Nightly News   |  March 12, 2011

Meltdown fears at damaged nuclear reactor

Of all the aftershocks that could hit Japan, nothing frightens the world more than the possibility of a devastating nuclear disaster. NBC’s Anne Thompson.

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This content comes from Closed Captioning that was broadcast along with this program.

>> that. japanese officials say they have kal cau lighted that 160 people have been exposed to radio activity . 160,000 people have been evacuated from around two nuclear plants most from the one that suffered the explosion. more on this from nbc's anne thompson .

>> reporter: nothing frightened the world more than this. an explosion at the troubled fukushima one power plant . the japanese government declared emergency when the systems failed. now fear of a meltdown. throughout the day japanese tv reporters used diagrams and maps to help a near versus nation. the blast explosioned the ex exterior building and blamed the explosion on a buildup of hydrogen. this official said after the explosion the radiation leaking from the plant actually decreased as did the pressure.

>> i think they are being cautious and conservative and given the conditions there, i think that is wise.

>> reporter: when the earthquake struck, the reactor shut down and systems failed. without water to cool it, the core could over heat and melt down releasing radio activity into the environment. tonight, the japanese utility took the unusual step of pumping sea water into the reactor to prevent a melt down.

>> if you do so, you have decided that the consequences of not doing so are very serious. the evidence in this case is that they are worried about significant melting of the core.

>> reporter: more signs of concern. people living within the evacuation zone are scanned for radiation and the interational atomic energy agency said japan is preparing to give iodine to those living near the plant.

>> reporter: this was reported on a scale of 1 to 7. at pennsylvania's three mile island, there was a partial core melt down but not a significant release of radiation. japan now wait to see if it its fate is the same. anne thompson .