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.