Three of Japan’s oldest nuclear reactors will be decommissioned, the start of what is expected to be a series of shutdowns because of costs to meet tougher safety standards introduced after the Fukushima disaster.

Kansai Electric Power Co., the utility most dependent on nuclear power, will close two reactors in western Japan’s Fukui prefecture. Japan Atomic Power Co. will scrap a reactor in its Tsuruga plant, also in Fukui, the utilities announced Tuesday in statements.

The three reactors -- Kansai’s No. 1 and No. 2 units at its Mihama nuclear plant and Japan Atomic’s No. 1 unit -- date back to the 1970s. The three units had a combined operating capacity of 1,197 megawatts.

As of this year, seven of the country’s reactors are at least 40 years old, considered the operational age limit by regulators. The reactors are small by modern standards and would have faced costly safety upgrades to continue operating.

Other utilities will probably announce two more reactor shutdowns this week, the Yomiuri newspaper reported last week. That will bring the total to five, with all having exceeded 40 years of operation by July 2016, according to the newspaper, which didn’t say where it got the information.

Utilities must seek government approval to operate nuclear reactors beyond the 40-year limit.

Kansai Electric President Makoto Yagi met with Fukui’s governor on Tuesday to explain the utility’s decision, the company said.

Also on Tuesday, Kansai Electric applied to restart the No. 3 unit at the Mihama plant and the No. 1 and No. 2 units at its Takahama plant. The new applications bring the number of reactors under safety review at the regulator to 24.

Japan relied on nuclear energy for more than a quarter of its electricity before the Fukushima disaster four years ago. The nation’s 48 operational reactors have since been shut for inspections and safety improvements. (Bloomberg)