Leachability of technetium from concrete

Abstract

Technetium-bearing medium-level waste is in the Federal Republic of Germany supposed to be conditioned in concrete and stored in a salt mine repository. An accidental ingress of water into the repository leads to the formation of a brine solution which may corrode the barrier system components and leach radionuclides from the concrete. The long half-life of 99Tc may then permit for a slow migration of this radionuclide into the biosphere. The leachability of technetium was investigated with samples prepared from portland, aluminous and blast furnace cement with or without the additives bentonite and Microsilica. 99Tc was used as NH 4TcO 4 resp. TcO 2. A quinary brine solution and distilled water were applied as leachants. 137Cs and 36Cl were also used for comparative measurements. With aluminous cement/bentonite only unfavourable results were obtained: 20% of the technetium had been leached into brine after 60 days. Later, but in water already after 20 days with 40% of the technetium leached, the slope of the leaching curve turned negative, which was possibly due to the formation of new mineral phases with Tc as a constituent. Bentonite-free aluminous cement behaved somewhat better than the mixture. Portland cement with or without bentonite showed an overall medium leachability. The best results were obtained with blast furnace cement. Leachability generally remains below 0.1%; bentonite is of no significance. Here Tc is possibly rapidly fixed as a species of low solubility, whose solubility product is being reached within short time. Leachability of TcO 2 is higher than expected, it amounts to about 10% as compared with pertechnetate, which is probably due to a transformation of TcO 2 into a more soluble species. Cs and Cl are leached by a factor of >100 faster than Tc from blast furnace cement, supporting the assumption of a mineralogical fixation of Tc and a diffusion-controlled migration of Cs- and Cl-ions.