Radioactive Tc in deep nuclear waste repositories can be immobilized by reduction of the highly soluble TcO₄− anion by agents like magnetite (Fe₃O₄) and H₂, which create an amorphous TcO₂⋅x H₂O precipitate. EXAFS measurements combined with DFT calculations reveal that the precipitate is an inorganic polymer formed of zigzag TcO₂⋅2 H₂O chains and give evidence of an increasing polymerization over time.