Researchers in Denmark have found a naturally occurring substance called green rust can help protect groundwater against radioactive contamination from stored reactor waste. Bo Christiansen’s findings are published in the March issue of the journal Geochimica et Cosmochimica Acta (paid subscription required).
Christiansen (pictured left) is a geochemist at the University of Copenhagen who has made green rust his specialty. Green rust was long considered a nuisance, at best, that formed on reinforced concrete. In recent years however, a group of chemists, physicists, and geologists including Christiansen in the university’s Nano-Geoscience Research Group have been studying the substance’s beneficial properties.
One of green rust’s beneficial properties is its ability to control pollutants, in some cases dangerous substances. Green rust is a type of clay known as an anionic clay. Because it consists of iron which has not entirely rusted, green rust has an electron deficit. This makes it react very readily with other pollutants.
One drawback of green rust is its high level of reactivity. While it’s easy to make — green rust will form if iron sulphate and caustic soda are present in water — it won’t last long. As soon as oxygen is added to the mix, green rust becomes ordinary red rust.
Spent reactor fuel needs millions of years to stabilize. The element neptunium, a waste product from uranium reactors, could pose a serious health risk should it ever find its way into groundwater. Typically, reactor waste with neptunium is disposed in iron-lined copper canisters, which is suitable as long as the canisters are surrounded by water.
However, any draw-down or disruption of the water table could remove or reduce the surrounding water. In these circumstances, should the copper dry, the reactor waste will begin to decay. And as the copper disappears, it would take a short time for the iron to begin rusting away, leaving the reactor waste exposed to the groundwater.
Christiansen’s team conducted experiments demonstrating green rust’s ability to immobilize neptunium at the Swedish Nuclear Fuel and Waste Management’s pilot research facility at Okskarshamn on Sweden’s east coast and at the Karlsruhe Institute of Technology in Germany. “Our study shows that even the safest encapsulation of radioactive waste could be made safer if radioactive waste canisters are buried in a place where green rust will form,” explains Christiansen.
To ensure the security of radioactive waste, green rust could be established to surround the canisters. Christiansen points out that green rust is not a quick fix for cleaning up radioactive pollutants, but rather part of the defenses built into the waste sites protect the groundwater.
* * *