The government has plans for tons of excess plutonium: burn some and bury the rest. The Energy Department is due today to announce its $2 billion plan for disposing of the deadly material. The plutonium, nearly 50 tons of it, is left over from nuclear warheads. The Energy Department said in March that it was considering mixing the weapons-grade plutonium with uranium and then burning it in commercial atomic power plants. It’s not clear how much the plutonium would be encased in glass and buried. A source says about a third of the plutonium has impurities that make it unsuitable for burning. The disposal process could take decades.