Nukes were actually provided by Israel, and the flying of a US spyplane (SR-71 Blackbird) prevented a nuclear test. Basically a Soviet spy leaked the date and details of the proposed nuclear test, and forced the USA, in front of the international community, into a position where they had to find out what was happening with regards to the situation. Otherwise US support of apartheid would become too blatant, if they refused to gather intelligence on a nuclear test by South Africa.

South Africa tested the nuclear weapons at a different location, see https://en.wikipedia.org/wiki/Vela_incident