Download Geology of the Proposed Radioactive Waste Repository at Yucca

Yucca Mountain from the Ground Up: Geology of the Proposed Radioactive Waste
Repository at Yucca Mountain, Nevada
Christopher J. Potter, U.S. Geological Survey, Denver, Colorado.
The U.S. Department of Energy began studying Yucca Mountain, Nevada, in 1978 to
determine whether it would be suitable for the nation's first long-term geologic repository
for spent nuclear fuel and high-level radioactive waste. Currently stored at 131 sites
around the nation, these materials are a result of nuclear power generation and national
defense programs.
On July 23, 2002, President Bush signed House Joint Resolution 87, allowing the DOE to
take the next step in establishing a safe repository in which to store our nation's nuclear
waste. The Department of Energy is currently in the process of preparing an application
to obtain the Nuclear Regulatory Commission license to proceed with construction of the
repository.
Yucca Mountain is located in a remote desert on federally protected land within the
secure boundaries of the Nevada Test Site in Nye County, Nevada. It is approximately
100 miles northwest of Las Vegas, Nevada.
[preceding paragraphs are from a Department of Energy web site,
www.ocrwm.doe.gov/ymp/]
In cooperation with the Department of Energy, the U.S Geological Survey has completed
numerous geologic and hydrologic studies to characterize the proposed repository site.
The site lies within volcanic rock, in the unsaturated hydrologic zone above a deep water
table. This presentation will provide a geologic tour of the Yucca Mountain area and a
discussion of some of the scientific issues related to the proposed repository.
The following page contains a geologic summary of Yucca Mountain, published in 2003
as an abstract in:
Geological Society of America Abstracts with Program, v. 34, no. 7, p. 353.
GEOLOGIC SETTING OF THE PROPOSED HIGH-LEVEL RADIOACTIVE WASTE
REPOSITORY, YUCCA MOUNTAIN, NEVADA
POTTER, C.J. ([email protected]), DAY, W.C., and SWEETKIND, D.S., U.S.
Geological Survey, Denver Federal Center, Denver, CO 80225-0046.
Yucca Mountain (YM), in southern Nevada, is the proposed site of the nation’s sole
high-level radioactive waste repository and lies within the middle to upper Miocene
southwestern Nevada volcanic field (SWNVF) in the Walker Lane tectonic zone near the
SW margin of the Basin and Range province. Rocks of the SWNVF were erupted from
the Timber Mountain caldera complex; they unconformably overlie an older Tertiary
nonmarine section, which in turn overlies a highly deformed Paleozoic and Proterozoic
section along a profound angular unconformity. The Paintbrush Group dominates the
near-surface stratigraphy at YM, comprising two thick, mainly densely welded rhyolite
tuffs – the 12.8-Ma Topopah Spring Tuff and the 12.7-Ma Tiva Canyon Tuff – and a
thinner intervening interval of nonwelded and bedded tuffs. The Tiva Canyon is the most
extensively exposed bedrock unit at YM, whereas the underlying Topopah Spring
includes the “repository host horizon,” which would be excavated for waste storage.
The proposed repository volume (in north-central YM) is in the structurally simplest
part of YM: a 4-km-wide fault block of gently E-dipping strata. This “central block” is
one of several 1- to 4-km-wide blocks that are delineated by moderately to steeply Wdipping, block-bounding normal faults. Within the central block, intrablock faults are
commonly short and discontinuous, except for one prominent 7-km–long fault. Block
margins are more intensely deformed, particularly in the hanging-walls of blockbounding faults. The mesoscale fracture network in the tuffs is strongly controlled by
lithologic properties of the volcanic stratigraphy (degree of welding, lithophysae
development, etc.), and locally the fracture network has a strong influence on the nature
of intrablock faulting.
In the southern part of YM, the intensity of deformation increases markedly, and
block-bounding faults commonly are linked by northwest-striking relay faults. Blockbounding faults were active at Yucca Mountain during and after eruption of the
Paintbrush Group, and significant motion on those faults postdated the 11.6-Ma Rainier
Mesa Tuff. Roughly half of the stratal tilting in the repository-site area occurred after
11.6 Ma. In addition, 0 to 4 m of Quaternary displacement are associated with each
block-bounding fault.