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The Fukushima Daiichi plant originally comprised six boiling water reactors of a half-century-old General Electric design. The installation was one of the world’s 25 largest nuclear power stations. TEPCO currently believes that after the tsunami, units 1-3 suffered meltdowns, perhaps even through the floors of the buildings and into the soil.
However, the situation in unit 4 merited the most immediate attention. There, the building had partly collapsed. New and spent (used) fuel rods stored in temporary containment 100 feet above ground level were emitting radiation to the atmosphere.
In danger of internal collision due to seismic events or spillage from the damaged secondary containment, this exposed fuel has threatened Japan with a massive uncontrollable nuclear chain reaction. After completing precautionary activities, on November 18, 2013, the company began a delicate task of rearrangement. Plant workers started moving the assemblies of nuclear fuel rods racked in the upper-story pool to safer ground-level storage in a centralized pool common to all the damaged reactors.
As well as the hazard of spontaneous explosions, spent fuel rods can release considerable heat and quickly lethal levels of radiation. Robot cranes and other human-directed machinery are necessary to move these very hazardous materials. The unit 4 cooling pond contains 1,533 fuel assemblies, of which 1,331 have been used and 202 are unirradiated (fresh) ones. Some of the fuel assemblies are known to be damaged.
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