The fate of Tc in a UK intermediate-level nuclear waste repository

摘要

In the UK, tetraphenylphosphonium bromide (TPPB) has been used to remove technetium prior to disposal from some waste-streams by forming TPPBTc fioc. If this fioc were to be disposed of in a cementitious repository the TPPBTc would degrade by alkaline hydrolysis to triphenylphosphonium oxide, or by radiolysis to triphenylphosphine, releasing the Tc into solution. Its chemistry would be dominated by TcO4 in aerobic waters and sparingly soluble TcO2(s) in anaerobic. Repository heterogeneity could mean that both Tc(VII) and Tc(IV) are present simultaneously. If TcO_4~- migrates into reducing conditions, ligands in the waste may complex with Tc during reduction to form water-soluble complexes. Also possible, is increased Tc solubility when organic ligands react with TcO2(s). Comparisons of Tc solubilities at high pH starting from TcO2 and from TcO_4~- reduced in the presence of ligands were made. With EDTA and NTA no difference was observed, suggesting that Tc(IV)-ligand complexes were formed. For ISA and gluconic acid the Tc solubility starting from TcO_4~- was higher than in systems with TcO2 as the starting point. This suggests that the TcO_4~- was not reduced to TcO2, but an intermediate oxidation state complex was formed, e.g. Tc(V). The conditional stability constant for the Tc(IV)-gluconic acid complex has been determined to be log β = 26.6 ± 0.2.