Work continued Wednesday to restore electricity to key equipment at the crippled nuclear reactors in the Fukushima No. 1 nuclear power plant, a day after electricity was restored to all six reactors via outside power sources.

In a foreboding sign, however, black smoke again spewed from the No. 3 reactor building at around 4:20 p.m., Tokyo Electric Power Co. said. Black smoke also rose Monday.

The smoke forced workers to evacuate but died down after about an hour and radiation levels in the unit mostly remained unchanged, Tepco said. However, it remained unclear when work would resume.