The seawater pump in the cooling system for the Fukushima power plant's No. 5 reactor broke down Saturday evening, prompting repair crews to install a backup pump 15 hours later on Sunday afternoon, Tokyo Electric Power Co. said.
Tepco discovered the pump had stopped at 9 p.m. Saturday but didn't announce it to the public until Sunday morning.
The beleaguered utility said it notified the local and central governments of the situation on Saturday evening.