Log in to your account. Advanced search Tag cloud. Login: Password:. Contributor s : Bart, Florence [editor. Tags from this library: No tags from this library for this title. Total holds: 0. Log in to your account to post a comment. The crystallization pressures resulting from the interaction between growing monosulfate crystals and the surrounding C-S-H matrix are assumed to cause the observed macroscopic swelling. A macroscopic bulk strain tensor calculated from the volume of formed ettringite is introduced for directly reproducing these expansions.
Explicit upscaling techniques applied on a simplified representation of the materials allow estimating both mechanical and diffusive properties of the evolving microstructure. Finally, simulations of laboratory tests investigating the effects of ESA on mortar specimens are performed and analyzed. Comparison of the numerical results with experimental ones is performed and discussed. Micro-XRF maps reveal a heterogeneous distribution of the actinides.
The present study is complementary to earlier ones carried out in our research group and demonstrates that information on the coordination environment of actinides is essential with the aim of developing a mechanistic understanding of actinide interaction with cementitious materials. The micro-scale studies show that actinide uptake by cementitious materials has to be interpreted in terms of solid solution formation rather than surface binding ion exchange, surface complexes. Mainly on the basis of literature data, best estimate sorption values in addition to upper and lower bound values were determined for a so-called benchmark cement—the unperturbed cement without effects of organics, high chloride content or other chemical components that might adversely impact radionuclide sorption.
Effects of perturbing components are discussed separately. The geochemical evolution of the cementitious engineered barriers was also addressed to clarify the conditions under which sorption values are applicable.
A substantial part of the scientific basis supporting the data selection was established at several meetings of an international panel of experts who reviewed and endorsed the data selection. To this end, the sorption data were checked for reliability, appropriateness for the conditions expected for the Dessel disposal facility, data quality, time frames i. Wang, M. Ochs, D. Mallants, L. Vielle-Petit, E. Martens, D. Jacques, P.
Cement-Based Materials for Nuclear Waste Storage : Florence Bart :
Berry, B. In this chapter we propose to develop an experimental way to obtain the diffusion coefficient of chloride through porous materials. The method, based on the application of the Nernst—Einstein equation, requires the determination of the electrical conductivity of the specimen.
On that purpose, an electrochemical impedance spectroscopy test is used. An experimental setup has been established and carefully controlled in order to improve the accuracy of the measurements. The resulting impedance spectra are analyzed by means of two different electrical models. A conventional equivalent circuit is first applied to get the main spectrum shape features and the material characteristics. Another physical-phenomenon-based equivalent circuit is then used and intended to provide better explanation of the material microstructure and its electrical behavior.
The results obtained are validated with data previously found, arising from an electrokinetic test based on the Nernst—Planck equation. This paper reviews the multi-phase modeling of moisture transport process in pore structure of cement-based materials used as engineered barriers in radioactive waste disposal. The emphasis is put on the fundamental relationship of moisture isotherm and the related hysteresis phenomenon.
A typical cement-based material is retained for study and its properties for moisture transport were measured.
The pore structure was characterized by mercury intrusion porosimetry MIP and gravimetry method. The moisture isotherm was measured in laboratory by humidity equilibrium method and the predicted isotherm from MIP pore structure is confronted with the measured isotherm. Afterwards, a numerical scheme is set up for the multi-phase transport model and the model is applied to the moisture transport process of engineered barriers exposed to natural drying and drying—wetting cycles.
It is observed that the ratio between drying and wetting periods has strong influence on the depth of surface convection zone. This work documents the relationship between geopolymers, which are materials with a quasi-monomodal pore network, and their gaseous diffusivity capacities.
Cement-Based Materials for Nuclear Waste Storage (2012, Hardcover)
Using a monomodal material allows studying a specific pore size contribution to gaseous diffusion. The pore network is characterized by mercury porosimetry. The modeling work consists of creating a virtual pore network. Then, water layers are deposited in this network to simulate variable water saturation levels. Finally, hydrogen is transported through the virtual network using a combination of ordinary diffusion and Knudsen diffusion.
