Troubles at Ameren Missouri's Taum Sauk hydroelectric project seemed over last spring when the 440-megawatt plant returned to service more than four years after its mountaintop reservoir ruptured, unleashing a torrent into a state park.

But the plant about 100 miles southwest of St. Louis has been running at half-capacity since early June when one of its two generating units failed, caught fire and suffered $10 million in damage. The fire only became public in a Dec. 16 staff memo to the Public Service Commission.

PSC staff were unaware what had happened when members visited the plant eight days after the fire to discuss the rebuilding of the upper reservoir and saw the generator damage firsthand.

Soon after the visit, the commission staff filed a complaint against the utility for failing to notify the PSC. Rules require Missouri utilities to report within a day "any accident or event at a power plant" that involves serious injury, death or property damage in excess of $200,000. Ameren classified the incident as an outage that didn't require immediate notification.

The rebuilt upper reservoir at the Taum Sauk plant was completed in early 2010 at a cost of $490 million.

The concrete kidney-shaped pool atop Proffit Mountain replaced an earthen dike, which ruptured on Dec. 14, 2005, releasing a flood that scoured the mountainside of trees, boulders and dirt and swept away the home of the Johnson Shut-Ins State Park superintendent.

The family survived with minor injuries. Ameren has paid nearly $200 million under settlements with federal regulators and the state.

Federal energy regulators are now considering whether to issue a new license for the plant.

Details of the staff's complaint about the unreported fire are highly redacted, so the incident and the extent of the damage was only recently made public in the Dec. 16 staff memo to the commission.

According to the report, the Taum Sauk generating unit tripped on June 6 and suffered an "extensive fire" that melted some of the stationary parts. Fire crews were dispatched to the plant's powerhouse, but employees had already extinguished the fire when they arrived, the report said.

Ameren Missouri's vice president of power operations, Mark Birk, said in an e-mailed response to questions that an insulated copper winding inside the generator failed. He said the generators had been inspected while the plant's upper reservoir was being rebuilt, but showed no signs that repairs were imminent.

"Much like a light bulb, these windings will age and fail over time," Birk said. "It is difficult to predict when they will fail."

The St. Louis-based utility, which sells electricity to 1.2 million Missouri customers, said the generating unit is being repaired and should be back in service by late March.

And the staff report to the PSC said Ameren and commission staff are in discussions to settle the complaint. A followup on the status of the settlement discussions is due by Dec. 30.

Taum Sauk began operations in 1963. It is the only pumped storage plant in Missouri, generating electricity during the afternoon when water from the upper reservoir flows through a tunnel to spin turbines. At night, when electricity is cheaper, electric pumps move water back up the mountain to refill the reservoir.