Radiation from the quake-crippled reactors at the Fukushima nuclear power plant has seeped into the surrounding atmosphere. Invisible clouds of low-level fallout traveled as far south as Tokyo, 135 miles away. Radiation levels posed serious health risks closer to the plant, forcing authorities to order 140,000 people to seal themselves indoors while crews worked to stave off the evolving crisis.
A meltdown on the scale of Chernobyl remains unlikely, so far, but Japan's nuclear emergency has surpassed Three Mile Island;in magnitude, according to an International Atomic Energy Agency evaluation. If the fire burning inside Fukushima's reactor spreads to the spent fuel pools, the 2011 Sendai earthquake may well be remembered for having triggered one of history's worst nuclear disasters.