The sequence of events at Unit 3 seems to be following the pattern that occurred at Unit 1. When the pressure becomes too high and risks the integrity of the containment vessel, it is necessary to vent steam, which is what TEPCO did. When the water level drops below the top of the fuel rods, there is the risk of Cesium, Iodine, and H2 and O2, getting into the vapor, and when it is vented, the atmosphere. When TEPCO vented steam at Unit 3, I expect that as with Unit 1 they vented it into the concrete structure covering the containment vessel. The build-up of hydrogen and oxygen there resulted in an explosion that blew out the cladding, but left the containment vessel intact.
Unit 3 uses a different fuel called MOX, which is comprised of mixed oxides of plutonium and natural uranium, reprocessed uranium, or depleted uranium.
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