A map of radiation-contaminated areas around Fukushima: Radiation from the accident has spread dozens of miles, but most of the area (the blue) is still showing very low levels ("low, but not insignificant," according to the National Nuclear Safety Administration.)Courtesy NNSAThe tsunami- and earthquake-caused disaster at Japan’s Fukushima nuclear power plant has been upgraded to a 7, a “major accident.” 7 is the highest (worst) rating on the International Nuclear Events Scale, and Fukushima and Chernobyl are the only two events to have received the rating. As Liza pointed out, it should be said that Fukushima has so far only released about a tenth of the radiation released from the Chernobyl disaster.

While there were explosions in the reactor housing at Fukushima in the days after the earthquake, none of them were on the scale of Chernobyl, where the reactor itself exploded, and where graphite fires continued to burn for days, spreading huge quantities of radioactive materials long distances. At Fukushima, the vessels containing the reactors are largely intact, and the plant is not producing tons of radioactive smoke.

Still, where authorities seemed to have been giving overly optimistic assessments of the situation before, now it looks like they’re doing a better job of acknowledging potential worst-case scenarios—since the plant is still releasing radioactive materials, plant operators say that it could eventually equal Chernobyl in terms of radioactive output.

The situation was upgraded to a seven after officials extended the evacuation area around the plant from a twelve-mile radius to a region that includes five towns, home to tens of thousands of people. The affected region isn’t a perfect bull’s-eye, like you might see on some maps—the radiation is spreading unevenly due to wind and the shape of the land, meaning some areas will be less affected, while others are receiving a greater dose of radiation. The map posted above shows the contamination as of April 3—radiation has spread dozens of miles, but most of it (in the blue) is still at very low levels. The units are for radiation absorbed in an hour. As a reference point for those numbers, the Nuclear Regulatory Commission estimates that the average American absorbs about 620 mRem a year, which equals about 0.071 mRem/hr.

Authorities are saying that it will take months to totally get the reactors under control, and years to clean up the plant and surrounding area. No one has died from acute radiation exposure (although several workers have been hospitalized), but it could be months or years before the impact on those exposed reveals itself.