University of Manchester develops new methods for treating radioactive waste

New methods for treating radioactive effluents, remediating contaminated groundwater, and stabilizing waste for long-term disposal have been reported by University of Manchester researchers working at the National Nuclear User Facility Radioactive Waste Management and Environmental Remediation (RADER) laboratories in Manchester, England. The research focuses on converting liquid nuclear waste containing the long-lived radioisotope technetium-99 into stable solid waste forms, as well as locking the radioisotope into phosphate minerals and immobilizing it in contaminated groundwater systems.

Tc-99 is a byproduct of nuclear fuel production and reprocessing. When released into the environment, Tc-99 is highly mobile, moving easily through water and soil and spreading radioactive contamination. These characteristics present clean-up challenges at many legacy nuclear sites, including Sellafield in the United Kingdom and Hanford in the United States.

Waste effluents: The methods developed by the University of Manchester researchers are described in two papers published in Environmental Science & Technology. In "Reductive Incorporation of Technetium-99 in Fe(II)-Doped Layered Double Hydroxide," the researchers developed a technique to remove Tc-99 from radioactive waste effluents by incorporating it into a type of engineered environmental-remediation mineral known as layered double hydroxides (LDHs).

The investigators used LDHs to remove 99 percent of dissolved Tc-99 from a solution, demonstrating that the radioisotope can be incorporated directly into the mineral structure of LDH during the formation of the mineral.

This study provides the first evidence of Tc-99 incorporation into LDH, which has been used in other areas of environmental remediation. This finding may be applicable to treating the effluents that are generated during the operation of nuclear power plants, as well as to producing stable inorganic waste during decommissioning activities.

Samuel Shaw, a professor of environmental mineralogy at the University of Manchester, noted that in the study, "we're not simply removing technetium from the effluent; we're locking it up directly into a mineral structure to provide much greater long-term stability. Understanding and optimizing these processes could open up new routes for treating radioactive wastes, and we are working with Sellafield through the Game Changers program to determine whether this new tool can help with the decommissioning of this legacy nuclear site."

Phosphate-based options: In "Novel Pathways to Treat Radioactively Contaminated Land: Bisphosphonate Ligands Enhance ⁹⁹Tc Sequestration by Phosphate Minerals," researchers evaluated whether methylene diphosphonic acid (MDP) could immobilize Tc-99 contaminants in groundwater systems. MDP is a compound used in nuclear medicine to bind the short-lived medical radioisotope Tc-99m to hydroxyapatite, a calcium phosphate mineral in human bone.

The University of Manchester researchers-who worked with researchers from Pacific Northwest National Laboratory and the Christie NHS (National Health Service) Foundation Trust-were able to form a soluble complex of MDP with chemically reduced Tc-99. They then then used hydroxyapatite to capture and bind 99 percent of the Tc-99-MDP complex from a solution. The experiment was conducted under environmental conditions similar to those found at nuclear legacy sites, and further experiments suggested the technique would work in flowing groundwater conditions.

According to the study, this Tc-99-MDP technique, when combined with phosphate mineral-forming remediation technology, could result in the targeted, long-term removal of technetium from contaminated land and groundwater. Callum Robinson, a radionuclide research associate at the University of Manchester, said, "This study demonstrates new ways of capturing and immobilizing technetium in stable minerals, helping to build the toolkit needed for the safe cleanup of nuclear legacy sites."