(421f) Gas-Bubble Retention and Release Behavior in Deep Sediment Layers

Gas generation in Hanford’s underground waste storage tanks may, under certain conditions, lead to gas accumulation within the layer of sediment (sludge) at the tank bottom.  The gas, which typically has hydrogen as the major component together with other flammable species, is formed by thermal- and radiation-driven chemical reactions.  Accumulation of these gases within the sludge in a waste tank is undesirable and limits the amount of tank volume for waste storage.  Further, accumulation of large amounts of gas in the sludge may potentially result in an unacceptable sudden release of the accumulated flammable gas if the sludge-layer density is reduced to less than that of the overlying sludge or that of the supernatant liquid.  A large, sudden flammable gas release can potentially result in a deflagration that is a hazard to personnel and equipment near the tank.  For this reason, a thorough understanding of the circumstances that can lead to a potentially problematic gas accumulation in sludge layers is needed. 

Previous work has demonstrated that for sufficiently strong sludge, bubbles form as slits and cracks that interconnect to create channels for the generated gas to be released.  One previous series of studies presented a model for gas transport in channels where the bubble retention mechanism changed at a specific sludge depth (denoted as d_max and measured from the sludge surface) that marked a transition between two types of gas retention behavior.  At depths less than d_max (the region of the sludge between the surface and d_max), connected cracks and channels allow gas to escape the sludge and gas is prevented from accumulating beyond the level needed to establish the connected channels.  At depths greater than d_max, however, the previous work argues that overburden stress is sufficient to prevent formation of the connected-channel network and any generated gas will tend to accumulate in the sludge until the sludge bulk density is low enough that the layer becomes unstable and the gas is released.  The purpose of the current study is to evaluate whether gas retention and release behavior changes at depths below d_max.

Gas retention and release tests were performed in a 14 m tall column and supporting tests were also conducted in a 6 m tall and bench-scale columns.  These tests used aqueous pastes of kaolin clay with shear strengths ranging from 350 to 1000 Pa where the layer depths during testing were at least 50% deeper than estimates of d_max.  In situ gas generation was accomplished by adding small quantities of zero valent iron powder that corroded to generate hydrogen gas.  Results will be shown for the retained gas fraction as a function of depth and total retention, and video images of gas bubble transport throughout the column will also be presented.  The results show no difference in gas retention and release behavior for depths above and below the sludge depth d_max.