South Africa, in an effort to assure that it will have a ready source of fuel for two nuclear powder plants now under construction, has decided to convert its experimental uranium enrichment plant into a facility capable of producing enriched uranium in commercial quantities.

The decision, disclosed this week, is part of a broader move by Pretoria to became self-sufficient in all strategic supplies so that opposition abroad to South Africa's racial policies cannot strangle its industrial development.

South Africa has planned since 1975 to construct a commercial enrichment plant, and had indicated that it would be ready to sell enriched uranium to foreign customers by the mid-1980s. By converting the experimental plant, South Africa hopes to be able to begin producing at least enough fuel to meet its own needs sooner.

No date has been announed for completion of the project, but the assumption is that fuel from the converted plant would be available by 1981 , when two French-built nuclear power generating plants now under construction near Cape Town will bw ready to go into operation.

The decision, announced by Minister of Mines J. S. F. Botha, comes during a long-smidering dispute between the United States, formely South Africa's main supplier of enriched uranium, and the Pretoria government over nuclear policies.

Because South Africa has resisted signing the nuclear nonproliferation treaty and opening its nuclear facilities to inspection by the International Atomic Energy Agency, Washington has stalled the delivery of 57 pounds of weapons grade uranium that South Africa requested during the Ford administration for its Safari-I nuclear reactor.

The U. S. government has also told Pretria that unless it signs the treaty, it will not receive the fuel for its first nuclear power plants. The U. S. had originally agreed to supply 3 percent enriched uranium hexaflouride for these plants for their first three years of operations. 1981-84.

Threats by Washington to cut off future nuclear supplies clearly gave impetus to the Government's decision, which some reports indicate was made soon after the United Nations imposed an arms embargo on South Africa in October.

"This striving for independence (in supplies) is further strengthened by the question of future availability nuclear fuel for South Africa. "Botha said in announcing the expansion.

The pilot enrichment plant which will be expanded is situated at Pelindaba just west of this country's administrative capital, Pretoria, and was completed in 1976. Its production capacity is secret and it is not known whether weapons grade uranium can also be processed there.

The South Africans have planned since 1975 to construct a commercial uranium enrichment facility to take advantage of their abundant raw uranium resource, which amount to 17 percent of the non-Communist world's holdings.