microbial processes is required to fully consider their effects of microbial processes on the safety case for the disposal of radioactive wastes. Numerical models may be used to provide quantitative inputs to dose assessment calculations or may be used to demonstrate a phenomenological understanding of processes. In this chapter, a modeling approach is described that represents microbial growth and metabolism that are important to processes of; microbial gas generation, hydrogen consumption, sulfate reduction and sulfide generation, the evolution of pH and redox potential (Eh), and hence the chemical speciation of radionuclides. The model described is based on that used to support the safety case for the disposal of low-level radioactive waste (LLW) in the United Kingdom and which has been validated by modeling long-term (20 year) experimental studies of gas generation from LLW. The modeling approach described has a wider application to model microbial processes in higher activity wastes, including processes of sulfide generation that is important to the corrosion of copper canisters used for spent fuel disposal.

Over 60 years of nuclear activities have resulted in a global legacy of radioactive wastes, with uranium considered a key radionuclide in both disposal and contaminated land scenarios. With the understanding that U has been incorporated into a range of iron (oxyhydr)oxides, these minerals may be considered a secondary barrier to the migration of radionuclides in the environment. However, the long-term stability of U-incorporated iron (oxyhydr)oxides is largely unknown, with the end-fate of incorporated species potentially impacted by biogeochemical processes. In particular, studies show that significant electron transfer may occur between stable iron (oxyhydr)oxides such as goethite and adsorbed Fe(II). These interactions can also induce varying degrees of iron (oxyhydr)oxide recrystallization (<4% to >90%). Here, the fate of U(VI)-incorporated goethite during exposure to Fe(II) was investigated using geochemical analysis and X-ray absorption spectroscopy (XAS). Analysis of XAS spectra revealed that incorporated U(VI) was reduced to U(V) as the reaction with Fe(II) progressed, with minimal recrystallization (approximately 2%) of the goethite phase. These results therefore indicate that U may remain incorporated within goethite as U(V) even under iron-reducing conditions. This develops the concept of iron (oxyhydr)oxides acting as a secondary barrier to radionuclide migration in the environment.

An integrated waste management approach for irradiated graphite was developed during the European Commission project ‘Treatment and Disposal of Irradiated Graphite and other Carbonaceous Waste’. This included the identification of potential options for the management of irradiated graphite, taking account of storage, retrieval, treatment and disposal methods. This paper describes how these options can be assessed using multi-criteria decision analysis (MCDA) for a case study relating to a generic power reactor. Criteria have been defined to account for safety, environmental, economic and socio-political factors, including radiological impact, resource usage, economic costs and risks. The impact of each option against each criterion has been assessed using data from the project and the wider literature. A linear additive approach has been used to convert the calculated impacts to scores. To account for the relative importance of the criteria, example weightings were allocated. This application has shown that MCDA approaches can be used to support complex decisions regarding irradiated graphite management, accounting for a wide range of criteria. Use of this approach by individual countries or organisations will need to account for the specific options, scores, weightings and constraints that apply, based on their national strategies, regulatory requirements and public acceptability.

Over 60 years of nuclear activities have resulted in a global legacy of radioactive waste and contaminated land. Higher activity wastes are destined for disposal in a deep underground geological facility (GDF), with iron (oxyhydr)oxide phases expected to be ubiquitous in and around the repository. Additionally, uranium will be a significant radionuclide in many of these wastes. Given that a range of uranium-incorporated iron (oxyhydr)oxides have been reported in the literature, these mineral phases may be considered a secondary barrier to the migration of uranium in the environment. However, the long-term stability of these phases under fluctuating geochemical conditions is unknown. Stable iron oxyhydroxides (e.g. goethite) have undergone extensive recrystallisation (>90%) during Fe-atom exchange, with incorporated species released and/or reduced during the recrystallisation process. Here, the stability and fate of uranium-incorporated goethite during Fe-atom exchange was investigated. A U(VI)-goethite species was hydrothermally synthesized and reacted with aqueous Fe(II). The system was monitored using geochemical analysis and X-ray absorption spectroscopy (XAS), with an aqueous 57 Fe(II) tracer used to track the extent of Fe-atom exchange. This revealed that only ~2% of structural Fe(III) was exchanged with aqueous Fe(II), with the retention of incorporated U confirmed by acid digestions. Despite this, M IV -edge HR XANES and L III -edge EXAFS revealed an ingress of near-surface U(V) as the Fe-atom

