Under stricter rules implemented after the 2011 Fukushima disaster, Japanese utilities have been mandated to build emergency off-site control rooms to serve as back-up bases that can keep nuclear reactors cooled and prevent meltdowns in the event of a terrorist attack.

Kansai Electric's Takahama No. 3 reactor will be shut down from August 2 to Dec. 22, and No. 4 will be off-line from Oct. 7 to Feb. 10, 2021.

Japanese utilities are expected to face hundreds of millions of dollars in extra fuel costs in 2020 as they buy extra liquefied natural gas (LNG) and coal. Nearly half of the country's working nuclear reactors will go off-line for required security upgrades.

Kansai Electric declined to comment on what alternative power sources it plans to use to replace output from the two nuclear reactors. It had said last April that it could save fuel costs of 9 billion yen ($82 million) a month from operating both Takahama No. 3 and No. 4 reactors.

The company also said it plans to restart Takahama No. 4 on Thursday. The plant has been shut since Sept. 18, 2019, for scheduled maintenance.

(Reporting by Yuka Obayashi; Editing by Tom Hogue)