After spending 16 years and $489 million on part a plan to safely store millions of gallons of highly radioactive waste from nuclear weapons production, the Energy Department has abandoned its efforts, according to a report released yesterday. Researchers had discovered in the early 1980s that the procedure being developed to concentrate the waste produced high levels of explosive benzene gas, but instead of abandoning the procedure, engineers tried, unsuccessfully, to make the process safe. Now it could take another eight years and billions of dollars to develop an alternative plan. The General Accounting Office report was released by Rep. John Dingell (D-Mich.), who criticized the Energy Department for mismanagement and wasting taxpayer money.