The No. 6 reactor at Kashiwazaki-Kariwa resumed power generation and transmission to the Tokyo metropolitan area on Sunday, according to Tokyo Electric Power Company Holdings Inc. The restart came a day after TEPCO replaced a damaged component that had forced the reactor offline for 10 days.

An alarm sounded on March 12 indicating a possible electricity leak. TEPCO reduced output to 20% and halted transmission to investigate. The culprit: a cracked electric conductor linking the generator to an earthing device, damaged by vibrations. No actual leak occurred — just a faulty sensor signal.

The plant, located about 220 kilometers northwest of Tokyo in Niigata Prefecture, is the world’s largest nuclear facility. Reactor No. 6 became the first TEPCO-operated reactor to restart since the 2011 Fukushima disaster when it came back online in January.

Commercial operation, originally scheduled for this week, has been pushed to April at the earliest.

The restart matters beyond TEPCO’s grid. Japan’s government under Prime Minister Takaichi Sanae is pivoting back to nuclear as a core energy source, aiming to lift nuclear’s share of electricity generation from 9% to 20% by 2040. The policy shift comes as countries worldwide reconsider nuclear’s role in energy security.

For TEPCO, the stakes are financial as well as operational. The utility carries 11.9 trillion yen ($75 billion) in liabilities and projects a 641 billion yen ($4.1 billion) net loss for the current fiscal year, weighed down by Fukushima decommissioning costs.

Sources