TEPCO to shut reactor at world's biggest nuclear plant a day after resumption

In the early hours of Thursday, an alarm was triggered during work to withdraw control rods from the reactor, the utility said, adding it would halt the reactor to investigate the cause of the malfunction.

"I have determined it is necessary to temporarily suspend operations and conduct a thorough investigation into the cause," Takeyuki Inagaki, superintendent of the Kashiwazaki-Kariwa station, told a news conference.

Inagaki said TEPCO replaced electrical components in the control rod operation and monitoring panel following the alarm, but the situation did not improve.

The duration of the investigation and the schedule for restarting operations have yet to be determined, he said.

TEPCO restarted the No.6 unit at Kashiwazaki-Kariwa, the world's biggest nuclear power plant, on Wednesday evening, turning on its first nuclear reactor since the Fukushima disaster in 2011.

The process had been delayed from Tuesday as TEPCO investigated another alarm malfunction. As of early Wednesday, the equipment in question was functioning normally, TEPCO said at the time.

Shares in TEPCO lost 3.5% on Thursday, underperforming the benchmark Nikkei share average, which gained 1.7%.