Vladimir Putin, the Russian president, on the first visit by a Russian leader since the fall of apartheid, said on Tuesday that the nuclear deal would last until 2010, but gave no further details.
"We propose the widening of this co-operation and making our co-operation in the sphere of developing atomic energy for peaceful aims long term and large scale," he said.
Russia is reorganising its civilian nuclear sector as it seeks to widen sales of nuclear technology abroad, expanding into the energy-hungry markets of Asia and Africa.
South African officials have said that the Koeberg plant, which imports all its fuel, was considering processing its own uranium to boost power generation and envisages building up to six new nuclear reactors.
Pretoria abandoned its nuclear arms programme before the end of apartheid in 1994.
The country has been suffering power blackouts due to an ageing energy infrastructure and a recent shutdown at its nuclear power station.
The Koeberg plant, in the Western Cape province, is Africa's only nuclear-fired facility and produces about six per cent of South Africa's electricity.
Deepening trade ties
Putin went quickly into talks with his counterpart Thabo Mbeki after arriving in Cape Town from Greece for a two-day visit on Tuesday.
Mbeki said after the talks that Pretoria and Moscow wanted to build on strong political ties to deepen economic and trade relations and had signed several agreements.
Victor Vekselberg, one of 100 business leaders accompanying Putin, said his Renova investment firm would invest more than $1 billion in the building of a ferro-alloy plant in South Africa's Eastern Cape region.
The plant would be "a big part of the world market", Vekselberg said.
Ferro-Alloy is used in the production of steel.
Putin is also set to oversee the signing of a memorandum of understanding between De Beers and Alrosa, the South African and Russian firms that account for around 75 per cent of the world's diamond mining.
The two countries will also sign a deal on banking, the Kremlin said.