Jaif said the loading of 157 fuel assemblies in the core of the 780-MW pressurised water reactor plant was completed on 22 May at 22:25 local time.
Fuel loading at the plant, owned and operated by Kansai Electric Power, began on 20 May.
Jaif said Kansai Electric plans to physically start up Mihama-3 on 23 June and have it fully operational on 27 July.
In March 2021, Jaif said Kansai Electric was struggling to confirm restart dates for Mihama as the utility tried to meet deadlines for post-Fukushima safety improvements.
According to Jaif, the deadline for the Mihama-3 improvements is 25 October. However, plans for the work were only approved by the nuclear regulator in April.
Most of the work at Mihama-3 has been related to improving the unit’s seismic resistance.
Mihama-3 could become the first commercial nuclear power plant in Japan to operate beyond 40 years.
It began commercial operation in December 1976 and has been authorised by the regulator to run beyond the usual 40-year limit, a change to legislation which the government introduced after the 2011 Fukushima disaster.