Working with American scientists and American-supplied enriched uranium, South Africa has developed an independent nuclear industry that is within two to four years of manufacturing an atomic bomb.

That estimate is labeled as "the outside range" by a well-informed American government source who is convinced that the South Africans can cut it to a matter of months if they concentrate funds and manpower in a crash version of their present program.

At least one British expert feels that the South Africans may have already produced an atomic weapon.

Connie Mulder, the information and interior minister in the white-minority government that confronts increasing black pressure on its borders and unrest at home, refused to confirm or deny that his country was close to having the bomb.

"Let me just say that if we are attacked, no rules apply at all if it comes to a question of our existence," Mulder said. "We will use all means at our disposal, whatever they may be. It is true that we have just completed our own pilot plant that uses very advanced technology, and that we have major uranium resources."

South Africa has never signed the nuclear nonproliferation treaty, a fact that Prime Minister John Vorster himself called attention to last year when he told an interviewer thot while "we are only interested in the peaceful applications of nuclear power," South Africa "can enrich uranium and we have the capability" of mounting a nuclear defense.

Evidently developed without direct American involvement, the pilot plant at Valindaba, near Pretoria, can enrich uranium to weapons-grade level, according to American sources. A "moderate" rate of production would provide South Africa with at least 20 to 40 pounds of plutonium, enough raw fissionable material to put together as a weapon by 1981, according to these sources.

Military experts discount the usefulness of a nuclear arsenal against a black urban revolt or a guerrillo war, the most likely threats to the white government.

But recent guarded comments by South African officials indicate that they see the high level of nuclear technology they have developed as giving them both strategic bargaining power with the United States and the Soviet Union, and a future corner on much of the world's enriched nuclear fuel market now dominated by the United States.

These possibilities were hinted at in an unusually frank talk delivered last October at a Johannesburg seminar by A.J.A. Roux, president of the South African Atomic Energy Board, who also paid a rare open tribute to the usually screened American role in developing South Africa's nuclear technology.

"We can ascribe our degree of advancement today in large measure to the training and assistance so willingly provided by the United States of America during the early years of our nuclear program when several of the Western world's nuclear nations cooperated in initiating our scientists and engineers into nuclear science," Roux told the seminar.

He noted that a research reactor at Pelindaba, also in the Pretoria area, "is of American design (based on the Oak Ridge research reactor)" and that "much of the nuclear equipment installed at Pelindaba is of American origin, while even our nuclear philosophy, although unmistakably our own, owes much to the thinking of (American) nuclear scientists."

Roux did not mention a quietly arranged American commitment to supply enriched nuclear fuel between 1981 and 1984 for two French-manufactured nuclear-power generating reactors now being installed in the Cape Province at Koeberg, nor American sales or weapons-grade enriched uranium to South Africa in 1975 and 1976 for use in the Pelindaba research reactor.

In addition to giving South Africa the chance to gain something like $500 million a year in foreign exchange once a planned commercial enrichment plant is opened in the mid-1980s, the technology will also make South Africa "completely independent of overseas sources of supply" in the future, Roux stressed.

An emerging pattern of statements by white leaders who say they will resist all external pressures, including American moves to change their apartheid policies, have led some analysts to conclude that the potential development of atomic warheads is intended more as a diplomatic weapon than a battlefield one.

The Soviet Union is supporting African guerrilla movements in this region and Cuban military units are stationed in Angola. Prime Minister Vorster and other officials warn their followers increasingly that they cannot count on the United States to protect them from a Soviet strike and that the whites will have to rely on their own resources to dissuade any superpower intervention.

The government - for which only whites, who form 17 per cent of the 26 million population, can vote - has wrapped its nuclear program in deep secrecy for years. Now, as its need for diplomatic leverage rises, it is allowing a few details to filter out.

The nuclear program is divided into four distinct stages:

Raw uranium exports. South Africa is one of the four largest suppliers of uranium in the non-Communist world, having earned $1.4 billion in foreign currency since 1952 from foreign sales, largely to the United States and Britain.

State Department officials have indicated publicly that American willingness in recent years to sell nuclear material to South Africa stems in part from U.S. access to South African uranium in building up its nuclear arsenal during the Cold War.

Research done since 1964 on the Safari-I reactor system at Pelindaba. The American government approved the shipping of 112 pounds of weapons-grade enriched uranium in 1975 and 1976 to this facility, which American scientists at Oak Ridge helped train South Africans to run.

The United States required strict safeguards that prohibited South Africa from diverting the uranium out of the research reactor, and there is no evidence that any of the fissionable material recovered from the research reactor has been used for weapons research or building by South Africa.

Expert sources feel, however, that the operation of the Pelindaba plant was an essential element in the training of South African nuclear scientists who developed the enrichment process that now opens the way to nuclear weapons production.

Nuclear power production. South Africa agreed last summer to buy two 922-megawatt nuclear electricity generators from a French consortium, despite an energetic effort by the Nixon and Ford administrations and the embassy here to sell General Electric reactors to the South Africans.

The 1981-84 American commitment to process South Africa's low-grade uranium into 3 per cent enriched fuel and ship it back carries South Africa up to the time when its own commercial enrichment plan will be in operation, according to American and South African sources.

Arms control agencies do not see the sale of electricity generators and 3 per cent enriched fuel as a direct link to a country obtaining nuclear-weapons technology, and the Koeberg deal has arousedlittle concern outside of black Africa, which has condemned France for the sale.

Independent enrichment of uranium. The most dangerous part of the South African effort, from a weapons-manufacturing point of view, has been under way since the early 1960s.

Vorster and Roux lifted the veil over the Valindaba project slightly by announcing in August 1970 that South Africa had perfected a "new and unique process" that would enrich fuel at a much lower cost than the two existing technologies used by the United States and its Western European allies. Then they quickly dropped the veil again, until October when Roux confirmed that Valindaba plant was in operation.

Since the 1970 South African announcement, speculation has grown that the pilot enrichment plant at Valindaba is based on a process developed by West Germany's Erwin Becker, who has said that Roux and other South African scientists had free access to his research and may have succeeded in adapting it.

The process is also used in an enrichment plant that West Germany is seeking to sell to Brazil. The Carter administration has tried to stop the sale.

The Becker process requires huge amounts of electricity to power the motors that drive it. South Africa has one of the world's largest supplies of cheap coal, which presumably it would burn to make the electricity in its uranium enrichment.

Roux noted in his seminar talk that the abundant coal deposits here made it unlikely that nuclear energy would be used much outside the cape coast area and local consumption of enriched uranium would be "modest."