Screening the Hanford tanks for trapped gas

Whitney, Paul D.; Chen, Guang

doi:10.1002/env.533

Cited by 3 publications

(4 citation statements)

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“…Since 1981, the waste surface level has subsided about 5 inches and the ILL has increased about 10 in. (Whitney 1995). This indicates a gradual gas accumulation, probably in pore-filling bubbles.…”

Section: Conditions In Candidate Tanksmentioning

confidence: 95%

Gas Releases During Saltcake Dissolution for Retrieval of Single-Shell Tank Waste, Rev. 1

Stewart¹

2001

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It is possible to retrieve a large fraction of soluble waste from the Hanford single-shell waste tanks (SSTs) by dissolving it with water. This retrieval method will be demonstrated in Tanks U-107 and S-112 in the next few years. If saltcake dissolution proves practical and effective, many of the saltcake SSTs may be retrieved by this method. Many of the SSTs retain flammable gas that will be released into the tank headspace as the waste dissolves. This report describes the physical processes that control dissolution and gas release. Calculation results are shown and describe how the headspace hydrogen concentration evolves during dissolution. The observed spontaneous and induced gas releases from SSTs are summarized, and the dissolution of the crust layer in SY-101 is discussed as a recent example of full-scale dissolution of saltcake containing a large volume of retained gas. The report concludes that the dissolution rate is selflimiting and that gas release rates are relatively low. iv v Executive SummarySaltcake dissolution is a method that has been proposed for retrieving water-soluble salts from the Hanford single-shell tanks (SSTs). Water will be sprayed on the surface of the waste to dissolve the soluble fraction, and the resulting brine will be pumped out of the tank with an existing saltwell pumping system. Because a large fraction of typical saltcake waste is soluble, most of the waste in an SST can, in principle, be retrieved for nearly the same expenditure of time and money as a typical interim stabilization campaign. A proof-of-concept test (limited to approximately 100,000 gallons of brine) is planned in Tank Many of the SSTs contain flammable gases that will be released into the tank headspace as the waste dissolves. U-107 contains 180 ± 60 cubic meters (6,400 ± 2,000 scf) of retained gas, and S-112 is estimated to retain 122 ± 61 cubic meters (4,300 ± 2,200 scf). This report investigates the potential hazard of the flammable gas that will be released during the dissolution process.Dissolution is expected to release this gas in proportion to the fraction of the waste in which the soluble solids are dissolved. This process is self-limiting and controllable. Water or dilute brine capable of dissolving solids is less dense than the saturated liquid in equilibrium with the solids. Therefore, the solvent cannot penetrate below the pre-existing interstitial liquid in the waste, and dissolution can occur only in waste that is not saturated with liquid. Once the solvent has become saturated, it is no longer capable of further dissolution and forms a barrier to the less dense liquid from above.This means that dissolution, and the associated gas release, can proceed no faster than the interstitial liquid can drain away. This is the primary mechanism that limits dissolution-induced gas releases. This self-limiting behavior makes gas release by dissolution controllable. The dissolution rate can be reduced relatively quickly by terminating pumping (accumulating brine forms a barrier to fresh solvent)...

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Section: Conditions In Candidate Tanksmentioning

confidence: 95%

Gas Releases During Saltcake Dissolution for Retrieval of Single-Shell Tank Waste, Rev. 1

Stewart¹

2001

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“…The interstitial liquid level (ILL) is about 123 inches, 66 inches below the waste surface level. Since 1981, the waste surface level has subsided about 5 inches while the ILL has increased about 10 inches (Whitney 1995). This indicates a gradual gas accumulation, probably in pore-filling bubbles.…”

Section: Conditions In Candidate Tanksmentioning

confidence: 99%

“…The best estimate of the retained gas volume is 180 ± 60 cubic meters (6,400 ± 2,000 scf) . The waste level has risen 6 inches since 1981 (Whitney 1995) indicating a gas accumulation of 75 m 3 at 1 atm assuming an in situ pressure of 1.2 atm.…”

Section: Conditions In Candidate Tanksmentioning

confidence: 99%

Gas Releases During Saltcake Dissolution for Retrieval of Single-Shell Tank Waste

