(CNN) -- Japan's only operating nuclear reactor will be shut down for maintenance before Monday, leaving the country with no nuclear power supply for the second time in 40 years. Kansai Electric Power Co. will shut down reactor no. 4 at its Oi plant in Fukui prefecture in western Japan. With the shutdown, all 50 of the country's reactors will be offline. The government hasn't said when or if any of the reactors will come back on.