Japanese authorities say that it is "highly possible" that the damaged reactors at Fukushima Daiichi's unit 1 is melting down while they are proceeding on the assumption that unit 3 has melted down as well.

Washington Post:

Japan's Kyodo News reported that authorities began evacuating more than 450,000 residents from a 12.5-mile radius around Fukushima Daiichi and another nuclear power complex, made preparations to distribute potassium iodide pills, and warned people in the vicinity to stay inside and cover their mouths if they ventured outdoors.

Federal safety agency officials said that as many as 160 people had been exposed to radiation from the plants. "Only the gravest danger would justify an evacuation at such a moment," said Peter Bradford, a former commissioner at the U.S. Nuclear Regulatory Commission.

It was not the earthquake that caused this potential catastrophe, but rather the Tsunami that followed. When the earthquake hit, the reactors automatically shut down. But the fuel rods continued to generate tremendous heat. A loss of power off the grid triggered the operation of diesel generators that continued to supply power to the pumps that were furiously trying to cool off the reactor.

But it is believed - not confirmed - that water from the subsequent Tsunami severely damaged the generators, thus throwing the plant on to battery power. That power supply was limited and eventually went out for about 2 hours during which time pressure built up in the reactor as tremendous volumes of steam coming from exposed fuel rods threatened to blow up the reactor and the containment building. One containment building did blow up but the reactor is apparently still intact - for now.

Officials say there are 5 different reactors at risk:

Another indication that the fuel rods in Fukushima Daiichi unit 1 were exposed is that Japan's Nuclear and Industrial Safety Agency (NISA) said Saturday that the reactor could be nearing a meltdown and that two radioactive substances, cesium and radioactive iodine, had already been detected nearby.

The explosion also rattled public confidence, sparking a run on bottled water in Tokyo.

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Meanwhile, Tokyo Electric was still trying to get control over reactors at the second complex, Fukushima Daini. A water condensate system used to supplement the cooling system at Fukushima Daini unit 1 stopped working when temperatures reached 100 degrees Celsius.

Tokyo Electric also announced that it would carry out controlled releases to ease pressure in the containments of all four units at Fukushima Daini.

Nuclear safety experts were seeking answers to other questions about Japan's nuclear facilities that have been obscured by the focus on the Fukushima reactors. The nuclear plants also have spent fuel pools that some experts say may have spilled during the earthquake and its aftershocks. Tokyo Electric has not commented yet on those pools, which in the case of the GE-designed reactors are located on the roof, possibly making them vulnerable.

Similar pools are found at other nuclear power plants around the country.

No use speculating how bad it could get. Chernyobl damaged an area of hundreds of square miles, making much of it uninhabitable (although several thousand people still live in areas that are considered dangerous). The design of the reactors has apparently prevented an even more dangerous situation. The wind is blowing out to sea - for the moment - which has spared the population from being exposed to elevated levels of radiation.

But no one knows at this point how the runaway nuclear reaction can be stopped except by continuing to pour massive amounts of water on the core of the reactor. Trying to slow the flow of neutrons by adding boric acid to the water will help some, but it may be days before the reaction can be brought under full control.