Waste forms for actinides: borosilicate glasses

Abstract

This high level liquid radioactive reprocessing waste will become vitrified in order to obtain a stable borosilicate waste glass matrix that provides protection against environmental dispersion. Since vitrified waste is in a solid form, transportation, storage and final geological disposal are facilitated. The glass products contain the non-extracted fraction of uranium and plutonium together with the minor actinides Am, Np and Cm and fission products such as (90Sr, 99Tc ou 137Cs). In the long term, the safest way to deal with actinide containing nuclear waste (vitrified waste or spent fuel) is geological disposal. The disposal locations are isolated from biosphere by engineered barriers while dense rock characteristics and low groundwater flow assure that any potential return of radionuclides to biosphere by groundwater transport will be retarded for many hundreds of thousands of years. If the actinides (and other radionuclides) are conditioned within a glass matrix and cast into metal canisters, actinides can only be released to the ground water if the canister and the glass matrix becomes dissolved or corroded and this alteration process will take many hundreds of thousands of years. Even if the glass finally becomes dissolved or corroded entirely in ground water, large quantities of actinides will in all likelihood remain immobile by being bound to secondary phases, which form as solid corrosion products on the glass surface during the alteration process. Typical glass corrosion products are gel like and crystalline phases such as silicates and oxy-hydroxides.To predict the performance of the actinide containing waste glass, one needs to understand the dissolution process of the glass matrix, the thermodynamic and kinetic constraints which lead to actinide fixation in secondary reaction products as well as the short and long term transport processes.