A non-isothermal reactive transport model of the long-term geochemical evolution at the disposal cell scale in a HLW repository in granite

Abstract

The assessment of the long-term performance of the engineered barrier systems of high-level radioactive waste (HLW) repositories requires the use of reactive transport models. Here a non-isothermal reactive transport model of the long-term geochemical evolution of a HLW disposal cell in a granitic host rock is presented. The model includes the vitrified waste (40 cm in diameter), the carbon-steel canister (5 cm thick), the saturated FEBEX bentonite buffer (75 cm thick) and the reference granitic rock. The model accounts for the thermal transient stage and assumes generalized steel corrosion under anaerobic conditions with a corrosion rate equal to 1.41 m/y. Canister failure is assumed to occur when the remaining canister thickness is equal to 3.5 cm at t = 25,000 years. Canister corrosion caused an increase in pH. The computed pH in the canister just before canister failure (t = 25,000 years) was equal to 9.25 and ranged from 7.82 to 9.25 in the bentonite. Magnetite, the main corrosion product, precipitated in the bentonite and especially in the canister. The thickness of magnetite precipitation band in the bentonite was $\approx$ 1 cm. Siderite precipitated at both sides of the canister/bentonite interface. The precipitation front penetrated >1 cm into the bentonite. Nuclear glass started dissolving after canister failure (t > 25,000 years). The concentration of dissolved silica increased in the inner part of the glass until t = 30,000 years and decreased in the outer part of the glass due to the out diffusion of dissolved silica into the canister and the bentonite. This diffusive flux caused the precipitation of greenalite at the glass/canister and canister/bentonite interfaces. The pH at the end of the simulation (t = 50,000 years) ranged from 7.93 to 7.89 in the glass, from 7.89 to 8.66 in the canister and from 7.87 to 8.6 in the bentonite. Magnetite precipitated in the canister while there was carbon steel to corrode. Once the canister was fully corroded, magnetite redissolved near the glass/canister interface. Greenalite precipitated in the canister and the bentonite, especially at the glass/canister interface and siderite precipitated at the canister/bentonite interface. The simulation results should be useful for the performance assessment of engineered barriers of radioactive waste repositories in granitic host rocks.