A total of 7 reactors are being considered for decommissioning in Japan. Each unit is at or nearing the 40 year operational limit and would be required to pass additional safety screenings in order to continue operation. This is on top of meeting the new standards enacted after the Fukushima disaster.
The main reason for considering shutting down these plants is costs. One plant was going to need over $800 million in upgrades with no promise this would get the plant past the needed inspections. Meanwhile in the US the current cost of electricity is forcing some of their aging nuclear plants to require double the going rates for power just to turn a profit. This is before any of those US plants have been hit with safety upgrades or major repairs.
The plants in Japan that may close are:
Genkai unit 1
Mihama unit 1 and 2
Shimane unit 1
Tsuruga unit 1
Takahama unit 1 …
Source: Fukushima Sub