Feasibility analysis for a radioactive waste repository tunnel

This paper addresses the feasibility of constructing deep tunnels in the Opalinuston rock (Opalinus Clay Shale) formation in Switzerland. Laboratory and field tests show that the characteristics of Opalinuston vary over a wide range, influenced by different geologic conditions, by inherent spatial variability even under similar geologic conditions, and by the testing conditions. Tunnelling feasibility is examined through an initial elastic analysis followed by an elasto-plastic analysis. The objective of the elastic analysis is the investigation of the effect of a wide variety of parameters, particularly regarding the ground but also the liner material and a range of liner thicknesses. Since these analyses assume elastic behavior and, most importantly, the simultaneous excavation of the opening and installation of the liner which rarely corresponds to reality, high to very high liner stresses are produced. What becomes quite clear from these analyses is the importance of modeling the actual ground behavior, which the elastic analysis can do to quite a limited extent only, and to consider the construction procedure with support installation following excavation with a delay. This is done in the elasto-plastic analysis in which a specific material model for Opalinuston with somewhat conservative ground parameters is used together with the realistic condition of delayed support installation. The results of the elasto-plastic analysis show that in most cases liner stresses are in a range that can be handled with normal to high strength concrete. In addition, we also investigate the effect of drained versus undrained conditions behind the liner where the latter, as expected, lead to higher liner stresses. Finally, the effect of ground stiffness and of permeability on the results is studied. As expected a greater ground stiffness and higher permeabilities produces lower liner stresses.     

On the CVD of MoSi2: an experimental study from the MoCl4–SiCl4–H2–Ar precursor with a view to the preparation of C/MoSi2/SiC and SiC/MoSi2/SiC microcomposites

The chemical vapour deposition of MoSi2 on plane substrates (graphite or sintered-SiC) and ceramic fibres has been studied from MoCl4–SiCl4–H2–Ar gas mixtures at 900相似文献   

Hydrogen sorption properties of MgH2–NiCo2O4 composites activated mechanically under argon and hydrogen atmospheres

The effect of the mechanical activation medium on the hydrogen absorption–desorption properties of MgH₂ with NiCo₂O₄ additives is investigated. The composite 90 wt.% MgH₂–10 wt.% NiCo₂O₄ mechanically activated for 180 min under hydrogen reaches a higher absorption capacity as compared to the composite ground for the same time in an argon medium. At T = 573 K and P = 1 MPa the composite activated mechanically in a reactive medium shows a value of 5.67 wt.% while for the composite ground under argon the value is 4.36 wt.% only, both samples preserving a high absorption capacity at temperatures below 573 K. Addition of nickel cobaltite is found to have a favorable effect on the hydriding kinetics of magnesium. In order to elucidate this effect, a composite containing a large amount of NiCo₂O₄ (50 wt.%) is also investigated.     

Liner stresses in deep tunnels below the water table

The primary support of a tunnel must be designed to sustain the loads that are transferred from the surrounding ground during excavation. The loads are originated from the ground itself and from the groundwater, if any. For deep circular tunnels, assuming that both ground and support remain within their elastic regime, the load on the primary support does not change with drainage conditions; it is the same whether there is flow towards the tunnel (drained tunnel) or the pore pressure behind the support is hydrostatic (no-drainage tunnel). Stresses and deformations in the ground, however, are quite different, with larger stresses and deformations occurring for the drainage case. In tunnels where there is an impermeable layer between the primary and secondary supports, as the secondary support is placed there is a load transfer from the primary to the secondary support. The primary support unloads and moves outwards, while the secondary support takes load and moves inwards. In tunnels where there is a drainage layer between the primary and secondary supports, the pressure behind the support depends on the discharge capacity of the drainage system relative to the water inflow from the ground. Within the range of cases investigated, the relative permeability factor, r₀K_g/t_fK_f, can be used to evaluate the magnitude of the pore pressure buildup behind the secondary support. Numerical experiments combined with analytical solutions provide a rational approach for a preliminary design of the primary and secondary supports in deep tunnels below the water table, and contribute to identify the load-transfer mechanisms between ground, water, and support.     

Enhancement of hydrogen-storage properties of Mg by reactive mechanical grinding with oxide, metallic element(s), and hydride-forming element

In order to improve the hydrogen-storage properties of magnesium, oxides, metallic element(s) and a hydride-forming element were added to Mg by grinding under a hydrogen atmosphere (reactive mechanical grinding). As the oxides, Fe₂O₃ purchased, Fe₂O₃ prepared by spray conversion, MnO purchased, SiO₂ prepared by spray conversion, and Cr₂O₃ prepared by spray conversion were chosen. As the metallic elements, Ni, Fe, and Mo were selected. In addition, as the hydride-forming element, Ti was selected. Samples with the compositions of Mg-10 wt%oxide, 76.5 wt%Mg-23.5 wt%Ni, 71.5 wt%Mg-23.5 wt%Ni-5wt% Fe₂O₃, 71.5 wt%Mg-23.5 wt%Ni-5 wt%Fe, and Mg-14 wt%Ni-2 wt%Fe-2wt%Ti-2 wt%Mo were prepared. The hydrogen-storage properties and changes in phase and microstructure after the hydriding-dehydriding cycling of the prepared samples were then investigated.     

