Strontium Isotopes in Pore Water as an Indicator of Water Flux at the Proposed High-Level Radioactive Waste Repository, Yucca Mountain, Nevada (original) (raw)
Related papers
Uranium-Series Constraints on Subrepository Water Flow at Yucca Mountain, Nevada
2006
Mineral abundances and whole-rock chemical and uranium-series isotopic compositions were measured in unfractured and rubble core samples from borehole USW SD-9 in the same layers of variably zeolitized tuffs that underlie the proposed nuclear waste repository at Yucca Mountain, Nevada. Uranium concentrations and isotopic compositions also were measured in pore water from core samples from the same rock units and rock leachates representing loosely bound U adsorbed on mineral surfaces or contained in readily soluble secondary minerals. The chemical and isotopic data were used to evaluate differences in water-rock interaction between fractured and unfractured rock and between fracture surfaces and rock matrix. Samples of unfractured and rubble (fragments about 1 centimeter) core and material from fracture surfaces show similar amounts of uranium-series disequilibrium, recording a complex history of sorption and loss of uranium over the past 1 million years. The data indicate that fractures in zeolitized tuffs may not have had greater amounts of water-rock interaction than the rock matrix. The data also show that rock matrix from subrepository units is capable of scavenging uranium with elevated uranium-234/uranium-238 from percolating water and that retardation of radionuclides and dose reduction may be greater than currently credited to this aspect of the natural barrier. Uranium concentrations of pore water and the rock leachates are used to estimate long-term in situ uranium partition coefficient values greater than 7 milliliters per gram.
Journal of Contaminant Hydrology, 2003
The evolution of fluid chemistry and mineral alteration around a potential waste emplacement tunnel (drift) is evaluated using numerical modeling. The model considers the flow of water, gas, and heat, plus reactions between minerals, CO 2 gas, and aqueous species, and porositypermeability-capillary pressure coupling for a dual permeability (fractures and matrix) medium. Two possible operating temperature modes are investigated: a "high-temperature" case with temperatures exceeding the boiling point of water for several hundred years, and a "low-temperature" case with temperatures remaining below boiling for the entire life of the repository. In both cases, possible seepage waters are characterized by dilute to moderate salinities and mildly alkaline pH values. These trends in fluid composition and mineral alteration are controlled by various coupled mechanisms. For example, upon heating and boiling, CO 2 exsolution from pore waters raises pH and causes calcite precipitation. In condensation zones, this CO 2 redissolves, resulting in a decrease in pH that causes calcite dissolution and enhances feldspar alteration to clays. Heat also enhances dissolution of wallrock minerals leading to elevated silica concentrations. Amorphous silica precipitates through evaporative concentration caused by boiling in the high-temperature case, but does not precipitate in the low-temperature case. Some alteration of feldspars to clays and zeolites is predicted in the high-temperature case. In both cases, calcite precipitates when percolating waters are heated near the drift. The predicted porosity decrease 2 around drifts in the high-temperature case (several percent of the fracture volume) is larger by at least one order of magnitude than in the low temperature case. Although there are important differences between the two investigated temperature modes in the predicted evolution of fluid compositions and mineral alteration around drifts, these differences are small relative to the model uncertainty and the variability of water compositions at Yucca Mountain.
YUCCA MOUNTAIN: Earth-Science Issues at a Geologic Repository for High-Level Nuclear Waste
Annual Review of Earth and Planetary Sciences, 2004
Key Words unsaturated flow in fractures, nuclear waste transport, spent fuel chemistry, fracture flow and transport, Nuclear Waste Policy Act s Abstract The nation has over 40,000 metric tonnes (MT) of nuclear waste destined for disposal in a geologic repository at Yucca Mountain. In this review, we highlight some of the important geoscience issues associated with the project and place them in the context of the process by which a final decision on Yucca Mountain will be made. The issues include understanding how water could infiltrate the repository, corrode the canisters, dissolve the waste, and transport it to the biosphere during a 10,000-year compliance period in a region, the Basin and Range province, that is known for seismic and volcanic activity. Although the site is considered to be "dry," a considerable amount of water is present as pore waters and as structural water in zeolites. The geochemical environment is oxidizing, and the present repository design will maintain temperatures at greater than 100 • C for thousands of years. Geoscientists in this project are challenged to make unprecedented predictions about coupled thermal, hydrologic, mechanical, and geochemical processes governing the future behavior of the repository and to conduct research in a regulatory and legal environment that requires a quantitative analysis of repository performance. . Downloaded from arjournals.annualreviews.org by University of British Columbia Library on 12/19/05. For personal use only.
Earth and Planetary Science Letters, 2010
Keywords: fluid inclusions stable isotopes hypogene Yucca Mountain paleohydrogeology Secondary calcite residing in open cavities in the unsaturated zone of Yucca Mountain has long been interpreted as the result of downward infiltration of meteoric water through open fractures. In order to obtain information on the isotopic composition (δD and δ 18 O) of the mineral-forming water we studied fluid inclusions from this calcite. Water was extracted from inclusions by heated crushing and the δD values were measured using a continuous-flow isotope-ratio mass spectrometry method. The δ 18 O values were calculated from the δ 18 O values of the host calcite assuming isotopic equilibrium at the temperature of formation determined by fluid-inclusion microthermometry.
1999
Initial scoping numerical simulations, using FEHM, evaluate perturbed groundwater behavior associated with underground nuclear tests in the Tuff Pile 1 area of Yucca Flat. Because many of these tests were conducted below the water table, we direct our simulations to a preliminary study of the sensitivity of the saturated pressure response to an instantaneous pressurization event caused by a nuclear test when different permeability and porosity configurations are considered.. Geologic and hydrostratigraphic data were digitized for the area to create a 3-D simulation mesh. We modeled underground nuclear tests with sufficient numerical resolution to resolve spherical regions within the mesh with radii scaled to reported yields and surrounding disturbed zone extending to 2 cavity radii. Ranges of appropriate rock permeability and porosity values allow a number of different model cases to be studied. Of these cases, ones that considered the disturbed zone to be contained within low permeability rocks may best model observations of water mounding in the area. For these cases, hydraulic head increases in rocks up to 4 cavity radii away from tests for up to 100 years after the test and require over 1000 years to return to a pretest state. For deep tests, this pressurization extends into the regional aquifer, indicating a possibility that fluids originating near the boundary of the disturbed zone will eventually move into the regional aquifer. In cases where the disturbed zone extends into higher permeability rocks, there is a rapid decay of overpressure. Future work requires detailed hydrologic analysis of shot cavities and disturbed zones, consideration of unsaturated rocks, solute transport modeling, and testing with observed water rise heights and rates.
1997
At Yucca Mountain, which is currently under consideration as a potential permanent underground repository for high-level radioactive wastes, the present-day water table is 500 to 700 rn deep (Luckey et al., 1996). This thick unsaturated zone (UZ) is part of the natural barrier system and is regarded as a positive attribute of the potential site. The USCS has studied the stable isotopes and petrography of secondary calcite and silica minerals that coat open spaces in the UZ and form irregular veins and masses in the