(Kyodo, 02/07/12) – Reactor 3 at the Oi power plant in Fukui Prefecture attained a self-sustaining chain reaction, or criticality, early Monday after being restarted the previous night, becoming the first in the nation reactivated since the Fukushima meltdown crisis effectively shut down the nuclear power industry.

Kansai Electric Power Co.’s reactor 3 in Oi, Fukui Prefecture, was taken offline for mandatory checks in March last year. It is now slated to gradually increase output and resume power transmission as early as Wednesday, and reach full operation Sunday.

The utility is firing up the 1.18-million-kw reactor to help supply power this summer, despite the presence of hundreds of protesters there since Saturday.

The reactor’s 53 control rods, which moderate the fission, were pulled out from 9 p.m. Sunday through early Monday, while the concentration of boron, which has the same function in the primary cooling water, was gradually reduced. Criticality was reached at 6 a.m. Monday.

Senior Vice Minister of Economy, Trade and Industry Seishu Makino witnessed the work as part of efforts to enhance monitoring amid high public concern about the safety of nuclear power since three cores melted at Tokyo Electric Power Co.’s Fukushima No. 1 plant last year.

Kepco is also preparing to restart reactor 4 on July 17 with an eye to full operation on July 24.

Nuclear power had been put on hold as the government mulled its options after the 9.0-magnitude earthquake and tsunami led to the world’s worst nuclear crisis since Chernobyl and renewed fears that more giant quakes were on the way.

But on June 16, Prime Minister Yoshihiko Noda gave the green light to restarting the two Oi reactors to head off a summer power crunch amid warnings of a huge electricity shortfall.

Prior to Fukushima, nuclear power supplied almost a third of the nation’s electricity needs.

A series of minor glitches have been reported at the Oi plant since June 16, with alarms going off at locations where the plant receives external power and on equipment used to monitor power transmission lines.

The government had set an energy-saving target of up to 15 percent from 2010 levels for the summer in Kepco’s service area until the Oi reactors resume.

Sunday’s antinuclear protest followed a rally outside the plant Saturday night by around 650 people, reports said.

“If the reactor is reactivated . . . other reactors will be restarted in quick succession,” said 40-year-old designer Ikuyo Hattori, who came with her two children.

“We can’t accept a forcible restart when the Fukushima crisis hasn’t been settled,” she added.

In the worst nuclear accident since the 1986 Chernobyl disaster, radiation spread over homes and farmland in a large swath of the northeast in March 2011.

Nobody is officially recorded as having died as a direct result of the meltdowns, but tens of thousands were evacuated and many remain so, and some areas will be uninhabitable for decades.

On Friday, tens of thousands of people gathered on streets outside the prime minister’s office, with organizers estimating the turnout at up to 180,000.

Police estimates were around 20,000, according to media reports. No explanation was given for the large disparity.