Flammable Gas Release Estimates for Modified Sluicing Retrieval of Waste from Selected Hanford Single-Shell Tanks
The 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 Barker and Hedengren (2003), 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 475 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. This report is Revision 1 of PNNL-14271. This revision expands the analysis of interstitial liquid drainage-induced gas releases to address a general retrieval scenario (the previous version of this report assumed a center-out retrieval approach and conditions). Tank waste conditions (waste volumes, interstitial liquid levels, temperatures, retained gas void fractions, etc.) have also been updated from the previous version.
- Research Organization:
- Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
- Sponsoring Organization:
- US Department of Energy (US)
- DOE Contract Number:
- AC06-76RL01830
- OSTI ID:
- 15006899
- Report Number(s):
- PNNL-14271 Rev. 1; 820101000; TRN: US0401146
- Resource Relation:
- Other Information: PBD: 5 Mar 2004
- Country of Publication:
- United States
- Language:
- English
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