So I can answer this.

The context of South Africa disarming was two fold.

Firstly, South Africa developed nuclear weapons for both the international prestige of acquiring them independently, as well as a deterrent against any intervention by former colonial powers who were against Apartheid. The National Party, dominated by former Boers in South Africa whose grandparents remember the British colonial rule treating them not equitably after the capitulation of the Orange Free State and Transvaal after the Second Boer War. Nuclear weapons for South Africa was essentially a deterrent for any power who would invade, with a paranoia being instilled in National Party leadership since its inception, compounded by British talks of repossession of leadership during the Labor government in the UK after the Second World War. Compounded by the prestige of obtaining nuclear devices in spite of the “black African burden” that was used to explain away deficiencies in South African society in many unrelated occasions.

What led to the disarmament of South Africa was an increasing realization that although South Africa had been a valuable ally acting as a check on African socialism in the Cold War, that was coming to an end as the Soviet Union lost power in the diplomatic world. International socialism had lost much of its credibility by the late eighties, with many diplomats feeling that it had been sufficiently contained. This led to some administrations like the first Bush Administration to push democracy rather than anti communism in some policies, in Africa but more importantly in Latin America. With National Party leadership knowing that the rest of the community had not much use for them, and the poor optics of Apartheid policies weighing heavily in the eyes of the international community, State President de Klerk made structural changes to the presidency that would enable democratic transitioning to a more stable, less repressive society. The National Party, even with de Klerk didn’t see leaving a black majority in control of nuclear weapons as being advantageous to the white minority, and as such disarmed.

Let me find some sources when I get to my desktop in twenty mins.

Edit:

de Kock, Leon. "Freedom on a frontier? The double bind of (white) postapartheid South African literature." Ariel no. 3 (2015): 55. Literature Resource Center, EBSCOhost(accessed September 5, 2017).

Dunbar, Danielle, and Sandra Swart. "‘No less a foe than Satan himself’: The Devil, Transition and Moral Panic in White South Africa, 1989–1993." Journal Of Southern African Studies 38, no. 3 (September 2012): 601-621. Sociological Collection, EBSCOhost (accessed September 5, 2017).

Leonard, Pauline. "Making whiteness work in South Africa: A translabour approach." Women's Studies International Forum 36, no. Gender, Culture and Work in Global Cities (January 1, 2013): 75-83. ScienceDirect, EBSCOhost (accessed September 5, 2017).

Scheidegger, Ursula. 2015. Transformation From Below? White Suburbia in the Transformation of Apartheid South Africa to Democracy. Basel, Switzerland: Basler Afrika Bibliographien, 2015. eBook Collection (EBSCOhost), EBSCOhost (accessed September 5, 2017).