Once the rods reach more than 1200 degrees Celsius, the zirconium will interact with the steam and split the hydrogen from the water. If no fresh water is introduced to cool the rods then they continue to heat up. The high temperatures that the fuel rods create boil water and continually turn it into steam. If the fuel rods are no longer being cooled-as has happened at all three reactors at the Fukushima Daiichi power plant operating at the time of the earthquake-then the zirconium cladding will swell and crack, releasing the uranium fuel pellets and fission byproducts, such as radioactive cesium and iodine, among others. Though control rods have stopped the uranium fission process that drives normal operation of a nuclear reactor, the byproducts of that continue to split and generate heat. The 3.7-meter-long nuclear fuel used at Fukushima is composed of uranium oxide pellets encased in a zirconium cladding. "The cesium and iodine showing up in releases shows the fuel has been damaged." You get enough and some spark source and you get an explosion," explains nuclear engineer Michael Golay of the Massachusetts Institute of Technology. "The hydrogen accumulates outside of containment but inside the reactor building. in 1979 as a result of the meltdown there-caused by nuclear fuel rods experiencing extremely high temperatures stripping the hydrogen out of the plant's steam. The culprit in all three cases is likely a build-up of explosive hydrogen gas-as occurred at Three Mile Island in the U.S. This followed an explosion March 11 that ripped the roof off reactor No. 2 at the stricken Fukushima Daiichi nuclear power plant. Just after 6 AM local time on Tuesday in Japan, a sound like an explosion was heard near the suppression pool of reactor No.
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