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Temperature in the reactor vessel apparently kept rising, heating the zirconium cladding that makes up the fuel rod casings. Once the zirconium reached 2,200 degrees Fahrenheit, it reacted with the water, becoming zirconium oxide and hydrogen.

When the hydrogen-filled steam was vented from the reactor vessel, the hydrogen reacted with oxygen, either in the air or water outside the vessel, and exploded.

If the temperature inside the Fukushima reactor vessel continued to rise even more — to roughly 4,000 degrees Fahrenheit — then the uranium fuel pellets would start to melt.

According to experts interviewed by AP, any melted fuel would eat through the bottom of the reactor vessel. Next, it would eat through the floor of the already-damaged containment building. At that point, the uranium and dangerous byproducts would start escaping into the environment.

At some point in the process, the six-inch stainless steel walls of the reactor vessel would melt into a lavalike pile, slump into any remaining water on the floor, and potentially cause an explosion much bigger than the one sparked by the hydrogen. Such an explosion would enhance the spread of radioactive contaminants.