PNNL

Abstract

The Department of Energy plans to vitrify approximately 60,000 metric tons of high level waste sludge from underground storage tanks at the Hanford Nuclear Reservation. To reduce the volume of high level waste requiring treatment, a goal has been set to remove a significant quantity of the aluminum, which comprises nearly 70 percent of the sludge. Aluminum is found in the form of gibbsite and sodium aluminate, which can be easily dissolved by washing the waste stream with caustic, and boehmite, which comprises nearly half of the total aluminum, but is more resistant to caustic dissolution and requires higher treatment temperatures and hydroxide concentrations. Chromium, which makes up a much smaller amount (~3%) of the sludge, must also be removed, as there is a low tolerance for chromium in the HLW immobilization process. In this work, the coupled dissolution kinetics of aluminum and chromium species during caustic leaching of actual Hanford high level waste samples is examined. The experimental results are used to develop a model that provides a basis for prediction of dissolution dynamics from known process temperature and hydroxide concentration.