Following an intersection of rising magma with drifts of the potential Yucca Mountain nuclear waste repository, a pathway
is likely to be established to the surface with magma flowing for days to weeks and affecting the performance of engineered
structures located along or near the flow path. In particular, convective circulation could occur within magma-filled drifts
due to the exsolution and segregation of magmatic gas. We investigate gas segregation in a magma-filled drift intersected
by a vertical dyke by means of analogue experiments, focusing on the conditions of sustained magma flow. Degassing is simulated
by electrolysis, producing micrometric bubbles in viscous mixtures of water and golden syrup, or by aerating golden syrup,
producing polydisperse bubbly mixtures with 40% of gas by volume. The presence of exsolved bubbles induces a buoyancy-driven
exchange flow between the dyke and the drift that leads to gas segregation. Bubbles segregate from the magma by rising and
accumulating as a foam at the top of the drift, coupled with the accumulation of denser degassed magma at the base of the
drift. Steady-state influx of bubbly magma from the dyke into the drift is balanced by outward flux of lighter foam and denser
degassed magma. The length and time scales of this gas segregation are controlled by the rise of bubbles in the horizontal
drift. Steady-state gas segregation would be accomplished within hours to hundreds of years depending on the viscosity of
the degassed magma and the average size of exsolved gas bubbles, and the resulting foam would only be a few cm thick. The
exchange flux of bubbly magma between the dyke and the drift that is induced by gas segregation ranges from 1 m3 s−1, for the less viscous magmas, to 10−8 m3 s−1, for the most viscous degassed magmas, with associated velocities ranging from 10−1 to 10−9 m s−1 for the same viscosity range. This model of gas segregation also predicts that the relative proportion of erupted degassed
magma, that could potentially carry and entrain nuclear waste material towards the surface, would depend on the value of the
dyke magma supply rate relative to the value of the gas segregation flux, with violent eruption of gassy as well as degassed
magmas at relatively high magma supply rates, and eruption of mainly degassed magma by milder episodic Strombolian explosions
at relatively lower supply rates. 相似文献
The greatest natural threats to the integrity of the geological barriers to nuclear wastes isolated in cavities mined at depths between 400 and 800 m are likely during rapid retreats of future ice sheets. The next major glacial retreat is expected at ca 70 ka, well within the lifetime of high grade nuclear waste, but it is not yet clear how long man's greenhouse effect may delay it.
This contribution discusses the potential problems posed to European waste isolation sites during erosion by ice and over-pressurizing of meltwater and gasses in a lithosphere flexed by major ice sheets. These depend on the target rocks and the location of the site with respect to the ice-streams and margins of future ice sheets of particular size.
No sites are planned under the centres of future ice sheets in Europe where end-glacial earthquakes can be expected to reactivate major faults, nor where ice can be expected to deepen and lengthen fjords along the Atlantic coast. Sites in the Alps may be vulnerable to radical changes in the patterns of glacial troughs. The stability and geohydrology of sites in coastal areas beyond future ice margins are threatened by river gorges when sea level falls ca 125 m or, in enclosed basins like the Mediterranean, ever lower. The greatest problems are likely in lowland regions exposed by the rapid retreat of thick ice fronts where large lakes on or under thick warm-based ice are dammed by more distal cold-based ice. Groundwater in subhorizontal fractures dilated by glacial unloading may reach over-pressures capable of hydraulically lifting megablocks of bedrock with fracture permeability and/or the ice damming them so that less permeable substrates are susceptible to incisions eroded to depths of ca 360 at locations controlled mainly by ice topography, kinematics and history. 相似文献
Elements and natural radionuclides in the contact zone of two granites with different ages would migrate from one to the other because of the difference in their chemical contents and later water-rock interactions. This migration could serve as an analogue for the near-field process of radwastes in a high-level radwaste deep geological disposal repository.In the contact between the Indosinian granite (whole-rock Rb-Sr isochron age at 214@3 Ma) and Hercynian granite (zircon U-Pb isochron age at 296@31 Ma) located in Ziyuan County, Guangxi, the O and Pb isotope characteristics and the activity ratios of 234U/238U, 230Th/238U, 230Th/234U and 226Ra/230Th show that, based on the whole-rock chemical contents, both of the two granites have maintained a relatively open chemical system in their evolution processes. However, as there is no obvious open fault, the migration of major elements, trace elements and natural U-series nu-clides takes place within only 1-2 m in the contact zone, and water-rock interaction 相似文献