The Rancho Seco nuclear power plant, shut down since Jan. 31 by a mechanical pump failure, was expected to have resumed operation by early today, a plant spokesman said. “It’s hard to provide an exact start-up time. It could be before midnight. It could be after midnight in the early morning,” said Kerry Shearer, a spokesman for the Sacramento Municipal Utility District, owner and operator of Rancho Seco. Authorities at the power plant 25 miles southeast of Sacramento received permission from the Nuclear Regulatory Commission over the weekend to restart the facility, which was shut down when a mechanical governor for an auxiliary feed-water pump failed.