Investigation of some process parameters using microwave plasma technology for the treatment of radioactive waste

• Published in: Radiation effects and defects in solids 2017-02, Vol.172 (1-2), p.23-31
• Main Authors: Trnovcevic, J., Schneider, F., Scherer, U.W.
• Format: Article
• Language: English
• Subjects: Investigations; Melting; Melts; Microwave plasmas; Nuclear power generation; Ovens; Plasma; Plasma torches; plasma treatment; Radioactive waste; Radioactive wastes; vitrification; volume reduction; Wastes
• ISSN: 1042-0150; 1029-4953
• DOI: 10.1080/10420150.2017.1290092

The production of nuclear energy and the application of other nuclear technologies produce large volumes of low- and intermediate-level radioactive wastes. To investigate a novel means of treating such wastes, plasma is investigated for its efficacy. Plasma treatment promises to simultaneously treat all waste types without any previous sorting or pre-treatment. Microwave-driven plasma torches have the advantage of high-energy efficiency and low-electrode wear. In small-scale experiments, several design variations of an open plasma oven were assembled in order to investigate constraints caused by the materials and oven geometry. The experimental set-up was modified several times in order to test the design characteristics and the variation of plasma-specific proprieties related to the radioactive waste treatment and in order to find a suitable solution with the minimum complexity that allows a representative reproducibility of the results obtained. A plasma torch controlled by a 2.45 GHz microwave signal of up to 200 W was used, employing air as the primary plasma gas with a flow rate of ∼2 L/min. Different organic and inorganic materials in different shapes and sizes were treated besides a standardized mixture resembling mixed wastes from nuclear plants. The results prove that the chosen microwave plasma torch is suitable for a combined combustion and melting of organic and in-organic materials. Investigation of the specimen size to be treated is influential in this process: the power is still too low to melt larger samples, but the temperature is sufficient to treat all kinds of material. When glass particles are added, materials melt together to form an amorphous substance, proving the possibility to vitrify material with this plasma torch. By optimization of the oven configuration, the time needed to combust 25 g of standard sample was reduced by ∼50%. Typical energy efficiencies were found in the range of 8-20% for melting of metal chipping, and ∼90% for melting of zinc powder.