Carbonation Kinetics of Cementitious Materials Used in the Geological Disposal of Radioactive Waste PDF Download

Author: Jia Sun
Publisher:
ISBN:
Category :
Languages : en
Pages : 0

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Author: J. Sun
Publisher:
ISBN:
Category :
Languages : en
Pages :

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The use of cement based materials could be widespread in the long term management of radioactive materials in the United Kingdom. In the Geological Disposal Concepts proposed by the Radioactive Waste Management Directorate of the Nuclear Decommissioning Authority (NDA), several cement based materials are used in the long-term management of intermediate-level wastes. Much of the waste will be immobilised within stainless steel containers using cement grouts based on ordinary Portland cement (OPC) blended with blast furnace slag (BFS) or pulverised fuel ash (PFA). The resulting waste packages will be placed underground in a Geological Disposal Facility (or Repository) after a period of storage at the waste producers' sites. The repository will then be filled with cement based backfill. The encapsulation grouts and the backfill materials will perform as both a physical barrier and chemical barrier for confining the radioactive wastes. During storage and disposal, some wastes may generate carbon dioxide from the degradation of organic materials and this will react with the cement based materials. Therefore, carbonation of the cementitious encapsulation grouts and backfill materials is of interest because of the resulting changes to their physical and chemical properties and also because of its ability to remove carbon-14 labelled carbon dioxide from the gas phase. It is also important to understand the reaction kinetics under a range of conditions, due to the long-term nature of storage and disposal. In this work, the carbonation progress of one backfill material and of two encapsulation grouts used in the UK has been studied in batch reactors. These materials are known as Nirex Reference Vault Backfill (NRVB), 3:1 PFA/OPC and 3:1 BFS/OPC. Based on the single dimensional carbonation experiments, fundamental parameters affecting the rate of carbonation were investigated and the carbon dioxide uptake capacity of each material was determined. For these three materials, an increase in relative humidity (75% to 100%) decreases the carbonation rate. A higher reaction pressure can facilitate the carbonation, but its effect was less obvious than the effect of relative humidity. The progression of the carbonation fronts have also been observed by various techniques and the shape of carbonation front was proved to be influenced by the relative humidity. Special attention was given to the modelling of the kinetics and mechanism of the carbonation reaction of these materials. This work provides fundamental understanding of the carbonation reaction of NRVB, 3:1 PFA/OPC and 3:1 BFS/OPC of relevance to the future optimization of a geological disposal facility in the UK and to assessments of the performance of such a facility.

Author: Rehab O. Abdel Rahman
Publisher: John Wiley & Sons
ISBN: 1118512006
Category : Science
Languages : en
Pages : 245

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Cementitious materials are an essential part in any radioactive waste disposal facility. Conditioning processes such as cementation are used to convert waste into a stable solid form that is insoluble and will prevent dispersion to the surrounding environment. It is incredibly important to understand the long-term behavior of these materials. This book summarises approaches and current practices in use of cementitious materials for nuclear waste immobilisation. It gives a unique description of the most important aspects of cements as nuclear waste forms: starting with a description of wastes, analyzing the cementitious systems used for immobilization and describing the technologies used, and ending with analysis of cementitious waste forms and their long term behavior in an envisaged disposal environment. Extensive research has been devoted to study the feasibility of using cement or cement based materials in immobilizing and solidifying different radioactive wastes. However, these research results are scattered. This work provides the reader with both the science and technology of the immobilization process, and the cementitious materials used to immobilize nuclear waste. It summarizes current knowledge in the field, and highlights important areas that need more investigation. The chapters include: Introduction, Portland cement, Alternative cements, Cement characterization and testing, Radioactive waste cementation, Waste cementation technology, Cementitious wasteform durability and performance assessment.

Author: Michael Ochs
Publisher: Springer
ISBN: 3319236512
Category : Technology & Engineering
Languages : en
Pages : 322

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Cementitious materials are being widely used as solidification/stabilisation and barrier materials for a variety of chemical and radioactive wastes, primarily due to their favourable retention properties for metals, radionuclides and other contaminants. The retention properties result from various mineral phases in hydrated cement that possess a high density and diversity of reactive sites for the fixation of contaminants through a variety of sorption and incorporation reactions. This book presents a state of the art review and critical evaluation of the type and magnitude of the various sorption and incorporation processes in hydrated cement systems for twenty-five elements relevant for a broad range of radioactive and industrial wastes. Effects of cement evolution or ageing on sorption/incorporation processes are explicitly evaluated and quantified. While the immobilisation of contaminants by mixing-in during hydration is not explicitly addressed, the underlying chemical processes are similar. A quantitative database on the solid/liquid distribution behaviour of radionuclides and other elements in hydrated cement systems is established on the basis of a consistent review and re-evaluation of literature data. In addition to recommended values, all underlying original experimental data and key experimental info rmation are provided, which allows users to trace the given recommendations or to develop their own set of key values. This database is closely tied to the safety analysis of near surface disposal of radioactive waste in Belgium. It focuses on radioelements, toxic stable elements and heavy metals, which makes it relevant for investigations involving the interaction of radioactive and conventional contaminants with cement-based barriers.

