The nation's new nuclear regulator ignored earthquake risk and its own rules in approving the safety of the Sendai nuclear plant, said Kobe University professor and seismologist Katsuhiko Ishibashi.

Ishibashi, well-known in Japan for books and papers on earthquake threats that later became reality, said he has filed a formal complaint to the Nuclear Regulation Authority challenging the legality of its decision.

The NRA's approval in September of Kyushu Electric Power Co.'s Sendai nuclear plant in Kyushu opens the way to restart two reactors at the station, possibly this year. They are the first of Japan's viable reactors to pass the new standards introduced since the Fukushima nuclear crisis in 2011. All reactors in the country have been shut for safety checks for at least 18 months.