NEW YORK, June 1 (Xinhua) -- From 1950 to 1990, the U.S. Energy Department produced an average of four nuclear bombs every day, turning them out of hastily built factories with few environmental safeguards that left behind a vast legacy of toxic radioactive waste, reported The New York Times (NYT) on Wednesday.

"Nowhere were the problems greater than at the Hanford Site in Washington State, where engineers sent to clean up the mess after the Cold War discovered 54 million gallons of highly radioactive sludge left from producing the plutonium in America's atomic bombs, including the one dropped on the Japanese city of Nagasaki in 1945," said the report.

Engineers thought they had solved it years ago with an elaborate plan to pump out the sludge, embed it in glass and deposit it deep in the mountains of the Nevada desert, it noted.

"But construction of a five-story, 137,000 square-foot chemical treatment plant for the task was halted in 2012 -- after an expenditure of 4 billion U.S. dollars -- when it was found to be riddled with safety defects," it said. "The naked superstructure of the plant has stood in mothballs for 11 years, a potent symbol of the nation's failure, nearly 80 years after the Second World War, to deal decisively with the atomic era's deadliest legacy."

The U.S. government now appears to be seriously considering whether it will be necessary to leave thousands of gallons of leftover waste buried forever in Hanford's shallow underground tanks, and protect some of the waste not in impenetrable glass, but in a concrete grout casing that would almost certainly decay thousands of years before the toxic materials that it is designed to hold at bay, it added.