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 9 to 19 kilometres. What the ultimate fallout will be is anyone's guess.
While the Daiichi plant is one of the two that experienced cooling failure early after the devastating quake. According to Tokyo Electric Power Company, the explosion happened "near" but not in the Unit 1 reactor. Radiation levels are currently 1,000 times above normal in a reactor control room at the plant, and more troublingly levels have 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 Daiichi is currently walking that line very tightly.