The impacts of the pore network arrangement or the pore network discretization are also studied. In order to be able to simulate the behaviour of radionuclides RN in waste repositories in space and time it is important to know their chemical speciation. Similarly, tritium in the reduced gaseous form, HT, does not at all behave as its oxidised form liquid water, HTO.
Carbonation kinetics of cementitious materials used in the geological disposal of radioactive waste
For other RN such as U, Se, Tc, Np and Pu the impact is less striking as the change in redox state does not generate a phase change but a change in the sorption behaviour. As a rule of thumb the oxidised form is more mobile than the reduced form. Nuclear waste repositories for both low and intermediate level wastes are characterised by the presence of cementitious phases and zero-valent metals as part of waste, waste containers or engineered materials; organic matter is also likely present in both waste and engineered barrier.
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Hydrogen gas can be formed either via radiolysis or anaerobic corrosion. Oxygen, present during the exploitation phase, is quickly consumed and not considered to diffuse significantly into deep or near-surface repositories. There is more and more evidence for the microbial control of reactions implying electron transfer within H and C species [Hoehler TM Biogeochemistry of dihydrogen H2. Furthermore, the impact of microbial activity on the degradation of complex organic matter i. Waste Manag 93—].
There are many analogues for testing simulation approaches for microbial catalysis of related redox reactions, but few are in alkaline systems. With H 2 almost omnipresent as an energy source, essential and trace nutrients most likely present in the heterogeneous waste cell environment, with space and water available depending on depth, architecture and re-saturation, the high pH may become the most critical parameter controlling microbial activity in space and time.
In this chapter, we will review the importance of oxyanions in the nuclear industry and their impact together with concrete, steel and organic matter on the redox state in the near field of a waste storage cell. Particular consideration will be given to the knowledge in relation to alcaliphilic microbial activity in some cases derived from existing natural analogues.
Case studies will consider specific redox-sensitive radionuclides in both near surface and deep storage settings.
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In the French geological repository concept, intermediate-level vitrified wastes could be disposed of in a cement medium. The glass dissolution mechanisms and kinetics, expected to depend strongly on the chemical composition and pH of the leaching water, were studied in various cement pore water compositions corresponding to different stages of cement aging. A decrease in the maximum glass dissolution rate due to the effect of calcium was clearly observed compared to the reference medium, i.
At higher reaction progress, calcium in solution was almost totally consumed after a few days, probably due to the formation of Calcium Silicate Hydrate C-S-H phases with silicon leached from the glass. It has long been common practice to solidify and stabilize low- and intermediate-level radioactive wastes with calcium silicate cements ordinary Portland cement, or composite cement.
However, the quality of the final product may be noticeably reduced by adverse cement—waste interactions. This article reviews the potential of three kinds of alternative inorganic binders to treat problematic wastes: 1 calcium aluminate and sulphoaluminate cements, 2 magnesium and calcium phosphate cements, and 3 alkali-activated binders. Their setting and hardening process is briefly presented, and their potential for waste conditioning is discussed. This chapter reports the first leaching experiments performed on a simulated ZnCl 2 -rich waste stabilized with calcium sulfoaluminate cement according to a standard test developed to understand and model the degradation processes of the cement paste.
Zinc was never detected, showing the excellent confining properties of the cement matrix. The hydrated phase evolution along depth was qualitatively reproduced with a model coupling transport by diffusion and chemical reactions.
https://rinrayreperdau.tk XRD and TGA analysis were carried out on pastes with increasing hydration degrees up to 90 days to specify the mineralogy and to figure out the mechanisms of borate immobilization. It has been shown that the retarding effect of borate anions is due to the precipitation of the amorphous calcium borate C 2 B 3 H 8 ; borate anions were then incorporated in Aft-type phases. The macroscopic properties of hydrated binders compressive strength, length change were also followed during days.
Champenois, C. Cau-dit-Coumes, A.