Salt Disposal of Heat-Generating Nuclear Waste

other possible modes of interference are inconsequential. After the repository is
filled with emplaced waste and horizontal panels are closed, seals would be
installed in the access shafts.
Similar to the WIPP shaft seal design, from the disposal horizon upward, the shaft
seal system would include the following components:
• Shaft Station Monolith—The base of the shaft will be sealed with saltsaturated,
Portland cement-based concrete. It will be placed by tremie line
techniques against nonremovable (committed material) forms, which could be
fabricated from concrete block or other abutments.
• Clay Column—A sodium bentonite compacted clay component is placed on
top of the mass of concrete. Alternative construction methods including block
placement and dynamic compaction are viable. Clay columns effectively limit
formation water movement from the time they are placed. The stiffness or
swelling pressure associated with the clay column is sufficient to promote
healing of fractures in the surrounding rock near the bottom of the shafts, thus
effectively removing the proximal damage zone as a potential pathway.
• Salt Column—A crushed salt element for the shaft seal system has been
thoroughly evaluated for salt application. The performance has been
established from the microstructural scale to the full construction scale. Very
low permeability is attained by the crushed salt in laboratory time, which is an
excellent basis for concluding the crushed salt element will become
impermeable in a matter of decades.
• Asphalt Column—Asphalt is a widely used construction material with
properties considered desirable for sealing applications. Asphalt is readily
adhesive, highly waterproof, and durable. Furthermore, it is a plastic
substance that provides controlled flexibility to mixtures of mineral
aggregates with which it is usually combined. It is highly resistant to most
acids, salts, and alkalis. A number of asphalts and asphalt mixes are available
that cover a wide range of viscoelastic properties and which can be tailored to
design requirements.
• Earthen Fill—The upper shaft is filled with locally available earthen fill. Most
of the fill is dynamically compacted (e.g., by the method used to construct the
compacted clay column) to a density approximating the surrounding
lithologies. The uppermost earthen fill is compacted with a sheep-foot roller
or vibratory plate compactor.
As described among the analogues summarized in Section 1.4, three atomic
explosions were detonated in salt. No special seal provisions were made in any
case. In the fifty years since the nuclear tests in salt, there is no detectable
release. For sealing a salt HLW nuclear waste repository, the engineering and
material specifications developed for the WIPP can be translated with minor
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