Mixing processes in high-level waste tanks. Progress report, September 15, 1996--September 14, 1997

'U.C. Berkeley has made excellent progress in the last year in building and running experiments and performing analysis to study mixing processes that can affect the distribution of fuel and oxygen in the air space of DOE high-level waste tanks, and the potential to create flammable concentrations at isolated locations, achieving all of the milestones outlined in the proposal. The DOE support has allowed the acquisition of key experimental equipment, and has funded the full-time efforts of one doctoral student and one postdoctoral researcher working on the project. In addition, one masters student and one other doctoral student, funded by external sources, have also contributed to the research effort. Flammable gases can be generated in DOE high-level waste tanks, including radiolytic hydrogen, and during cesium precipitation from salt solutions, benzene. Under normal operating conditions the potential for deflagration or detonation from these gases is precluded by purging and ventilation systems, which remove the flammable gases and maintain a well-mixed condition in the tanks. Upon failure of the ventilation system, due to seismic or other events, however, it has proven more difficult to make strong arguments for well-mixed conditions, due to the potential for density-induced stratification which can potentially sequester fuel or oxidizer at concentrations significantly higher than average. This has complicated the task of defining the safety basis for tank operation. The author is currently developing numerical tools for modeling the transient evolution of fuel and oxygen concentrations in waste tanks following loss of ventilation. When used with reasonable grid resolutions, standard multi-dimensional fluid dynamics codes suffer from excessive numerical diffusion effects, which strongly over predict mixing and provide nonconservative estimates, particularly after stratification occurs. The National Institute of Standards and Technology (NIST) has developed useful codes for predicting stratification and mixing due to fires in enclosures, but these codes are not supported by appropriate experiments for waste tanks, and do not consider mixing induced by injected jets, or the detailed distribution of fuel and oxygen concentration. The UCB Thermal Hydraulics Group model BMIX (Berkeley Mechanistic Mixing Model) is being developed to mechanistically predict mixing processes in large waste-tank volumes, where mixing processes can be driven by hot and cold vertical and horizontal surfaces and injected buoyant jets. The author is supporting the model with scaled experiments using water/salt solutions, as well as separate experiments using air with simulant fuels (helium and refrigerant-22 for hydrogen and benzene) to study the specific mixing processes which occur in waste tanks, and will also support the implementation of the code for use in waste tank operations. This year''s experimental efforts have focused on scaled water systems to study the exchange flows which occur through tank ceiling openings following loss of ventilation, as well the mixing processes that occur below the ceiling. These water experiments are providing data and insight for the modeling effort while construction of the larger air/benzene simulant experiment is underway.