The restart of the world’s largest nuclear power plant was suspended in Japan on Thursday just a day after it went online for the first time in about 14 years, with the operator saying it does not know when the problem will be solved.

The Kashiwazaki-Kariwa plant in Niigata province had been closed since the 2011 Fukushima disaster, but operations to relaunch it began on Wednesday after it received the final green light from the nuclear regulator.

However, its operator the Tokyo Electric Power Company (Tepco) said on Thursday that “an alarm from the monitoring system … sounded during the reactor startup procedures”, causing it to suspend operations.

“We don’t expect this to be solved within a day or two. There is no telling at the moment how long it will take,” site superintendent Takeyuki Inagaki told a news conference.

“We will for now fully focus on trying to identify the cause of what happened,” he said.

Spokesperson Takashi Kobayashi told the AFP news agency that “once it became clear that it would take time, we decided to reinsert the control rods in a planned manner”, he said, adding that the reactor “is stable and there is no radioactive impact outside”.

Control rods are a device used to control the nuclear chain reaction in the reactor core, which can be accelerated by slightly withdrawing them, or slowed down or stopped completely by inserting them deeper.

Kashiwazaki-Kariwa is the world’s biggest nuclear power plant by potential capacity, although just one reactor of seven was restarted. The facility was taken offline in 2011 when Japan pulled the plug on nuclear power after a colossal earthquake and tsunami sent three reactors at the Fukushima atomic plant into meltdown.

However, resource-poor Japan now wants to revive atomic energy to reduce its reliance on fossil fuels, achieve carbon neutrality by 2050 and meet growing energy needs from artificial intelligence.

Kashiwazaki-Kariwa is the first Tepco-run unit to restart since 2011. The company also operates the Fukushima Daiichi plant, now being decommissioned.

Public opinion in Niigata is deeply divided: About 60% of residents oppose the restart, while 37% support it, according to a survey conducted in September.

Earlier this month, seven groups opposing the restart submitted a petition signed by nearly 40,000 people to Tepco and Japan’s Nuclear Regulation Authority, saying that the plant sits on an active seismic fault zone and noted it was struck by a strong earthquake in 2007.