According to the Associated Press, A partial meltdown was likely under way at a second nuclear reactor, a top Japanese official said Sunday, as operators frantically tried to prevent a similar threat from a nearby unit at the same facility following a catastrophic earthquake and tsunami that may have killed 1,000 people.
From Time: Japan’s Nuclear Emergency: How to Stop a Meltdown
The reported presence of Cesium 127 was disturbing because the element is usually evidence that the reactor core has overheated, if only for a portion of time. The radioactive debris is produced when the core is exposed above the coolant water level and then overheats. All it takes is 20 to 50 minutes of exposure to produce enough heat to let off Cesium 127. One of the other potential by-products of such overheating is hydrogen, which is produced when the zirconium sheaths around the uranium rods form a chemical reaction with the heat and steam. Hydrogen is believed to be the cause of the explosion at Fukushima on Saturday that was captured on videotape and alarmed not only Japan but most of the watching world.