Generator problems have shut down the Fort Calhoun Nuclear Station north of Omaha — the second time the troubled plant has stopped producing power since restarting in December.

Omaha Public Power District employees were working on the plant's generator about noon Monday when the turbine cooling system began losing water, according to an event report filed with the Nuclear Regulatory Commission.

This tripped an automatic failsafe system that shut down the turbines, then the reactor.

It's not known how or whether the maintenance work caused the problems, OPPD spokesman Jeff Hanson said.

“That's what we're investigating now,” he said Tuesday.

Hanson said the shutdown is temporary and won't affect electrical service.

OPPD also had to power down Jan. 9 when workers discovered a damaged sluice gate. This happened two weeks after the plant finally was allowed to restart following nearly three years in a cold shutdown.