[en] Approximately 60 million gallons of high-level radioactive waste--caustic liquids, slurries, saltcakes, and sludges--are stored in underground tanks at the Department of Energy's Hanford Site. At least one-third of the tanks are known to have leaked waste into the enviroranent, and there are many unresolved tank safety issues. In order to resolve the environmental and safety concerns, the Department plans to retrieve the waste, immobilize it, and dispose of it in a permanent geologic repository. Processing all of the tank waste in this manner could cost $40 billion, including $1.2 billion to construct the Hanford Waste Vitrification Plant. The purpose of our audit was to examine the reasons for cost estimate increases and schedule delays on the Hanford vitrification program. We also wanted to report on outstanding technical, safety, and environmental issues that could make the project even more costly and further delay its completion. We found that the Department managed the Hanford remediation system as a number of separate projects not fully integrated into one major system acquisition. Total costs have, therefore, been obscured, and the Department has not yet clearly defined system requirements or developed overall cost and schedule baselines. This lack of visibility could result in additional cost growth and schedule delays. We also noted a vast array of technical uncertainties, including tank safety and inadequate information about the makeup of tank waste, that could significantly affect the program's cost and ultimate success. To increase visibility of program cost and schedule, we are recommending that all separate projects relating to tank waste be included in a single major system acquisition, and that the Department complete its ongoing baselining effort to the extent practical before making major funding commitments. Management concurred with our finding and recommendations