Yucca Mountain Site Characterization Project Research Articles

The saturated zone at Yucca Mountain: an overview of the characterization and assessment of the saturated zone as a barrier to potential radionuclide migration

The US Department of Energy is pursuing Yucca Mountain, Nevada, for the development of a geologic repository for the disposal of spent nuclear fuel and high-level radioactive waste, if the repository is able to meet applicable radiation protection standards established by the US Nuclear Regulatory Commission and the US Environmental Protection Agency (EPA). Effective performance of such a repository would rely on a number of natural and engineered barriers to isolate radioactive waste from the accessible environment. Groundwater beneath Yucca Mountain is the primary medium through which most radionuclides might move away from the potential repository. The saturated zone (SZ) system is expected to act as a natural barrier to this possible movement of radionuclides both by delaying their transport and by reducing their concentration before they reach the accessible environment. Information obtained from Yucca Mountain Site Characterization Project activities is used to estimate groundwater flow rates through the site-scale SZ flow and transport model area and to constrain general conceptual models of groundwater flow in the site-scale area. The site-scale conceptual model is a synthesis of what is known about flow and transport processes at the scale required for total system performance assessment of the site. This knowledge builds on and is consistent with knowledge that has accumulated at the regional scale but is more detailed because more data are available at the site-scale level. The mathematical basis of the site-scale model and the associated numerical approaches are designed to assist in quantifying the uncertainty in the permeability of rocks in the geologic framework model and to represent accurately the flow and transport processes included in the site-scale conceptual model. Confidence in the results of the mathematical model was obtained by comparing calculated to observed hydraulic heads, estimated to measured permeabilities, and lateral flow rates calculated by the site-scale model to those calculated by the regional-scale flow model. In addition, it was confirmed that the flow paths leaving the region of the potential repository are consistent with those inferred from gradients of measured head and those independently inferred from water-chemistry data. The general approach of the site-scale SZ flow and transport model analysis is to calculate unit breakthrough curves for radionuclides at the interface between the SZ and the biosphere using the three-dimensional site-scale SZ flow and transport model. Uncertainties are explicitly incorporated into the site-scale SZ flow and transport abstractions through key parameters and conceptual models.

An International Peer Review of the Biosphere Modelling Programme of the U.S. Department of Energy’s Yucca Mountain Site Characterization Project,

An International Peer Review of the Biosphere Modelling Programme of the U.S. Department of Energy’s Yucca Mountain Site Characterization Project,

Shear-enhanced compaction in sandstone under nominally dry and water-saturated conditions

We have developed an extensometer for measurement of distance and displacements in geotechnical and civil engineering applications. This is an optical extensometer that measurem distance using a modulated laser beam. In this design, reflecting targets are placed at desired measurement locations, and distance between each target and an optical head are measured repeatedly using the modulated laser beam. Moreover, all electronic and moving parts are located outside of the hostile or difficult environment as the optical head is connected to the laser, switching and signal analysis hardware using optical fibers. A reference beam is utilized to provide direct correction of system behavior. The system also utilizes movable reflective anchors that can be repositioned if desired. We have installed the system in a field experiment currently being conducted for the Yucca Mountain Site Characterization Project, and report preliminary results.

Deformation and velocity measurements at elevated temperature in a fractured 0.5 M block of tuff

This paper presents preliminary results of laboratory tests conducted on small block samples of Topopah Spring tuff, in support of the Yucca Mountain Site Characterization Project. The overall objective of these tests is to investigate the thermal-mechanical, thermal-hydrological, and thermal-chemical response of the rock to conditions similar to the near-field environment (NFE) of a potential nuclear waste repository. We present preliminary results of deformation and elastic wave velocity measurements on a 0.5-m-scale block of Topopah Spring tuff tested in uniaxial compression to 8.5 MPa and at temperatures to 85°C. The Young's modulus was found to be about 7 to 31 GPa for vertical measurements (parallel to the stress direction) across parts of the block containing no fractures or a few fractures, and 0.5 to 0.9 GPa for measurements across individual fractures, at ambient temperature and 8.5 MPa maximum stress. During stress cycles between 5 and 8.5 MPa, the deformation modulus values for the matrix with fractures were near 15–20 GPa at ambient temperature but dropped to about 10 GPa at 85°C. Compressional wave velocities were found to be about 3.6 to 4.7 km/s at ambient temperature and stress. After the stress was cycled, velocities dropped to values as low as 2.6 km/s in the south end of the block where vertical cracks developed. Heating the block to about 85°C raised velocities to as much as 5.6 km/s in the upper third of the block.

Discrete element modeling of sand production

This paper presents preliminary results of laboratory tests conducted on small block samples of Topopah Spring tuff, in support of the Yucca Mountain Site Characterization Project. The overall objective of these tests is to investigate the thermal-mechanical, thermal-hydrological, and thermal-chemical response of the rock to conditions similar to the near-field environment (NFE) of a potential nuclear waste repository. We present preliminary results of deformation and elastic wave velocity measurements on a 0.5-m-scale block of Topopah Spring tuff tested in uniaxial compression to 8.5 MPa and at temperatures to 85°C. The Young's modulus was found to be about 7 to 31 GPa for vertical measurements (parallel to the stress direction) across parts of the block containing no fractures or a few fractures, and 0.5 to 0.9 GPa for measurements across individual fractures, at ambient temperature and 8.5 MPa maximum stress. During stress cycles between 5 and 8.5 MPa, the deformation modulus values for the matrix with fractures were near 15–20 GPa at ambient temperature but dropped to about 10 GPa at 85°C. Compressional wave velocities were found to be about 3.6 to 4.7 km/s at ambient temperature and stress. After the stress was cycled, velocities dropped to values as low as 2.6 km/s in the south end of the block where vertical cracks developed. Heating the block to about 85°C raised velocities to as much as 5.6 km/s in the upper third of the block.

A new reflective optical extensometer (rox) system for geomechanical deformation measurements

We have developed an extensometer for measurement of distance and displacements in geotechnical and civil engineering applications. This is an optical extensometer that measures distance using a modulated laser beam. In this design, reflecting targets are placed at desired measurement locations, and distances between each target and an optical head are measure repeatedly using the modulated laser beam. Moreover, all electronic and moving parts are located outside of the hostile or difficult environment as the optical head is connected to the laser, switching and signal analysis hardware using optical fibers. A reference beam is utilized to provide direct correction of system behavior. The system also utilizes movable reflective anchors that can be repositioned if desired. We have installed the system in a field experiment currently being conducted for the Yucca Mountain Site characterization Project, and report preliminary results.

An investigation of thermal release of carbon-14 from PWR Zircaloy spent fuel cladding

An investigation to study the thermally-activated release of carbon-14 from irradiated PWR Zircaloy spent fuel cladding to potential repository environments for the Yucca Mountain Site Characterization Project was performed. Experimental parameters included release temperature, atmosphere and oxide thickness. Thirteen experiments measuring carbon-14 release from irradiated Zircaloy-4 cladding samples with thin pretransition and thicker posttransition oxide films were completed. Experiments were performed in air at 373, 473, 548, and 623 K, for up to 24 h and in argon at 623 K for 8 h. The data were examined using classical diffusion models, diffusion from a semiinfinite plane and diffusion from a finite thickness plane. Diffusion coefficients were calculated at each of the four temperatures in air and for both the pre- and posttransition oxide cladding. The diffusion coefficients measured ranged from 1 × 10 −19 to 8 × 10 −14 m 2/s with activation energies from 78–103 kJ/mol in the temperature range of 373–623 K. The data indicate that carbon-14 is released primarily from the oxide layer on these cladding samples.