But the process had to be suspended hours later due to a malfunction related to control rods, which are essential to safely starting up and shutting down reactors, the Tokyo Electric Power Company Holdings said.
TEPCO, which also manages the wrecked Fukushima plant, said there was no safety issue from the glitch.
Kashiwazak-Kariwa plant chief Takeyuki Inagaki told a news conference that he has decided to shut down the reactor to ensure safety. The operation had to stop when an alarm went off after 52 of the 205 control rods were removed from the core, he said. Inagaki said he hoped to start putting them back in later Thursday to bring the No. 6 reactor to a shutdown.
“The equipment is essential to safe operation, and we will examine it inside out," he said, adding that the reactor will not be restarted until the cause is found and measures are taken.
“I don't think this is going to be resolved in a couple of days," Inagaki said.
The restart at the Kashiwazaki-Kariwa plant was being watched closely since TEPCO also runs the Fukushima Daiichi plant that was ruined in the 2011 quake and tsunami. Resource-poor Japan is accelerating atomic power use to meet soaring electricity needs.
All seven reactors at Kashiwazaki-Kariwa have been dormant since a year after the meltdowns of reactors at the Fukushima plant contaminated the surrounding land with radioactive fallout so severe that some areas are still uninhabitable.
TEPCO is working on the cleanup at the Fukushima site that’s estimated to cost 22 trillion yen ($139 billion). It's also trying to recover from the damage to its reputation after government and independent investigations blamed the Fukushima disaster on TEPCO’s bad safety culture and criticized it for collusion with safety authorities.
Fourteen other nuclear reactors have restarted across Japan since 2011, but the Kashiwazaki-Kariwa plant, about 220 kilometers (135 miles) northwest of Tokyo, is the first TEPCO-run unit to resume production.
A restart of the No. 6 reactor could generate an additional 1.35 million kilowatts of electricity, enough to power more than 1 million households in the capital region.
The Kashiwazaki-Kariwa plant’s combined output capacity of 8 million kilowatts makes it the world’s largest, though TEPCO plans to resume only two of the seven reactors in coming years.