The Experts below are selected from a list of 10398 Experts worldwide ranked by ideXlab platform
Susan A Bernal - One of the best experts on this subject based on the ideXlab platform.
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gaseous carbonation of cementitious backfill for Geological Disposal of radioactive waste nirex reference vault backfill
2019Co-Authors: N C Collier, Susan A Bernal, David W Heyes, Ed J Butcher, Jason Borwick, A E Milodowski, L P Field, S J Kemp, I MounteneyAbstract:The ability of Nirex Reference Vault Backfill (NRVB), a cement backfill material, to capture carbon dioxide from Intermediate Level Radioactive waste packages after repository backfilling, has been assessed. Large-scale trials assessed the physical and chemical reaction of carbon dioxide with the hardened backfill grout. A carbonation front, radial in nature, was observed extending into the grout and three distinct regions were identified in the hardened grouts. A carbonated region, a carbonation front, and a partially carbonated zone were discerned. Potassium, and to a lesser extent sodium, were concentrated in the carbonated region just behind of the main reaction front. The area just ahead of the carbonation front was enriched in both sulphur and aluminium, while sulphur was found to be depleted from the carbonated material behind the main reaction front. Within the main carbonated region, virtually all of the hydrated cement phases were found to be carbonated, and carbonation extended throughout the grout, even within material indicated by phenolphthalein solution to be uncarbonated. Importantly, carbonation was observed to impact both the mineral assemblage and porosity of the cement backfill; it is therefore important to understand these characteristics in terms of the long term evolution of NRVB and its groundwater buffering safety function within the Geological Disposal facility near-field.
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thermodynamic modelling of bfs pc cements under temperature conditions relevant to the Geological Disposal of nuclear wastes
2019Co-Authors: Dale P Prentice, Brant Walkley, Susan A Bernal, Mark Bankhead, Martin Hayes, John L. ProvisAbstract:Abstract Intermediate level waste produced in UK nuclear power generation is encapsulated or immobilised in blended cements comprising blast furnace slag (BFS) and Portland cement (PC), to be emplaced in a proposed Geological Disposal facility (GDF). The wasteforms are expected to be exposed to temperatures from 35 to 80 °C during the initial 150 years of GDF operation. Thermodynamic modelling is applied here to describe the phase assemblages of hydrated 1:1, 3:1 and 9:1 BFS-PC blends, with the participation of hydrogarnet as an important phase above 60 °C. The chemical composition of the main phase forming in these systems, an aluminium rich calcium silicate hydrate (C-A-S-H), was well described by a solid-solution model with explicit Al incorporation, although the Al/Si ratio was systematically slightly under-predicted. The developed thermodynamic model predicts the correct phase assemblage across varying temperature regimes, making it a valuable tool to assess the effects of temperature on cements.
Rodney C Ewing - One of the best experts on this subject based on the ideXlab platform.
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mission impossible socio technical integration of nuclear waste Geological Disposal systems
2018Co-Authors: Francois Diazmaurin, Rodney C EwingAbstract:We present a new perspective on Geological Disposal systems for nuclear waste. Geological Disposal systems encompass all the processes required for the permanent isolation of highly-radioactive materials from humans and the biosphere. Radioactive materials requiring Geological Disposal are created by commercial nuclear power plants, research reactors, and defense-related nuclear activities, such as spent nuclear fuel from commercial reactors and high-level waste from reprocessing to reclaim fissile material for weapons. We show that Disposal systems are so complex that new methods of representation are required. Despite the common call for a systems approach, a broader perspective is needed to obtain an integrated view of Disposal systems. We introduce a conceptual formalism of Geological Disposal systems based on a multi-scale integrated analysis approach. This ‘metabolic’ representation allows one to account for the technical complexity of Disposal systems in relation to their broader societal context. Although the paper is conceptual, the integrated formalism can improve the understanding of the complexity of Disposal systems and their policy requirements by connecting technical solutions with societal constraints. However, the paper also reveals the limits to efforts to integrate technical and social dimensions of Geological Disposal systems into a single formalism.
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Geological Disposal of nuclear waste a primer
2016Co-Authors: Rodney C Ewing, Robert A Whittleston, B W D YardleyAbstract:The back-end of the nuclear fuel cycle has become the Achilles Heel of nuclear power. After more than 50 years of effort, there are, at present, no operating nuclear waste repositories for the spent nuclear fuel from commercial nuclear power plants or for the high-level waste from the reprocessing of spent fuel. The articles in this issue of Elements describe the status of Geological Disposal in salt, crystalline rock, clay, and tuff, as presently developed in five countries.
