Concrete structures that house nuclear power reactors soak up radionuclides into their surfaces over a plant’s few-decades-long life. Chemical washing used to clean these surfaces when a plant is decommissioned can release radioactive material, and if not done immediately, that radioactive material may penetrate deeper into the concrete.
The layer could later be mechanically separated for long-term disposal and the concrete underneath reused, says Susan A. Cumberland, an environmental geoscientist at the University of Leicester.
Hydroxyapatite is a natural calcium phosphate mineral found in human bones and teeth. Powdered hydroxyapatite is known to soak up radioactive strontium and uranium, because strontium substitutes for calcium and uranium for phosphates in the mineral structure.
Cumberland and her colleagues devised a simple way to make hydroxyapatite coatings a few micrometers thick on cement. They soak cement blocks in a silica-based solution for 2 days, followed by a 2-day soak in calcium phosphate solution. Silica molecules attach to the surface of the block, creating a negative charge. Meanwhile, calcium ions leach out of the concrete and form calcium hydroxide, which raises the pH of the solution to 11. The alkaline, negatively charged environment is conducive to the formation of hydroxyapatite.
After the blocks were kept for a week in a solution containing strontium, X-ray spectroscopy showed that the metal accumulated in the hydroxyapatite coatings. The researchers have found the same results for uranium and other radioactive materials. Cumberland and her colleagues are now working on developing a separation method for the layer. Removing it would leave a “very tiny volume to put in deposits for long-term disposal,” she says.
Concrete treated like this in new-build nuclear could help simplify decommissioning “if it is demonstrated to work at scale both physical and temporal,” says Ian Farnan, chair of the Cambridge Nuclear Energy Centre at the University of Cambridge. The initial results are promising, but longer-term studies will be needed to confirm that strontium does not diffuse through the layer.