PULLMAN – Efforts to design nuclear waste facilities should take into account the tendency of plutonium to attach itself to tiny particles called colloids that are suspended in the groundwater, according to a new study by an international research team that included Washington State University chemist Sue Clark and scientists from Moscow (Russia) State University, the University of Michigan and Cameca in France.
Working on samples from a highly-contaminated site in Russia, the researchers found that plutonium that leached into groundwater over the past 55 years can be detected more than a mile and a half from the site, and that at distances of a mile or more from the site, most of the plutonium was carried by colloids of iron oxides. Their report appears in the Oct. 27 issue of Science magazine.
The finding that plutonium became attached to particles of iron oxides is significant, said Clark, because iron oxides are present in almost all soils worldwide. Since they are ubiquitous, their potential to help plutonium spread from containment sites should be addressed whenever new nuclear waste sites are being planned.
“The impact of this research is that we must consider colloidal transport of plutonium as a potential mechanism for release, and then design the facility to address that sort of a transport mechanism,” said Clark, a professor in the WSU Department of Chemistry. Furthermore, she said, efforts to remediate existing contamination should consider the possibility of colloids, especially when dealing with groundwater in the “far field,” or outside the immediate vicinity of the waste site.
“In the case of the Russian site, you would want to remediate the far field groundwater based on that colloidal form. The plutonium there is not in the dissolved form. You could expend lots of resources trying to remediate the dissolved form and not actually fix the problem, because you didn’t address the colloids,” she said.
The study area in Russia’s southern Ural Mountains was the site of the Mayak Production Association, the former Soviet Union’s first facility devoted to converting spent nuclear fuel to weapons-grade plutonium. Starting in the 1950s, highly radioactive waste from the facility was discharged directly into Lake Karachai. From there, it seeped into the groundwater. The lake is currently undergoing remediation efforts.
The researchers drew water from test wells dug at distances up to 3.9 km (1.6 miles) from Lake Karachai. Each sample was tested for chemical characteristics such as its pH (acidity or alkalinity); the presence of substances commonly dissolved in groundwater, such as carbonates and phosphates; and the presence or absence of plutonium and other elements.
The plutonium was further evaluated to determine whether it was simply dissolved in the water or was attached to colloids. Colloids are particles, hundreds of nanometers or smaller in diameter, that are suspended in the groundwater rather than being dissolved in it. They are solids that move with the natural flow of the groundwater.
The research team found that within about two and a quarter kilometers (1.4 miles) of Lake Karachai, nearly 30 percent of the plutonium present was dissolved in the water, and the rest was attached to colloidal particles; but at greater distances, less than 15 percent of the plutonium was dissolved in the water. The vast majority of it was attached to particles of iron oxides that were smaller than 15 nanometers in diameter. At the greatest distance tested, plutonium levels were still detectable but were about a thousand-fold lower than at the test well closest to the lake.
Clark said colloid-aided transport of plutonium could be reduced by changing the chemistry of a site to prevent plutonium from associating with colloids, or to prevent iron colloids from forming in the first place. Engineering barriers to prevent release of radioactive substances from a containment site is far preferable to trying to remediate after materials escape containment, she said.
Clark cautioned that containment plans for each nuclear waste site must be tailored for the site. In addition to differences in the type and amount of radioactive waste and how the material is contained, each site will have unique geology, chemistry, groundwater flow, and other physical characteristics that can affect how fast and how far plutonium and other contaminants will travel.
“It’s all very site-specific,” she said. “Where you have a problem that was created 40 years ago or more [as at Lake Karachai], you’re stuck with trying to do something to protect human populations from that environmental insult that happened a long time ago. And when you have planned repositories that are under construction, which Yucca Mountain is, you can then go ahead and plan on the front end to include consideration of colloids in the overall facility design, if there is reason to believe that the chemical conditions could lead to the formation of colloids.”
The research was supported by the US Department of Energy, the Russian Academy of Sciences and the Russian Basic Research Foundation.