Structure of a new ternary compound with high magnesium content,so-called Gd13Ni9Mg78

The magnesium-rich composition Gd₁₃Ni₉Mg₇₈ was synthesized from its constituent elements in sealed tantalum tubes in an induction furnace. X-ray diffraction, electron probe microanalysis and dark-field transmission electron microscopy (TEM) images revealed a new compound with a composition ranging from Gd_10–15Ni_8–12Mg_72–78 and low crystallinity. In order to increase the crystallinity, different experimental conditions were investigated for numerous compounds with the initial composition Gd₁₃Ni₉Mg₇₈. In addition, several heat treatments (from 573 to 823 K) and cooling rates (from room temperature quenched down to 2 K h^?1) have been tested. The best crystallinity was obtained for the slower cooling rates ranging from 2 to 6 K h^?1. From the more crystallized compounds, the structure was partially deduced using TEM and an average cubic structure with lattice parameter a = 4.55 Å could be assumed. A modulation along both a¹ and b¹ axis with vectors of modulation q1 = 0.42a¹ and q2 = 0.42b¹ was observed. This compound, so-called Gd₁₃Ni₉Mg₇₈, absorbs around 3 wt.% of hydrogen at 603 K, 30 bars and a reasonable degree of reversibility is possible, because after the first hydrogenation, irreversible decomposition into MgH₂, GdH₂ and NiMg₂H₄ has been shown. The pathway of the reaction is described herein. The powder mixture after decomposition shows an interesting kinetics for magnesium without ball milling.     

Catalytic effect of monoclinic WO3, hexagonal WO3 and H0.23WO3 on the hydrogen sorption properties of Mg

The H sorption properties of mixtures Mg + WO₃ (having various structures) and Mg + H_0.23WO₃ are reported. First, the higher conversion of Mg into MgH₂ during reactive mechanical grinding (under 1.1 MPa of H₂) for higher WO₃ content is due to the improvement of the milling efficiency. Then, it is shown that the hydrogen absorption properties are almost independent of the crystal structure of the catalyst and that only the particles' size and the specific surface play a major role. Finally, for the desorption process, it appears that the chemical composition and structure of the catalyst, together with the particle size and specific surface have an effect.     

Analytical Solutions for Shallow Tunnels in Saturated Ground

Estimates of ground deformations and liner stresses in a tunnel are usually obtained from empirical correlations or from past experience on similar tunnels. These correlations account for only a few of the significant factors, and extrapolation to other cases is questionable because similitude conditions are not generally fulfilled. In this paper, complete analytical solutions for a shallow tunnel in saturated ground are obtained. Two different drainage conditions have been considered: full drainage at the ground-liner interface, and no drainage. The solutions cover different construction processes and soil conditions: (1) dry ground; (2) saturated ground with and without air pressure; (3) with and without a gap between the ground and the liner; and (4) applicability for short term analysis (i.e., undrained excavation and liner installation) and for long term analysis. Since the ground and the liner are assumed to behave elastically, the solutions obtained are restricted to cases where ground deformations are small, such as stiff clays and rocks, or when the excavation method prevents large deformations of the ground.     

Hydrogen-storage performance of an Mg–Ni–Fe alloy prepared by reactive mechanical grinding

The 71.5%Mg–23.5%Ni–5%Fe alloy prepared by reactive mechanical grinding for 4 h does not need activation. The activated sample has the hydriding rate of 0.494 wt%/min for 5 min and absorbs 3.32 wt% for 60 min at 593 K under 1.2 MPa H₂. It has the dehydriding rate of 0.330 wt%/min for 5 min and desorbs 2.42 wt%H for 20 min at 593 K 0.1 MPa H₂. The XRD pattern of 71.5 wt%Mg–23.5 wt%Ni–5 wt%Fe after reactive mechanical grinding exhibits MgH₂ in addition to starting elements Mg, Ni, and Fe. 71.5 wt%Mg–23.5 wt%Ni–5 wt%Fe after hydriding–dehydriding cycling contains Mg, Mg₂Ni, MgO, and Fe. The reactive mechanical grinding of Mg with Ni and Fe is considered to facilitate nucleation by creating many defects on the surface and in the interior of Mg, by the additive acting as active sites for the nucleation and shorten diffusion distances of hydrogen atoms by reducing the particle size of Mg. The MgH₂ formed in the as-milled 71.5 wt%Mg–23.5 wt%Ni–5 wt%Fe alloy is considered to lead to the creation of more defects and finer particle size.     

Improvement of hydrogen storage characteristics of Mg by planetary ball milling under H2 with metallic element(s) and/or Fe2O3

Among samples of Mg-Ni, Mg-Ni-5Fe₂O₃, and Mg-Ni-5Fe, Mg-Ni-5Fe had the highest hydriding and dehydriding rates. For the as-milled Mg-Ni-5Fe alloy and the hydrided Mg-Ni-5Fe alloy after activation, the weight percentages of the constituent phases were calculated using the FullProf program. The creation of defects and the diminution of Mg particle size through reactive mechanical grinding and hydriding-dehydriding cycling, and the formation of the Mg₂Ni phase are considered to increase the hydriding and dehydriding rates. Mg-14Ni-2Fe-2Ti-2Mo had higher hydriding and dehydriding rates than did any of the other samples (Mg-Ni, Mg-Ni-5Fe₂O₃, Mg-Ni-5Fe, and Mg-14Ni-6Fe₂O₃) prepared in this work.