Salt Disposal of Heat-Generating Nuclear Waste

epositories using thermodynamic models. For example, some of the constituents
listed in Table 3 differ from what is included in the current geochemical models
for salt repositories. When additional, major constituents (i.e., those that expand
the chemical system) are introduced, the emphasis of geochemistry research
would be the consistent compilation/derivation of the relevant thermodynamic
data and other parametric values needed for representing concentrated brine
systems (e.g., Pitzer parameters).
One of the overriding advantages of salt disposal is the absence of brine.
However, it may be possible to develop a scenario by which the repository
resaturates after the thermal period. The volume of brine depends on the
mechanism bringing brine into the repository and whether or not the repository
has removed nearly all the void space by plastic deformation and reconsolidation.
For a disturbed scenario where an aquifer in the vicinity of the repository may be
breached in some way, open and connected repository space could fill with brine.
Important chemical conditions occur only if ample brine is available to the
system.
Chemical conditions in a repository can be described based on the predicted
chemical reactions among the solids, liquids, and gases present in the system. For
a salt repository, this translates into characterizing the reactions among the solid
phases found in the salt formation, the solid phases that are the waste, the waste
package and the emplacement materials, the brines that are in equilibrium with
the salt formation solid phases, and the gases that may be present as part of the
formation or may be introduced into the repository during construction.
The host rock of a bedded salt repository is, typically, dominantly composed of
halite (NaCl) with significant amounts of anhydrite (CaSO 4 ), polyhalite
(K 2 MgCa 2 (SO 4 ) 4 •2H 2 O), and clay minerals as minor phases. The thermodynamic
properties of halite and anhydrite are well known. To the extent that polyhalite
and clay minerals are present in the formation, their thermodynamic properties
would be part of a geochemical model, and their associated uncertainties would
contribute in a fractional manner to the overall uncertainty in the results.
Salt formation brines tend to have high concentrations of sodium, calcium, and
chloride. Lesser amounts of sulfate and carbonate are present. Some brines also
have high magnesium concentrations. Recently, ongoing research has shown that
borate is an important component in WIPP brines.
The in situ pH of brines is slightly acidic (i.e., about 6.0 to 6.5). Mineral
components of the salt formation buffer the pH to their in situ values. The pH of
brines after interaction with steel waste packages at low temperatures would
probably be similar to that characteristic of the brucite “half buffer” used at WIPP
(pH of about 9). The pH of brines in contact with steel waste packages at high
temperatures may need to be determined. Heating the brine would change the pH.
If buffered by mineral or gas equilibria, the pH would shift with temperature
according to the applicable equilibrium constants in a predictable manner. Buffer
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