Waste Preparation and Transport Chemistry: Results of the FY 2002 Studies

The initial step in the remediation of nuclear waste stored at Hanford and the Savannah River Site (SRS) involves the retrieval and transfer of the waste to another tank or to a treatment facility. The retrieved waste can range from a filtered supernatant to a slurry. Nearly all of the recent solid formation problems encountered during waste transfers and subsequent treatment steps have involved decanted or filtered supernatants. Problems with slurry transfers have not yet surfaced, because tank farm operations at Hanford and the SRS have focused primarily on supernatant transfers and treatment. For example, the interim stabilization program at Hanford continues to reduce the level of supernatants and interstitial liquids in its single-shell tanks through saltwell pumping of filtered liquid. In addition, at present, the cross-site transfer lines at Hanford can be used only to transfer liquids. Another reason for fewer problems with slurry transfers involves the additions of large quantities of dilution water prior to the transfer. When the waste is transferred, a drop in temperature is expected because most transfer lines are not heated. However, the dilution water reduces or eliminates solid formation caused by this temperature drop. In sharp contrast, decanted or filtered supernatants are near or at saturation for certain compounds. In such cases, tank farm operators must continue to evaporate their liquid waste since available tank space is quite limited. Solid formation can occur when the temperature of saturated solutions drops even slightly. The evaporation step can also lead to the formation of problematic solids. At the SRS, the evaporation of a relatively dilute waste stream was suspended due to the formation of deposits in the evaporator system. Therefore, small drops in temperature or evaporation can lead to problematic solid formations