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less geology in the Geological Disposal of nuclear waste
1999Co-Authors: Rodney C EwingAbstract:New rules and standards substantially decrease the role of geologic barriers in nuclear waste repositories. The reliance on probabilistic performance assessment to provide the sole quantitative criterion for compliance, in conjunction with the elimination of performance standards for individual barriers, the Geologically short compliance period, and the extended distance to the point of compliance, reduce substantially the role of the geologic properties of the repository in the waste containment strategy.
Claire L. Corkhill - One of the best experts on this subject based on the ideXlab platform.
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Characterisation of a high pH cement backfill for the Geological Disposal of nuclear waste: The Nirex Reference Vault Backfill
2018Co-Authors: Rita G. W. Vasconcelos, Nicolas Beaudoin, Andrea Hamilton, Neil C. Hyatt, John L. Provis, Claire L. CorkhillAbstract:In a conceptual UK Geological Disposal facility for nuclear waste within a high-strength, crystalline geology, a cement-based backfill material, known as Nirex Reference Vault Backfill (NRVB), will be used to provide a chemical barrier to radionuclide release. The NRVB is required to have specific properties to fulfil the operational requirements of the Geological Disposal facility (GDF); these are dependent on the chemical and physical properties of the cement constituent materials and also on the water content. With the passage of time, the raw materials eventually used to synthesise the backfill may not be the same as those used to formulate it. As such, there is a requirement to understand how NRVB performance may be affected by a change in raw material supply. In this paper, we present a review of the current knowledge of NRVB and results from a detailed characterisation of this material, comparing the differences in performance of the final product when different raw materials are used. Results showed that minor differences in the particle size, surface area and chemical composition of the raw material had an effect on the workability, compressive strength, the rate of hydration and the porosity, which may influence some of the design functions of NRVB. This study outlines the requirement to fully characterise cement backfill raw materials prior to use in a Geological Disposal facility and supports ongoing assessment of long-term post-closure safety.
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dissolution of uk high level waste glass under simulated hyperalkaline conditions of a colocated Geological Disposal facility
2013Co-Authors: Claire L. Corkhill, Nathan J Cassingham, Paul G Heath, Neil C. HyattAbstract:We report analysis of chemical durability of UK HLW MW+25% simulant glass under model hyperalkaline conditions of a colocated Geological Disposal facility. Glass powders and monoliths were dissolved for 168 days in saturated Ca(OH). Dissolution in the presence of high concentrations of Ca (>200 mg/L) was an order of magnitude lower than dissolution in water. Dissolution of Si did not occur until a Ca:Si ratio of <2 was achieved. The mechanism of dissolution involved the incorporation of Ca into the hydrated surface (initial, incubation regime), the precipitation of C-S-H phases, including a range of compositions in the C-(N)-(A)-S-H and M-S-H systems (intermediate regime), and the precipitation of C-S-H phases (the residual regime). Thermodynamic analysis and consideration of the CaO-SiO-HO phase diagram suggest that the rate-limiting step of glass dissolution in Ca-rich solutions is Ca-Si equilibrium, involving the precipitation of C-S-H phases, which change in chemical composition as a function of solution chemistry. In low SA/V ratio experiments, the dissolution progressed only to the initial incubation regime, resulting from fewer surface sites for Ca incorporation. Overall, these results suggest that Ca and Si in solution play an important role in the long-term durability of UK HLW in Ca-rich solutions. © 2013 The Authors. International Journal of Applied Glass Science published by John Wiley & Sons Ltd on behalf of The American Ceramic Society and Wiley Periodicals, Inc.
Jonathan R. Lloyd - One of the best experts on this subject based on the ideXlab platform.
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bentonite barrier materials and the control of microbial processes safety case implications for the Geological Disposal of radioactive waste
2021Co-Authors: Haydn M Haynes, Matthew T Bailey, Jonathan R. LloydAbstract:Abstract Higher activity radioactive wastes represent a significant long-term human and ecological hazard. There is an international consensus that Geological Disposal of these materials is the most responsible approach to their long-term management, in order to safely contain and isolate them from people and ecosystems. Some higher radioactivity Disposal concepts use a bentonite barrier to surround metallic waste containers. For bentonite to satisfy its function as a barrier material, it is required to protect waste containers from corroding agents, limit the release of radionuclides, provide stability against rock displacements, and ensure excess gas pressure build-up does not occur. Microbial processes within bentonites, if they develop, have the potential to alter the properties of the material and associated pore waters. This review therefore focuses on the microbial colonization of bentonite buffers, and in particular examines the roles of (i) sulphide-producing bacteria (SPB) and (ii) iron(III)-reducing bacteria (IRB). These groups are significant, since sulphide production is implicated in container corrosion and longevity and, the reduction of structural iron(III) in bentonite could affect its geo-chemical/physical properties. Conversely, microbial activity may have positive safety case-related functions, reducing radionuclide transport by transforming radionuclides into insoluble forms and reducing gas build-up by consuming hydrogen, for example. On balance the review indicates that preventing microbial activity within bentonite buffers, to avert any potential deleterious effects, is a higher priority than harnessing any potential benefits which may arise. In order to do this, bentonites used in a Geological Disposal context, e.g., in barrier systems, should be engineered so as to be able to attain a swelling pressure, on re-saturation, to an extent proven to disrupt microbial activity. Some uncertainty remains, however, in very long-term evolution of the bentonite, where degradation and loss of swelling pressure may occur in localized areas. Further research should consider the rates of microbial growth and metabolism under repository relevant conditions, through experimentation, study of natural analogues and numerical modelling.
