A spent fuel pool. Highly radioactive “spent fuel rods” must be kept submerged under at least 20 feet of constantly circulating cold water for at least five years after being removed from the reactor core.
Arnie Gunderson, a former engineer employed in the nuclear power industry, explains the dangers of spent fuel rods catching fire at Fukushima Daiichi’s reactor #4, and at other Mark 1 reactors internationally.
The U.S. has at least 23 Mark 1 reactors of identical design, with spent fuel pools storing many more times the spent fuel rods.
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