Radiation inside a Japanese nuclear reactor surged to 1,000 times its normal level after today's 8.9-magnitude earthquake knocked out power to a cooling system, and tsunami floods have hampered efforts to get it restored.
It reportedly was one of five Japanese nuclear reactors that lost cooling ability, prompting a race against the clock to install fixes.
In the worst case, at the Fukushima Daiichi power plant's No. 1 reactor, heat-induced pressure built up inside the crippled reactor, prompting widespread evacuations within a 10 kilometer radius and stoking fears of a potentially catastrophic radioactive event.
Officials declared a "nuclear emergency" at the plant, about 200 miles northeast of Tokyo, amid the cooling system failure after the No. 1 reactor lost power and automatically shut down.
Later, officials announced cooling ability also had been compromised at a second reactor at the site and in three of four reactors at the nearby Fukushima Daini plant, the Associated Press reported. Both plants are operated by the Tokyo Electric Power Co.
There was an evacuation order in effect for residents living within a mile of the Daini plant.
Scientists said that even though the No. 1 reactor at the Fukushima Daiichi site, in particular, had stopped producing energy, its fuel continued to generate heat and needed steady levels of coolant to prevent it from overheating and triggering a dangerous cascade of events.
"You have to continue to supply water. If you don't, the fuel will start to overheat and could melt," said Edwin Lyman, a senior staff scientist in the Global Security program at the Union of Concerned Scientists in Washington, D.C.
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However, U.S. nuclear experts told ABC News late Friday ET that they were regaining optimism as a high-level Japanese nuclear official told them that water levels were stabilizing.
On the other hand, a meltdown could lead to a breach of the reactor's steel containment vessel and allow radiation to escape into an outer, concrete containment building, or even into the environment.
"Up to 100 percent of the volatile radioactive Cesium-137 content of the pools could go up in flames and smoke, to blow downwind over large distances," said Kevin Kamps, a nuclear waste specialist at Beyond Nuclear, which is an advocacy group that opposes nuclear weapons and power.
"Given the large quantity of irradiated nuclear fuel in the pool, the radioactivity release could be worse than the Chernobyl nuclear reactor catastrophe of 25 years ago."
Japanese officials said radiation had not yet leaked from the Fukushima Daiichi plant, but ordered thousands of people living around the facility to evacuate their homes as a precaution.
The Kyodo News Service has reported, however, that some radioactive material already may have escaped, citing reports from the Japanese Nuclear Safety Agency that radiation levels outside the plan have been eight times the normal level. Experts said that level of exposure is not dangerous to the general population.
"You've got to take all potential precautions," President Obama told reporters Friday when asked about vulnerability of the Japanese nuclear power facilities. "And I've asked Steve Chu, our energy secretary, to be in close contact with their personnel to provide any assistance that's necessary, but also to make sure that if in fact there have been breaches in the safety system on these nuclear plants, that they're dealt with right away."
Experts say cooling the reactor's core to minimize pressure inside the containment structure is a top priority. Japanese authorities have been trying to connect diesel-powered generators to restore the water pumps inside the reactor but have been hampered by the floods.
"If you have something that generates heat and you don't cool it off or release the steam, you're in trouble," said nuclear consultant Mycle Schneider, who compared the situation to a pressure cooker.
The risk is a rapid rise in heat that would leave the core uncovered.
"If it's not covered with water, it can start melting very quickly," Schneider said.
Meanwhile, officials planned to perform a controlled release of some slightly radiocative vapor that has been building up inside the containment structure, the Associated Press reported.
The release would allow harmful material to escape into the environment, but not at levels as great as if there was a massive containment failure, Lyman said.
"It's good they evacuated -- let's put it that way," Lyman said. "All indications are that this is a very serious event."
U.S. nuclear experts say modern power plants are designed to withstand earthquakes and tsunamis and have several security layers in place in the event of lost power, including diesel fuel generators and battery systems.
"There are multiple redundancies to continue to feed water to the core to take the heat away at most facilities," said an official with the U.S. Nuclear Regulatory Commission, who asked not to be named because he is not familiar with details of the Fukushima plant.
But those back-up power sources may not have worked in this case, a development many international experts called troublesome.
"The Japanese are considered the best in the world," said Schneider. "They had several generators in place in case one of them doesn't work. This is the first time I've heard of where none of them worked. To me, that is a very deep concern."
Other Japanese nuclear plants also appeared compromised by the earthquake and tsunami.
Besides the loss of cooling systems at three of four Fukushima Daini reactors, the turbine building at the nearby Onagawa nuclear power plant burst into flames shortly after the earthquake, though it later was extinguished.
The International Atomic Energy Agency said it was closely monitoring the situation at the four Japanese nuclear power sites impacted by the earthquake and confirmed that all had been successfully shut down.
"It's a positive sign," Mitch Singer of the Nuclear Energy Institute, a U.S. industry trade group, said of initial reports of the power plants' performance and durability following the quake. "This industry more than all others depends on the safe operation of the plant, and it appears these robust facilities have operated as they were designed to do."
Japanese nuclear power plants have been tested repeatedly by earthquakes in recent years and operated effectively, according to the World Nuclear Association.
Worldwide, 20 percent of nuclear powerplants operate in areas of "significant seismic activity," according to the association.