"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.