Mihama-3 began commercial operation in December 1976 and Takahama-1 in November 1974.
Press reports in Japan said Kansai Electric plans to bring Mihama-3 back online in January 2021, and the other, Takahama-1, in March 2021, although approvals will be needed from local governments.
The two reactors have been authorised by the nuclear regulator to run beyond the usual limit of 40 years, which the government introduced after the 2011 Fukushima-Daiichi disaster.
In exceptional cases reactors can operate for 60 years if the facilities, and the operators’ plans to ensure their safety, are approved by the regulators.
Takahama-2 has also been approved to operate beyond 40 years. Kansai Electric said work to prepare for the extension is continuing.
Most of the work at Mihama-3 has been related to improving the unit's seismic resistance. At Takahama-1 and -2, work has involved reinforcements to their containment buildings. Kansai Electric said it has reinforced the existing concrete wall surrounding the containment vessel of each unit and installed secondary domes.
Japan has a total of 62 nuclear power units, but shut down all 42 reactors that were operating at the time after the Fukushima-Daiichi accident in 2011. Thirty-three units have a licence to operate although before units return to service they need to meet stricter safety standards.
The nine units that have returned to service since Fukushima are Sendai-1 and -2, Genkai-3 and -4, Ikata-3, Ohi-3 and -4 and Takahama-3 and -4. However, Sendai-1 and Sendai-2 were both shut down again earlier this year after failing to meet deadlines for safety improvements.