The Energy Department said yesterday that it is closing the uranium enrichment plant at Oak Ridge, Tenn., where the United States produced the uranium used in the two atomic bombs that were dropped on Japan in 1945 to end World War II.

The department also said it will halt construction of a uranium enrichment plant at Portsmouth, Ohio, on which it has spent $2.6 billion.

The Portsmouth plant would have used a high-speed centrifuge to produce the isotope U-235, which triggers and sustains the chain reaction that generates nuclear power. The department said it will abandon that process in favor of a newer one that uses laser light.

Energy Secretary John S. Herrington said the decision on the plants was based solely on economics and what he called the "current crisis" in the nuclear power industry and the U.S. uranium-enrichment business.

"We have lost half of a market we once dominated because we are now the world's highest-priced supplier of enriched uranium," Herrington said at a news conference. "To become competitive again, we must cut costs and develop the best technology available."

Herrington said the best technology to enrich uranium now is laser light, a process that was developed at the Lawrence Livermore Laboratory in California and that is still classified information because it is used in nuclear weapons.

Herrington said the U.S. government is expected to save between $400 million and $500 million over the next three years by focusing on one uranium-enrichment process rather than two and between $3 billion and $5 billion in funds that would have been required to complete the Portsmouth centrifuge plant.

"The laser is the way of the future, the uranium-enrichment technology of the 21st century," Herrington said. "It has the technical and economic potential for better performance and lower production cost than the gas centrifuge."

Herrington said the department decided to close Oak Ridge because the plant is so old and because it is no longer needed to satisfy the nation's needs for enriched uranium fuel for nuclear reactors and the Navy's atomic submarines. Two newer plants -- another facility at Portsmouth and one at Paducah, Ky. -- produce more than enough uranium for U.S. needs, even though they are now operating at less than half their capacity.

The three plants use a process known as gas diffusion that requires enormous amounts of electricity to produce enriched uranium. Gas diffusion works by passing uranium hexafluoride gas through thousands of chemical barriers that separate U-235 from the nonfissile isotope known as U-238.

"There comes a time to make some tough business decisions," Herrington said. "Keeping Oak Ridge open and continuing construction of the gas centrifuge plant became incompatible with the overriding need to reduce costs."

The decision will mean the eventual loss of 1,300 jobs, 800 at the Portsmouth gas centifuge plant and 500 at Oak Ridge.

"The plain facts are that the world has changed," Herrington said. "The demand for uranium enrichment is not what we expected at the same time that foreign competition has caught up with us and even passed us by with respect to new and lower-cost technology."

Herrington said that the department will still have to pay the Tennessee Valley Authority $400 million a year through 1992 for electricity it may never use at Oak Ridge. The department has a "take or pay" contract with the TVA for the electricity to run the Oak Ridge plant that the TVA has refused to renegotiate.