The geological disposal of radioactive waste is under investigation in several countries, especially in Europe. In this field, clay-rich sedimentary rocks are promising natural barriers to limit and delay the release of radionuclides toward the biosphere, due to their diffusion-driven transport and retention properties. In addition to laboratory studies, field studies in Underground Research Laboratories (URL) represent a significant value to strengthen safety demonstrations. In this context, the DIR experiment (Diffusion of Inert and Reactive tracers) was developed and implanted in the Meuse/Haute-Marne URL located in the Callovo-Oxfordian clay-rich formation. Over 6 years, several diffusion experiments were performed with radioactive tracers: HTO (tritiated water), Cl-36(-), I-125(-), Na-22(+), Cs-134(+), assessing the behaviour of neutral, anionic and cationic species. The evolution of each radiotracer activity was monitored in the injection borehole, from at least 100 days up to 3 years. Overcoring of the rock at the end of the experiments allowed the accurate characterization of radiotracer profiles. An overview of the DIR experiment is provided here, illustrating the large amount of data gathered in 4 specific boreholes at two different depths: -445 and - 495 m, within the Callovo-Oxfordian geological layer. The modelling of experimental results allowed the quantification of diffusion properties parallel to the bedding of the rock for the studied radionuclides. Diffusion parameters for HTO are in agreement with those determined on samples using the conventional through diffusion method. Anions are partially excluded from porosity with quantified values 0.3 < epsilon(a)(anion)/epsilon(H2O) < 0.6. On contrary, the diffusion of cations is limited by their adsorption on clay surfaces with the evidence of surface diffusion processes. The sensitivity analysis on adjusted parameters provided additional insights on the local properties, such as homogeneity of the diffusion plume and the limited extension of the damaged zone surrounding boreholes: L-EDZ similar to 1-3 cm. Still, further field experiments are necessary to measure diffusion in such EDZ and to improve accuracy on the diffusion anisotropy ratio.
This paper presents a detailed experimental investigation of the near-wall transport dynamics in Taylor-Couette-Poiseuille flows using measurements of the large-scale variations of local wall-shear-rate components. A three-segment electrodiffusion probe was used to conduct multiple experiments across a wide range of Taylor numbers (300 < Ta <2520) and Reynolds numbers (2 < Re <32) to capture laminar, transitional, and turbulent flow conditions. Both azimuthal and axial wall shear rate component measurements were made in order to directly correlate the effects of flow structures on the transport of momentum in the near-wall region. The results show that the average azimuthal wall shear rate is primarily influenced by the Taylor number, while its variation with the Reynolds number remains within the experimental uncertainty. On the other hand, the large-scale shear rate dynamics are strongly influenced by the flow structures. More specifically, vortical flows have almost identical average wall shear rates as helical flows but distinctly different temporal characteristics and fluctuation magnitude. The differences in large-scale wall shear dynamics between the two flow structures arise from distinct mechanisms of momentum transfer; vortices induce high shear rates through rapid radial inflow and outflow, whereas helices generate low shear rates via a much more uniform shear distribution due to their oblique configuration. These findings suggest that the average wall shear stress alone is insufficient to determine the transport processes occurring in the wall region. Localized measurements are required in order to establish the relationship between flow structure and transport dynamics within the measurement bandwidth. These findings provide new insight into shear-induced transport in rotating flows and establish experimental benchmarks for the development and validation of transport models.
This paper presents a comprehensive electromagnetic transient model for a switched reluctance motor to enable its simultaneous integration into an electromagnetic transient (EMT) type solver with surrounding drive components. In this regard, a circuit-based representation of the machine is developed and used. The proposed model seamlessly integrates a nonlinear permeance network. The machine’s electromagnetic model is automatically constructed using a mesh-based approach, enabling local studies within the machine while accounting for the complete drive system. The airgap is modeled with variable permeances, enabling transient studies without remeshing or reconnecting nodes during runtime. Comparison with the finite element-based model confirms the accuracy of the proposed model and demonstrates its lower computational time. The proposed model is used to simulate transient scenarios of the entire switched reluctance drive system under faulty conditions.
