If all the reactors at Tokyo Electric Power Co.'s Kashiwazaki-Kariwa nuclear power plant in Niigata Prefecture remain offline, and if approval is not given for restarting the Nos. 3 and 4 reactors at Kansai Electric Power Co.'s Oi nuclear power plant in Fukui Prefecture following their just-completed stress tests, Japan will enter the summer without any electricity being supplied from nuclear power plants.

That's because the No. 3 reactor at Hokkaido Electric Power Co.'s Tomari nuclear power plant in Hokkaido, which is currently Japan's only operating nuclear power plant, will stop operating in May for regularly scheduled inspection.

As of January, when two of Japan's 58 nuclear power plants were operating, electricity generated by nuclear plants accounted for just 4.3 percent of the total electricity supply — down from around 30 percent before March 11, 2011, when the earthquake-tsunami struck Tepco's Fukushima No. 1 nuclear power station.