Japan's Oi nuclear plant's No. 3 reactor attains criticality

After being idled for over 15 months for mandatory checks since March last year, the No. 3 reactor at the Kansai Electric Power Co. Plant in Fukui Prefecture is now slated to gradually increase output and restart power transmission as early as Wednesday.

It is due to go into full operation possibly by next Sunday, Kyodo news agency reported.

The utility serving western Japan continued work to resume operation of the 1.18 million kilowatt reactor to head off power crunch despite a noisy demonstration outside the plant.

It pulled out the reactor's 53 control rods that had contained fission reactions, while gradually reducing the concentration of boron that has the same function in the primary cooling water.

Criticality was reached at 6 am (local time) today.

Senior Vice Minister of Economy, Trade and Industry Seishu Makino witnessed the work as part of the government's efforts to enhance monitoring of the plant's resumption amid public concern over nuclear safety in the wake of Fukushima accident.

Fukui Governor Issei Nishikawa said in a statement that the plant's safe operations as the first facility to resume operations after checkups since the Fukushima crisis will be "the key to regaining confidence in nuclear power that was lost after the accident in Fukushima."

"I expect the utility and the central government to make utmost efforts to ensure safety and power supply to the Kansai region" of western Japan, he said.

Following government approval on June 16, Kansai Electric is also preparing to reactivate the plant's No. 4 reactor as early as July 17 to resume generating power three days later and return to full operation possibly on July 24.

Restarts had put been on hold as the government mulled its options following a 9.0-magnitude earthquake and subsequent tsunami that crippled reactor cooling systems at Fukushima nuclear power plant.

Nuclear power had supplied a third of Japan's electricity needs prior to the Fukushima crisis.