Salt Disposal of Heat-Generating Nuclear Waste

flow into the waste emplacement rooms. Because maximal stress differences
occur immediately upon creating the opening, the maximum extent of the DRZ
manifests quickly, and the brine migration (i.e., dewatering) process begins
shortly after excavation. During the operational period, the accessible brine will
tend to migrate down the stress gradient and evaporate into the ventilation air.
These processes are observed in underground openings at WIPP. Soon after
mining, moist areas were noticed by geotechnical personnel, and it was apparent
that not all brine was bound to hydrous minerals or sealed inside fluid inclusions.
The clear or purer salt contains the least water by weight (0.3%), argillaceous salt
contains somewhat more (1.5%), and clay by itself contains the most (>2.2%). A
brine sampling and evaluation program (BSEP) was adopted to investigate the
origin, hydraulic characteristics, extent, and chemical composition of brine in the
Salado Formation at the repository horizon and the seepage of that brine into
excavations at the WIPP. The BSEP program noted, however, that after 11 years
of study, brine was remarkably hard to find in the WIPP excavations (Deal et al.
1995).
Beauheim and Roberts (2002) performed an evaluation of Salado Formation
hydrology and hydraulic properties. They conclude,
On the time scale of the operational period of WIPP (decades), the far
field lacks the capacity to fill all of the newly created porosity in and around
the repository, much less pressurize it to near lithostatic pressure. After WIPP
is closed, far-field flow toward the repository will continue, but the overall
“healing” of the formation around the repository (closure) and compaction of
the crushed-salt backfill will act to reduce both the hydraulic gradient and the
porosity present near the waste. Thus, the amount of brine that ever comes
into contact with waste will be controlled by the relative rates at which brine
flow and repository closure occur.
As salt creep closes a disposal room, the stress gradient decreases, fractures heal,
and crushed salt (if present) reconsolidates. Thus conditions in a repository would
evolve to significantly limit brine flow to the waste disposal areas.
2.4.1.4 Laboratory Studies of Salt Damage
Laboratory studies provide the most readily attainable setting for testing material
properties. Extensive research has been conducted in the laboratory to determine
both the deformational and hydrological behavior of natural salt. Early work
focused on the characterization of the elastic and inelastic viscoplastic flow
properties of salt (e.g., Hansen and Mellegard 1977, Wawersik and Hannum 1979,
Hansen and Mellegard 1980, Senseny 1986, Mellegard and Pfeifle 1993). Most of
the early studies provided parameters for the constant-volume creep model, which
was validated against large-scale thermal-mechanical experiments.
Van Sambeek, Ratigan, and Hansen (1993) reexamined the extensive creep data
existing from years of experiments on salt and evaluated volumetric strain in
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