Tokyo Electric Power Company (TEPCO) has suspended operations at the Kashiwazaki-Kariwa nuclear power plant in Japan, just a day after a reactor was brought back online for the first time in more than a decade.

The pause came after an alarm sounded during startup procedures, and the operator said it did not yet know when the issue would be resolved.

The No. 6 reactor at the Kashiwazaki-Kariwa plant in Niigata prefecture began operating on 21 January 2026, marking its first operation since being shut down after the 2011 Fukushima nuclear disaster.

The plant is regarded as the world’s largest nuclear facility by potential capacity, though only this single unit was restarted.

Early on Thursday, an alarm from the reactor’s monitoring system was triggered while workers were withdrawing control rods, equipment crucial for regulating the nuclear fission process.

In response, TEPCO halted the startup and shut the reactor down again for safety checks and investigation. The company has not provided a timetable for when the reactor might resume operation.

TEPCO emphasised that there has been no abnormal release of radiation and that the reactor remains in a stable condition, but the suspension highlights ongoing challenges in bringing nuclear capacity back online in Japan’s highly regulated post-Fukushima environment.

The resumed operation of Kashiwazaki-Kariwa was seen as a significant step in Japan’s efforts to reduce dependence on fossil fuel imports and reintegrate nuclear energy into its power mix.

However, this early halt may delay those plans, and calls for clearer explanations of the cause and potential fixes have begun to emerge from analysts and energy policy observers.

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