The start of ebullition in quiescent, yield-stress fluids

Abstract

Non-Newtonian rheology is typical for the high-level radioactive waste (HLW) slurries processed in the Hanford Tank Waste Treatment and Immobilization Plant (WTP). Hydrogen and other flammable gases are generated in the aqueous phase by radiolytic and chemical reactions. HLW slurries have a capacity for retaining gas characterized by the shear strength holding the bubbles still. The sizes and degassing characteristics of flammable gas bubbles in the HLW slurries expected to be processed by the WTP are important considerations for designing equipment and operating procedures. Slurries become increasingly susceptible to degassing as the bubble concentration increases. This susceptibility and the process of ebullitive bubble enlargement are described here. When disturbed, the fluid undergoes localized flow around neighboring bubbles which are dragged together and coalesce, producing an enlarged bubble. For the conditions considered in this work, bubble size increase is enough to displace the weight required to overcome the fluid shear strength and yield the surroundings. The buoyant bubble ascends and accumulates others within a zone of influence, enlarging by a few orders of magnitude. This process describes how the first bubbles appear on the surface of a 7 Pa shear strength fluid a few seconds after being jarred.

Hydrogen Gas Retention and Release from WTP Vessels: Summary of Preliminary Studies

“…Although the release event characterized in Figure 6.2 may have been precipitated by a slight disturbance of the vessel (discussed below), video clearly shows that it was a BC gas release, as was also noted by Sherwood and Sáez (2014). Figure 6.3 shows a time sequence of photos before, during, and shortly after the main BC GRE.…”

“…In the earlier studies, the focus was on understanding bubble retention and release in much stronger materials that do not exhibit BCs, though it was noted that BCs would be important for situations in weak waste materials with relatively fast HGRs (Stewart et al 1996). A recent theoretical study (Sherwood and Sáez 2014) discusses BCs (referred to as ebullitions) and provides a model that describes a way in which a dense array of bubbles in a thixotropic fluid may be destabilized by creeping flow. The role of coalescence in the subsequent BC is emphasized.…”

“…Preliminary (unreviewed) retained gas fraction and release data and a video of the BC gas release event have been shown in a number of presentations to BNI, Hanford contractors, DOE, and others interested in tank farm and waste operations since 2004 Sherwood and Sáez (2014). also featured a photo from the video and discussed the BC gas release mechanism.…”

Enhanced gas migration through permeable bubble networks within consolidated soft sediments

“…10 Legacy nuclear waste sludge deposits, for example, at Sellafield, UK 11 and Hanford, USA, 12,13 also experience in situ gas (mainly hydrogen) generation due to corrosion of metallic cladding materials and exposed fuel elements, combined with radiolysis of aqueous liquors by water soluble fission products. Further understanding of gas migration and chronic release is required to mitigate against acute gas release events 13,14 with the potential to present a flammability hazard which could risk a breach of containment.…”

Numerical study of chemical reaction effects in magnetohydrodynamic Oldroyd-B: oblique stagnation flow with a non-Fourier heat flux model

“…Non-Newtonian liquids are encountered in many technological applications including polymer processing, biotechnology, lubrication of aerospace and automotive vehicles and nuclear thermo-hydraulics [1]. Such fluids may be delineated broadly into three classes, namely "rate type", "differential type" and "integral type".…”