The company has not made a final investment decision, but it is considering building 650 MW No.7 and No.8 plants, with gross thermal efficiency of about 63% - one of the highest levels anywhere in the world - to help reduce CO2 emissions, a JERA spokesman said.

If they go ahead, the two units would start operations in August and December 2027 respectively, he said.

JERA, a thermal power and fuel joint venture between Tokyo Electric Power Company Holdings and Chubu Electric Power, plans to decommission the older No.1-No.4 plants by March 2022 and the No.5 plant by March 2027, shutting 3.1 gigawatts(GW) capacity in total.

It will retain the 854 MW No.6 unit.

JERA, which is also the world's biggest LNG buyer, submitted a primary environmental impact consideration document to Japan's industry ministry and local authorities on Tuesday, the first step in four phases of the environmental impact assessment process.

(Reporting by Yuka Obayashi; Editing by Susan Fenton)