Johannesburg - Koeberg nuclear power station on Monday began a planned shut down of its unit two, Eskom said.
"The unit was disconnected from the power grid at 1:30, however the unit takes several days to cool down sufficiently for the workers to access the necessary areas," spokesperson Hilary Joffe
The shut down was expected to last about 55 days, and the station would be running again in early May.
This was to enable routine refuelling, maintenance and statutory inspections.
Eskom conducted these shut downs during spring, summer and autumn, as in winter the demand for power was higher and hence shut downs were kept to a minimum.
Unit one would continue to operate at full power, Joffe said.
Residents in the area were advised that steam would issue from the unit during the shut down process, but this was no cause for alarm.
"Given the events in Japan, we want to assure the public that perfectly clean steam will issue from next to the reactor during the routine shut," spokesperson Tony Stott said.
He said that this was part of the cool down process of unit two.
The unit would take two to three days to cool down enough for it to be opened for workers to begin the refuelling, maintenance and inspections.
Stott said that valves, pumps and high pressure components were among items scheduled for maintenance. One third of the fuel in the reactor would be replaced.
Once the unit had been reconnected to the power grid, it would take about four days to begingenerating electricity. In about a week, the unit would be back up to full power.
At various stages of the power up, tests would be conducted to ensure all was working correctly.
Stott said unit two provided between 2% and 2.5% of South Africa's energy capacity.