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Introduction
Thermodynamic data are essential for understanding and evaluating geochemical processes, as
by speciation-solubility calculations, reaction -path modeling, or reactive transport simulation.
These data are required to evaluate both equilibrium states and the kinetic approach to such
states (via the affinity term in rate laws). The development of thermodynamic databases for these
purposes has a long history in geochemistry (e.g., Garrels and Christ, 1965; Helgeson et al.,
1969; Helgeson et al., 1978, Johnson et al., 1992; Robie and Hemingway, 1995), paralleled by
related and applicable work in the larger scientific community (e.g., Wagman et al., 1982, 1989;
Cox et al., 1989; Barin and Platzki, 1995; Binneweis and Milke, 1999). The Yucca Mountain
Project developed two qualified thermodynamic databases for to model geochemical processes,
including ones involving repository components such as spent fuel. The first of the two (BSC,
2007a) was for systems containing dilute aqueous solutions only, the other (BSC, 2007b) for
systems involving concentrated aqueous solutions and incorporating a model for such based on
Pitzer's (1991) equations . A 25 C-only database with similarities to the latter was also
developed for WIPP (cf. Xiong, 2005).
The YMP dilute systems database is widely used in the geochemistry community for a variety of
applications involving rock/water interactions. It builds on the work of Prof. Helgeson and his
students (see BSC, 2007a for many applicable references), and covers a significant range of
temperature (25-300 C). The last version covers 86 chemical elements, 1219 aqueous species,
1156 minerals and other solids species, and 128 gas species. Many data for actinide species have
been adopted from the Nuclear Energy Agency (NEA) series of volumes on actinide
thermodynamics (see references given in BSC, 2007a), and the appropriate temperature
extrapolations have been applied. The YMP concentrated systems database covers a smaller
chemical system (40 chemical elements, 237 aqueous species, 470 minerals and other solids, and
11 gas species). It includes temperature dependence, which for many species extends to 200 C,
but for others extends to 250 C, to 110 C, or is restricted to 25 C. It is based on many sources
(see BSC, 2007b), but draws in particular from the work of Pabalan and Pitzer (1987) and
Greenberg and Msller (1989). In addition to their other characteristics, these databases have a
regulatory cachet as qualified products of the Yucca Mountain Project.
The purpose of the present task is to improve these databases for work on the Used Fuel
Disposition Project and maintain some semblance of order that will support qualification in
support of the development of future underground high level nuclear waste disposal. The Yucca
Mountain Project was based on disposal in volcanic stuff, in a thick vadose zone in which
oxidizing conditions were expected to prevail. A 50 year period of tunnel ventilation was
planned to limit maximum temperature. Concentrated solutions were not originally expected at
Yucca Mountain. Later concerns about dust deliquescence and evaporative concentration led to
the development of the YMP concentrated solutions thermodynamic database (see BSC, 2007b).
The Yucca Mountain Project design scenario was very different from those for planned
repositories in other countries, which envision disposal below the water table (generally under
reducing conditions) in clay, salt, granite or other hard rock, usually incorporating relatively low
maximum temperature in the designs. The Used Fuel Disposition program is investigating
potential disposal in mined repositories in these three rock types, plus a deep borehole option
(which appears to imply in granite or other hard rock). The UFD may consider higher maximum
