Mixing Processes in High-Level Waste Tanks - Final Report

Abstract

RESEARCH OBJECTIVE AND SUMMARY OF RESULTSMixing and transport in large waste-tank volumes is controlled by the multidimensional equations describing mass, momentum and energy conservation, and by boundary conditions imposed at walls, structures, and fluid inlets and outlets. For large enclosures, careful scaling arguments show that mixing is generated by free buoyant jets arising from the injection of fluid or buoyancy into the enclosure, and by temperature and/or concentration gradients generated near surfaces by heat and mass transfer at walls, cooling tubes, and liquid-vapor interfaces. For large enclosures like waste-tank air spaces, scaling shows that these free and wall jets are generally turbulent and are generally relatively thin.When one attempts to numerically solve the multi-dimensional mass, momentum, and energy equations with CFD codes, very fine grid resolution is required to resolve these thin jet structures, yet such fine grid resolution is difficult or impossible to provide due to computational expense. However, we have shown that the ambient fluid between jets tends to organize into either a homogeneously mixed condition or a vertically stratified condition that can be described by a one-dimensional temperature and concentration distribution. Furthermore, we can predict the transition between the well-mixed and stratified conditions. This allows us to describe mixing processes in large, complex enclosures using one-dimensional differential equations, with transport in free and wall jets modeled using standard integral techniques. With this goal in mind, we have constructed a simple, computationally efficient numerical tool, the Berkeley Mechanistic Mixing Model (BMIX), which can be used to predict the transient evolution of fuel and oxygen concentrations in DOE high-level waste tanks following loss of ventilation, and validate the model against a series of experiments. The experiments have been done with both water and air as working fluid.Using a scaled water tank experiment, the question of the dilution of a ceiling plume of ambient air entering the waste tank at the ventilation penetrations at the top of the waste tank is addressed (see reference [2] included with this final report). The water tank experiments model flow exchange similar to the exchange between the waste tank and the ambient air by partitioning the water tank in two equal sized horizontal compartments with two penetrations. The upper compartment is filled with denser sugar or salt water and the bottom compartment is filled with pure water. When the experiment is started the unstable density potential creates an upward buoyant jet though one penetration and a downward plume in the other. Measurements of buoyant jet dilution in the bottom compartment of the water experiment is accomplished using a new measurement technique using diffraction of a sheet of laser light to measure the density profile in the bottom compartment. Also flow patterns and mixing are observed using dye introduced into the downward directed buoyant jet. ...