The New York Times has an instructive graphic showing exactly what happens during a partial and full meltdown in a nuclear reactor.
Radiation levels spiked dramatically today after a new explosion at the Fukushima plant, which workers have been struggling to gain control of through flooding reactors with seawater. Over four days, three hydrogen explosions have rocked different reactors at the plant.
So far, most U.S. politicians have avoided calls for a slowdown of nuclear power plant development in the country, even as Swiss and German officials declared temporary moratoriums.
(Screenshot: New York Times graphic.)