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. Turning nuclear waste into glass is effective — but expensiveEncasing radioactive waste in glass — called “vitrification” — has been recognized since at least the 1980s as an effective method for neutralizing it. 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.