Cleaning up Cold War-era nuclearweapon sites remains the U.S. Energy Dept.’s single-largest financial liability, totaling an estimated $266 billion, says a Nov. 17 agency report.

The report, validated by accounting firm KPMG, classifies DOE’s environmental-management program as an “unfunded liability” because its budget is set through annual congressional appropriations, as opposed to a dedicated funding stream. The report claims that factors such as the delay in opening the planned Yucca Mountain repository in Nevada could drive the cost higher. “Estimating this liability requires making assumptions about future activities and is inherently uncertain,” the DOE report says. “The future course of the department’s environmental cleanup activities will depend on fundamental technical and policy choices, many of which have not been made.”