After spending billions of dollars over several decades to remove radioactive waste leaking from a plant where nuclear bombs were made, the Energy Department has come up with a new plan: leave it in the ground. The shuttered Hanford Nuclear Reservation, which produced plutonium for U.S. atomic weapons from World War II through the Cold War, is the nation’s largest nuclear cleanup site with about 56 million gallons of waste stored in leak-prone underground tanks in south-central Washington State. The Energy Department has proposed to effectively reclassify the sludge left in 16 nearly empty underground tanks from “high-level” to “low-level” radioactive waste.