Corrosion resistance of titanium canisters lined with lead for final disposal of reprocessed and vitrified waste from nuclear reactors

The nuclear Fuel Safety Project (KBS) has proposed that reprocessed and vitrified waste from nuclear reactors would be disposed of by enclosure in titanium canisters with 6 mm thick walls and a 100 mm thick lead lining. The canisters are to be placed in vertical drill-holes in rock, 500 m below ground, and embedded in a buffer of 80 - 90 % sand and 20 - 10 % bentonite. On estimation of the life of the titanium sheath a general corrosion rate of 0.25 μ/year has been taken as a conservative value, which would lead to a life of at least ten thousand years. Pitting and crevice corrosion have been considered very unlikely at the foreseen temperatures and salt contents. Further the risk of delayed fracture, due to hydrogen up-take, is considered as small but cannot be completely excluded at the present state of knowledge. For this reason the titanium sheath cannot absolutely be guaranteed an appreciable lifetime. If the titanium sheath were penetrated due to mechanical damage or localized corrosion, the exposed lead could suffer localized attack. The corrosion rate would then be determined by the supply of oxygen from the surrounding buffer to the canister surface. Conservative calculations have shown that perforation of the 100 mm thick lead lining would take about 4 500 years. In any case the life of the lead lining was estimated to at least a thousand years. In total a titanium canister with a lead lining was estimated to have a life of at least thousands of years, and probably tens of thousands of years. The expert group was unanimous in its judgement with the exception of professor Goesta Wranglen, who has delivered a statement of his own