The utility restarted the 846-MW pressurised water reactor unit on Saturday, a spokesperson said.
The reactor had been offline since 17 October when the company also started a special inspection that is required to have its operating life extended beyond 40 years.
The special inspection is continuing and the company has not decided whether or not it would apply for approval to extend operation beyond 40 years, Reuters reported.
The deadline for such an application for the plant, which began commercial operation in July 1984, is 4 July 2023.
With the restart, Japan has nine commercial nuclear reactors in operation. They are Genkai-3, Genkai-4, Ikata-3, Ohi-3, Ohi-4, Sendai-1, Sendai-2, Takahama-3 and Takahama-4.
Nine regional power utilities and a wholesaler, Japan Atomic Power Company (Japco), now have 33 reactors available for commercial use. The companies had 54 reactors operating before the March 2011 Fukushima disaster.
Before Fukushima, Japan’s nuclear fleet generated about 30% of the country’s electricity. According to the International Atomic Energy Agency that figure was about 5.1% in 2020.
Before plants can resume operation they need to meet stringent guidelines introduced following Fukushima.
Japan has adopted a new energy policy that promotes nuclear and renewables as sources of clean energy to achieve the country’s pledge of reaching carbon neutrality in 2050. It keeps the target for nuclear power unchanged at 20-22% and says reactor restarts are key to meeting emissions targets.
In a March 2021 report, the International Energy Agency called on Japan to speed up reactor restarts as a way to achieve its national climate commitments.