Explosion At Fukushima Nuclear Power Plant
The word from officials is that everything is Ok. The steel shell around the core is still intact.
Just guessing here... But if all of the cooling mechanism is gone, there is nothing to prevent the core from melting down. Reactor cores require constant cooling to prevent them from melting down.
In the video he discusses applying pressurized water to keep it cool. IF the core is exposed, there is nothing to pressurize the water in. The water will be turned to radioactive steam and jet back out.
This nuclear power plant is almost in the center of Japan. If this blows, will it be necessary to evacuate the entire nation of Japan?
BBC Podcast on this disaster.