South Africa began using locally enriched uranium fuel on Monday in its Safari I nuclear research reactor following a refusal by the United States to resupply it with the required fuel.

Minister of Mineral and Energy Affairs, F.W. DeKlerk announced last night that South Africa's U.S.-made research reactor is now operating on South African-made fuel elements of 45 percent enriched uranium. The disclosure draws back slightly the veil of secrecy South Africa has put over its uranium-enrichment program.

De Klerk's announcement came on the eve of national elections. Early returns indicated that Prime Minister Pieter W. Botha would retain a strong majority in Parliament but with a sharp drop in the popular vote for his National Party. [Details on Page A25.]

The chairman of South Africa's Atomic Energy Board, Wynand De Villiers, said 45 percent enriched uranium could be used to make a bomb but "it would be a clumsy bomb." Weapons-grade uranium is usually enriched to more than 90 percent. Having reached the current level of enrichment, the South Africans could, at great expense, go on the higher levels.

The 45 percent enriched uranium is the lowest possible grade on which the research reactor can operate. Highly refined uranium is needed in research reactors to produce isotopes for medical and other investigations. The U.S.-supplied fuel was enriched to about 93 percent.

U.S.-built reactor comes under international safeguards, but South Africa has refused to allow inspection of its pilot enrichment plant at Valindaba because it claims to have developed a unique process of enrichment and fears it will be discovered by other nations.

South Africa's revelation that it has made enriched uranium into fuel elements, a process that requires considerable effort and financial investment, signals its determination not to allow its atomic energy program to be jeopardized by international nuclear sanctions.

Public disclosure of the information, which was made known to representatives of the international Atomic Energy Agency on a visit here a few weeks ago, comes as the United States and South Africa are engaged in high-level diplomatic exchanges to break an impasse over supply of enriched uranium for two nuclear power plants under construction here.

South Africa's refusal to sign the Nuclear Nonproliferation Treaty or to allow inspection of its enrichment facilities is holding up plans for U.S. supply of nuclear fuel for the power plants at Koeberg near Cape Town. The topic is likely to be on the agenda of South African Foreign Minister Pik Botha when he meets U.S. Secretary of State Alexander M. Haig Jr. in Washington May 14.

South Africa's announcement that home-made fuel is being used to run Safari 1 is out of character with its usual reticence about disclosing developments in its atomic energy program until forced by news leaks to do so. It may be an attempt to strengthen its bargaining position in the nuclear talks.

One South African said this latest development might make it "abundantly clear that this pressure is counterproductive as far as nonproliferation is concerned. One should not deny fuel or technology, because it is counterproductive."

Prodded by suspicions that South Africa was developing atomic weapons, the U.S. government told Pretoria in the late 1976 that unless it signed the Nuclear Nonproliferation Treaty and accepted inspection of its pilot enrichment plant at Valindaba it would not get any more uranium for Safari 1.

Since then, the South Africans slowed down their atomic research program, which they say is mainly aimed at medical problems, in an effort to make the remaining fuel for Safari 1 last as long as possible. Over 18 months ago, Atomic Energy Board Chairman De Villiers admitted the reactor was operating well below capacity. Today he said that in recent weeks it had operated only three days every three weeks.

De Klerk said yesterday that the reactor will still only operate at a quarter of its full capacity of 5 megawatts.

South Africa is enlarging the Valindaba enrichment plant to produce commercial quantities of uranium that it says will be used mainly for its own needs.

De Villiers said he thinks the Americans were aware for some time that South Africa was making "a special effort" to produce fuel for Safari 1 in order to keep it going. De Klerk said "a limited quantity" of 45 percent enriched uranium was produced. Both remarks suggest that the amount produced is small enough to keep Safari 1 from going cold.