Revision 0 iv EXECUTIVE SUMMARYProcesses for the removal of residual sludge from SRS waste tanks have historically used solutions containing up to 0.9 M oxalic acid to dissolve the remaining material following sludge removal. The selection of this process was based on a comparison of a number of studies performed to evaluate the dissolution of residual sludge. [1] In contrast, the dissolution of the actinide mass, which represents a very small fraction of the waste, has not been extensively studied. The Pu, Np, and Am in the sludge is reported to be present as hydrated and crystalline oxides. [2][3][4] To identify aqueous solutions which have the potential to increase the solubility of the actinides, the alkaline and mildly acidic test solutions shown below were selected as candidates for use in a series of solubility experiments. 8 wt % (0.9 M) oxalic acid 2 wt % (0.22 M) oxalic acid 1.8 wt % (0.2 M) oxalic acid/0.1 M citric acid 0.18 M HNO 3 /0.5 wt % (0.056 M) oxalic acid 0.18 M HNO 3 /0.2 M NaMnO 4 0.18 M HNO 3 10 M NaOH 10 M NaOH/0.2 M NaMnO 4 1 M NaHCO 3 /Na 2 CO 3 at pH 9.5 1 M NaHCO 3 /Na 2 CO 3 /0.2 M NaMnO 4 at pH 9.5 0.05 M DTPA at pH 2-4 0.18 M HNO 3 blank 10.0 M NaOH blank 1 M NaHCO 3 /Na 2 CO 3 at pH 9.5 blank The efficiency of the solutions in solubilizing the actinides was evaluated using a simulated sludge prepared by neutralizing a HNO 3 solution containing Pu, Np, and Am. The hydroxide concentration was adjusted to a 1.2 M excess and the solids were allowed to age for several weeks prior to starting the experiments. The sludge was washed with 0.01 M NaOH to prepare the solids for use. Following the addition of an equal portion of the solids to each test solution, the concentrations of Pu, Np, and Am were measured as a function of time over a 792 h (33 day) period to provide a direct comparison of the efficiency of each solution in solubilizing the actinide elements. Although the composition of the sludge was limited to the hydrated actinide oxides (and did not contain other components of demonstrated importance), the results of the study provides guidance for the selection of solutions which should be evaluated in subsequent tests with a more realistic surrogate sludge and actual tank waste.The results from the solubility experiments showed that the test solutions containing 0.2 M NaMnO 4 were highly effective in solubilizing the Pu. The order of the solutions from the highest to the lowest concentrations achieved was as follows: 0.18 M HNO 3 /0.2 M NaMnO 4 , > 10 M NaOH/0.2 M NaMnO 4 , > 1 M NaHCO 3 /Na 2 CO 3 /0.2 M NaMnO 4 . SRNL-STI-2011-00521Revision 0 v The concentrations after 792 h (33 days) fell in the range of 0.4-2.0E-02 mol/L (1.0-4.8 g/L); although, the Pu in each solution was at or near its maximum concentration after 168 h (1 week). The concentrations of Pu in these solutions were higher than the concentration measured in the baseline 0.9 M oxalic acid solution at the conclusion of the experiments (5.2E-03 mol/L or 1.2 g/L) except for the concentration in NaHCO 3 /Na 2 CO 3 /NaMn...

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