Nearly 400 pounds of uranium are unaccounted for at two federal plants, but Energy Department officials say they doubt the radioactive material has been stolen.

Inventory checks between October 1983 and March 1984 uncovered a 70-pound deficit at the Oak Ridge Gaseous Diffusion Plant and a 325-pound shortfall at a plant in Paducah, Ky., according to the department's "Semiannual Report on Strategic Special Nuclear Material Inventory Differences."

Sen. James Sasser (D-Tenn.) said Monday he wants a report from Secretary John S. Herrington on the missing uranium, which is enriched at the plants for use as fuels at nuclear power plants.

"Frankly, I'm surprised we were not given a report earlier given the size and the amount of the uranium that is unaccounted for," Sasser said.

Officials do not believe the material was stolen, said Bob Harris of DOE's Nuclear Materials Control Division.

The discrepancy at the Oak Ridge plant, which is being shut down in a cost-cutting move, probably is the result of mistakes in estimating the amount of uranium gases inside pipes used in the gaseous diffusion process, he said. Rounding off figures and converting U.S. measurements to metric units also may be responsible, he added.

"If there is a reasonable explanation, then I think the Department of Energy has just dropped the ball for not making a full and complete explanation of it to Congress," Sasser said.

"There is always the possibility," he said, "that the uranium has been diverted for purposes contrary to the public policy of the United States."