Mechanisms of gas bubble retention and release: results for Hanford Waste Tanks 241-S-102 and 241-SY-103 and single-shell tank simulants

Abstract

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...

Strong-Sludge Gas Retention and Release Mechanisms in Clay Simulants

“…For these tests, there was insufficient gas generated to reach a peak in retention in the 55 wt% (680 Pa) kaolin. Gas retention reaching a peak value and then holding steady or decreasing slightly with continued gas generation is equivalent to observations made in previous studies with similar strength bentonite simulants (Gauglitz et al 1996). To determine whether the gas generation rate has an effect on gas retention, the gas retention results that were shown as a function of time in Figure 3.2 can instead be shown as a function of gas volume collected.…”

“…The average void fraction peaks at about 7% and then decreases to about 5%, which are the average gas void fractions reported by . This is strikingly low gas retention in comparison with smaller-scale tests of bubble retention reported by Van Kesteren (see Figure 11.5 of Winterwerp and Van Kesteren 2004), Van Kessel (1998), and similar studies focused on bubble retention in actual waste and simulants for Hanford waste (Gauglitz et al 1996;Rassat et al 1998). …”

“…After the solids had been mixed sufficiently to obtain a uniform mixture based on visual observation of the absence of clay clumps, (typically more than 5 min), hydrogen peroxide or small iron particles were added and mixing continued for about a minute. The hydrogen peroxide is used because it decomposes into water and oxygen, which then becomes gas bubbles (Gauglitz et al 1996) and the corrosion of iron particles is known to generate hydrogen gas (Reardon 1995). Upon completion of the mixing, the slurry was transferred to the test vessel.…”

“…The small-scale studies on the mechanisms of gas retention and bubble behavior in tank waste have been the subject of a number of studies (see for example, Gauglitz et al 1994Gauglitz et al , 1995Gauglitz et al , 1996Gauglitz et al , 2001Gauglitz et al , 2009Stewart et al 1996;Rassat et al 1997Rassat et al , 1998Rassat et al , 1999Bredt et al 1995;Bredt and Tingey 1996;and Walker et al 1994). Gauglitz et al (2009) have recently summarized these studies.…”

In situ rheology and gas volume in Hanford double-shell waste tanks

“…The waste yield stress affects the way bubbles grow and how much is stored (Gauglitz et al 1996). The apparent viscosity should also have some effect on the dynamics of a GRE.…”