Remediation of low-level mixed waste: cellulose-based materials and plutonium

Abstract

Low-level mixed radioactive wastes containing cellulose-based materials and plutonium have been generated during various nuclear processing activities. Biological digestion of the organic- or cellulose- based material was examined as an environmentally acceptable and effective method of treatment for these and other similar wastes. Cellulase enzyme was used to initiate biodegradation prior to 90% destruction of the cellulose material by a sewage sludge consortium. Plutonium did not significantly effect the biodegradation. Bench-scale experimental data were used to design a batch treatment system. A cost and sensitivity analysis was performed to determine the optimal reactor size, materials of construction and media type. The sensitivity analysis indicated that while a 12-month treatment scenario using a carbon steel ball mill, sludge digester and vacuum thickener was the least expensive scenario evaluated on a levelized cost basis ($800 per ton of waste degraded per month), the 12-month scenario using stainless steel construction and the alternative dewatering system offered the most cost-effective treatment alternative and better corrosion resistance (levelized cost of $1130 per ton per month). The dewatering system consisting of a disk centrifuge and sludge dryer is capable of doubling the sludge solids content and produce an overall waste reduction of 67%. The proposed waste treatment system offers a cost savings of up to 31% compared to conventional disposal practices.