One of the reactors at Takahama nuclear plant in the central Japanese prefecture of Fukui has been restarted for the first time in 15 months.
Plant workers began pulling control rods out of the No.4 reactor at 5 PM on Wednesday.
It runs on MOX fuel, which is a mixture of plutonium extracted from spent nuclear fuel and uranium.
The reactor is to reach criticality, a self-sustained nuclear reaction, in about 13 hours by around 6 AM on Thursday. The operator, Kansai Electric Power Company, plans to start power generation and transmission to the grid on Monday next week.
The power utility plans to resume commercial operations with the reactor in mid-June.
Wednesday's restart brought the number of reactors operating in Japan to 4, including 2 reactors at Sendai plant in Kagoshima Prefecture and one at Ikata plant in Ehime Prefecture.
Takahama's No.4 reactor was reactivated in February of last year, but shut down automatically due to a glitch that occurred 3 days after the restart.
A district court later issued an injunction that suspended operations of the No.4 and No.3 reactors of the Takahama plant, but the Osaka High Court nullified the injunction in March this year.