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DISCLAIMERPortions of this document may be illegible in electronic image products. Images are produced from the best available original document. Executive SummaryResearch at Pacific Northwest National Laboratory (PNNI,)@) has probed the physical mechanisms and waste properties that contribute to the retention and release of flammable gases fiom radioactive waste stored in underground tanks at Hanford. This study was conducted for Westinghouse Hanford Company as part of the PNNL Flammable Gas Project. The wastes contained in the tanks are mixes of radioactive and chemical products, and some of these wastes are known to generate mixtures of flammable gases, including hydrogen, nitrous oxide, and ammonia. Because these gases are flammable, their retention and episodic release pose a number of safety concerns.Previous investigations of bubble retention focused on bubbles retained in settled solids that are submerged beneath a supernatant liquid layer. This configuration is typical of waste stored in double-shell tanks (DSTs). In this situation, when the retention of bubbles causes the solids to become buoyant, the waste undergoes a buoyancy-induced rollover. While the rollover gas release mechanism in DSTs is well documented, the mechanism of bubble retention is not as well understood. In single-shell tanks (SSTs), in contrast, the settled solids are often not completely submerged, and buoyant rollovers similar to those in DSTs are not possible. For SST waste, neither the mechanisms of bubble retention nor those of bubble release are well understood.The objective of this study is to quantify the pertinent mechanisms of bubble retention and release by measuring and observing bubble retention both in actual waste samples and in simulated wastes. M a x i m u m gas retention and release data were obtained fiom actual waste samples fiom the SST 241-S-102 (S-102) and the DST 241-SY-103 (SY-103), both of which are on the Flammable Gas Watch List. In addition to the retentiodrelease studies, the ability of waste particles to armor and stabilize gas bubbles was investigated using an SY-103 waste sample.The simulants studied in this work were chosen to mimic the behavior of actual SST waste. SST wastes have a wide range of physical properties that range fiom clay-like, plastic sludges to hard salt cake. In this work, experiments focusid on fine-particle simulants composed of bentonite clay and water, because these are believed to mimic the sludge-like waste contained in SSTs. Because the actual properties of SST waste are not well-known, simulants with a wide range of strengths were prepared and tested. The experimental results quantified the ability of these simulants to retain gas and indicated how the gas is released. For comparison-with these simulants, some previously reported results for particulate simulants were reevaluated. In addition, new gas retention results were obtained for partially drained particulate simulants to aid in our understanding of gas retention in SSTs that have been salt-well pumped. Together, th...

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.

Executive SummaryThe 177 storage tanks at Hanford contain a vast array of radioactive waste forms resulting, primarily, from nuclear materials processing. Through radiolytic, thermal, and other decomposition reactions of waste components, gaseous species including hydrogen, ammonia, and the oxidizer nitrous oxide are generated within the waste tanks. Many of these tanks are known to retain and periodically release quantities of these flammable gas mixtures. The primary focus of the Flammable Gas Project is the safe storage of Hanford tank wastes. To this end, we strive to develop an understanding of the mechanisms of flammable gas retention and release in Hanford tanks through laboratory investigations on actual tank wastes. These results support the closure of the Flammable Gas Unreviewed Safety Question (USQ) on the safe storage of waste tanks known to retain flammable gases and support resolution of the broader Flammable Gas Safety Issue.The overall purpose of this ongoing study is to develop a comprehensive and thorough understanding of the mechanisms of flammable gas retention and release. The first objective of the current study was to classify bubble retention and release mechanisms in two previously untested waste materials from Tanks 241-AN-103 (AN-103) and 241-AW-101 (AW-101). Results were obtained for retention mechanisms, release characteristics, and the maximum gas retention. In addition, unique behavior was also documented and compared with previously studied waste samples. The second objective was to lengthen the duration of the experiments to evaluate the role of slowing bubble growth on the retention and release behavior. Results were obtained for experiments lasting from a few hours to a few days. Waste Materials and MethodThe types of wastes in Hanford tanks are varied, and many must be studied to probe the full range of gas retention and release characteristics. Although each tank is unique, broad classification and categorization is attempted to reduce the number of waste types that must be studied individually. In previous laboratory studies of gas retention and release in Hanford tank wastes, the waste types included double-shell tank (DST) wastes from Tanks 241-SY-101 (SY-IOl), and 241-SY-103 (SY-103), both complex concentrate wastes, and single-shell tank (SST) waste from Tank 241-S-102, a saltcake waste. In the current study, the specific materials investigated include waste from AN-103, a double-shell slurry waste, and from Tank AW-101, a double-shell slurry feed waste. Both of these DST waste types are concentrated evaporator wastes, but the double-shell slurry in AN-103 is in a more concentrated form. These two tanks are of interest because they have a history of retaining large quantities of flammable gas. continues to retain gas and has yet to exhibit large gas release events. The two tanks also represent waste types not previously investigated in laboratory gas retention studies.The general experimental approach applied in the current and previous laboratory gas retention and rel...