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the microbial ecology of a hyper alkaline spring and impacts of an alkali tolerant community during sandstone batch and column experiments representative of a Geological Disposal facility for intermediate level radioactive waste
2016Co-Authors: Sarah L Smith, Athanasios Rizoulis, Julia M West, Jonathan R. LloydAbstract:Naturally occurring hyper-alkaline springs and associated hyper-alkaline environments may have components that are analogous to a cement-based deep Geological Disposal facility (GDF) for intermediate level radioactive waste (ILW). Such high pH environments could give insights into the biogeochemical processes that could occur in the region of a GDF environment after the ingress of GDF-derived groundwater leads to the formation of a hyper-alkaline plume in the surrounding rock mass. This study focuses on the microbial community composition found at a highly alkaline spring near Buxton, Derbyshire, England, and the variation in community structure across spatially separated sample points of contrasting pH values (ranging from pH 7.5–13). Communities containing alkaliphilic and alkalitolerant bacteria were observed across the site by PCR amplification and 16S rRNA gene pyrosequencing and included members of the families Comamonadaceae and Xanthomonadaceae. At pH 13, the sequence library was dominated by Gammaproteobacteria of the families Pseudomonadaceae and Enterobacteriaceae. Bacterial communities from the site demonstrated the ability to reduce Fe(III) in microcosm experiments up to pH 11.5, suggesting the potential to reduce other metals and radionuclides of relevance to cement-encapsulated intermediate level radioactive waste (ILW) Disposal. In laboratory column flow-through experiments, microbial communities present at the field site were also able to colonize crushed sandstone. Bacterial community composition varied between columns that had been supplied with alkali surface waters from the site amended with carbon (lactate and acetate, as proxies for products of cellulose degradation from ILW), and control columns that were not supplied with added carbon. Members of the family Clostridiaceae dominated the sequence library obtained from the carbon amended column inlet (45.8% of library), but became less dominant at the outlet (20.8%). Members of the family Sphingomonadaceae comprised 11.8% of the sequence library obtained from the control column inlet, but were not present in sediments collected from the column outlet, whereas the relative abundance of members of the family Comamonadaceae increased from the column inlet (35.2%) to the column outlet (57.2%). The spatial variation in community composition within the columns is indicative of discrete biogeochemical zonation in these flow-through systems.
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microbial reduction of fe iii under alkaline conditions relevant to Geological Disposal
2013Co-Authors: Adam J Williamson, Katherine Morris, Samuel Shaw, James M Byrne, Christopher Boothman, Jonathan R. LloydAbstract:To determine whether biologically mediated Fe(III) reduction is possible under alkaline conditions in systems of relevance to Geological Disposal of radioactive wastes, a series of microcosm experiments was set up using hyperalkaline sediments (pH ~11.8) surrounding a legacy lime working site in Buxton, United Kingdom. The microcosms were incubated for 28 days and held at pH 10. There was clear evidence for anoxic microbial activity, with consumption of lactate (added as an electron donor) concomitant with the reduction of Fe(III) as ferrihydrite (added as the electron acceptor). The products of microbial Fe(III) reduction were black and magnetic, and a range of analyses, including X-ray diffraction, transmission electron microscopy, X-ray absorption spectroscopy, and X-ray magnetic circular dichroism confirmed the extensive formation of biomagnetite in this system. The addition of soluble exogenous and endogenous electron shuttles such as the humic analogue anthraquinone-2,6-disulfonate and riboflavin increased both the initial rate and the final extent of Fe(III) reduction in comparison to the nonamended experiments. In addition, a soluble humic acid (Aldrich) also increased both the rate and the extent of Fe(III) reduction. These results show that microbial Fe(III) reduction can occur in conditions relevant to a Geological Disposal facility containing cement-based wasteforms that has evolved into a high pH environment over prolonged periods of time (>100,000 years). The potential impact of such processes on the biogeochemistry of a Geological Disposal facility is discussed, including possible coupling to the redox conditions and solubility of key radionuclides.
