President Carter yesterday signed into law legislation to tighten restrictions of the supply of nuclear fuels of foreign countries.

The legislation, the Nuclear Nonproliferation Act of 1978, calls for a halt to the supply of nuclear fuels to countries that do not now have nuclear weapons but which explode nuclear devices in the future or violate international nuclear safeguards.

It will also require nations receiving nuclear fuels from the United States to comply fully with all International Atomic Energy Agency (IAEA) safeguards.

Two years from now, the same IAEA safeguard requirements would be applied in existing contracts for the supply of nuclear fuel.

The legislation will also impose additional restrictions in new nuclear supply contracts, including a provision requiring the U.S. consent for the retransfer or reprocessing of spent nuclear fuels.

The measure, which was strongly advocated in Congress by Rep. Jonathan B. Bingham (D-N.Y.) and Sen. Charles H. Percy (R-Ill.), is a major part of the administration's nuclear nonproliferation policy.

At a White House signing ceremony yesterday, the president called the legislation "a major step forward in clarifying our own nation's policy."

Carter also predicted that the new safeguard requirements, particularly when they are applied to nations already receiving nuclear fuel from the United States, will require "some of our friends abroad . . . to readjust their policy."

The president recently told the nation's governors that the administration will develop a plan by the end of the year for the disposal of nuclear wastes. He said yesterday he hopes next week to send Congress legislation that would expedite the licensing of atomic power plants.

Carter also used the signing ceremony as an occasion to reaffirm his support for nuclear power and to repeat his objections to the development of a nuclear fast-breeder reactor at Clinch River, Tenn.

"I feel very strongly that we should continue to use in an increasing way atomic power in our country, as a major element of energy production," he said. "Our light water nuclear plants, using enriched uranium, are adequate for the time being.

"We have a heavy research and development program going on for future production, including the liquid metal fast-breeder reactors," he continued. "We do not need to waste our money at this time in production models of the breeder reactor."

Breeder reactors produce plutonium, which can be used to make nuclear weapons.