The South African miner's ACP phase A plant at Waterval smelter in Rustenburg, 141 km (88 miles) from Johannesburg, was damaged after an explosion inside the converter on Feb. 10. The phase B unit was set to take over but water was detected in the furnace.

"This poses a high risk of explosion and the company has determined that it has no other option but to temporarily shut down the phase B unit to ensure the safety of all employees and avoid a catastrophic event," Amplats said on Friday.

The company, which is majority owned by Anglo American, said repair works to fix the phase B unit would take about 80 days, and that it had to declare force majeure as it is unable to process material during the converter repair.

Amplats cut its total platinum group metals production guidance for 2020 from between 4.2 million and 4.7 million ounces to between 3.3 million and 3.8 million ounces.

It cut its platinum production guidance to between 1.5 million and 1.7 mln ounces, from 2 million to 2.2 million ounces. Palladium guidance was cut to between 1.1million and 1.2 million ounces, down from 1.4 million to 1.5 million ounces.

Amplats' Johannesburg-listed shares were down 19.7% at 890 rand by 1155 GMT, set for a record one-day loss.

(Reporting by Helen Reid; Editing by David Goodman)