President Carter has decided to build a uranium enrichment plant at Portsmouth, Ohio, using a new technique called the gas centrifuge process, the administration announced yesterday.

The plant will cost an estimated $4.2 billion to $4.5 billion and will increase U.S. capacity to process nuclear fuel by almost one-third, officials said.

Carter's decision was announced by Robert W. Fri, acting administrator of the Energy Research and Development Administration, which manages the nation's three urnium enrichment plants.

Fri said construction on the new plant is expected to start near the end of 1978. The plant is expected to begin production in 1986 and reach full production 1988.

The expansion of the facilities will permit the United States to provide enrichment services for new nuclear power plants in the United States and abroad.

ERDA said the expansion will "reestablish internaional credibility for the U.S. as a supplier of enrichment services, thereby assisting the country's international objectives."

The statement referred to Carter's policy of discouraging construction of plutonium-producing "breeder reactors" throughout the world in order to limit the potential spread of nuclear weapons.

Carter also wants to halt the export of uranium enrichment and fuel reprocessing plants, which could also be used to produce nuclear weapons materials.

However, the United States has been unable to fill new orders for enriched uranium, prompting foreign nations seeking nuclear power to develop their own fuel processing plants and breeder reactors.

Expanding the U.S. capacity will enable the nation to offer enrichment service to other nations and reduce the chances of their developing their won processing facilities, officials hope.

The Portsmouth plant will use a new technology in which uranium gas is whirled in high-speed centrifuges to separate U-235, which is used in nuclear reactors from the heavier and more common U-238.

Former President Ford favored a proposal by a group of companies headed by the Bechtel Corp. to build the next enrichment plant at Dothan, Ala., instead of Portsmouth, Ohio, and to place it in private rather than government ownership.

But the plan, requiring large government guarantees and price supports was rejected by Congress.

During his presidential campaign last year Carter pledged to build the next enrichment plantat Portsmouth.