Kansai Electric Power Company took unit 3 of its Mihama nuclear power plant in Japan's Fukui Prefecture offline on 23 October as it could not meet today's deadline set by regulators to implement antiterrorism measures. The reactor - which was restarted in June - is expected to remain offline for 13 months whilst the measures are completed.

The Mihama plant (Image: NRA)

Antiterrorism measures, including bunkered back-up control centres, are a requirement of new regulations introduced in July 2013 in response to the March 2011 accident at the Fukushima Daiichi plant. The Nuclear Regulation Authority (NRA) ruled in November 2015 that such measures must be completed within five years after regulatory approval of each plant's engineering and construction work programme. That programme is the second step in NRA's three-step process of assessing reactor safety prior to restart. The third and final stage includes pre-operational inspections to ensure the unit meets new safety requirements.

If a back-up control centre is not completed by one week before the deadline, the NRA can order a utility to suspend operation of a reactor. Under the new rules, an operator must place a unit in cold shutdown - where coolant water is at less than 100ºC - within a day of the unmet deadline.

Mihama 3 - a 780 MWe pressurised water reactor that entered commercial operation in 1976 - was restarted on 23 June after having been idle since May 2011. It became the first Japanese power reactor to operate beyond 40 years. In November 2016, Mihama 3 was given permission by the Nuclear Regulation Authority to operate until 30 November 2036.

However, Kansai has now taken Mihama 3 back offline as it was going to miss the 25 October deadline for completing the antiterrorism measures.

After undergoing safety checks, the utility expects to complete the required facilities around September 2022. It anticipates restarting the reactor in mid-October and for normal operation to be resumed one month later.

In November 2020, unit 1 of Kyushu Electric Power Company's Sendai plant in Kagoshima prefecture was the first Japanese unit to resume operation after building a bunkered back-up control centre.

Kansai suspended operation of units 3 and 4 of its Takahama plant in Fukui Prefecture in August and October 2020, respectively, due to delays in completing back-up facilities at those units. Other utilities have also said there will be delays of up to two-and-a-half years in completion of back-up centres at their reactors.

Researched and written by World Nuclear News