After an early-morning explosion at the Fukushima Daiichi Nuclear Power Station damaged one reactor's steel containment vessel and spurred the removal of emergency workers, officials expanded the evacuation zone and braced for "much larger emissions of radioactive materials." How bad are we talking? "It's way past Three Mile Island already," physicist Frank von Hippel tells The New York Times.
Those hoping for good news should look elsewhere: Earlier reports that the explosion damaged the No. 2 reactor's steel containment vessel have been confirmed, making the explosion far more critical than the two that hit reactors No. 1 and No. 3 over the weekend.
Reactor No. 2 is now the worst-off of the plant's three active reactors, having experienced some amount of melting fuel, which would lead in turn to more radioactive material during controlled releases of steam; the damage to the containment vessel could lead to "uncontrolled release of radioactive material and full meltdown of the nuclear fuel inside." This could be made far more likely in absence of emergency workers:
If all workers do in fact leave the plant, the nuclear fuel in all three reactors is likely to melt down, which would lead to wholesale releases of radioactive material—by far the largest accident of its kind since the Chernobyl disaster 25 years ago...
Even if a full meltdown is averted, Japanese officials have been facing unpalatable options. One was to continue flooding the reactors and venting the resulting steam, while hoping that the prevailing winds, which have headed across the Pacific, did not turn south toward Tokyo or west, across northern Japan to the Korean Peninsula. The other was to hope that the worst of the overheating was over, and that with the passage of a few more days the nuclear cores would cool enough to essentially entomb the radioactivity inside the plants, which clearly will never be used again. Both approaches carried huge risks.
Everyone within 20 km (12.4 miles) of the plant has been ordered to evacuate; those within 30 km (18.6 miles) are advised to stay indoors.