Saturday’s restart of the Kashiwazaki-Kariwa nuclear power plant in Niigata Prefecture is likely to revive public concerns about safety but relieve political concerns harbored by the government and the power industry.

Tokyo Electric Power Co. restarted the No. 7 reactor at2 p.m. by pulling out a control rod to trigger a fission reaction. The reactor reached criticality in 90 minutes, Tepco said.

The restart apparently went smoothly. The test will last for 40 to 50 days before commercial operation resumes and the plan actually begins to generate electricity.

Concerns about safety have been mounting ever since a series of fires broke out at the world’s biggest nuclear power station.

The fires raised safety concerns even higher because they kept occurring despite repeated apologies and safety pledges from Tepco.

The government is pinning its hopes on the restart because it is eager to reduce greenhouse gas emissions in its stated battle against climate change.

“We’ve had to wait a long time” for the restart, Economy, Trade and Industry Minister Toshihiro Nikai said recently about the 22 months since a major earthquake in July 2007 damaged the plant and forced it to close.

“I would like to urge all of those concerned to reflect seriously,” Nikai said, referring apparently to lax management at Tepco, the nation’s largest power utility.

A total of nine fires have occurred at the plant since the quake. The latest, a minor fire in a warehouse, occurred in April after the government gave it permission to restart.

Niigata Gov. Masataka Shimizu, who gave the final nod Friday, demanded the utility “continue to improve” safety measures.

Experts say restarting the nuclear power plant is crucial for the government’s environmental policy of seeking to establish a “low-carbon society.” It is also keen to curb industrial emissions of carbon dioxide and other greenhouse gases as part of its obligation under the 1997 Kyoto Protocol.

Nikai’s frustration underlines the government’s sense of urgency as it seeks to achieve Japan’s the Kyoto emissions targets.

Under the Kyoto pact, Japan is required to cut greenhouse gas emissions during the 2008-2012 period by 6 percent from the 1990 level.

According to government figures, Japan emitted 1.37 billion tons of greenhouse gases in fiscal 2007 through March 2008 — up 2.4 percent from the year before and 9 percent from 1990.

Excluding projected forest absorption of carbon dioxide and emission rights purchased by the government from other countries, Japan needs a net 9.6 percent emissions cut from the fiscal 2007 level by the end of 2012 if it is to achieve its target under the Kyoto protocol.

But Japan would only need a net reduction of 4.6 percent during the same period if output by nuclear power plants is maintained at record-high levels.

Nuclear power production in Japan has been falling due mainly to temporary shutdowns for safety checks. The operating capacity rate of the 53 reactors nationwide declined to around 60 percent in fiscal 2008, according to the Ministry of Economy, Trade and Industry. The rate peaked at 84.2 percent in fiscal 1998.

Amid the slowing output of nuclear plants, many utility firms have boosted thermal power generation to cover shortages in power supply. But this has led to increased use of fossil fuels, such as coal and fuel oil, releasing more carbon dioxide.

Japan must cut a net 120 million tons of greenhouse gases over the next 3 1/2 years to achieve the Kyoto target. But restarting the No. 7 reactor alone will only help cut 4 million tons, the ministry said based on an estimate by the utility.

“Restarting one or two reactors would not help much,” said Tomoko Murakami, nuclear power analyst at the Institute of Energy Economics Japan. “With the current level of nuclear energy output, it will be very difficult to reach the Kyoto goal.”

She added that the total operating capacity rate of the reactors throughout the country needs to remain stable at around 80 to 90 percent if the government is to work out a reliable medium- to long-term strategy for curtailing greenhouse gas emissions.

However, the government believes that even a small step by Tepco could lead to a significant change in the near future.

Even if only one reactor is reactivated it will be “meaningful for us as we are drawing up a road map,” a ministry official said.

If Tepco were to restart all of the seven reactors at the Kashiwazaki-Kariwa plant, it would reduce emissions by 30 million tons, a quarter of the total greenhouse gas emissions cut target under the Kyoto protocol.

Restarting the Kashiwazaki-Kariwa plant could “trigger the revitalization of the nuclear power industry in Japan,” Murakami said.