PNNL

Abstract

In Hanford Site underground waste storage tanks, a typical waste configuration is settled beds of solid particles beneath liquid layers. These sediment beds are typically also composed of layers due to incremental waste transfers into the vessel, for example, and these layers can have different physical and chemical properties. One postulated configuration within the settled bed is a more-dense layer lying atop a less-dense layer. The different densities can be a result of different gas retention in the layers or different degrees of settling and compaction in the layers. If the density difference between the layers is sufficiently high, this configuration can experience a Rayleigh-Taylor (RT) instability, in which the less-dense lower layer rises into the upper layer. The motion from the RT instability has the potential to cause the release of some portion of the gas retained in these layers, and, because hydrogen is a component in the retained gas, this results in a potential flammable gas safety hazard. Previous studies of gas retention and release in Hanford tank waste have not considered potential buoyant motion within a settled bed of solids. However, because future waste management operations may lead to sludge depths in double-shell tanks (DSTs) that are greater than in historical practice and this may create waste configurations where an RT instability could occur, consideration is being given to the RT instability. The postulated gas retention-release scenario is referred to as a deep-sludge gas release event (DSGRE). The overall objectives of the present report are to provide quantitative information for (a) predicting the conditions under which an RT instability might occur in waste stored in DSTs, and (b) estimating the size of the DSGRE should an RT instability occur. The overall effort includes conducting tests in different size vessels and developing a simple physical model (modified energy ratio) to support extrapolation of the results to estimate full-scale DST behavior.