Understanding anthropogenic radionuclide biogeochemistry and mobility in natural systems is key to improving the management of radioactively contaminated environments and radioactive wastes. Here, we describe the contemporary depth distribution and phase partitioning of 137Cs, Pu, and 241Am in two sediment cores taken from the Irish Sea (Site 1: the Irish Sea Mudpatch; Site 2: the Esk Estuary). Both sites are located ~10 km from the Sellafield nuclear site. Low-level aqueous radioactive waste has been discharged from the Sellafield site into the Irish Sea for >50 y. We compare the depth distribution of the radionuclides at each site to trends in sediment and porewater redox chemistry, using trace element abundance, microbial ecology, and sequential extractions, to better understand the relative importance of sediment biogeochemistry vs. physical controls on radionuclide distribution/post-depositional mobility in the sediments. We highlight that the distribution of 137Cs, Pu, and 241Am at both sites is largely controlled by physical mixing of the sediments, physical transport processes, and sediment accumulation. Interestingly, at the Esk Estuary, microbially-mediated redox processes (considered for Pu) do not appear to offer significant controls on Pu distribution, even over decadal timescales. We also highlight that the Irish Sea Mudpatch likely still acts as a source of historical pollution to other areas in the Irish Sea, despite ever decreasing levels of waste output from the Sellafield site.

Understanding interactions between iron (oxyhydr)oxide nanoparticles and plutonium is essential to underpin technology to treat radioactive effluents, in cleanup of land contaminated with radionuclides, and to ensure the safe disposal of radioactive wastes. These interactions include a range of adsorption, precipitation, and incorporation processes. Here, we explore the mechanisms of plutonium sequestration during ferrihydrite precipitation from an acidic solution. The initial 1 M HNO3 solution with Fe(III)(aq) and 242Pu(IV)(aq) underwent controlled hydrolysis via the addition of NaOH to pH 9. The majority of Fe(III)(aq) and Pu(IV)(aq) was removed from solution between pH 2 and 3 during ferrihydrite formation. Analysis of Pu-ferrihydrite by extended X-ray absorption fine structure (EXAFS) spectroscopy showed that Pu(IV) formed an inner-sphere tetradentate complex on the ferrihydrite surface, with minor amounts of PuO2 present. Best fits to the EXAFS data collected from Pu-ferrihydrite samples aged for 2 and 6 months showed no statistically significant change in the Pu(IV)-Fe oxyhydroxide surface complex despite the ferrihydrite undergoing extensive recrystallization to hematite. This suggests the Pu remains strongly sorbed to the iron (oxyhydr)oxide surface and could be retained over extended time periods.

U(IV) mobility can be significantly enhanced by colloids in both engineered and natural environments. This is particularly relevant in decommissioning and clean-up of nuclear facilities, such as legacy fuel ponds and silos at the Sellafield site, UK, and in long-term radioactive waste geodisposal. In this study, the product of metallic uranium (U) corrosion under anaerobic, alkaline conditions was characterised, and the interaction of this product with silicate solutions was investigated. The U metal corrosion product consisted of crystalline UO2 nanoparticles (5-10 nm) that aggregated to form clusters larger than 20 nm. Sequential ultrafiltration indicated that a small fraction of the U metal corrosion product was colloidal. When the uranium corrosion product was reacted with silicate solutions under anaerobic conditions, ultrafiltration indicated a stable colloidal uranium fraction was formed. Extended X-ray absorption fine structure (EXAFS) spectroscopy and high resolution TEM confirmed that the majority of U was still present as UO2 after several months of exposure to silicate solutions, but an amorphous silica coating was present on the UO2 surface. This silica coating is believed to be responsible for formation of the UO2 colloid fraction. Atomic-resolution scanning TEM (STEM) indicated some migration of U into the silica-coating of the UO2 particles as non-crystalline U(IV)-silicate, suggesting alteration of UO2 at the UO2-silica interface had occurred. This alteration at the UO2-silica interface is a potential pathway to the formation of U-silicates (e.g. coffinite, USiO4). (C) 2019 The Authors. Published by Elsevier B.V.

