The head of South Africa's Atomic Energy Board today labeled U.S. government suggestions that South Africa might have exploded a nuclear device as "complete nonsense" and denied that this country was responsible for the suspected explosion.

"If there was anything of the sort, my first reaction would be that some other power might have undertaken a test, but it was definitely not South Africa," J. Wynand De Villers, the Atomic Energy Board's chairman, said.

However, Foreign Minister Pik Botha offered a more circumspect reply when asked for comment, saying he had "no knowledge of such an event."

He added, "Why don't you ask the Russians or the Chinese or even the Americans?" Botha showed indignation that the United States first suspected South Africa when a U.S. satellite indicated that a low-grade explosion may have taken place Sept. 22 in an area including portions of the Indian and southern Atlantic oceans as well as parts of the Antarctic continent and southern Africa.

The United States is following up with other checks to try to corroborate that an explosion actually took place and to determine what type it might have been.

It is a particularly vast area in which the phenomenon is supposed to have taken place," Botha said. "If the Americans don't know what is going on, I suggest they first make sure of their facts before they run away with the idea. It might even have been the rebirth of Venus," he added, suggesting sarcastically that "the Americans ask Neptune for a report."

U.S. Ambassador William Edmondson confirmed that he met today with Botha to discuss the U.S. information about the possible blast, but he declined to disclose the substance of what was said.

Botha's refusal to categorically deny South Africa had tested a nuclear explosive device is seen by many observers as an attempt to maintain the ambiguity and uncertainty that currently surround South Africa's intentions in the atomic weapons field.

The doubts about this point give South Africa a psychological advantage over its potential enemies without risking the harsh international reaction that might come if South Africa openly admitted it had or intends to acquire nuclear weapons, most analysts here believe.

As a cartoon in a local paper today pointed out: "Who needs the real bomb? Rumors are a cheaper deterrent."

South Africa's nuclear energy program is a cause of concern to nonproliferation proponents because it is not subjected to the scrutiny of the International Atomic Energy Agency, from which South Africa was jettisoned in the 1960s. While there is no proof, there is widespread belief that South Africa already has a nuclear weapons capability that quickly could be activated to make it the world's seventh atomic power. The worry is aggravated by South Africa's refusal so far to sign the Nuclear Nonproliferation Treaty.

If this country did acquire atomic weapons, it would introduce an ominous new dimension to strife-torn southern Africa, already riven by racial conflict between blacks and whites.

Prime Minister Pieter W. Botha, who is also defense minister and chiefly responsible for building up South Africa's impressive domestic armament program, has described his country's military role as a "stabilizing factor" in southern Africa against what he terms the "Marxist onslaught."

South African officials are particularly worried about the Soviet naval buildup in the Indian Ocean.

U.S. concern about South Africa's nuclear plans was heightened in August 1977, when a Soviet intelligence satellite spotted what looked like a nuclear testing site under construction in South Africa's Kalahari desert.

Pressed by the United States for assurances that it would not test a nuclear device, South Africa reacted in an ambiguous and contradictory manner. President Carter said he had received these assurances, but then-prime minister John Vorster denied this, saying in an American television interview, "I am not aware of any promise that I gave to President Carter."

The administration countered by releasing portions of a letter to Carter from Vorster in which he had written that "South Africa did not have, nor did it intend to develop a nuclear explosive device for any purpose, peaceful or otherwise."

Covert U.S. efforts to confirm this or to discover whether this commitment had changed under Botha -- are believed to be at the root of an incident last April when South Africa expelled three American diplomats for allegedly taking aerial photographs of "strategic installations." Among them was thought to be South Africa's nuclear enrichment plant at Pelindaba near Pretoria. The "spy-plane affair" brought bilateral relations between the two countries to their lowest ebb.

The uranium enrichment plant, which is under construction, has fueled speculation that South Africa is producing weapons-grade material. It is not known for sure whether this plant has capability to produce highly enriched uranium needed for explosive devices.

The government has said the plant is meant to produce only the low-enriched 3 percent uranium hexafluoride that South Africa requires for two nuclear power plants near Cape Town that will come on stream at the end of 1981.

South Africa was originally to get that supply from the United States, but Washington cut it off when Pretoria stalled on signing the nonproliferation treaty. The United States also has stopped a consignment of 57 pounds of weapons-grade, highly enriched uranium that South Africa had requested for its Safari I nuclear research reactor.

However, some observers point out it is not necessary for South Africa to build an enrichment plant just to provide itself with fuel for its power plants, since the French consortium building them, Framatome, has already indicated it would supply the fuel if the United States did not, according to reliable sources.

Moreover, South Africa has no plans for more nuclear power stations.

In an interview with ABC television today, Foreign Minister Botha said the American refusal to furnish South Africa with the highly enriched fuel it needs for the Safari plant " is indeed encouraging my country to initiate its own program of enrichment."

His remark implied that South Africa is pursuing production of highly enriched uranium.

The chairman of South Africa's domestic arms program said late Thursday that his country was giving preference to the kind of arms needed in an "African war" rather than to highly sophisticated arms.

"In the African context, South Africa manufactures weapons for her own use, but certain highly sophisticated arms will probably never be produced in this country because of the economic implications and because of the lesser role that these arms would play in an African war," said P.G. Marais, chairman of Armscor. CAPTION: Picture, The South African government says this uranium enrichment plant at Pelindaba will produce only low-enriched uranium for use in nuclear power stations. Copyright (c) South African Atomic Energy Board