Emergency officials ordered the evacuation Saturday of all civilians within a six-mile radius of the Fukushima No. 1 plant, which is about 150 miles northeast of Tokyo, after its normal backup cooling systems failed and it became necessary to release radioactive steam to relieve pressure that could cause an explosion.
Several hours later, authorities revealed that cooling systems at the Fukushima No. 2 plant, a few miles south, had also failed, and evacuations were ordered around that plant as well.
In all, five reactors at the two plants were damaged.
Radiation levels in the control room at Fukushima No. 1 were reported to be as high as 1,000 times normal, while levels outside the plant were said to be about eight to nine times normal, indicating some leakage of radiation had already occurred.
Shaking from the magnitude-8.9 earthquake triggered an automatic safety procedure at 11 of Japan's 55 commercial nuclear reactors. At those plants, control rods were plunged into the cores, where the radioactive fuel rods are kept, to bring electricity production to a halt.
But the cores continued to produce massive amounts of heat that needed to be cooled.
The cooling system must continually pump water from a large pond that surrounds the core through a set of towers that keep the water at a safe temperature. Otherwise, the water will boil off, the fuel rods will melt, and there is a possibility that radioactive material will escape from the reactor's containment dome — a so-called meltdown.
"If they can't get adequate cooling to the core, it could be a Three Mile Island or worse," said nuclear physicist Edwin Lyman of the Union of Concerned Scientists, which is working to improve the safety of nuclear power. The loss of coolant at the Three Mile Island Nuclear Generating Station in Pennsylvania for only 30 minutes led to a 50% meltdown of the core in that 1979 accident.
The pumps normally obtain electricity from the grid. But the Fukushima prefecture's grid was badly damaged by the tsunami, cutting power needed to cool the fuel rods. Though reactors have diesel emergency generators to provide backup electricity, those at Fukushima No. 1 and No. 2 also were apparently damaged by the tsunami.
The reactors have backup batteries, and operators used them to restore the flow of coolant at the No. 1 plant. But those batteries have a life of no more than about eight hours, experts said.
Officials of Japan's Nuclear and Industrial Safety Agency said Saturday they had obtained more batteries and may use military helicopters to fly them to the site.
Authorities said that pressure had already built up inside the containment building at the No. 1 plant to about 50% above normal and that they had begun venting radioactive gas into the atmosphere, although they said there was no danger from the release.
"Rising containment pressure is not a good sign that they have things under control," Lyman said. "We need to get better information from the Japanese about what is going on, but the few things that are coming out are very worrisome."
The reactors at the two plants use some of the oldest nuclear technology, dating to the 1960s. "These first-generation boiling-water reactors have the least margin of safety of any reactor design," said Frank N. von Hippel, a Princeton University physicist and former White House advisor.
Without electrical power to circulate water inside the core, the cooling water would begin to boil off, he said.
But despite the dangers, some experts said the nuclear plant operators should be able to add new cooling water and keep the core fully immersed while it cools down.
"It doesn't sound like we are in meltdown mode," Von Hippel said.
Jim Walsh of MIT's Center of International Studies agreed that these reactors will probably be OK. But there are other facilities in Japan that produce enriched fuel for reactors and manage highly radioactive waste, some of which are in remote areas in the north, "and no one has said 'boo' about them," he said. "It's not inconceivable that some of them have had problems. The story may continue to unfold in the next few weeks."
Long before the problems created by the tsunami, a series of serious incidents in Japan's ambitious civilian nuclear power industry over the last decade have raised concerns about its attention to safety and the role of government regulators.
The history of Japan's nuclear incidents includes a pattern of problems being kept secret or passed off as far less serious than they actually were.
In 2007, several electric utilities admitted covering up accidents, including one that experienced an uncontrolled nuclear reaction inside a reactor while it was shut down for maintenance. The same year, Tokyo Electric Power Co. — which operates the two Fukushima plants — apologized for a radiation leak caused by an earthquake at a plant that had not been built to withstand a quake of that magnitude.
In 2004, five workers at a nuclear plant in western Japan were killed when a corroded pipe burst and sprayed them with boiling water and steam, revealing flaws in safety procedures.
The U.S. Nuclear Regulatory Commission said Friday that American nuclear reactors are not vulnerable to the sequence of events that overtook the Japanese reactors because regulations here take into account the specific vulnerabilities at each plant under the most extreme conditions possible.
In the case of the San Onofre Nuclear Generating Station, operated by Edison International in northwestern San Diego County, a 30-foot seawall protects the plant and its emergency generators from the maximum theoretical tsunami that could hit the site, said regulatory commission spokesman Scott Burnell.
Edison International spokesman Chris Abel said the seawall is just one of several redundant systems meant to ensure power for an orderly shutdown in the event of an emergency.
In Japan, the shutdown of the 11 nuclear reactors affects perhaps 8% of the country's electrical generating capacity. Most of those plants should be able to go back on line within a few days, though it may be longer before repairs can be made to damaged power lines that carry electricity from the plants to consumers.