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Severe damage to a chimney at Kusile is the latest blow to the beleaguered power station.
On Wednesday, Eskom said that the flue gas duct at Unit 1 of the Mpumalanga power station "failed" in two places on 23 October.
"On 23 October 2022, a section of the Kusile Unit 1 flue gas duct (the equivalent of a chimney in a household) exiting the sulphur dioxide absorber failed on the horizontal rubber expansion joint as well as the compensator (a bend to direct flue gas up the chimney and to allow for thermal expansion of the chimney) whilst the unit was on forced shutdown for Flue Gas Desulphurisation recirculating pump repairs," spokesperson Sikonathi Mantshantsha said.
A photo of the flue gas duct shows extensive damage, and Eskom said Unit 1 would remain offline for a few months.
Investigations are under way to figure out the cause of the failure. Access to the area has been restricted as a precaution, Mantshantsha said.
Eskom is consulting with specialists and the equipment manufacturer to establish how best the plant can be restored quickly, he added.
Unit 2 was not generating power at the time of the incident, but its return to service is on hold as a precaution, Eskom said. Unit 3 and Unit 4 are generating electricity.
In September, a fire broke out at a gas air heater at Unit 5, which is being commissioned and has not yet been synchronised to the grid.
Last week, Eskom chief operating officer Jan Oberholzer told Parliament that the fire would delay the unit's completion by a whole year, until December 2024. Oberholzer said the incident is projected to cost Eskom R150 million per month, which is related to the delays in getting the nearly 800MW unit online much later than expected.
No foul play is suspected.
Unit six was still on track to come online in the middle of 2024.
Kusile, which is years behind schedule and billions of rands over budget, will have six generating units once complete. Each unit is set to generate 800 MW, for a total of 4 800MW.
Kusile, one of the world's biggest coal-fired power plants, was commissioned in 2007 and was supposed to be completed in 2014. But severe design defects at the two power stations are expected to take until 2027 to rectify fully. Kusile is expected to cost more than R161 billion.