The New York Times Sunday reported the discovery of the higher plutonium levels at the 560-square-mile Hanford Nuclear Reservation will likely make long-term cleanup a greater challenge than previously thought.
The plutonium poses no immediate radiation danger because of "institutional controls" such as guards, weapons and gates, the Times said.
But scientists note plutonium, which can cause cancer in even small quantities, takes 24,000 years to lose half its radioactivity.
In coming generations, experts worry, plutonium could reach the underground water flow and enter the Columbia River.
The U.S. Energy Department said it is deciding whether to try to clean up 90 percent, 99 percent or 99.9 percent of the waste and now prefers the 99 percent option, the Times said.
Hanford, taken over by the federal government as part of the Manhattan Project, has produced most of the nation's plutonium. It stopped production in the 1980s.
Plutonium waste is also greater at other U.S. nuclear sites than the Energy Department's official estimates, Robert Alvarez, a former department official, told the Times.
Alvarez reanalyzed studies conducted by the department in the last 15 years for Hanford, the Idaho National Engineering Laboratory, the Savannah River Site, near Aiken, S.C.; and elsewhere, the newspaper reported.