Tokyo - A strong aftershock off Japan’s Pacific coast on Monday briefly set off a tsunami warning and knocked out cooling at the crippled Fukushima Daiichi nuclear power plant for almost an hour, underscoring the vulnerability of the plant’s reactors to continuing seismic activity along the coast a month after the devastating March 11 earthquake and tsunami.
The Japanese government said on Monday that it was preparing to expand the evacuation zone around the nuclear facility to address concerns over long-term exposure to radiation; the announcement was made before the aftershock struck.
The United States Geological Survey measured the aftershock’s magnitude at 6.6, only about one hundred-thousandth the strength of the March 11 quake, and the tsunami warning was lifted after 45 minutes when no sizable waves were detected. Still, the shake left 220,000 homes in three prefectures in the areas without power and caused a spate of injuries.
It also knocked out the external power supply to the Fukushima Daiichi plant, temporarily stopping pumps there from sending cooling water into the facility’s three most severely damaged reactors, according to Japan’s nuclear regulator. The tsunami warning also obliged workers at the plant to evacuate temporarily.
Using emergency pumps to cool the nuclear fuel rods within the reactors and in spent-fuel pools above the reactors has been a top priority for the operator of the plant, Tokyo Electric Power Company, since the March 11 quake and tsunami damaged the reactors’ usual circulation systems. But Monday’s aftershock appeared to have exposed a big vulnerability in that approach.
The backup power and pumping systems that have been brought to the plant since March 11, including emergency diesel generators, fire trucks on standby and other generator trucks — all require workers to operate them manually, according to the Nuclear and Industrial Safety Agency. That makes them useless when workers must evacuate away from the reactors.
Moreover, pumping hundreds of tons of water a day into the reactors has produced harmful runoff of highly contaminated water, some of which leaked into the Pacific Ocean earlier this month. Plant workers have now plugged that leak, and are capturing the runoff in various storage tanks at the plant. However, as the tanks fill up, Tokyo Electric has had to release lower-level radioactive water into the ocean to make room.
Monday’s quake was shallower than most of the previous aftershocks — only eight miles down, half the depth of the 7.1-magnitude temblor last Thursday. Its epicenter was farther south than earlier aftershocks; it is the most powerful recorded so far between Fukushima and Tokyo striking at a point about 50 miles south of Fukushima and 101 miles north-northeast of Tokyo.
Yukio Edano, the government’s chief cabinet secretary, said in the capital on Monday that the government would order parts of five villages and cities that are outside the current zone to prepare to evacuate. The fear is that these areas are being exposed to radiation equivalent to at least 20 millisieverts a year, he said, which could be harmful to human health over the long term. Evacuation orders will come within a month for Katsurao, Namie, Iitate and parts of Minamisoma and Kawamata, Mr. Edano said.
People in five other areas may also be told to evacuate if there is a worsening of the crisis at the Fukushima Daiichi station, Mr. Edano said. Those areas are Hirono, Naraha, Kawauchi, Tamura and other sections of Minamisoma.
“This measure is not an order for you to evacuate or take actions immediately,” he said. “We arrived at this decision by taking into account the risks of remaining in the area in the long term,” he added, appealing for calm.
The incremental ways in which the government has ordered evacuations from around the Fukushima complex has caused confusion on the ground.
In a different directive issued last month, residents within a 30-kilometer (18-mile) radius have already been advised — though not ordered — to evacuate, overlapping with some of the communities ordered to prepare for evacuation on Monday. Mr. Edano also said on Monday that pregnant women, children and hospital patients should stay out of the 30-kilometer radius, and that schools in that zone would remain closed.
The Japanese government had so far refused to officially widen the zone, despite being urged to do so by the International Atomic Energy Agency. Countries like the United States and Australia have advised their citizens to stay 50 miles (80 kilometers) away from the plant.
Mr. Edano said on Monday that the chance of a large-scale radiation leak from the Fukushima Daiichi plant had, in fact, decreased.
Michael Friedlander, a former senior nuclear plant operator for 13 years in the United States and a specialist in emergency responses to nuclear accidents, said that the Japanese decision to evacuate more areas made sense not just in terms of protecting their residents but also in terms of making easier the eventual decontamination of farms and communities.
Allowing people and non-emergency vehicles to continue moving through both radiation-contaminated areas and safer areas farther from the Fukushima Daiichi reactors runs the risk of spreading around radioactively contaminated particles, which could result in more square miles of territory ultimately being contaminated. “Unless you gain control, it will be like trying to mop your kitchen floor with the kids running in and out of the house,” Mr. Friedlander said.
The I.A.E.A., based in Vienna, said on Sunday that its team had measured radiation on Saturday of 0.4 to 3.7 microsieverts per hour at distances of 20 to 40 miles from the damaged nuclear reactors — well outside the earlier evacuation zone. At that rate of accumulation, it would take 225 days to 5.7 years to reach the Japanese government’s threshold level for evacuations: radiation accumulating at a rate of at least 20 millisieverts per year.
In other words, only the areas with the highest readings would qualify for the new evacuation ordered by the government.
But the former Soviet Union used a lower threshold — 5 millisieverts per year — in eventually offering resettlement to people who lived near the Chernobyl reactors in 1986. At the rates of radiation accumulation identified by the I.A.E.A., the new evacuation areas in Japan would take 56 days to 17 months to reach this level.
Across Japan at evacuation centers, at work and on the street, people observed a moment of silence at 2:46 p.m. to mark the passage of exactly one month after the March 11 quake.
“We will not let the tsunami defeat us,” the mayor of the coastal city of Miyako, overrun by waves that measured as high as 125 feet, said in a message broadcast across the city through emergency speakers on Monday.
In the city, a port famous for its salmon trade about 370 miles north of Tokyo, about 400 people have been confirmed dead from the March 11 disaster and 682 remain missing. Almost 3,500 people from Miyako remain in evacuation centers after losing their homes. “If we keep up our courage and hope,” said Mayor Masanori Yamamoto, “Miyako will surely recover.”
The overall death toll from the disaster, Japan’s worst since World War II, has surpassed 13,000, with more than 14,000 people still missing, according to Japan’s National Police Agency. The figures include the casualties from the March 11 quake and tsunami, as well as the several aftershocks that have since jolted northeast Japan.
Officials have said that many people may never be accounted for, because they were washed out to sea or buried under mountains of rubble. More than 150,000 people overall remain housed in emergency shelters, the national broadcaster NHK said.
Reporting was contributed by Moshe Komata, Kantaro Suzuki and Ken Ijichi.
This article "Strong Aftershock as Japan Urges More Evacuations" originally appeared at The New York Times.