Iron (oxyhydr)oxide nanoparticles are known to sorb metals, including radionuclides, from solution in various environmental and industrial systems. Effluent treatment processes including the Enhanced Actinide Removal Plant (EARP) (Sellafield, UK) use a neutralisation process to induce the precipitation of iron (oxyhydr)oxides to remove radionuclides from solution. There is a paucity of information on mechanism(s) of U(VI) removal under conditions relevant to such industrial processes. Here, we investigated removal of U(VI) from simulated effluents containing 7.16 mM Fe(III) with 4.2 x 10(-4)-1.05 mM U(VI), during the base induced hydrolysis of Fe(III). The solid product was ferrihydrite under all conditions. Acid dissolutions, Fourier Transform infrared spectroscopy and thermodynamic modelling indicated that U(VI) was removed from solution by adsorption to the ferrihydrite. The sorption mechanism was supported by X-ray Absorption Spectroscopy which showed U(VI) was adsorbed to ferrihydrite via a bidentate edge-sharing inner-sphere species with carbonate forming a ternary surface complex. At concentrations <= 0.42 mM U(VI) was removed entirely via adsorption, however at 1.05 mM U(VI) there was also evidence for precipitation of a discrete U(VI) phase. Overall these results confirm that U(VI) sequestered via adsorption to ferrihydrite over a concentration range from 4.2 x 10(-4)-0.42 mM confirming a remarkably consistent removal mechanism in this industrially relevant system.

One of the nuclear fuel storage ponds at Sellafield (United Kingdom) is open to the air, and has contained a significant inventory of corroded magnox fuel and sludge for several decades. As a result, some fission products have also been released into solution. 90Sr is known to constitute a small mass of the radionuclides present in the pond, but due to its solubility and activity, it is at risk of challenging effluent discharge limits. The sludge is predominantly composed of brucite (Mg(OH)2), and organic molecules are known to be present in the pond liquor with occasional algal blooms restricting visibility. Understanding the chemical interactions of these components is important to inform ongoing sludge retrievals and effluent management. Additionally, interactions of radionuclides with organics at high pH will be an important consideration for the evolution of cementitious backfilled disposal sites in the UK. Batch sorption-desorption experiments were performed with brucite, 90Sr and natural organic matter (NOM) (humic acid (HA) and Pseudanabaena catenata cyanobacterial growth supernatant) in both binary and ternary systems at high pH. Ionic strength, pH and order of addition of components were varied. 90Sr was shown not to interact strongly with the bulk brucite surface in binary systems under pH conditions relevant to the pond. HA in both binary and ternary systems demonstrated a strong affinity for the brucite surface. Ternary systems containing HA demonstrated enhanced sorption of 90Sr at pH 11.5 and vice versa, likely via formation of strontium-humate complexes regardless of the order of addition of components. The distribution coefficients show HA sorption to be reversible at all pH values studied, and it appeared to control 90Sr behaviour at pH 11.5. Ternary systems containing cyanobacterial supernatant demonstrated a difference in 90Sr behaviour when the culture had been subjected to irradiation in the first stages of its growth.

The European Treatment and Disposal of Irradiated Graphite and other Carbonaceous Waste project sought to develop best practices in the retrieval, treatment, and disposal of irradiated graphite including other irradiated carbonaceous waste such as structural material made of graphite, nongraphitized carbon bricks, and fuel coatings. Emphasis was given on legacy irradiated graphite, as this represents a significant inventory in respective national waste management programs. This paper provides an overview of the characteristics of graphite irradiated during its use, primarily as a moderator material, within nuclear reactors. It describes the potential techniques applicable to the retrieval, treatment, recycling/reuse, and disposal of these graphite wastes. Considering the lifecycle of nuclear graphite, from manufacture to final disposal, a number of waste management options have been developed. These options consider the techniques and technologies required to address each stage of the lifecycle, such as segregation, treatment, recycle, and ultimate disposal in a radioactive waste repository, providing a toolbox to aid operators and regulators to determine the most appropriate management strategy. It is noted that national waste management programs currently have, or are in the process of developing, respective approaches to irradiated graphite management. The output of the Treatment and Disposal of Irradiated Graphite and other Carbonaceous Waste project is intended to aid these considerations, rather than dictate them.