Salt Disposal of Heat-Generating Nuclear Waste

mechanical behavior and hydraulic properties must be described effectively by
constitutive models in repository simulations. Comparison of different
constitutive models and their theoretical bases is identified as one of the first
activities recommended if the U.S. opts for a salt disposal option. Modeling
collaborations can begin with coupled, thermal-mechanical three-dimensional
benchmark simulations. These calculations would compare the evolution of
stresses, strains, dilatancy (i.e., volumetric strains), damage, and permeability in
rock salt under the influence of elevated and changing temperatures.
To summarize, the developments at SNL with the SIERRA Mechanics codes and
the developments described by several German salt researchers provide ample
experimental data and simulation capability for mechanical deformation of rock
salt. Several advanced constitutive models have been developed and applied.
However, for elevated-temperature repository applications, the strong temperature
dependence of mechanical deformation behavior needs to be reevaluated if the
salt disposal option is selected.
2.5 Chemical Conditions in the Host Rock
Near-field geochemistry in a salt repository is controlled by the potential
interactions between the salt formation brine and the waste, packaging, and
emplacement materials. To the extent that brine is available to react with these
materials, the pH, oxidation/reduction conditions, gas fugacities, and dominant
aqueous species present would evolve over time. This evolution would be
influenced by temperature, especially during the thermal period of a HLW
repository in salt.
This section discusses potential geochemical interactions in the near field that
would determine the source term for radionuclide release if brine is available to
react with the waste.
2.5.1 The Thermal Period
It is only during the thermal period that the geochemistry of the near-field
environment of a HLW repository in salt has the potential of being substantially
different from what has already been studied within the context of the national
and international isothermal salt repository programs.
Definition of the thermal period is uncertain due to possible variations in the
geometry of the disposal room, the heat generating capacity of the waste, and the
emplacement configuration for the waste. In the most general sense, the thermal
period can be defined as the time when temperatures are elevated above those of
an isothermal repository. Elevated temperatures below 100°C cause aqueous salt
solubilities to increase, gas fugacities to change in a closed system, and reaction
rates to increase. These trends apply from ambient temperature up to dryout at
107°C (for NaCl brine at 1 atm) or higher temperatures (for Na-Ca-Cl and/or total
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