Atmospheric Pressure Plasma Cleaning Of Contaminated Surfaces 1998 Annual Progress Report

The object of this research program is to develop an atmospheric-pressure plasma jet for converting transuranic wastes (TRUs) into low-level radioactive wastes (LLWs). This plasma process will be used to efficiently decontaminate a wide range of structures and equipment. This report summarizes work after 1 year and 9 months of a 3-year project. A picture of the atmospheric-pressure plasma jet is shown in Fig. 1. This new plasma source consists of two concentric electrodes through which a mixture of helium and reactive gases flow. The plasma is ignited by applying 13.56 MHz RF power to the inner electrode. The characteristics of this discharge are different from other atmospheric-pressure plasmas, such as transferred arcs, torches, coronas and silent discharges. Shown in Fig. 2 is the current-voltage curve for the plasma jet. Spark breakdown occurs at 0.01 A, and is proceeded by a normal glow region, in which the voltage remains constant with increasing current, and an abnormal glow region, in which the voltage increases rapidly with current. At about 1.0 A and 225 V, the plasma begins to arc. The normal glow region is rarely observed in atmospheric pressure plasmas. They usually proceed directly from spark breakdown to arcing. The trend shown in the figure indicates that the plasma jet is stable over a wide range of operating conditions. The distribution of reactive species in a plasma jet, containing oxygen and helium, has been characterized by Langmuir probe measurements, optical emission spectroscopy, and ultraviolet absorption spectroscopy. The charged particle density ranges from about 5 x 1011 cm−3 inside the plasma to 1 x 101° cm−3 in the jet exit. The concentration of metastable oxygen molecules (a 1 Dg and b 1 Sg) is estimated to be between 1012 to 1013 cm−3 . By contrast, the ozone concentration increases from about 5 x 1014 cm−3 inside the plasma to 1 x 1016 cm−3 in the effluent. The ozone molecules are produced by the reaction of O atoms with O2 molecules: O + O2 + M a O3 + M. To generate the amount of ozone observed, the O atom concentration in the plasma must be near 1 x 1016 cm−3, or about 10% of the oxygen fed. These results are quite unexpected, because most non-equilibrium, low-temperature plasmas achieve a much lower degree of dissociation. The etching of actinide metals has been simulated by using tantalum as a surrogate material. Tantalum etching rates of up to 1.2 mm/min are achieved with the plasma jet using a CF4/O2/He gas feed. This is 10 times faster than conventional plasma systems, and indicates that the atmospheric-pressure plasma is a promising technology for decontamination of DOE sites. Finally, the plasma jet has been successfully modified to process larger areas, up to about 1.0 ft2 . Work is underway to increase the process scale further. The authors are confident that this technology is capable of being adapted for decontamination operations in the field.

Goals of the project are to (1) identify the key physics and chemistry underlying the use of high pressure plasmas for etching removal of actinides and actinide surrogates; and (2) identify key surface reactions and plasma physics necessary for optimization of the atmospheric pressure plasma jet. Technical description of the work decommissioning of transuranic waste (TRU) into low-level radioactive waste (LLW) represents the largest cleanup cost associated with the nuclear weapons complex. This work is directed towards developing a low-cost plasma technology capable of converting TRU into LLW, based upon highly selective plasma etching of plutonium and other actinides from contaminated surfaces. In this way, only the actinide material is removed, leaving the surface less contaminated. The plasma etches actinide material by producing a volatile halide compound, which may be efficiently trapped using filters. To achieve practical, low-cost operation of a plasma capable of etching actinide materials, the authors have developed a y-mode, resonant-cavity, atmospheric pressure plasma jet (APPJ). In contrast to conventional, low pressure plasmas, the APPJ produces a purely-chemical effluent free of ions, and so achieves very high selectivity and produces negligible damage to the surface. Since the jet operates outside a chamber, many nuclear wastes may be treated including machinery, duct-work, concrete and other building materials. In some cases, it may be necessary to first remove paint from contaminated surfaces using a plasma selective for that surface, then to switch to the actinide etching chemistry for removal of actinide contamination. The goal of this work is to develop the underlying science required for maturation of this technology and to establish early version engineering prototypes. Accomplishments to Date The authors have made significant progress in this program. The work conducted jointly at Los Alamos and at UCLA. This has been facilitated by exchange of people, equipment and designs between the two locations. The study can be divided into three (3) components: (1) source design, operation and physics, (2) effluent characterization and analysis, (3) surface reactions and decontamination. Some of the key accomplishments in each area are noted.

The purpose of this project is to develop a low-cost, environmentally benign technology for the decontamination and decommissioning of transuranic waste. In order to accomplish this goal, an understanding of the scientific principles of operating the atmospheric-pressure plasma jet must be achieved. This knowledge can then be applied to the design of a working tool for D & D applications within DOE.

The objective of this work is to demonstrate a practical, atmospheric pressure plasma tool for the surface decontamination of radioactive waste. Decontamination of radioactive materials that have accumulated on the surfaces of equipment and structures is a challenging and costly undertaking for the US Department of Energy. Our technology shows great potential for accelerating this clean up effort.

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This 3e, edited by Peter M. Martin, PNNL 2005 Inventor of the Year, is an extensive update of the many improvements in deposition technologies, mechanisms, and applications. This long-awaited revision includes updated and new chapters on atomic layer deposition, cathodic arc deposition, sculpted thin films, polymer thin films and emerging technologies. Extensive material was added throughout the book, especially in the areas concerned with plasma-assisted vapor deposition processes and metallurgical coating applications.