The nuclear threat facing Japan

Policemen wear gas masks and patrol near the nuclear power plant in Fukushima prefecture on March 12, 2011 a day after a massive 8.9 magnitude quake and tsunami hit the region. An explosion sent plumes of smoke spewing from the ageing Japanese nuclear power plant, raising fears of radioactive meltdown a day after the massive quake struck the facility's cooling system. JAPAN OUT RESTRICTED TO EDITORIAL USE AFP PHOTO /YOMIURI SHIMBUN (Photo credit should read YOMIURI SHIMBUN/AFP/Getty Images)

Of all the stories we're following in the aftermath of Friday's earthquake in Japan, perhaps the most worrisome just now comes from the city of Fukushima, north of Tokyo. It's believed a partial meltdown is under way at a nuclear complex there, and that's forced the evacuation of tens of thousands. Celia Hatton has the latest:

Friday's massive earthquake has unleashed a Japanese nuclear nightmare - with the government now saying that a partial meltdown is likely underway at a second of three reactors affected by the massive quake.

All three of the Fukushima reactors lost their cooling ability after Friday's temblor.

On Saturday an explosion blew off the outer enclosure of one of the reactor's shells. It's believed the blast was the result of an effort to cool its overheating core.

Thus far, the reactor core remains intact.

But Edwin Lyman of the Union of Concerned Scientists worries Japan's nuclear emergency is not yet over.

"You could have the fuel overheating and melting and to the extent that you can't recover from the situation, and then the possibility of large scale radiological release exists," he told Hatton.

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Tokyo Electric, the plant's operator, took the unusual step of using nearby seawater to help cool the reactors after an electricity outage caused a cooling system malfunction.

Government officials are protectively evacuating some 170,000 people from a 12-mile area around the plant, and giving iodine to nearby residents to help prevent absorption of fallout.

They're also scanning everyone for radiation exposure.

Some residents are already being treated for unusually high radiation levels.

"I still have people I haven't been able to contact, and there have been reports of a nuclear leak. I'm really concerned about their safety," said one man.

Quake-damaged roads are hampering evacuation efforts. Many have given up driving - hoping instead to get on flights out of the region.

Even before everyone has made it to safety, there are questions about the future of Japan's nuclear energy program. CBS News has confirmed the damaged nuclear reactor was scheduled to be mothballed in just a few weeks.

For decades, critics have worried that an earthquake-prone country like Japan is the wrong place for 54 active nuclear reactors. The trouble at Fukushima is all but certain to raise the level of concern.