At 3:40pm local time in Japan's Fukushima Prefecture, an explosion shook the Fukushima Daiichi nuclear plant. Four people were reported injured from the initial blast, but broader concerns over increased radiation leakage have lead officials to double the evacuation zone around the plant from 6 to 12 miles. What the ultimate fallout will be is anyone's guess.
The Daiichi plant is one of the two that experienced cooling failure early after yesterday's devastating quake, and was teetering on the brink of a meltdown just hours ago. According to Tokyo Electric Power Company, the explosion happened "near" but not in the Unit 1 reactor. Radiation levels had reached 1,000 times above normal in a reactor control room at the plant, and more troublingly levels had reached 8x normal near the main gate.
The important thing, though, is that it appears that the explosion—likely caused by a hydrogen build-up—only affected the wall around the reactor and not steel container housing the reactor itself. The important thing now is that cooling operations continue unhampered. If the cooling systems are inoperative for several hours, the reactor's water will boil away and the fuel will begin to melt. When that happens, the situation escalates from "manageable" to "Three Mile Island." And while there are indications that radiation levels have in fact declined since the explosion, Daiichi is still currently walking that line very tightly.