Strong-Sludge Gas Retention and Release Mechanisms in Clay Simulants

Abstract

Executive SummaryThe Hanford Site has 28 double-shell tanks (DSTs) and 149 single-shell tanks (SSTs) containing radioactive wastes that are complex mixes of radioactive and chemical products. The mission of the Department of Energy's River Protection Project is to retrieve and treat the Hanford tank waste for disposal and close the tank farms. A key aspect of the mission is to retrieve and transfer waste from the SSTs, which are at greater risk for leaking, into DSTs for interim storage until the waste is transferred to and treated in the Hanford Waste Treatment and Immobilization Plant. There is, however, limited space in the existing DSTs to accept waste transfers from the SSTs, and approaches to overcoming the limited DST space will benefit the overall mission.The current safety basis for managing the waste in the tank farms provides controls to prevent spontaneous buoyant-displacement gas release events (BDGREs) and provide for ongoing safe storage of the waste. The current mission plan for future waste transfers assumes a relaxed set of BDGRE controls for selected wastes and DSTs based on a new experimental finding that shows relatively low gas retention for sediment materials that are sufficiently strong (shear strengths greater than about 1000 Pa). Based on these relaxed controls, waste from additional SSTs could be retrieved and transferred to DSTs while still avoiding the potential for BDGREs.The purpose of this study is to summarize and analyze the key previous experiment that forms the basis for the relaxed controls and to summarize progress and results on new experiments focused on understanding the conditions that result in low gas retention. The previous large-scale test used about 50 m 3 of sediment, which would be unwieldy for doing multiple parametric experiments. Accordingly, experiments began with smaller-scale tests to determine whether the desired mechanisms can be studied without the difficulty of conducting very large experiments.The previous large-scale gas retention test was conducted in a 3.5 m diameter test vessel with a sediment layer about 5 m deep. The average retained-gas volume fraction grew over a 25 day period to a peak value of about 7% and then decreased to about 5%. This is much lower than in previous small-scale tests.A key objective for the current gas-retention tests is to conduct tests that have slower gas generation rates and are larger than the previous small-scale tests. In the current study, small-scale tests were conducted with various mixtures of kaolin clay and Min-U-Sil 30 ®1 , which is finely ground silica with a median diameter of about 8 microns, in primarily 12.7 cm diameter test vessels with gas generation provided either by the decomposition of hydrogen peroxide to form oxygen bubbles or by the corrosion of iron to form hydrogen bubbles. The gas generation rates varied and resulted in gas retention that peaked after about 1 to 10 days. The volumes of retained gas and total generated gas were measured over time to determine the retained-gas fraction. Tests we...

Evaluation Of Methods To Measure Hydrogen Generation Rate In A Shielded Cell Environment And A Method Recommendation

“…Although the method is capable of measuring hydrogen retention in the sample if the hydrogen generation in the sample is high enough to be detected, no attempt was made to scale the results obtained from the study illustrated in Figure 1, with the authors stating, "It is not possible to quantitatively extrapolate these retention fractions to large tanks." No methods were identified in the literature to support a scaling method for the results from the retention data; however tests with kaolin and lake sediments suggest that the lab-scale test will result in a very conservative and bounding estimate for retained gases [Gauglitz, 2012]. If the test protocol incorporates mixing into the apparatus, then the test results will not be impacted by hydrogen retention in a settled solids bed.…”

“…While Figure 1 above shows that a small sample can be impacted by solids and/or mixing during the test and that the results are measurable, it is not possible to determine how these results scale from the available literature. Based on studies with kaolin, the small scale results are likely bounding [Gauglitz, 2012] while studies at SRS indicate retention could be very high [Ledbetter, 2004]. The scaling of hydrogen retention in settled beds is a knowledge gap for the measurement of HGR.…”

Rayleigh-Taylor Instability within Sediment Layers Due to Gas Retention: Preliminary Theory and Experiments

“…The stability criterion (Equation (2.8)) together with Equation (2.9) gives the conditions of the onset of an RT instability as a function of the void fraction (difference), the strength and density of the sediment layers, and the vessel diameter. Figure 2.1 shows the result of using the stability criterion given by Equation (2.8), assuming the strengths of the two layers are the same, together with shear strength and density values for kaolin clay slurries given in Gauglitz et al (2012). The different lines represent three smaller vessels that are suitable for small-scale experiments (5, 10, and 23 in.…”

“…The results also show that, for a given shear strength, the void increase needed to initiate an RT instability decreases with increasing tank diameter. Figure 2.1 also shows the range of void fraction that can typically be reached in clay simulants as a function of shear strength (Rassat et al 1998, Gauglitz et al 2012). In the shaded region, bubble release occurs and a void fraction that high cannot be achieved.…”

“…For comparison, Meacham and Kirch (2013) have noted that the estimated maximum channel depth for DST waste sludges is between 4.4 and 9.8 m. To date, no experiments have been conducted that are deep enough to observe the potential difference above and below the maximum channel depth. Gauglitz et al (2012) studied gas retention behavior in kaolin clay simulants and conducted tests with sediment depths up to 1.5 m. In these tests, the clay simulants had very little consolidation during the time period of gas generation. Accordingly, in contrast to the Ketelmeer sediment tests ( Van Kessel and Van Kesteren 2002) in which water removal was a factor in channel formation, the kaolin clay tests did not have water release during consolidation to contribute to channel formation.…”