Nuclear mattersCement prison for old radioactive waste

The cold war ended long ago, but its radioactive legacy still lingers in the water and soil of the western United States. Between 1950 and 1990, nuclear weapons materials production and processing at several federal facilities generated billions of gallons of water contaminated with radioactive byproducts.

These sites have since updated their waste treatment practices to keep new contamination from entering the water supply. Some old contamination, though, is still present.

Though public drinking water near Department of Energy sites in Washington and Idaho does not contain strontium-90 at concentrations above the EPA’s safe drinking water limit, some monitoring wells at the sites do. At least some contamination in the monitoring wells is strontium-90 that slowly leached out of the soil.

“It’s not something you want to leave to wash out,” says James Henriksen, an INL microbiologist.

Researchers at Idaho National Laboratory, the Center for Advanced Energy Studies, and other national labs and universities are working together to test an inexpensive method to sequester strontium-90 where it lies. The researchers can coax underground microbes to form calcite, a white mineral form of calcium carbonate and the main ingredient in cement. Calcite should be able to trap strontium-90 until long after it has decayed into harmless zirconium. Strontium-90 has a nearly 30-year half-life, which means that a sample of the stuff will take around 300 years to decay more or less completely.

“Three hundred years is nothing for calcite,” Henriksen says.

When people consume strontium-90 in contaminated food or drinking water, the radioactive isotope can replace some of the calcium in living bone.

The bone-bound strontium-90 acts as a long-term source of damaging radiation that can spawn deadly cancers of blood, bone and skin.

Cleaning strontium-90 out of the ground and water hasn’t been easy. “There have been heroic efforts that cost enormous amounts of money,” Henriksen says. Some of the most contaminated dirt was excavated, but digging up all the contamination would be astronomically expensive. A project to pump out and treat contaminated groundwater only removed a tenth as much strontium-90 as did natural decay during the same period.

Pumping the groundwater was not effective because most of the contamination was stuck on solid surfaces underground, says INL environmental researcher Yoshiko Fujita. Remove the strontium-90 from the water, and more leaches off the solids to replace it.

So instead of trying to strip strontium-90 out of the ground, Fujita, Henriksen and their colleagues are trying to coax microbes