Solidification/stabilization of technetium in cement-based grouts

Mixed low-level radioactive and chemically hazardous process treatment wastes from the Portsmouth Gaseous Diffusion Plant are stabilized by solidification in cement-based grouts. Conventional portland cement and fly-ash grouts have been shown to be effective for retention of hydrolyzable metals (e.g., lead, cadmium, uranium, and nickel) but are marginally acceptable for retention of radioactive 99Tc, which is present in the waste as the highly mobile pertechnetate anion. Addition of ground blast furnace slag to the grout is shown to reduce the leachability of technetium by several orders of magnitude. The selective effect of slag is believed to be due to its ability to reduce Tc (VII) to the less soluble Tc (IV) species.