16 September 1991
Shortly after the signing of the Non-Proliferation Treaty (NPT) in 1991, the South African government entered into a Comprehensive Safeguards Agreement with the International Atomic Energy Agency (IAEA). The Safeguards Agreement ensures that member states comply with international obligations in containing the proliferation of nuclear weapons. The SA and IAEA agreement was only implemented in November of the same year it was signed. In 1995 the IAEA announced that it was satisfied that all South African nuclear materials were accounted for and that the weapons program had been terminated and dismantled.Click here to read the treaty on the Non-Proliferation of Nuclear Weapons signed in 1968. 
References

fas,' Treaty on the non-profileration of nuclear weapons ',[online],Available at www.fas.org [Accessed: 12 September 2013]|

world-nuclear,' International Atomic Energy Agency (IAEA) ',[online],Available at www.world-nuclea.org [Accessed: 12 September 2013]