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The damage to the Koeberg nuclear plant was deliberate and not accidental, Public Enterprises Minister Alec Erwin said on Tuesday.
"Let me be very clear on this. The bolt that caused the generator's destruction did not get there by accident," Erwin said.
One of two of Koeberg generators has been down since December, causing major power outages in the Western Cape over the past two months.
Erwin said the National Intelligence Agency (NIA) and the police were investigating.
"The investigation is ongoing and we will bring criminal charges against individuals soon," Erwin said.
The police were also looking at other power outages which could have been caused by sabotage.
"These events curiously coincide with an important process in the democratic calendar of the country," Minister of Minerals and Energy Lindiwe Hendricks said in reference to Wednesday's local government election.
"It has become clear that the recent event cannot just be linked to inadequate transmission or generation capacity. Clearly other forces are at play here," she said.
Erwin said negotiations were being concluded with French electricity supplier EDF for parts to fix the broken generator.
On Tuesday morning, Koeberg's remaining generator shut down automatically when a high-voltage line tripped.
It would take some time to bring the generator back up to full capacity, said Eskom Board chairman Valli Moosa.
Until then, Cape Town suburbs would go without power for periods of up to two hours.
"The Western Cape uses on average 3700 megawatts but we can currently only supply about 2200 megawatts. Once Koeberg is up, the shortage will decrease to about 600 to 800 watts," Moosa said.
Eskom teams were working on repairing the high-voltage line, he said. He hoped Koeberg would be running by the end of Tuesday. - Sapa