Operator Shikoku Electric Power Company restarted the plant's No.3 reactor at around 9 AM on Friday. It removed the control rods, which are designed to contain nuclear fission reactions.

The reactor is expected to achieve a self-sustaining nuclear chain reaction by Saturday morning if the process goes as scheduled.

The utility plans to begin electricity generation and transmission to the grid on Monday, and start commercial operation early next month.

The reactor had been undergoing final inspections after clearing the new government regulations last year.

It had been offline since a regular inspection one month after the 2011 nuclear accident in Fukushima. It was last rebooted in March 2010.

The utility says the plant's workforce includes personnel who have no experience operating a reactor, as well as veteran workers who have not rebooted a reactor for a long time.

The firm says it will spend more than 20 hours to achieve criticality of the reactor. That's 5 hours longer than the previous reboot.

Two reactors at the Sendai plant in Kagoshima Prefecture, southern Japan, have been operating since last year.

Two reactors at the Takahama plant in Fukui Prefecture, central Japan, were restarted earlier this year. But a court injunction has suspended their operation.

Applications for restarting 21 other reactors at 14 nuclear plants across Japan are being processed at the Nuclear Regulation Authority.