Now, after decades of planning and billions of dollars of investment, the site is turning liquid nuclear and chemical waste at the Hanford Nuclear Reservation into a much safer substance: glass. The long-awaited development is a key step in cleaning up the nation's most polluted nuclear waste site. Construction on the Hanford Waste Treatment and Immobilization Plant began in 2002. “We are at the precipice of a really significant moment in Hanford’s history,” said Casey Sixkiller, director of the Washington State Department of Ecology, in a video interview. There are plans for two facilities at Hanford: the one now approved to process low-level nuclear waste after repeated delays, and an adjacent facility for the high-level waste that remains under construction.