JOHANNESBURG, SOUTH AFRICA -- When the time came to destroy South Africa's secret arsenal of nuclear weapons, workers at a laboratory northwest of here withdrew the highly enriched uranium and loaded it piece by piece into the trunk of a Toyota sedan, according to government officials involved in the effort.

The Toyota, in a number of trips, ferried the bomb material to permanent storage at a tightly guarded nuclear complex known as Pelindaba, named after a Zulu word meaning "we don't talk about this anymore." On one evening drive, the Toyota and its chase car of security personnel had to veer suddenly around a fat puff adder snake stretched across the blacktop.

When the shipments ended in mid-1991, South Africa had become the first nation to give up its nuclear capability voluntarily, leaving the African continent again free of nuclear arms. But the event passed without public notice, because the government kept the decision to scuttle the weapons secret, having never acknowledged developing them in the first place.

Obsessive secrecy cloaked South Africa's nuclear program during the 16 years that it was officially active, from 1974 to 1990. The task was formidable, considering the program's estimated cost of $800 million, its total workforce of about 1,000 people and its production of six bombs weighing a ton apiece plus a seventh that was half-finished. Each bomb could have easily wrecked a large city and killed more than 100,000 people.

But senior government officials here recently agreed to explain for the first time publicly how and why they made and then destroyed the country's nuclear weapons, displaying unusual candor, partly to help convince Washington that they have not exported or squirreled away some bombs or nuclear materials.

The path South Africa pursued to the bomb was one that has long aroused the most concern among anti-proliferation specialists: It used the ruse of developing a civilian nuclear power program to accumulate the expertise and hardware to construct a weapon.

South Africa shrewdly took great pains to keep key nuclear assets -- such as the nation's principal bomb factory and storage site -- hidden from U.S. spy satellites for nearly two decades. Also, its bomb-building program was sufficiently low-tech to avoid extensive reliance on foreign assistance, officials said, an approach that effectively insulated the program from the restraints of international trade sanctions leveled against the apartheid system.

The tale of the South African nuclear program -- a story of success in the face of considerable obstacles -- is partly a caution to the world that the task of making atom bombs may be easier than is commonly believed.

'Operating Totally in Isolation'

Western sources said they understand the project's secret code name, at least for a time, was "Kraal" -- an Afrikaner word for the stone wall that farmers once constructed here to protect livestock, and an apt symbol of the circle-the-wagons psychology that pervaded the nuclear effort.

The effort succeeded, South African officials said, partly because of extraordinary determination by scientists who believed the nation's security was threatened by hostile neighbors opposed to its system of racial segregation.

"Remember that we are dealing with a situation in which we are operating totally in isolation," said Tielman de Waal, director of Armscor, the government-owned arms conglomerate that was secretly assigned to make the bombs in 1974 by then-Prime Minister John Vorster. "We were not trying to recruit . . . {scientists who} had worked overseas on a similar program because it was such a security risk."

The country's initial challenge was to transform its abundant reserves of natural uranium into a large stockpile of highly enriched uranium suited for use in nuclear weapons. For that purpose, engineers designed and constructed an elaborate network of pipes and caldrons at a large site west of Pretoria known as Valindaba -- a Zulu word meaning "we don't talk about this at all."

Valindaba is adjacent to Pelindaba, South Africa's main nuclear research center, where for nearly a decade scientists had operated a U.S.-built civilian reactor fueled by imported, highly enriched uranium.

Most of the work followed what officials said was a standard recipe for making either bombs or fuel rods, which could be pursued by virtually any country with a supply of uranium, skilled technicians, abundant energy supplies and extensive experience handling nuclear materials.

Natural uranium slurry, or yellowcake, collected from gold mines was converted to uranium hexafluoride gas. The gas was then fed into 112 "separating units" connected in parallel in three long buildings at Valindaba constructed of concrete deliberately colored light brown to blend in with surrounding terrain.

