Zeo19: A thermodynamic database for assessing zeolite stability during the corrosion of nuclear waste immobilization glasses

Abstract

Stable, durable immobilization of radioactive wastes requires robust understanding of the sub-surface geochemical processes that occur in repository environments. For example, the accelerated dissolution (corrosion) of nuclear waste immobilization glasses (i.e., the so-called “Stage III” corrosion) following the precipitation of zeolitic phases is a significant issue that could result in radionuclide release. However, current uncertainties in establishing the tendency for the persistence of zeolites results in difficulties in estimating the chemical environments and state variables that favor zeolite precipitation. To assess the tendency for Stage III corrosion, we compiled a unified, internally-consistent thermodynamic database to estimate zeolite stability under conditions relevant to nuclear waste repositories (namely, p = 1 bar and T < 95 °C), i.e., for compounds including: analcime, clinoptilolite, mordenite, erionite, thomsonite, bikitaite, brewsterite, dachiardite, epistilbite, ferrierite, gonnardite, harmotome, leonhardite, paranatrolite, tetranatrolite, yugawaralite zeolite X, and zeolite P(Ca). The database, which features both existing and/or newly-derived thermodynamic properties, is integrated with a Gibbs (free) energy minimization (GEM) solver to estimate stable zeolite phase equilibria and their partitioning at equilibrium. The database offers favorable predictions of the solubility of the zeolite phases as a function of temperature. The validity of the database is ascertained by comparing newly-constructed equilibrium activity diagrams with experimental observations of zeolite formation during glass dissolution across conditions encompassing diverse solution chemistries, pH’s, and temperatures.