A special geochemical environment exists within the Shelter (”Sarcophagus”)
erected in 1986 over the destroyed Unit-4 of Chernobyl nuclear power plant
(NPP). Based upon the available in situ and compositional
data, thermodynamic models of solid-aqueous interactions were developed to
clarify the leaching behaviour of various materials within the Shelter. The
“Selektor-A” code, based on a convex programming approach to Gibbs free
energy minimization, was used for the calculations. A built-in flexible
hybrid thermodynamic database for the system
Na-K-Ca-Mg-Cl-S-N-H-O-Si-P-Fe-Al-Sr-Cs was extended with the critically
selected and matched parameters for aqueous species and solid phases in the
U-Zr-Si-O-H subsystem, secondary U-minerals, mineral phases of fully
hydrated Portland cements and U-bearing zircons. Modeling results show that
the “Shelter waters” can selectively leach a significant quantity of U and
Si from the fuel-containing masses, while Zr, Fe, Ca, Mg and some other
components are rather insoluble. Serpentinite, assemblages of fully-hydrated
phases of Portland cements, and oxidation products of steel structural
elements are estimated to be sufficiently stable in the aqueous environment
of the Shelter. Our calculations also define some feasible pathways for
secondary mineral formation from evaporation of Shelter water solutions and
interactions between these waters with the mineral matter inside the
Shelter.