Solidification Tests for LLW Sludges

Oak Ridge National laboratory has about 350,000 gallons of remote-handled (RH) sludge in ten liquid low-level waste (LLLW) tanks that must be solidified and stabilized for disposal at the Nevada Test Site. Samples of the waste sludge were collected from four tanks, and a total of 36 small-scale grouting tests were performed. The presence of free water during curing was evaluated, and the cured grouts were analyzed using the Toxicity Characteristic Leaching Procedure (TCLP) to determine if any of the hazardous metals in the sludge (Cd, Cr, Hg, Pb, etc.) would leach above the Resource Conservation and Recovery Act (RCRA) limits.

The grouting formulation used for these tests, with ratios of grout-forming additives weight to waste slurry weight ranging from 0.75:1 to 1.2:1, produced wet grout mixtures that were easy to stir and were self leveling. The grout mixtures cured with no free water visible at any time. The cured grout matrix was very effective at retaining the hazardous metals in the sludge, with TCLP leachate concentrations well below the RCRA limits. The addition of ferrous sulfide (FeS) to some of the sludge samples resulted in, at most, a minimal reduction in the amount of mercury that leached from the grout samples, and had no detectable impact on the other heavy metals present in the sludge (Cd, Cr, and Pb). The TCLP extraction was performed on three samples after 1 day and 7 days of curing, compared to the >28 days for the remaining samples. The metal concentrations for the short cure time samples were similar to the remaining samples, and were all well below the RCRA limits.