Salt Disposal of Heat-Generating Nuclear Waste

coefficient models exist (e.g., the Davies equation, the B-dot equation, Pitzer’s
equations) (Wolery 1992). The use of Pitzer’s equations to model ionic
interactions is appropriate for salt repositories and has been applied at WIPP.
Pitzer’s equations have been incorporated into many geochemistry, atmospheric
chemistry, and chemical engineering codes. These equations and the models
based on them are widely used and accepted. In geochemistry, Pitzer-based
models have been incorporated into EQ3/6 (Wolery 1992, Wolery and Daveler
1992, Wolery and Jarek 2003), the U.S. Geological Survey codes PHRQPITZ
(Plummer et al. 1988) and its successor PHREEQC (version 2.12, see
http://wwwbrr.cr.usgs.gov/projects/GWC_coupled/phreeqc), and The Geochemist’s
Workbench (Bethke 1996; see also http://www.rockware.com). These
are some of the better-known examples and are used by geochemists worldwide.
They have been applied to a wide variety of problems in aqueous geochemistry
including mineral-water interactions in many different settings, brine generation
by evaporation, and deliquescence of salts. The Pitzer approach presently has
widespread application in the technical community although alternative
approaches such as Extended UNIQUAC (Thomsen 2005) do exist and are
drawing increasing interest.
The Fracture-Matrix Transport (FMT) code (Babb and Novak 1995, Babb and
Novak 1997, Wang 1998) was used for modeling geochemical interactions in the
WIPP repository. The geochemistry part of this software uses Pitzer’s equations
(Pitzer 1973, 1975, 1991) to represent the thermodynamic activity coefficients of
aqueous species, including both solutes and the solvent, water. The standard form
of the Pitzer equations is based on molalities and requires data for interaction
parameters for pairs and triplets of the solute species included in the model
(interaction parameters explicitly involving the solvent, water, are not employed).
The FMT model is based on the classic Pitzer model of Harvie, Møller, and
Weare (1984) for the “sea-salt” system at 25°C and has been extended by adding
data for organic complexants (e.g., oxalate, citrate, ethylenediaminetetraacetate
[EDTA]) and actinides.
The WIPP project has migrated to EQ3/6 in order to facilitate future analyses of
their salt repository system. The EQ3/6 package of geochemistry codes has been
well tested during its widespread usage. For equilibrium analyses, the package
includes the EQ3NR code, which accepts the usual kinds of inputs describing an
aqueous solution, such as solute component molalities and pH. The EQ6 code
allows evaluation of irreversible mass-transfer reactions among liquids, solids,
and gases with relative or specific kinetic rate considerations. These offer two
options to address charge balance, calculate and fix the imbalance, or adjust one
of the ionic components to achieve charge balance. Also, EQ3/6 uses the later
Harvie (1981) approximation, which is the one used in essentially all modern
work involving Pitzer’s equations, including the Harvie, Møller, and Weare
(1984) model for the sea-salt system that forms the core of the FMT CHEMDAT
database. The WIPP version of EQ3/6 is Version 8.0a. This is the version of the
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