hydrogen explosion Tuesday morning destroyed the outer building of a quake-damaged Unit 2 nuclear reactor at the Fukushima No. 1 power plant.

 hydrogen explosion Tuesday morning destroyed the outer building of a quake-damaged Unit 2 nuclear reactor at the Fukushima No. 1 power plant. Engineers had been struggling overnight to cool the nuclear core and stave off a meltdown that could release radioactivity over a wide area. It was the third reactor at the site whose external structure had been damaged by such an explosion.

Neither of the reactor containment vessels of Units 1 and 3 had been damaged in the earlier explosion and there is no evidence so far to suggest the vessel of no. 2 had been damaged either.

Officials had feared the possibility of such an explosion because the fuel core had been exposed to air for more than two hours, allowing it to overheat. When the zirconium cladding on the fuel rods was subsequently exposed to seawater used for cooling, it released hydrogen gas, which built up to dangerous levels in the plant and was most likely ignited by a spark.