The reduction of tetravalent manganese (Mn-(IV)) to trivalent manganese (Mn-(III)) by HEPES Good's buffer is often used to modify the reactivity of δ-MnO2 and to distinguish between the Mn-(III) and Mn-(IV) oxidants in redox reactions. However, the structure of HEPES-reacted δ-MnO2 has remained elusive, hindering a detailed understanding of interfacial electron transfer between adsorbed species and structural Mn. Here, we characterized the structure of δ-MnO2 reacted with HEPES at pH 6 and 8 under low and high NaCl ionic strength, using chemical analysis, high-energy X-ray diffraction, pair distribution function (PDF), extended X-ray absorption fine structure (EXAFS) spectroscopy, and high-resolution transmission electron microscopy (HRTEM) coupled with selected area electron diffraction (SAED). The average Mn oxidation state (AMOS) decreases from 3.92-3.87 to 3.71-3.59 after HEPES addition, depending on pH and ionic strength. HEPES-reacted δ-MnO2 has a distinctly different structure at low and high ionic strength. At low ionic strength, the δ-MnO2 HE crystallites are 3-6 nm across, and the MnO2 layers have approximately 23% vacant sites capped with mainly Mn-(III) and some Mn-(II). At high ionic strength and pH 8, δ-MnO2 HE contains large crystals, several hundred nanometers across, made up of crystallographically oriented nanodomains. Most SAED patterns show streaks along the [100]* direction, indicating a high degree of disorder in the close packing of the anionic sheets, in the Na position within the interlayer, and in the Mn-(IV)-Mn-(III) distribution within the layer. Some nanodiffraction patterns show distinct superstructure reflections along the streaks with A* = 3a*, as seen in well-crystallized triclinic birnessite, and A* = 6a*. High-ionic-strength δ-MnO2 HE has no interlayer Mn-(III), and the Na-(I) ions, along with the layer Mn-(III) and Mn-(IV) cations, are semiordered at the short- to medium-range scales and essentially disordered over longer distances. Identifying the two distinct structures of HEPES-reacted δ-MnO2 clarifies structural ambiguities reported in the literature and provides a solid foundation for exploring its redox reactivity and electrochemical performance.
Iron-titanium (Fe-Ti) charge transfer is mentioned in numerous articles as the source of the coloration of many natural minerals and some manufactured materials, but no global review of this phenomenon has been provided so far. Iron and titanium are ubiquitous in nature and are often found in the same material as Fe2+ and Fe3+, and Ti4+ (more rarely Ti3+). When Fe and Ti ions are in close geometric proximity in an oxide or (alumino)silicate structure, charge transfer can occur between the two ions, even though their concentration might be below 100 ppm. This results in a variety of absorption features that contribute to the color of minerals. A debate remains on the exact nature of Fe/Ti electronic transition, i.e., Fe2+ + Ti4+ -> Fe3+ + Ti3+ or the reverse, but solving this issue is not within the scope of the present work. Ascertaining a metal-metal charge transfer is often not straightforward. This review compiles existing knowledge on Fe-Ti charge transfer in both crystalline and amorphous materials and identifies several key characteristics in more than 40 different materials. A charge transfer is associated with broad, intense, optical absorption bands that decrease in intensity at elevated temperatures. It is also strongly pleochroic in non-isotropic materials. Until now, Fe-Ti charge transfer transitions have been primarily described in the 2.25 to 3.1 eV range, corresponding to yellow to orange to brown colors, with notable exceptions such as blue sapphire or kyanite, and green andalusite. This review suggests that Fe-Ti charge transfer can occur across the entire visible spectrum, and the position of the absorption band correlates with the Fe-Ti interatomic distance. This correlation highlights the presence of multiple crystallographic sites for both Fe and Ti in many oxides, leading to multiple Fe-Ti bands within these materials (e.g., sapphire, ilmenite, pseudobrookite). Finally, the use of metal-metal distances is suggested to differentiate this heteronuclear Fe-Ti charge transfer from the common homonuclear charge transfer Fe2+-Fe3+.
