Unit 4 of the Ohi nuclear power plant in Japan's Fukui Prefecture began supplying electricity to the grid again on 11 May, Kansai Electric Power Company has announced. The reactor is expected to re-enter commercial operation early next month.

Ohi units 3 and 4 (Image: Kansai)

Following the shutdown of all of Japan's reactors after the March 2011 accident at the Fukushima Daiichi plant, Ohi 3 and 4 were given permission to resume operation in August 2012. However, the two 1180 MWe pressurised water reactors were taken offline again for Nuclear Regulation Authority (NRA) inspections in September 2013.

The NRA announced in May 2017 that the two units meet safety standards introduced in July 2013. The NRA approved Kansai's plan for strengthening the units in August last year. The regulator subsequently conducted pre-operation inspections of the units to confirm that the safety countermeasure equipment complies with the approved construction plan at the plant. The governor of Fukui Prefecture approved the restart of Ohi units 3 and 4 in November.

Kansai loaded 193 fuel assemblies into the core of Ohi 3 in February. The reactor was restarted on 14 March and attained criticality - a sustained chain reaction - the following day. The unit returned to commercial operation on 10 April.

Fuel loading was completed at unit 4 in early April. The reactor was restarted on 9 May and reached criticality the following day. Kansai announced on 11 May that the unit had begun power generation at 5.00pm that day.

In a statement, Kansai President and Director Shigeki Iwane said: "It is planned to increase the generator output in a stepwise manner while confirming the plant status at each power output. We will continuously give sincere and deliberate support to the subsequent inspections to be performed by the NRA."

Ohi 4 is the eighth of Japan's 39 operable reactors which have so far cleared inspections confirming they meet the new regulatory safety standards and have resumed operation. The others are: Kyushu's Sendai units 1 and 2 and Genkai unit 3; Shikoku's Ikata unit 3; and Kansai's Takahama units 3 and 4. Another 17 reactors have applied to restart.

Kyushu Electric Power said today that it had withdrawn its application to the NRA for test operation of unit 4 at its Genkai plant in Saga Prefecture. The company applied on 25 April to carry out test operation of the reactor, necessary for the completion of final inspections ahead of its restart. However, Kyushu company said today it had "temporarily stopped the start-up process" to check the seal on the primary coolant pump.