Kansai Electric Power Co. on Friday restarted the 48-year-old No. 1 reactor at its Takahama nuclear plant in Fukui Prefecture for the first time in 12 years.

The Takahama No. 1 reactor is the oldest of Japan's nuclear reactors not set for decommissioning.

It is the second time that a reactor over 40 years old has been reactivated, after the restart of the No. 3 reactor at Kansai Electric's Mihama plant, also in Fukui.

The Takahama No. 1 reactor may become the first reactor in the country to operate more than 60 years after its lifespan began, which is allowed under a law enacted in May.

The Takahama plant's No. 2 reactor is scheduled to go back online in mid-September.

Kansai Electric started removing the control rods of the No. 1 reactor at around 3 p.m. Friday. The reactor is expected to reach criticality, or a self-sustaining nuclear chain reaction, around 6 a.m. Saturday and begin generating and transmitting electricity on Wednesday.

The reactor is slated to shift to commercial operation after a comprehensive inspection on Aug. 28.

The Takahama No. 1 reactor, put into operation in November 1974, had been offline for routine checkups since January 2011, two months before the triple meltdown at Tokyo Electric Power Company Holdings Inc.'s Fukushima No. 1 nuclear plant.

In June 2016, the Nuclear Regulation Authority approved the extension of the reactor's operating life to 60 years, and the prefectural government of Fukui gave its consent in April 2021 to the reactor's reactivation.

However, the restart of the reactor has been delayed due to the construction of required anti-terrorism facilities.