Salt Disposal of Heat-Generating Nuclear Waste

4.4 Availability and Movement of Brine and Vapor
Phases
The availability and movement of brine and water vapor in a thermally driven salt
repository remain uncertain despite some scientific investigation of these
phenomena. If it can be shown that the environment in the vicinity of the waste
and/or waste packages is dry through the regulatory period, then issues of
chemical degradation of the waste package and/or waste forms could be screened
out in the FEPs analysis.
The conceptual model for brine migration involves a fluid inclusion trapped
within the crystal structure, which tends to migrate toward the heat source
because of enhanced solubility on the “hot” end and relatively less solubility on
the “cool” end. The fluid inclusion preferentially dissolves salt away from the hot
end and deposits the dissolved salt on the cool end, thus the crystal void migrates
toward the heat. The phenomenon of brine migration has been observed in the
laboratory and in field experiments.
Introduction of heat into the salt formation will drive moisture movement. Schlich
(1986) describes some relevant results obtained by computer simulation of water
transport in the heater experiments conducted at the Asse mine. The main result
was that an evaporation model with Knudsen-type vapor transport combined with
fluid transport by thermal expansion of the adsorbed water layers in the nonevaporated
zone showed the best agreement with experimental evidence. Based
on his studies it appears that vapor transport processes dominate moisture
movement.
Potentially, the hydraulic gradient between the far field host salt formation and
the excavated room is initially large. If the permeability is high enough to allow
brine flow, nearly all the brine contained in the DRZ can flow into the waste
emplacement openings. These processes are observed in underground openings in
salt. Stress differences occur immediately upon excavation, so the dewatering
process begins quickly. Brine migrates down the stress gradient and evaporates
into the ventilation air. Understanding the rate and extent of moisture removal by
these processes is important for advanced modeling and PA.
Description of the evolution and movement of brine and water vapor in a hot salt
repository requires multiphysics modeling for all three of the brine movement
mechanisms described above.
4.5 Geochemical Environment
The importance of the geochemical environment near the waste or waste packages
depends on thermal-hydrologic-mechanical coupled processes. If the repository is
dry and the salt has consolidated around the waste, there is no dissolution
mechanism for the waste package and/or waste and no mechanism for movement
of contaminants away from the waste.
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