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.
The seawater pump was set up after the reactor's original pumps were knocked out by the March 11 earthquake and tsunami. It was part of the critical Residual Heat Removal System that was later used to safely ease the reactor into a cold shutdown on March 20.
Unlike reactors 1 through 4, No. 5 is less at risk of meltdown because it was not damaged by hydrogen blasts as some of the others were and because workers managed to restore external power to its cooling system.
The pump had been taking in seawater for the RHRS's heat exchanger, which uses it to cool down fresh water being used to regulate the temperature of the reactor and its spent fuel pool.
The cause of the pump's failure was not immediately known but was likely caused by seawater fouling some of its parts, Tepco spokesman Junichi Matsumoto told a news conference Sunday morning.
By noon Sunday, the core had reached a temperature of 93.6 degrees and the fuel pool had reached 46 degrees, compared with 68 degrees and 41 degrees, respectively, at 9 p.m. Saturday.
The backup pump kicked in at 12:31 p.m., bringing the core back down to 83 degrees by 1 p.m., a Tepco spokeswoman said by phone later Sunday.
The temperature of the core must stay below 100 degrees to maintain cold shutdown status. Anything above might cause the water to evaporate and expose the fuel rods.
Tepco began setting up the backup pump at 8 a.m. Sunday. Matsumoto said Tepco didn't start until morning because the temperatures of both facilities were rising slowly and an emergency water-injection system was available for unit 5.
"We judged that it's better to wait for the morning, rather than to start working while it's dark. We didn't think it would pose any immediate danger," Matsumoto said.
"Even if the temperature (of the core) reaches 100 degrees, the alternative water-injection system is available," he said.
Still, Matsumoto admitted that "it might have been better" to notify the media sooner about the pump's failure.
Tepco is under fire for failing to disclose information fully and quickly since the nuclear crisis began on March 11. The quake and tsunami severely damaged units 1, 2, 3 and 4 at Fukushima No. 1, but Nos. 5 and 6 were already shut for regular inspections at the time of the disaster.
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