The nation's nuclear watchdog granted approval Nov. 16 for the aging No. 3 reactor at the Mihama nuclear plant to operate for an additional 20 years, making an exception for the second time.
The plant in Fukui Prefecture operated by Kansai Electric Power Co. will reach its 40-year-lifespan at the end of the month.
The Nuclear Regulation Authority previously allowed the No. 1 and No. 2 reactors at the Takahama nuclear plant, also in the prefecture, to extend operations for 20 years. That authorization was given in June.
Limiting nuclear plant operations to 40 years, in principle, was decided on in the aftermath of the 2011 accident at the Fukushima No. 1 nuclear power plant.
The decision, made while the Democratic Party of Japan was in control of government, was supported by the Liberal Democratic Party and Komeito, then in opposition but now ruling as a coalition. Under that new rule, there was a provision for a one-time exemption to allow extension of operations for 20 years.
But the DPJ government said such extensions would be "extremely exceptional."
The latest approval does not mean the Mihama No. 3 reactor will be resuming operations any time soon. Kansai Electric Power must first implement the measures it promised to heighten safety.
Anti-quake measures and the switching of electric cables that total about 1,000 kilometers in length is expected to take more than three years, meaning that operations will only resume by March 2020 at the earliest.
In order to allow for an extension, a nuclear plant operator has to pass three screenings based on tougher safety standards before the 40-year deadline is reached.
Kansai Electric Power submitted an application to the NRA in November 2015 for an extension of the Mihama reactor.
Under safety screening based on new standards, the estimated size of the shaking of a possible earthquake that might hit the area around the plant was raised from 750 gal to 993 gal. A gal is a measure of ground acceleration related to seismic shaking.
The higher standard meant Kansai Electric Power had to demonstrate that equipment at the Mihama plant could withstand shaking of that size. The utility was also told to switch the electric cables used on the plant site to ones that were less flammable.
With the November deadline approaching, the NRA pushed the Mihama No. 3 reactor safety screening to the top of its list. In October, the agency approved the utility's basic policy for safety measures. It later also approved the detailed design plans for plant equipment.
The latest decision by the NRA means Kansai Electric Power now has the three approvals it needs to extend operations for 20 years.