The separators isolated and concentrated a key fissionable isotope, uranium-235, and roughly a year after getting started, according to de Waal and other officials, produced significant quantities of a uranium gas enriched to slightly less than 90 percent. Instead of fashioning the material into fuel rods, however, South Africa secretly converted it into dense, heavy blocks and machined it into bomb parts with imported, high-precision tools.

An engineer who worked at the enrichment plant recalled in an interview that a portion of the process was worked out by South African scientists who kept a U.S. article debunking their technique tacked to a bulletin board over their desks. "It was a challenge we met," said Hendrik Hendriks.

Modeled on Hiroshima Bomb

De Waal, the Armscor director, said the highly enriched uranium was installed in fairly primitive weapons that were much easier to engineer than, say, a self-propelled howitzer Armscor completed last year.

Armscor designed the weapon to operate much like the bomb detonated by the United States over Hiroshima during World War II, officials said. It was fashioned out of two chunks of highly enriched uranium, one in the shape of a sphere with a hole part way through the middle, and the other in the shape of a cylinder designed to fit in the hole.

The chunks were separated by a superbly made gun barrel. When the bomb was detonated, an explosive charge would propel the cylinder-shaped uranium down the barrel and into the hole in the sphere-shaped uranium, creating a critical mass of fissionable material so suddenly that it would promptly explode with a force of 10,000 to 18,000 tons of TNT.

Most of the work on the bombs was performed at Advena laboratory, a large windowless structure nestled against a hill roughly 25 miles west of Pretoria, at the edge of a sprawling test range for drivers of fast automobiles. Signs warn the curious not to take photos and to approach "at your own risk."

Officials said that the government took great pains to keep satellite photo analysts from learning the building's function. The building's green roof, for example, was installed before its internal walls were constructed or any telltale equipment was brought to the site.

Still, "we were surprised that between the International Atomic Energy Agency and the CIA, this had never been identified" before its disclosure by Pretoria in March, said Waldo Stumpf, executive director of the South African Atomic Energy Corporation.

Officials in Washington instead focused their suspicion on another building closer to Valindaba where South African officials now swear no weapons work was ever done.

Officials said only those physicists, chemists and engineers who were South African-born citizens or had lived in the country for at least 15 years were allowed to join the effort, and no more than 10 people were permitted to learn all of its secrets. Roelof F. Botha, the country's minister of foreign affairs, was not informed until four years after he joined the cabinet.

Roughly 150 people worked inside the main Advena lab and a nearby building that housed top weapons designers, with some working on bomb components at long tables and others guarding vaults where halves of each finished weapon -- containing a "sub-critical" portion of enriched uranium -- were stored on trolleys in airless canisters.

De Waal said a crude prototype was completed by 1977. That was the year the United States, Soviet Union, Britain, France and West Germany successfully pressured Pretoria to halt what appeared to be preparations for an underground nuclear test at a remote site in the Kalahari Desert.

"South Africa has informed us that they do not have and do not intend to develop nuclear explosive devices for any purpose, either peaceful or as a weapon; {and} that the Kalahari test site . . .is not designed for use to test nuclear explosives," then-President Jimmy Carter said at the time.

Pretoria finally admitted in March that it had dug two shafts, each roughly 200 meters deep, at the desert site for a potential nuclear blast. But the country never conducted any nuclear test there.

Nuclear Poker

Just as Washington created an elaborate system of safeguards to ensure proper command and control of U.S. nuclear weapons, so too did Pretoria take unusual steps to ensure that its arsenal was not misused, officials said. To get into the vaults, for example, required approval from the country's president and knowledge of at least three combinations for mechanical and electronic locks.

South Africa decided from the outset to build seven bombs, officials said, because that was the number judged to present a "serious" deterrent to conventional attack by Angola and Mozambique, two nearby black-ruled, communist-led nations that achieved independence the year the bomb project began.

The government never wanted to use the weapons, officials said in interviews, because it suspected from the beginning that one superpower or the other would retaliate massively.

