Solidification/stabilization of simulated uranium and nickel contaminated sludges

Abstract

Research missions in nuclear energy conducted by the U.S. Department of Energy facilities have generated large volumes of mixed wastes with hazardous and radioactive components. Uranium and nickel are the primary contaminants of concern in this research which focused on better understanding the Solidification/Stabilization (S/S) chemistry, complex waste-binder interactions, and the suitability and effectiveness of additives in the Portland cement based treatment systems. The treatability was investigated with a simulated waste representative of the actual Oak Ridge K-25 pond waste. Screening tools such as the short term slurry tests were used to ascertain the behavior of the contaminants in the cement based systems for a range of binder-to-waste ratios, and optimal substitutions of cement with additives. This was used in combination with the Acid Neutralizing Capacity of the S/S system components to design solid waste form mixes to be evaluated over long curing periods. Attempts at estimating the extent of sorption and other immobilization mechanisms were made using dry and hydrated cement matrices. Portland cement based systems with flyash, silica fume and sodium sulfide as additives were studied. Porewater uranium and nickel concentrations, leachability, physical immobilization in terms Of MacMullin number and Unconfined Compressive Strength, regulatory compliance, and risk reduction were evaluated in the solid waste forms ranging over three binder-to-waste ratios. The 900 mg/L uranium and 3,000 mg/L nickel in the untreated sludge were reduced to less than 10 mg/L uranium and 30 mg /L nickel in the TCLP extract of the solidified waste form.