The high-level radioactive wastes in many single-shell tanks (SSTs) at the Hanford Site are to be retrieved by a modified sluicing method. Retrieval operations will hydraulically erode and dissolve the saltcake waste, and the resulting brine will then be pumped to a double-shell tank (DST). Waste gases residing in the solid waste matrix will be released into the tank headspace when the matrix is eroded or dissolved. These retained waste gases include the flammable species hydrogen, methane, and ammonia, and there is a concern that these flammable gases could produce a flammable mixture in the tank headspaces during the retrieval operations. 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 waste retrieval operations. The SSTs considered here are ten of the twelve 241-S farm tanks (tanks 241-S-107 and 241-S-111 are excluded from consideration here) and tank 241-U-107 (U-107).
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