Research yields new tool for surface decontamination

Through the Environmental Management Science Program (EMSP), DOE’s Office of Environmental Management (EM) and Office of Science (SC) collaborate to fund basic research to solve intractable problems that threaten the successful closure of DOE sites. As one of the programs within the Office of Science and Technology, EMSP ensures that OST’s projects cover the full spectrum of R&D. EMSP’s Web site is at http://emsp.em.doe.gov.

The decontamination and decommissioning of buildings, structures, and materials is one of the most expensive and challenging components of the DOE cleanup effort. Baseline methods using wet chemical solvents and mechanical blasting generate substantial secondary waste and expose workers to risk. With funding from EMSP, researchers at the University of California at Los Angeles and Los Alamos National Laboratory have invented a plasma jet that provides a cost-effective and environmentally safe method of removing radioactive contamination.

The device selectively removes heavy metals from surfaces, making objects and structures radiation free. Volatile reaction products are captured on filters, enabling buildings and equipment to be safely decommissioned while dramatically reducing the volume of waste to be treated as contaminated. In addition, because the process doesn’t overheat the surface or require the addition of large quantities of chemicals, it doesn’t threaten the health and safety of D&D personnel.

The plasma jet consists of two concentric electrodes separated by 1–3 mm. The inner electrode is driven with 50–500 watts of radio frequency power; the outer electrode is grounded. Feed gas containing helium and other gases is ionized as it flows between the electrodes. The electrons in this plasma are not in equilibrium with the rest of the gas: their energy is 2–3 electron volts, or more than 20,000°C. As the plasma emerges from the nozzle and hits a substrate 2–10 mm downstream, the material is “etched” (cleaned).

With the addition of fluorine-containing compounds to the feed gas, the atmospheric-pressure plasma can strip a wide range of radioactive elements from contaminated structures and equipment. Tests on tantalum, a surrogate for plutonium, have demonstrated satisfactory etch rates under relatively mild conditions. For example, with a feed of 97.5% helium, 2% carbon tetrafluoride, and 0.5% oxygen, the plasma jet etches tantalum foil placed 5 mm downstream at 6 micrometers per minute. In this case, the plasma is driven with 400 W, and the metal sample is heated to 250°C. Alternative methods either have much higher power requirements or must operate in a vacuum chamber.

During its first three-year project, the research team made numerous improvements in the design of the plasma jet, which can operate for hundreds of hours without degradation in performance. Several source designs are scalable to large areas and can treat a variety of objects, including gloveboxes and building walls. Compact units have also been developed to facilitate field deployment. The research team, which won an R&D 100 Award in 1999, has obtained one U.S. patent and filed additional applications. Several companies have expressed an interest in acquiring patent rights and commercializing the plasma jet technology.

EMSP recently extended the research program to fully characterize the discharge physics and chemistry, engineer the exhaust containment system, and perform field tests. Tests on actinide contamination are planned at LANL’s plutonium facility. And if research exploring the removal of zirconium and strontium is successful, the plasma device will be tested at Idaho National Engineering and Environmental Laboratory to remove contamination with those materials.

In the first-generation atmospheric-pressure plasma device, process gas is ionized between two coaxial electrodes. The nozzle directs the reactive gas onto the target surface.

For more information on this research, contact principal investigator Robert Hicks, University of California at Los Angeles, (310) 206-6865, hicks@ea.ucla.edu.