Author: Charlotte Dewitte
Publisher:
ISBN:
Category :
Languages : en
Pages : 0

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IRSN is in charge of assessing the safety of the design of the French deep geological repository project for radioactive waste (Cigéo). One of the major safety issues concerns the closure systems. The cementitious materials used in civil engineering structures (underground geological disposal of radioactive waste, dams, etc.) are designed to withstand various stresses and strains for several decades or even hundreds of years. However, in contact with soft or sea water, the leaching of the cement matrix is accompanied by an enrichment in magnesium leading to the formation of brucite (filling the porosity and forming a protective layer) and/or magnesium silicate hydrates (M-S-H), the properties of which are little studied. With the emergence of new binders, the mineralogical composition of hydrated concrete changes and the mechanisms of alteration in the presence of magnesium are different from those encountered in ordinary portlandite-rich cements. The formation of M-S-H is favoured over that of brucite. The objectives of this thesis are: (i) to understand the reaction mechanisms of magnesium attack and M-S-H formation in low-calcium cementitious matrices, (ii) to study the influence of M-S-H formation on microstructural and mechanical properties and (iii) to propose microstructural and mechanical data of M-S-H that can be integrated in chemo-mechanical models.Firstly, the influence of the composition of the cementitious paste on the phases formed during magnesium attack was studied. The presence of portlandite implies the formation of brucite. Incomplete hydration of low C/S binders, even in the absence of portlandite, can also lead to the formation of brucite. M-S-H is formed within the paste and corresponds to a deep magnesium enrichment. Secondly, a characterisation of M-S-H was carried out on M-S-H pastes in order to acquire data on the intrinsic physical and mechanical properties of M-S-H and thus to be able, via small-scale chemo-mechanical modelling, to understand the mechanisms of damaged properties under these attacks. Finally, a multi-physical study, at the mesoscopic scale, of the damaged properties of low-calcium matrices by waters containing magnesium was carried out. Two low C/S cementitious pastes (model paste based on colloidal silica and real paste based on silica fume and slag - T3) were exposed to solutions with different Mg concentrations (5 and 50 mM). The chemical and mineralogical characterisation showed that a deep decalcification takes place in parallel with a strong magnesium enrichment of the paste, corresponding to the formation of M-S-H. Micro-structural and mechanical characterisations (by indentation), carried out to observe the evolution along this degradation front, showed an increase in porosity and a strong loss of local elastic properties despite the formation of M-S-H. Mechanical homogenisation, based on previously determined M-S-H paste data, confirms that degraded M-S-H have low elastic properties. Increasing the Mg concentration does not change the mineralogy and chemistry of the degraded zone but only the degraded depth. The results on the real paste (T3) are similar to those on the model paste, validating the results on the latter. In addition, the model paste was also studied in pure leaching to compare the impact of leaching with that of magnesium attack on the low C/S binders. A study using a reactive transport code was carried out to better understand the differences in kinetics and phenomenology. It confirms that the presence of magnesium accelerates the degradation.

Author: Altmaier, M.
Publisher: KIT Scientific Publishing
ISBN: 3731508257
Category :
Languages : en
Pages : 342

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Author: Michael J Apted
Publisher: Elsevier
ISBN: 1845699785
Category : Technology & Engineering
Languages : en
Pages : 789

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Geological disposal has been internationally adopted as the most effective approach to assure the long-term, safe disposition of the used nuclear fuels and radioactive waste materials produced from nuclear power generation, nuclear weapons programs, medical, treatments, and industrial applications. Geological repository systems take advantage of natural geological barriers augmented with engineered barrier systems to isolate these radioactive materials from the environment and from future populations. Geological repository systems for safe disposal of spent nuclear fuels and radioactive waste critically reviews the state-of-the-art technologies, scientific methods, regulatory developments, and social engagement approaches directly related to the implementation of geological repository systems. Part one introduces geological disposal, including multiple-barrier geological repositories, as well as reviewing the impact of nuclear fuel recycling practices and underground research laboratory activities on the development of disposal concepts. Part two reviews geological repository siting in different host rocks, including long-term stability analysis and radionuclide transport modelling. Reviews of the range of engineered barrier systems, including waste immobilisation technologies, container materials, low pH concretes, clay-based buffer and backfill materials, and barrier performance are presented in Part three. Part four examines total system performance assessment and safety analyses for deep geological and near-surface disposal, with coverage of uncertainty analysis, use of expert judgement for decision making, and development and use of knowledge management systems. Finally, Part five covers regulatory and social approaches for the establishment of geological disposal programs, from the development of radiation standards and risk-informed, performance-based regulations, to environmental monitoring and social engagement in the siting and operation of repositories. With its distinguished international team of contributors, Geological repository systems for safe disposal of spent nuclear fuels and radioactive waste is a standard reference for all nuclear waste management and geological repository professionals and researchers. Critically reviews the state-of-the-art technologies, scientific methods, regulatory developments, and social engagement approaches related to the implementation of geological repository systems Chapters introduce geological disposal and review the development of disposal concepts Examines long-term stability analysis, the range of engineered barrier systems and barrier performance

