The miner said it took the pre-emptive action because the leaks threaten the plant's integrity as well as to maintain a safe operating environment.
Amplats CEO Natascha Viljoen previously called the unit "fragile" and said this week the rebuild of the main ACP Phase A unit, set for completion by the end of the year, will continue during the temporary shutdown of the B unit.
This is not the first time ACP has been ordered to idle; it closed in June after just one month. It had been closed prior to that after a March explosion that impacted both the Phase A and Phase B units.
"Increased monitoring, controls and systems were implemented to continually assess the ACP Phase B unit, and further water leaks were discovered," Amplats said in the new update. "The ACP Phase B unit underwent a planned and measured shutdown.
"Whilst the ACP management team has been able to complete repairs to previous water leaks, after a comprehensive assessment, and with further water leaks discovered, the ACP Phase B unit now requires more extensive repair work. As a result, the ACP Phase B unit has been closed for a full rebuild in 2021.
Amplats said that, as a result of the Phase B closure, it has reduced its whole-year refined production and sales guidance to 2.5 million PGM ounces from its prior estimate of 3.1-3.3 million PGM oz. It has put contingency plans in place, it added, and will be having discussions with customers to keep contract disruptions to a minimum.