Executive SummaryThe effects of heating and dilution on the rheological and physical properties of a composite sample taken from the nonconvective slurry region of Hanford waste Tank 241-SY-101 were investigated. The composite sample was prepared by combining three samples from segments 16, 16R, and 18 taken during the Window C event, core sample 22. Before preparation of the composite sample, the shear strength of each of these samples was determined and found to be much lower than the shear strength observed during previous characterization activities. This lower shear strength was probably observed because the original analyses were performed on unmixed samples, whereas the samples used for these dilution tests had been hand-stirred before shipment to Pacific Northwest Laboratory (PNL).(~) The composite sample was divided into five samples. Four of the divided samples were diluted to 10, 20, 35, and 50 volume percent 2 M NaOH, respectively; the dilutions were prepared by mixing the NaOH with the waste. Due to the amphoteric nature of aluminum hydroxides, a 2 M NaOH diluent was usedto avoid significantly decreasing or increasing the pH of the sample and precipitating dissolved salts.Selected rheological and physical properties were determined on the one undiluted and the four diluted (10, 20, 35, and 50 volume percent 2 M NaOH) samples at three temperatures (50, 70, and 90°C). The analyses were performed on duplicate samples from each of the five dilution levels. Shear stress as a function of shear rate, yield stress, settling behavior, slurry density, filtered solids and filtrate densities, and weight percent filtered solids were the properties obtained.The rheological properties (yield stress and shear stress as a function of shear rate) of samples from Tank 241-SY-101 are strongly dependent upon sample history. A decrease in the yield stress of the various samples was observed after the waste was subjected to mixing or shearing. This decrease in the yield stress appears to be irreversible, at least in a time span of two years. Because of this dependence upon the sample shear history, rheological studies should be performed on undisturbed samples if data on undisturbed in-tank properties are desired. Alternatively, in situ rheological measurements may be preferable.The one undiluted and all four of the diluted samples exhibited sample history dependent yield pseudoplastic behavior. The yield stress of these samples decreases with increasing temperature and dilution. Most of the decrease in the yield stress is observed between 50 and 70°C and 0 and 10% NaOH dilutions.The apparent viscosities of the one undiluted and the four diluted samples at shear rates higher than 150 s" are weakly dependent upon sample shear history. No significant changes in apparent viscosity were observed over multiple runs at the higher shear rates. The apparent viscosities of the four diluted and one undiluted sample are dependent upon both temperature and dilution, with the temperature dependence most evident in those samples with hi...

During the first four days of irradiation, additional settling was observed in aI1 of the 0.5: 1, 0.75: 1, and 1:l diluted samples and in one each of the composite and 3:l samples.Volume measurements showed no significant gas retention in the settled solids over the 27-day irradiation period. Gas generation was greatest for the 0.5:l samples followed by the 0.75:l samples and then the whole tank composite. The volume of gas generated by the 1 : 1 and 3: 1 samples was . below that produced by the composite.After 28 days of irradiation, a vacuum was applied to the head space of the sample vessels. The drop in pressure caused retained bubbles to expand and could also have caused other gases in solution to be drawn into bubbles. The composite and 0.5:l and 0.75:l samples experienced gas retention and events similar to an actual tank rollover. The' 1:l and 3:l samples did not experience rollovers or gas retention. These results support the recommendation that a 1:l dilution of Tank 241-SY-101 waste will mitigate hazardous-gas-release-event behavior (Hudson et al.