By Wednesday morning Japan time, the BBC confirmed that a "criticality event" was being reported in the No. 4 fuel pond after it caught fire. The fire was initially thought to have been extinguished, but then reignited. Tokyo Electric Power, which has come in for intense criticism for failing to keep government officials informed on the condition of the Fukushima reactors, has now admitted that "criticality" was reached during the first fire, and, according to reports, “The possibility of re-criticality is not zero."
"The explosion at the spent fuel pond at reactor No. 4 was a criticality explosion," Moglen asserted, "which means that the fuel in that pond has begun to interact on a nuclear level--that there was a nuclear chain reaction going on in the pond."
A "criticality event" means that the melted fuel rods accumulated enough mass to exceed the "critical" stage and start a nuclear chain reaction similar to the controlled reaction in a normal reactor. Without a containment vessel, the nuclear reaction emits large amounts of radiation while potentially releasing radioactive particulate directly into the air.
"The spent fuel ponds are even more dangerous than the inside of the reactors because the fuel pools are open to the air. At least there is a containment vessel around the reactor cores," said Moglen. "All six pools are in jeopardy, since they need to be constantly supplied with water to replace that which the fuel rods are constantly boiling off."
So far, nuclear reactions have only been reported in the No. 4 reactor fuel pond, but Tokyo Electric Power said fuel pools in the Nos. 5 and 6 reactors are becoming superheated, and that the No. 3 fuel pool is on fire.
"It’s like a cascade event, resulting in crisis after crisis without being able to resolve any of them," Moglen said.
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