So Pretoria's strategy in the case of overwhelming attack from Soviet-backed forces to the north was not to respond directly but to play a form of nuclear poker with extraordinarily high stakes: By threatening to explode a bomb, Pretoria expected to coerce the United States, Britain or France into intervening on South Africa's behalf before the country was lost.

"You would say, I am now going to pick up the phone, phone the American president or the British prime minister, and tell them, 'Look, if you do not apply pressure on the U.S.S.R. to withdraw these forces, we are going to activate the bomb,' " de Waal said. "And the reply would probably be, 'You have no bomb.' "

So South Africa would explode one or two weapons to demonstrate its nuclear capability, de Waal said. "Then you would say, 'We have got five more. Do you really want to look for trouble?' "

If all else failed, South Africa was prepared to drop the weapons from its Buccaneer, Mirage or Canberra jet aircraft, officials said.

This plan nearly became reality in 1987, when Pretoria responded to some victories by Cuban troops in Angola by ordering the Kalahari site -- which had been dormant for 10 years -- reopened, inspected and covered by a hangar in preparation for a possible nuclear blast.

Dismantling the Bombs

Wynand L. Mouton, a former dean and professor of nuclear physics at Stellenbosch University in South Africa, was invited to join the small circle privy to the country's top nuclear secrets in March 1990, about six months after President Frederik W. DeKlerk had decided the bombs should be dismantled.

The decision stemmed from foreign pressures, the end of the Cold War, the withdrawal of Soviet-backed troops from Angola and Pretoria's desire to keep the bombs from falling into the hands of the African National Congress, which by then was expected to assume control of the country eventually in a gradual transition to black-majority rule, officials said.

As a moderate, respected member of South Africa's social and scientific elite, Mouton, then 61, had been tapped before for special governmental projects. DeKlerk now said he wanted him to audit the process of dismantling the nuclear weapons.

The primary reason for Mouton's appointment, officials said last month, was to guard against any diversion of bomb materials by South African conservatives who might seek to brandish them in a last-ditch defense of the apartheid system.

Working as DeKlerk's personal emissary, Mouton was given access to voluminous classified documents on the program, shown slides of the bomb's inner workings and given a tour of the Advena lab where he got a first look at the 6-foot long, torpedo-shaped devices.

"I really got quite cold standing next to one," he recalled in an interview. "I remember thinking that you could use these things to destroy Pretoria or Johannesburg that evening, and it was not a nice feeling."

DeKlerk's decision aroused substantial controversy among those who had labored for years on the secret project and in the process become "a very, very close family," one official said. So workers at Advena were instructed to dismantle only one bomb at a time "to give people more time to get used to the idea," another official said. Two workers had to be severed from the program and kept under continuous surveillance when they threatened to abscond with the bomb materials.

Mouton said he watched as some of the bombs were dismantled, and he accompanied several shipments of the highly enriched uranium from Advena to Pelindaba. The South African Defense Force was instructed to monitor the route taken by the uranium-loaded Toyota, but was kept ignorant about the nature and timing of the shipments.

Officials said Armscor wanted to save a single set of bomb design documents, but the government decided in the end to burn them all. One reason given was that if world events ever prompted South Africa to re-create its nuclear arsenal, the new weapons would surely be modeled on a more sophisticated design.

The final bomb parts were crushed in July 1991, Stumpf said, shortly before Pretoria agreed to adhere to the nuclear Non-Proliferation Treaty. The accord bars development or retention of nuclear arms and authorizes international inspections of nuclear-related facilities in signatory states.

Stumpf said DeKlerk waited until 1 1/2 months ago to disclose the creation and destruction of the bombs because he was worried that the news would invite excessive foreign scrutiny and enrage right-wing opponents of the transition to black majority rule -- considerations that eventually gave way to a desire for improved relations with Washington and others.

"I trust that South Africa's initiative will inspire other countries to take the same steps," DeKlerk said in his speech March 24.

Staff researcher Lucy Shackelford in Washington contributed to this report.