Abstract. According to the ongoing site selection process for a repository for
high-level radioactive waste in Germany, rock salt, clay and crystalline rock
are possible host rocks. The pore water of these rocks contains saline
solutions with high ionic strengths. To model the speciation and/or migration
of radionuclides in long-term safety analyses for nuclear waste disposal, a
geochemical code that includes thermodynamic data suitable for saline
solutions is needed. Thermodynamic equilibrium in saline solutions with high
ionic strengths is usually modelled using the Pitzer approach (Pitzer,
1991). Within the context of nuclear waste disposal, the THEREDA project (Moog et
al., 2015) provides thermodynamic data for some widely used geochemical codes
(PHREEQC, Geochemist's Workbench, ChemApp, and EQ 3/6) using the Pitzer
approach; however, for modelling in long-term safety analyses for nuclear
waste disposal, another geochemical code, TOUGHREACT, is used. Therefore,
scripts were developed to convert thermodynamic data of the THEREDA project to
be applicable in TOUGHREACT. The scripts were validated by benchmark tests and by comparing calculations
using PHREEQC and TOUGHREACT (Weyand et al., 2021). In total, 50 different
benchmark tests were performed considering 3 specific geochemical systems,
which are relevant to long-term safety analyses: (1) oceanic salt system,
polythermal: K, Mg, Ca, Cl, SO4, H2O(l), (2) actinide system,
isothermal: Am(III), Cm(III), Nd(III), Na, Mg, Ca, Cl, OH, H2O(l) and
(3) carbonate system, isothermal: Na, K, Mg, Ca, Cl, SO4,
HCO3/CO2(g), H2O(l). Each benchmark test considered specific
ion concentrations in solution and in gaseous phases in the presence of
specific minerals. The benchmark tests derived the geochemical equilibria and
the results of both codes were compared to each other and to experimental
data. The results of the calculations using both codes showed a good
correlation. Remaining deviations can be explained by technical differences of
the codes.