Here's a look at where things stand.
A buildup of hydrogen in the disabled cooling system of one of the reactors at the Fukushima No. 1 (Daiichi) plant about 150 miles north of Tokyo. The explosion damaged the building the houses the reactor and its cooling system. However, the reactor containment vessel, which houses the radioactive fuel rods, remained intact, officials told the International Atomic Energy Agency, the U.N.'s nuclear monitor.
Photos: Scenes of destruction after the earthquake
Was there a meltdown?
Not according to Ichiro Fujisaki, Japan's ambassador to the United States, who spoke on CNN. But other officials said two of the reactors at the plant may have experienced meltdowns. Engineers have not been able to get close enough to the plant to rule that out, said Toshihiro Bannai of Japan's Nuclear and Industrial Safety Agency.
"We see the possibility of a meltdown," he said on CNN.
How serious is it?
The explosion itself does not appear to have caused radiation to leak into the environment. But experts said the problems stemming from Friday's magnitude 8.9 earthquake and the resulting tsunami rank as the third most severe nuclear accident in recent history, following the 1986 Chernobyl disaster and the 1979 accident at Three Mile Island.
Water from the tsunami disabled diesel generators supplying power to emergency cooling systems for five reactors at the Fukushima No. 1 plant and a nearby plant known as Fukushima No. 2 (Daini). Without power from the generators, the cooling system pumps cannot circulate cool water around the radioactive fuel rods. As a consequence, the water will boil off and the fuel rods will overheat and melt, which could lead to a massive escape of radiation. Already, some radiation has apparently escaped from one reactor at the No. 1 plant, as well as some hydrogen, which was the source of the explosion.
Authorities said the cooling system at a sixth reactor, located at Fukushima No. 1, broke down Sunday morning.
Why were people evacuated?
Officials ordered the evacuation to protect civilians in the event that radiation leaked from the plants.
"You want to err on the side of being cautious," Chesser.
But the decision to evacuate is not one that officials take lightly, since putting so many people on the roads at once might raise the risk for traffic accidents, said Peter Bradford, former commissioner of the U.S. Nuclear Regulatory Commission.
Have civilians been exposed to radiation?
Yes, though not many, officials say. Japanese officials have scanned thousands of evacuees for radiation exposure, and at least nine of them have tested positive, although they do not appear to have any immediate health problems. Japan's Nuclear and Industrial Safety Agency estimated that as many as 160 people may have been exposed.
The threat to civilians was diminished by the evacuation, and the fact that the prevailing winds at the site generally blow out to sea. They have been dispersing the small amount of radiation that has already been released.