A statement at the weekend from the state-owned arms firm Armscor said a nuclear test site that had been closed down in 1977 after intense international pressure had been reopened 10 years later. The statement, which did not elaborate on the reasons save to note that heavy battles had been raging at the time in Angola, followed President F W de Klerk's disclosure on Wednesday that Pretoria had, until recently, possessed six Hiroshima-sized nuclear bombs.
Even before Armscor's suprising admission about the test site, nuclear weapons experts had pointed out that the most likely target of a South African nuclear attack would have been Luanda, the Angolan capital, a city of 1 million people.
'If, as they say, they reopened their test site in 1987, it's quite frightening. It's a clear indication that they were getting ready to use the thing,' said Renfrew Christie, who is now Dean of Research at the University of the Western Cape but was sentenced to 10 years in jail in 1980 for passing on South African nuclear secrets to the African National Congress (ANC).
In 1987, South Africa was run by the volatile president P W Botha and a military establishment, all of whose judgements were coloured by a feverish conviction that the Russians were coming. Towards the end of the year the army was in desperate straits in the war in Angola against the Soviet- assisted Angolan army and Cuban air and ground forces.
South Africa had been fighting in Angola since 1975, convinced that the government there, as the one in Mozambique to the east, formed part of a Soviet-inspired 'total onslaught' against Pretoria spearheaded by Nelson Mandela's ANC and their Communist Party allies.
The broad fear, at a time of seething internal unrest and international isolation, was that if one front collapsed all the rest would follow. The specific fear, which in fact generated panic at the time in government circles, was that the Cuban air force was preparing to strike into South Africa.
Reinforcing long-standing perceptions that it was the former Soviet Union, and not the black majority, the white establishment ultimately had to fear, it was Tass news agency that first blew the whistle on South Africa's nuclear weapons capacity. On 21 August 1977, Tass reported that, according to information obtained from a Soviet surveillance satellite, South Africa was about to conduct a nuclear test in the Kalahari. The governments of the United States, France, Germany - all of whom had contributed to South Africa's commercial nuclear programme - and Britain suddenly faced an international crisis.
The US ambassador in Pretoria immediately called on the Foreign Minister, Pik Botha, who assured him the Tass report was 'wholly and totally unfounded'. The next day the French foreign minister repeated the Tass claim that South Africa was poised to conduct a nuclear explosion. The day after that the German foreign minister contacted Mr Botha, who reiterated that his country's nuclear energy was being used for exclusively peaceful purposes.
On 25 August the South African prime minister, John Vorster, gave a speech which he addressed, not entirely reassuringly, these fears. 'The time will arrive when South Africa will have no option - small as it is - but to say to the world: 'So far and no further]'
At this point President Carter and other Western leaders intervened, and Mr Vorster blinked. The planned test, as Armscor confirmed in its statement, was called off. 'The Republic of South Africa was preparing for capability to do a scientific underground nuclear test in 1977. This preparation work was terminated by the head of state when it became known that international opinion turned against him.'
Armscor revealed that a test site had been constructed in the Kalahari, in the country's north-western corner, consisting of two deep shafts. At the end of 1977 the shafts were sealed and abandoned, Armscor said. But in 1987 one of the shafts was reopened and a hangar was erected over it - presumably to avoid satellite detection again.
Professor Christie noted that the Botha government had to have felt under extreme pressure at the time to take such a step. 'The crisis in 1977 was a matter between Russia and the United States. Russia said 'There will not be a South African test'. In what terms the Russians said this I don't know. But the West clearly did not mean to challenge it and they told the South Africans not to do it. That's why it's so interesting that that the South Africans later reopened the test site. It meant they were acting in the face of what the Americans had said.'
What exactly might the South Africans have done? President de Klerk provided one possible answer last week. 'The strategy was that if the situation in Southern Africa were to deteriorate seriously, a confidential indication of the deterrent capability would be given to one or more of the major powers . . . in an attempt to persuade them to intervene.'
And if that attempt had failed? Contingency strategy two, Professor Christie believes, would have been to bomb Luanda. Again, it was intense diplomatic activity, led by the US, that saved the day.