Optimal Pump Elevation and Configuration for Mixing Erosion-Resisting Radioactive Tank Waste

At the U.S. Department of Energy’s Hanford Site, 300-hp mixer pumps will be installed in the 4,000-m3 double-shell tanks (DSTs) to stir the radioactive sludge and supernatant liquid waste so that it can be retrieved for subsequent treatment and disposal. A baseline mixer pump that has been used in the DSTs has an intake at the bottom and two injection nozzles at the top. The pump is placed approximately 0.46 m above the bottom of the tank. We compared the predicted mixing performance of this baseline pump with an alternative pump—an inverted pump with the inlet at the top and injection nozzles at the bottom. Model predictions were compared with some measured data. We evaluated 16 cases of baseline and inverted pump conditions with mixer pumps situated at levels ranging from right on the tank bottom to 0.83 m above the tank bottom. Model results generally indicated sludge erosion is greater the closer the pump’s injection nozzle is to the tank bottom. These results provided an evaluation of mixing performance for use in a cost, risk, and benefit analysis to determine which pump configuration should be used for DST waste retrieval activities.