Stewart¹

2001

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It is possible to retrieve a large fraction of soluble waste from the Hanford single-shell waste tanks (SST) by dissolving it with water. This retrieval method will be demonstrated in U-107 and S-112 in the next few years. If saltcake dissolution proves practical and effective, many of the saltcake SSTs may be retrieved by this method. Many SSTs retain a large volume of flammable gas that will be released into the tank headspace during retrieval as the waste dissolves. This report describes the physical processes that control dissolution and gas release. Calculation results are shown describing the headspace hydrogen concentration transient during dissolution. The observed spontaneous and induced gas releases from SSTs are summarized, and the dissolution of the crust layer in SY-101 is discussed as a recent example of full-scale dissolution of saltcake containing a very large volume of retained gas. The report concludes that the dissolution rate is self-limiting, and gas release rates are relatively low. iv v Executive SummarySaltcake dissolution is a proposed method for retrieving water-soluble salts from the Hanford SSTs. Water will be sprayed on the waste surface to dissolve the soluble fraction of the waste while the resulting brine is pumped out of the tank with an existing saltwell pumping system. Because a large fraction of typical saltcake waste is soluble, most of the waste in an SST can, in principle, be retrieved with almost the same time and cost as a typical interim stabilization campaign. A proof-of-concept test (limited to approximately 100,000 gal of brine) is planned in U-107 during the fall of 2001. A full-tank retrieval demonstration system is being planned for S-112 in 2002. If saltcake dissolution proves practical and effective, many of the saltcake SSTs may be retrieved by this method.Many of the SSTs contain a large volume of flammable gas that will be released into the tank headspace as the waste dissolves. U-107 contains 180 ± 60 cubic meters (6,400 ± 2,000 scf) of retained gas, and S-112 is estimated to retain a much smaller volume of 46 ± 23 cubic meters (1,600 ± 800 scf). This report investigates the potential hazard of flammable gas that will be released during the dissolution process.Dissolution is expected to release this gas in proportion to the fraction of the waste in which the soluble solids are dissolved. This process is self-limiting and controllable. Water or dilute brine capable of dissolving solids is less dense than the saturated liquid in equilibrium with the solids. Therefore, the solvent cannot penetrate below the pre-existing interstitial liquid in the waste; and dissolution can only occur in waste that is not saturated with liquid. Once the solvent has become saturated, it is no longer capable of further dissolution and forms a barrier to the less dense liquid from above.This means that dissolution, and the associated gas release, can proceed no faster than the interstitial liquid can drain away. This is the primary mechanism that limits dissolution-induced g...

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“…Barometric pressure changes cause the retained gas in the waste to expand or compress, causing the waste surface level and/or the interstitial liquid level to fluctuate. The quantity of retained gas can be estimated from the magnitude of the level fluctuations (Whitney 1995). This technique is not applicable to all tanks because many have a solid waste surface that does not fluctuate with barometric pressure changes, and changes in interstitial liquid level below the surface cannot be measured with sufficient precision to be used.…”

Section: Retained Gas Inventories and Compositionmentioning

confidence: 99%

Flammable Gas Release Estimates for Modified Sluicing Retrieval of Waste from Selected Hanford Single-Shell Tanks

Huckaby¹,

Wells²

2003

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Executive SummaryThe high-level radioactive wastes in many single-shell tanks (SSTs) at the Hanford Site are to be retrieved by a modified sluicing method that uses water jets to dissolve the water-soluble waste and mobilize the water-insoluble waste. Retrieval operations will liberate any waste gases trapped in the wetted solid waste matrix, and these gases will be released into the tank headspaces. Because the trapped gases include the flammable species hydrogen, methane, and ammonia, a concern exists that a flammable mixture could be formed in the tank headspaces. This report combines conservative retained gas inventory estimates and tank data with anticipated waste retrieval rates to estimate the potential headspace flammability of selected SSTs during modified sluicing waste retrieval operations. Considered here are nine of the 12 tanks from the 241-S tank farm (241-S-107, 241-S-111, and 241-S 112 are not considered) and Tank 241-U-107.This report is intended to support the specification of process controls that ensure flammable conditions do not develop in the tank headspaces. Consequently, the physical scenarios considered, the models developed to estimate retained gas releases and the tank headspace compositions under these scenarios, and the model input data are intended to conservatively assess the potential to reach headspace flammability. The analyses are intended to address worst-case conditions and establish reasonable upper bounds on the achievable flammability of the tank headspaces. Flammable retained gas inventories, for example, are based on the 95th percentile developed by , giving 95% confidence that actual inventories are smaller than those used in the calculations.Gas releases and headspace flammability were evaluated for three general scenarios: a very aggressive dissolution and erosion of saltcake waste by water jets impinging on the waste surface, the drainage of interstitial liquids from saltcake during a shutdown of the retrieval process, and the dissolution of saltcake by unsaturated liquids during a shutdown of the retrieval process.The simple model of waste retrieval using the modified sluicing approach indicated that the flammable gas headspace concentrations can rapidly approach the action level of 25% of the lower flammability limit (LFL) when the tank is passively ventilated. While it is not necessary to use the portable exhauster to maintain the headspace hydrogen concentration below this action level, retrieval rates would probably be limited by the slow removal of flammable gases by passive ventilation. It was determined that using a portable exhauster anywhere in the assumed operating range of 270 to 450 cfm would prevent the headspaces from reaching the 25% of LFL action level even if the water jets are very effective at eroding the saltcake. Specific guidelines are developed to ensure that, in the event of a catastrophic loss of the retrieval pump and portable exhauster, headspace flammability will not reach the LFL. iv This report is Revision 2 of PNNL-14271. Revision 1 ...

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Screening the Hanford tanks for trapped gas

Cited by 3 publications

References 9 publications

Gas Releases During Saltcake Dissolution for Retrieval of Single-Shell Tank Waste, Rev. 1

Gas Releases During Saltcake Dissolution for Retrieval of Single-Shell Tank Waste, Rev. 1

Gas Releases During Saltcake Dissolution for Retrieval of Single-Shell Tank Waste

Flammable Gas Release Estimates for Modified Sluicing Retrieval of Waste from Selected Hanford Single-Shell Tanks

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