The plant is 120 miles southwest of Tokyo, about 40 miles closer than the Fukushima Daiichi plant, which has been spewing radioactive material since a magnitude-9.0 quake and a tsunami devastated the country’s northeast coast on March 11.
Safety activists say the problems at the plant include inadequate protections against tsunamis; its operator has said it relies on sand dunes to block waves.
Mr. Kan’s decision to suspend operations at the Hamaoka plant was made relatively quickly by the standards in Japan, where leaders generally prefer to build a consensus before announcing such policy changes.
But Mr. Kan has faced withering criticism for his handling of the Fukushima disaster, including accusations that a slow government response in the early hours of the crisis made it worse.
In explaining his decision on Friday, he said it was “the result of considering the tremendous repercussions a major accident at Hamaoka would have on the entire Japanese society.”
The government’s own experts have estimated that there is a close to a 90 percent chance of an earthquake of about magnitude 8.0 hitting the area within the next 30 years. Much less severe earthquakes have damaged at least one nuclear plant in Japan in recent years.
In 2009, Hamaoka’s operator, the Chubu Electric Power Company, decommissioned the site’s two oldest reactors after deciding that upgrading them to withstand earthquake risks would be too costly. Those reactors were built in the 1970s, around the same time as those at the Fukushima Daiichi plant.
The power company argued that the remaining three reactors, built in the 1980s, were safe enough to withstand a major earthquake. But the crisis at the Fukushima plant has heightened concerns over the risks posed by quakes and tsunamis to the nearly 50 nuclear reactors in operation in Japan.
Mr. Kan said that Chubu Electric would be asked to strengthen its defenses before a restart could be authorized. The company said that constructing an adequate sea wall would take about two years.
“More than anything, I have the safety and security of the Japanese people in mind,” Mr. Kan said.
To brace for any shortfall in electricity, he called for Japanese citizens to do even more to conserve energy.
Offices and factories in eastern Japan have already dimmed lights and altered operating hours after the government requested that they reduce their energy use by 15 percent this summer. The three operating reactors at Hamaoka have a combined capacity of about 3,500 megawatts, or about 7 percent of Japan’s nuclear power generating capacity.
“I am certain that with an even stronger energy-saving effort by both local residents and the entire Japanese nation, we will overcome the power shortage,” he said.
One of the three remaining reactors at Hamaoka had already been shut down for scheduled inspections. The company released a statement late Friday saying it would “promptly consider” Mr. Kan’s request.
Environmental groups, including Greenpeace, quickly lauded Mr. Kan’s decision and urged the Japanese government to consider halting operations at all of the reactors in Japan, which is prone to earthquakes.
“Greenpeace welcomes Prime Minister Kan’s request to close Hamaoka, one of the most dangerous nuclear reactors in Japan,” said Junichi Sato, the organization’s executive director in Japan.
“The government must continue to close and decommission existing plants and cancel all new reactors,” he said. “Only then can the Japanese people feel their government is truly putting their safety first.”
But Katsuhiko Ishibashi, a seismologist at Kobe University who has long argued for more stringent seismic standards for Japan’s nuclear power plants, said that simply halting operations at the Hamaoka plant was not enough. The reactors would remain vulnerable to quakes and tsunamis while they cooled down, he said, as would the spent fuel rods stored at the plant. “Work must be carried out immediately to make sure the plant is protected,” he said.
He also said that the government had moved too slowly in recognizing the risks posed by earthquakes to the country’s nuclear reactors. In 2006, Mr. Ishibashi urged a government committee on which he served to adopt more stringent earthquake guidelines for nuclear power operators. He later resigned when his calls were ignored.
“If Japan had faced up to the dangers earlier, we could have prevented Fukushima,” he said.
At the Fukushima plant, workers have been struggling to bring the plant’s damaged reactors and fuel rod pools under control since the March 11 tsunami knocked out all power to the plant and shut down crucial cooling systems. Three of the reactors subsequently overheated and suffered hydrogen explosions. The plant is operated by the Tokyo Electric Power Company, known here as Tepco.
Tepco has said it will take at least six to nine months to bring all of the plant’s reactors to a stable state known as a cold shutdown.
Another nuclear plant operated by Tepco, the Kashiwazaki-Kariwa nuclear power station on the coast of the Sea of Japan, was damaged in a magnitude-6.6 earthquake in 2007. Fire broke out at one of the reactors, though it was quickly put out, and Tepco officials said there had not been a widespread release of radioactive material.
The plant was shut down for almost two years for repairs and inspections. Since then, four of its seven reactors have been restarted.
