The operator of the crippled Fukushima nuclear plant cleared a major regulatory hurdle Wednesday to restart two reactors in Japan, its first since the 2011 tsunami sparked the worst atomic accident in decades.

The Nuclear Regulation Authority gave Tokyo Electric Power Co. (TEPCO) preliminary approval to restart the two reactors at the Kashiwazaki-Kariwa plant, one of the world's biggest and the largest in Japan.

The plant, in the central Japan prefecture of Niigata, has been idle since the disaster as have been many other nuclear power plants in Japan.

Triggered by a 9.0-magnitude earthquake in March 2011, a massive tsunami overwhelmed reactor cooling systems at the Fukushima Daiichi plant in northeastern Japan.

It caused reactor meltdowns, releasing radiation in the most dangerous nuclear disaster since Chernobyl in 1986.

On Wednesday, TEPCO won safety approval as the authority judged the two reactors meet the stricter safety standards introduced after the disaster.

The decision is expected to be formalised after a month of public hearings but TEPCO still needs to get local consent to bring the reactors online, which could take years.

Niigata Governor Ryuichi Yoneyama, who won the local election in 2016 for a four-year term, is known to be cautious about restarting Kashiwazaki-Kariwa.

Nuclear power is one of key issues at the October 22 general election in Japan, with Prime Minister Shinzo Abe favouring gradual restarts while his main opponent and currently Tokyo governor, Yuriko Koike, campaigning to cease nuclear power by 2030.