5 research outputs found
Use of groundwater lifetime expectancy for the performance assessment of a deep geologic waste repository: 1. Theory, illustrations, and implications
Long-term solutions for the disposal of toxic wastes usually involve
isolation of the wastes in a deep subsurface geologic environment. In the case
of spent nuclear fuel, if radionuclide leakage occurs from the engineered
barrier, the geological medium represents the ultimate barrier that is relied
upon to ensure safety. Consequently, an evaluation of radionuclide travel times
from a repository to the biosphere is critically important in a performance
assessment analysis. In this study, we develop a travel time framework based on
the concept of groundwater lifetime expectancy as a safety indicator. Lifetime
expectancy characterizes the time that radionuclides will spend in the
subsurface after their release from the repository and prior to discharging
into the biosphere. The probability density function of lifetime expectancy is
computed throughout the host rock by solving the backward-in-time solute
transport adjoint equation subject to a properly posed set of boundary
conditions. It can then be used to define optimal repository locations. The
risk associated with selected sites can be evaluated by simulating an
appropriate contaminant release history. The utility of the method is
illustrated by means of analytical and numerical examples, which focus on the
effect of fracture networks on the uncertainty of evaluated lifetime
expectancy.Comment: 11 pages, 8 figures; Water Resources Research, Vol. 44, 200
Use of groundwater lifetime expectancy for the performance assessment of a deep geologic radioactive waste repository:2. Application to a Canadian Shield environment
Cornaton et al. [2007] introduced the concept of lifetime expectancy as a
performance measure of the safety of subsurface repositories, based upon the
travel time for contaminants released at a certain point in the subsurface to
reach the biosphere or compliance area. The methodologies are applied to a
hypothetical but realistic Canadian Shield crystalline rock environment, which
is considered to be one of the most geologically stable areas on Earth. In an
approximately 10\times10\times1.5 km3 hypothetical study area, up to 1000 major
and intermediate fracture zones are generated from surface lineament analyses
and subsurface surveys. In the study area, mean and probability density of
lifetime expectancy are analyzed with realistic geologic and hydrologic shield
settings in order to demonstrate the applicability of the theory and the
numerical model for optimally locating a deep subsurface repository for the
safe storage of spent nuclear fuel. The results demonstrate that, in general,
groundwater lifetime expectancy increases with depth and it is greatest inside
major matrix blocks. Various sources and aspects of uncertainty are considered,
specifically geometric and hydraulic parameters of permeable fracture zones.
Sensitivity analyses indicate that the existence and location of permeable
fracture zones and the relationship between fracture zone permeability and
depth from ground surface are the most significant factors for lifetime
expectancy distribution in such a crystalline rock environment. As a
consequence, it is successfully demonstrated that the concept of lifetime
expectancy can be applied to siting and performance assessment studies for deep
geologic repositories in crystalline fractured rock settings.Comment: 14 pages, 14 figures; Water Resources Research, Vol. 44, 200
Hybrid 1-D dielectric microcavity: Fabrication and spectroscopic assessment of glass-based sub-wavelength structures
Two different 1-D multilayer dielectric microcavities are presented, one activated by Er3+ ions fabricated by rf-sputtering and other one containing [email protected] quantum dots obtained by a hybrid radio frequency-sputtering/solution deposition process. The rare-earth activated cavity is constituted by an Er3+ -doped SiO2 active layer inserted between two Bragg reflectors consisting of 10 pairs of SiO2/TiO2 layers. Starting from the deposition procedure used for this cavity a fabrication protocol was defined with the aim to combine the high reproducibility allowed by the sputtering deposition for the fabrication of multilayers structures with the ability of fabricate films activated with highly luminescent quantum dots dispersed in polymeric matrix. In this case the cavity was constituted by poly-laurylmethacrylate host matrix containing [email protected] quantum dots inserted between two Bragg reflectors consisting of 10 pairs of SiO2/TiO2 layers fabricated by rf-sputtering on SiO2 substrate. The thicknesses of the films of the Bragg reflectors were tailored in order to reflect the visible radiation at around 650 nm. Transmittance spectra were employed to assess the optical features of the single Bragg gratings and whole samples. Luminescence measurements put in evidence that emissions strongly influenced by the presence of the cavities for both the samples