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Shale Disposal of U.S. High-Level Radioactive Waste « Facilities « Downloads

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Date postedJanuary 29, 2013
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CategoriesFacilities, ECIS, Technical Paper, Nuclear Energy, Waste Isolation Pilot Plant (WIPP), Defense Waste Management, Advanced Nuclear Energy, Nuclear Energy Safety

Description

This report evaluates the feasibility of high-level radioactive waste disposal in shale within the
United States. The U.S. has many possible clay/shale/argillite basins with positive attributes for
permanent disposal. Similar geologic formations have been extensively studied by international
programs with largely positive results, over significant ranges of the most important material
characteristics including permeability, rheology, and sorptive potential. This report is enabled by
the advanced work of the international community to establish functional and operational
requirements for disposal of a range of waste forms in shale media. We develop scoping
performance analyses, based on the applicable features, events, and processes identified by
international investigators, to support a generic conclusion regarding post-closure safety.
Requisite assumptions for these analyses include waste characteristics, disposal concepts, and
important properties of the geologic formation. We then apply lessons learned from Sandia
experience on the Waste Isolation Pilot Project and the Yucca Mountain Project to develop a
disposal strategy should a shale repository be considered as an alternative disposal pathway in
the U.S.
Disposal of high-level radioactive waste in suitable shale formations is attractive because the
material is essentially impermeable and self-sealing, conditions are chemically reducing, and
sorption tends to prevent radionuclide transport. Vertically and laterally extensive shale and clay
formations exist in multiple locations in the contiguous 48 states. Thermal-hydrologicmechanical
calculations indicate that temperatures near emplaced waste packages can be
maintained below boiling and will decay to within a few degrees of the ambient temperature
within a few decades (or longer depending on the waste form). Construction effects, ventilation, and the thermal pulse will lead to clay dehydration and deformation, confined to an excavation
disturbed zone within a few meters of the repository, that can be reasonably characterized.
Within a few centuries after waste emplacement, overburden pressures will seal fractures,
resaturate the dehydrated zones, and provide a repository setting that strongly limits radionuclide
movement to diffusive transport. Coupled hydrogeochemical transport calculations indicate
maximum extents of radionuclide transport on the order of tens to hundreds of meters, or less, in
a million years. Under the conditions modeled, a shale repository could achieve total
containment, with no releases to the environment in undisturbed scenarios. The performance
analyses described here are based on the assumption that long-term standards for disposal in
clay/shale would be identical in the key aspects, to those prescribed for existing repository
programs such as Yucca Mountain.
This generic repository evaluation for shale is the first developed in the United States. Previous
repository considerations have emphasized salt formations and volcanic rock formations. Much
of the experience gained from U.S. repository development, such as seal system design, coupled
process simulation, and application of performance assessment methodology, is applied here to
scoping analyses for a shale repository. A contemporary understanding of clay mineralogy and
attendant chemical environments has allowed identification of the appropriate features, events,
and processes to be incorporated into the analysis. Advanced multi-physics modeling provides
key support for understanding the effects from coupled processes. The results of the assessment
show that shale formations provide a technically advanced, scientifically sound disposal option
for the U.S.

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