Japan has suspended operations at the world's largest nuclear power plant just hours after restarting a reactor. Tokyo Electric Power Company (Tepco) said an alarm sounded "during reactor-start-up procedures" at the Kashiwazaki-Kariwa plant in Tokyo. Spokesperson Takashi Kobayashi confirmed the reactor remained "stable" with no radiation impact outside. Reactor number six, the first to restart since the 2011 Fukushima disaster, began operating on Wednesday after a one-day delay due to an alarm malfunction. The plant had all 54 reactors shut down after the Fukushima meltdown caused by a 9.0 magnitude earthquake 15 years ago. That disaster forced over 150,000 people to evacuate, many of whom have not returned. Tepco is investigating the cause of the incident but has not said when the reactor will restart. Reactor six is set to begin commercial operation next month. Meanwhile, reactor seven is not expected to restart before 2030, and five other reactors may be decommissioned. Local residents protested the restart over safety concerns, with crowds gathering outside Tepco's headquarters and Niigata prefectural assembly. Before 2011, nuclear power made up nearly 30% of Japan's electricity, with plans to increase to 50% by 2030. After shutting all reactors post-Fukushima, Japan has restarted 15 of its 33 operable reactors since 2015 as part of its net zero emissions goal by 2050.