A nuclear reactor in central Japan's Fukui Prefecture was halted on Saturday, just four months after its restart as it could not meet a deadline set by regulators to implement antiterrorism measures.

The suspension of Kansai Electric Power Co.'s Mihama No. 3 unit came ahead of the deadline on Monday for such measures to be completed. After undergoing safety checks, the utility aims to make the necessary changes to its facilities around September next year and restart operations in mid-October.

File photo taken June 23, 2021, shows (from front) the No. 3, No. 2 and No. 1 reactors of Kansai Electric Power Co.'s Mihama nuclear power plant in Fukui Prefecture, central Japan. (Kyodo)

In June, the No. 3 unit went online for the first time in about 10 years and became the first nuclear unit to operate beyond the government-mandated 40-year service period introduced under new rules set after the 2011 Fukushima disaster.

The reactor had stopped operations after the crisis at the Fukushima Daiichi plant triggered by a massive earthquake and tsunami. Regulators gave the green light to the aging No. 3 unit after screening the utility's safety measures.

The Nos. 1 and 2 units of the Mihama plant were terminated in April 2015 in line with the 40-year limit.