Atmospheric-pressure Plasma Cleaning of Contaminated Surfaces. 1998 Annual Progress Report

Download or read book Atmospheric-pressure Plasma Cleaning of Contaminated Surfaces. 1998 Annual Progress Report written by and published by . This book was released on 1998 with total page 3 pages. Available in PDF, EPUB and Kindle. Book excerpt: 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.