Sendai-2 went offline ahead of a 21 May deadline for the completion of anti-terrorism steps, which were introduced as part of revised safety measures following the March 2011 Fukushima-Daichi accident.
The utility aims to bring the reactor back online on 26 January 2021 after making the required changes to facilities and undertaking regular safety inspections.
Sendai-1 was shut down in March after also failing to meet a deadline for safety improvements.
In the wake of the Fukushima crisis, Japan’s Nuclear Regulation Authority obliges plant operators to build facilities that can withstand terrorist attacks such as planes being flown into them without succumbing to major damage such as massive leakages of radioactive materials.
Equipment such as coolant pumps and emergency power sources are supposed to be placed roughly 100 metres away from a reactor to avoid being affected by a terror attack.
Kyushu Electric said that as of late April, about 70% of electric and machinery work was completed, while 90% of civil engineering work was done.
The Japan Times reported that Kansai Electric Power Company is also expected to suspend operations from later this year at its Takahama-3 and -4 plants in Fukui Prefecture, western Japan, for the same reason.
The Sendai-2 shutdown means Japan now has seven commercial nuclear units in operation.
Japan has a total of 62 nuclear power units, but shut down all 42 reactors that were operating at the time after the Fukushima-Daiichi accident. Thirty-three units have a licence to operate although before units return to service they need to meet the stricter safety standards.
The nine units that have returned to service are Sendai-1 and -2, Genkai-3 and -4, Ikata-3, Ohi-3 and -4 and Takahama-3 and -4.