The Department of Energy can't account for more than a ton of weapons-grade plutonium and 688 pounds of enriched uranium at its Rocky Flats plant.

The agency said most is thought to be in pipes, ducts and sealed cases where workers used to handle hazardous material through glove holes.The amount of unaccounted-for materials is expected to drop once buildings and equipment at the weapons plant are decontaminated, the Energy Department said Monday in a statement.

Plutonium and highly enriched uranium often are in forms not easily inventoried, such as dispersed in a solution, the department said.

The review of records from 1953 to last year came up short 2,620 pounds of plutonium.

In May, the Clinton administration ordered a study of safety problems posed by tons of plutonium left over from the Cold War. The review is covering 14 department operations, from laboratories to weapons plants.

The Rocky Flats plant, which made plutonium triggers for nuclear weapons, is closed.