Salt Disposal of Heat-Generating Nuclear Waste

Healing conditions are created once the salt begins to compress the waste in the
rooms. Models of the creation, growth, and healing of the DRZ replicate
observations and are based on firm physical understanding of the salt mechanics
involved. Hansen and Stein (2006) elaborated on how room evolution can be
modeled to more accurately represent the empirical evidence, including an
updated DRZ treatment.
Based on the technical information discussed here, it is possible that the DRZ of a
HLW repository in salt could heal completely within the 100-year period when
administrative controls are assumed to prevent intrusion into the repository. Heat
from the waste has been postulated to create a dry halo that could severely limit
corrosion of the waste packages. Rapid healing of the DRZ would prevent brine
from resaturating the salt around the package. Also, without brine, production of
gas through radiolysis could be limited.
2.4.1.2 Principles of Salt Deformation
The geologic settings for many of the salt-bearing regions shown in Figure 1 are
tectonically stable and aseismic, and given the viscoplastic behavior of salt, the in
situ stress condition prior to excavation is lithostatic (i.e., σ 1 ≈ σ 2 ≈ σ 3 ). Upon
excavation, the rock closest to the opening experiences deviatoric (or shear) states
of stress (σ 1 > σ 2 > σ 3 ). In salt, deviatoric states of stress activate elastic, inelastic
viscoplastic flow and damage-induced deformation. Elastic deformation occurs
instantaneously (time-independent) in response to changes in stress state, while
the other mechanisms are time-dependent, which gives salt its well-known creep
characteristics.
Viscoplastic flow of salt has been extensively measured and characterized by U.S.
and German salt repository programs and other salt-based programs such as the
Strategic Petroleum Reserve or the Solution Mining Research Institute. Plastic
deformation occurs by dislocation motion within the salt lattice and includes
processes of dislocation multiplication, glide, cross slip, and climb. Because
viscoplastic flow is an isochoric or incompressible process, it does not induce
damage to the salt matrix. Damage occurs when the deviatoric stresses are
relatively high compared to the applied mean stress and manifests through the
time-dependent initiation, growth, and coalescence of microfractures. Modeling
approaches have been suggested for the microfracturing process. Predictions of
damage zones surrounding openings in salt represent directly what can be
observed underground. Point-wise geophysical measurements often validate the
geometry and other characteristics predicted by damage models.
For HLW repository applications, one of the most important features of salt as an
isolation medium is its ability to heal previously damaged areas. Healing arises
when the magnitude of the deviatoric stress decreases relative to the applied mean
stress. The healing mechanisms include microfracture closure and bonding of
fracture surfaces. Microfracture closure is a mechanical response to increased
compressive stress applied normal to the fractures, while bonding of fracture
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