NL's technology removes contaminants from surfaces.
Cleaning Up Radioactivity
August / September 2007
Volume 5 Number 4
The world media recently reported about an intercept of weapons-grade uranium in the former Soviet republic of Georgia. In Kazakhstan, a man was sentenced for attempting to sell radioactive isotopes that could be used to make a "dirty bomb." These and other events demonstrate the potential risk of small-scale nuclear terrorist acts. A "dirty bomb" and other kinds of radioactive dispersal devices (RDDs) are primarily economic and psychological weapons. American strategists fear that buildings that are precious national icons could be targeted for terrorist attack. The U.S. Defense Advanced Research Projects Agency (DARPA) and the Department of Homeland Security have actively sought the development of novel decontamination methods for RDDs. In response, Idaho National Laboratory has developed a new technology to safely and effectively remove radioactive contamination from various types of urban surfaces.
Idaho researchers have devised a technology that will remove radioactive contaminants from building materials to levels approaching 1 mSv/year (background radiation is about 3 mSv/year). Radiation exposure is measured by the Sievert (Sv) or milliSievert (mSv), which takes into account the particular biological effects of different types of radiation. Non-destructive removal of radioactive materials challenged the research group's ingenuity, because these isotopes have a high affinity to the composition of most building materials. Removing contaminants without injuring the surface was the primary objective of this project. In addition to the need to make the affected areas safe as quickly as possible, preserving the appearance and structure of buildings and monuments that Americans revere was very important.
INL researchers, Julia Tripp and Rick Demmer, put together a team that included Dean Peterman and Laurence Hull that responded to DARPA's research call. Tripp and Demmer had worked for several years on radionuclide decontamination techniques. Tripp says, "Rick and I worked on minimizing waste going to our tank farm. We did tests on all kinds of different decontamination techniques. We performed research on concrete because that was a problem there. Concrete's always bad to decon because the radionuclides like to go in the pores and there are a lot of pores. You can clean the surface, but the contamination leaches back out over time." In addition, Peterman's experience with solvent extraction chemistry and Hull's experience with modeling of geological systems was needed to create a winning approach.
The INL team's experience with decontaminating radionuclides from concrete and other surfaces was relevant to DARPA's focus. Counterterrorism strategists worry most about RDDs using highly reactive metal radionuclides that have an affinity for the surface of building materials that is greater than that for water. In that case, radionuclides cannot be washed off with water. Any radionuclides that are dislodged do not remain suspended in the water but return to bind on the surface of the building materials or move into the pores. DARPA asked the INL team to develop a foam-based decontamination approach that "shifted the affinity" of the radionuclide from the building surface to the decontamination solution. They defined success as capturing and removing radioactive particles without further contamination of the soil or ground water, in a manner that was safe for first responder use, and that would not remove or deface building surfaces (as typical scabbling or physical removal processes do).
The INL team, which now included Karen Wright from the geosciences group, developed an advanced chemistry that was applied in a foam chemical system to capture and remove the surface radiological contaminants. The long-lasting decontamination foam they invented is sprayed onto concrete, granite, marble or other building surfaces and will remain on the surface for several hours without breaking down. Then the foam is vacuumed off and safely disposed. Extensive testing revealed that the laboratory's decontamination foam removed the contaminants on the outer layer without damaging the surfaces, which met one of DARPA's goals for cleanup technology. However, the team realized that they would have to enhance the method because radioactive contaminants were distributed farther into the various porous building surfaces and those contaminants that had migrated into the building surface pores were not removed. Their challenge was to develop a two-step process: capture and remove the surface contaminants and remove the contaminants from the substrate pores.
INL researcher Craig Cooper was brought in to assist with modeling the decontamination behavior. Wright and Cooper, with their background in geosciences, helped to provide insight into decontamination of porous materials. Since removal of radionuclides from the clay layer in soils is always difficult, it was thought that clay might help trap and remove radionuclides from the building surface. So, the team developed a clay-containing mixture to be used after the foam to remove the remaining radioactive contamination. This advanced clay paste is applied and left on the surface for an extended period (days to weeks) to allow the contaminants to diffuse to the surface and be captured by the paste. Clay minerals absorb the radioactive contaminant because of their unique structures. By using the clay paste, the radioactive contaminants are "packaged" into a relatively small volume that can be easily removed and processed. Contrasted with decontamination carried out by destroying and hauling away buildings, recovery from the effects of an RDD is enormously enhanced by INL's cleaning technology.
After the combination treatment of foam/clay paste, the radioactive contaminants are removed without harm to the building surfaces. Says Tripp, "The technology actually does better than anything else I've ever seen at getting certain radionuclides out of concrete." On marble, up to 88 percent of the contamination was removed by the foam and within six weeks after paste application, approximately 97 percent of the radioactive contaminant was removed. On concrete, the foam removed about 30 percent of the radioactive contaminant and within six weeks after paste application, approximately 89 percent removal was obtained. Additional optimization of the decontamination system is expected to be completed in the near future. Patents have been filed on these technologies.
Colleen Thompson is a member of the technical writing staff at Idaho National Laboratory
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