Hydrogen gas exploded at one of the reactors. Japanese reactors are built to tolerate severe ground shaking. The earthquake of a magnitude of 8.9 was too much. At the Dai-ichi plant, workers attempt to cool down the reactors that are overheated. The reactors had to be flooded with seawater, which creates steam and pressure that had to be vented. This released small amounts of radiation. 180,000 people were evacuated from the areas around the plants, not knowing how much radiation people were exposed to. None of the reactors were close to the point of meltdown.
http://www.studentnewsdaily.com/daily-news-article/japan-works-to-contain-nuclear-reactor-meltdowns/
