I read (via a TIL today) that they started their nuclear program in 1948, had the bomb by 1979, and gave it up shortly before the end of Apartheid. This made me think, perhaps erroneously, that white South Africans didn't trust a black South Africa with the bomb, which in turn made me wonder... Is it possible that all the way up through 1979, many white South Africans may have thought Apartheid was indefinite, and more importantly, that it would be accepted indefinitely by the international community?
Generally the realisation that it was unsustainable came about in the late 1970's under Botha. This was kept very quiet though as they did not want to make any radical changes. The first big step in the direction of reform was in 1983 where Colourds (a distinct ethnicity in South Africa) and Indians (an umbrella term for willing and unwilling migrants from Asia) but not Blacks were given their own parliament. This failed miserably but was a big sign that the NP realised that pure Apartheid was not working.
They also began to recognise the fact that black resistance was becoming more violent as oppression increased and once the majority was radicalised they would not be controllable.
Another very important fact that is often overlooked is the profound difference between Botha and his predecessors. They were all idealogues, particularly BJ Vorster who directly preceded him, whereas Botha was a hardcore pragmatist.
Sources:
State Security in South Africa: Civil-military Relations Under P.W. Botha By James Michael Roherty
South Africa's Options: Strategies for Sharing Power by Frederik Van Zyl Slabbert
The system and the struggle: Reform, revolt, and reaction in South Africa by Frederik Van Zyl Slabbert
There's another book about Bothas Presidency that really goes into this particular issue but I can't for the life of me remember it (I worked on a research project on this topic during my Graduate studies), will edit if I remember.
EDIT: Remembered, it was P W Botha: The Last Betrayal by Eschel Rhoodie