Author: Altmaier, M.
Publisher: KIT Scientific Publishing
ISBN: 3731506602
Category :
Languages : en
Pages : 326

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Author: Jennifer Druhan
Publisher: Walter de Gruyter GmbH & Co KG
ISBN: 1501512005
Category : Science
Languages : en
Pages : 514

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Open system behavior is predicated on a fundamental relationship between the timescale over which mass is transported and the timescale over which it is chemically transformed. This relationship describes the basis for the multidisciplinary field of reactive transport (RT). In the 20 years since publication of Review in Mineralogy and Geochemistry volume 34: Reactive Transport in Porous Media, RT principles have expanded beyond early applications largely based in contaminant hydrology to become broadly utilized throughout the Earth Sciences. RT is now employed to address a wide variety of natural and engineered systems across diverse spatial and temporal scales, in tandem with advances in computational capability, quantitative imaging and reactive interface characterization techniques. The present volume reviews the diversity of reactive transport applications developed over the past 20 years, ranging from the understanding of basic processes at the nano- to micrometer scale to the prediction of Earth global cycling processes at the watershed scale. Key areas of RT development are highlighted to continue advancing our capabilities to predict mass and energy transfer in natural and engineered systems.

Author: Christoph Borkel
Publisher:
ISBN:
Category :
Languages : en
Pages : 239

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Cementitious materials are used to condition or stabilise waste and to build infrastructure in disposal sites. Moreover, they are envisaged to form part of engineered barrier systems as container, backfill or liner materials in radioactive waste disposal concepts. In the event of contact with water, contaminants dissolve and their mobility is influenced by the employed cementitious materials. Therefore, sound understanding of the interactions between contaminants and degrading cementitious materials in flowing water is essential for safety assessment. The aim of this study was to identify the processes affecting retention of caesium, nickel and selenium on Hardened Cement Paste (HCP) during its degradation, from sane to severely degraded states. The focus was put on the underlying mechanisms and possible remobilisation of previously retained contaminants due to the changing composition of the HCP. Caesium, as Cs(I), nickel, as Ni(II) and selenium, as Se(VI), were chosen because they are considered as safety relevant radionuclides for nuclear waste disposal, represent different chemical characteristics and their stable isotopes can be used in experiments. To address shortcomings of previous studies in this field a combined approach was developed. First, a previously used thin-layer flow-through reactor was adapted and improved for the needs of studying contaminant retention and release during degradation of the multiphase material HCP. Second, retention and degradation were studied in equilibrated batch systems as well. Regarding degradation of HCP the following results were obtained: (1) The thin-layer flow-through setup was established for degradation of HCP at far-from-equilibrium conditions and a number of relevant experimental data were obtained. (2) A kinetic degradation model satisfactorily reproduced experimental results on HCP degradation. For this, a set of dissolution rate constants of cement phases was optimised which can also be used for other modelling studies. (3) The same model also satisfactorily reproduces results from experiments with different aqueous CO2 concentrations and with different solution types, i.e. synthesised granitic groundwater (GG water) and deionised (DI) water. (4) When quantitatively comparing different solution types, degradation of HCP equilibrated with GG water is stronger than after equilibration with DI water, due to higher aqueous CO2 concentration. Further, the effects of carbonate buffering and carbonation on HCP at far-from-equilibrium conditions were identified and quantified. (5) Four characteristic stages of HCP degradation in flow-through conditions were classified, taking into account carbonate buffering effects. The different stages can be discerned on-line by measurement of pH, Ca and Si concentrations in outflow solutions. Regarding retention of Cs, Ni and Se(VI) in HCP conditioned systems the following results were obtained: (1) Caesium and selenate distribution coefficients were determined in equilibrated systems at different degradation states of HCP. (2) Caesium and selenate retention was quantified at flow conditions during continuous degradation of HCP in DI and GG water and likely retention mechanisms were narrowed down.(3) In the case of nickel, the solubility limiting phases formed in presence of HCP were identified at different degradation states. (4) The formation of a so far non-described nickel-silicate-hydrate was observed in the more degraded system at pH around 11.6. (5) The influence of different aqueous CO2 concentrations on Cs, Ni and Se(VI) retention was demonstrated to be minor. This study showed that the persistency of contaminant retention by adsorption in degrading cementitious systems is not only a question of distribution coefficients at different degradation states, but also a question of how fast these degradation states are reached.