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The potential impact of anaerobic microbial metabolism during the Geological Disposal of intermediate-level waste
2012Co-Authors: Athanasios Rizoulis, Katherine Morris, H. M. Steele, Jonathan R. LloydAbstract:Microbial metabolism has the potential to control the biogeochemistry of redox-active radionuclides in a range of geoDisposal scenarios. In this study, sediments from a high pH lime workings site were incubated under carefully controlled anaerobic conditions, at a range of alkali pH values with added electron donors and electron acceptors, to explore the limits and rates of bioreduction in a sediment system analogous to intermediate-level nuclear waste. There was a clear succession in the utilization of electron acceptors (in the order nitrate > Fe(III)-citrate > Fe(III) oxyhydroxide > sulfate), in accordance with calculated free energy yields and Eh values over the pH range 10� 12. The rate and extent of bioreduction decreased at higher pH, with an upper limit for the processes studied at pH 12. The biochemical limits for such processes are discussed, alongside the potential impact of such forms of microbial metabolism on the solubility of a range of redox active radionuclides that will feature heavily in the safety case for the Geological Disposal of intermediate-level nuclear waste.
J A Lawrence - One of the best experts on this subject based on the ideXlab platform.
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a sensitivity study of the factors affecting the risks associated with the Disposal of spent nuclear fuel in a Geological Disposal facility in a clay environment
2021Co-Authors: Ashley I Marsh, Freng Laurence Williams G Obe, J A LawrenceAbstract:Abstract The Disposal route for the UK's inventory of spent nuclear fuel has yet to be decided upon, however if the UK follows the approach taken by many nations, its spent nuclear fuel will be destined for Geological Disposal. This paper provides a simplified, approach to the calculation of radiological risk posed to members of the public, associated with the Disposal of spent advanced gas-cooled reactor (AGR) nuclear fuel in a Geological Disposal facility (GDF) located in a hypothetical clay Geological setting. Simulating the release of radionuclides at differing levels of complexity shall aid in the development of aspects of the design, construction, operation, decommissioning and post-closure phases of Geological Disposal and the development of a safety case for Geological Disposal. This paper presents a high-level, 1-D model built in GoldSim, to provide an initial indication of the radiological risks to the public, the nature of which would require further development into a complex total system model in order to facilitate risk evaluation supplementary to a safety case. The model was validated against benchmark calculations provided by Radioactive Waste Management Ltd. The base case calculations suggest that the predicted risks to the potential exposed groups are well below, approximately 2 orders, the recommended risk guidance level of 1 × 10−6 per year. A number of sensitivity studies were carried out to identify the importance of various factors that could influence the predicted risks. Sensitivity analysis indicated that the most influential sensitivities on the annual risk posed by Geological Disposal were the depth and rate at which spent nuclear fuel dissolved in contact with groundwater. The pathways available for groundwater flow to the biosphere were also noted to significantly alter the peak risk observed; crucially almost all sensitivities did not increase the peak risk to within 1 order of magnitude below the recommended risk guidance level. Validation for a second hypothetical high strength rock Geological setting resulted in higher projected predicted risks to the potential exposed groups although risk levels remained below the recommended risk guidance level. This suggests a clay environment may have favourable characteristics for the final Disposal of spent nuclear fuel compared to a high strength rock alternative.
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the important role and performance of engineered barriers in a uk Geological Disposal facility for higher activity radioactive waste
2021Co-Authors: Ashley I Marsh, Laurence G Williams, J A LawrenceAbstract:Abstract The effective management of radioactive waste is a necessary prerequisite to the use of nuclear energy. The UK's policy for the long-term management of higher activity radioactive waste (HAW), and potentially spent nuclear fuel (SNF), is Disposal in a deep underground Geological Disposal facility (GDF). A GDF will isolate HAW from mankind until the radioactivity has decayed to levels where any risk to future generations is acceptably low. It is likely, therefore, that a GDF will need to safely contain radioactive materials for hundreds of thousands of years. The necessary isolation will be provided by a combination of natural (Geological) and engineered barriers. A multi-layered engineered barrier system will provide the defence-in-depth that is required to give the public confidence in the long-term performance of the GDF. This paper identifies the significant role each engineered barrier or “layer” plays in ensuring that long-lived radionuclides remain isolated from the biosphere and receptors within the vicinity of a GDF. Receptors include human and animal populations, and the natural environment. The paper also explores the characteristics and performance of a number of suitable candidate materials for use in the UK GDF engineered barriers. An indication of the lifetime of potential barriers under conditions pertinent to each of the UKs proposed Geological settings is given. As the performance of the engineered barriers will be vital to the GDF post-closure safety case, several areas for further work are proposed.