Radiation inside the reactor surged to 1,000 times its normal level after Friday's earthquake knocked out power to its cooling system, and the tsunami floods have hampered efforts to get it restored.
Heat-induced pressure built up inside the crippled reactor after the reactor lost power, automatically shut down and the cooling system failed. The situation at the reactor and the four others with compromised cooling systems prompted officials to declare nuclear emergencies.
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.
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."
The Kyodo News Service reported Friday 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.
Meanwhile, officials performed a controlled release of some slightly radioactive 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."
ABC News' Devin Dwyer and Leezel Tanglao contributed to this report.