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BBC: If Cesium is Being Emitted, Core May be Melting
This is produced during the nuclear reaction, and should be confined within the reactor core.
If it has been detected outside the plant, that could imply that the core has begun to disintegrate.
"If any of the fuel rods have been compromised, there would be evidence of a small amount of radioisotopes in the atmosphere [such as] radio-caesium and radio-iodine," says Paddy Regan, professor of nuclear physics at the UK's University of Surrey.
"The amount that you measure would tell you to what degree the fuel rods have been compromised."
It is an important question - but as yet, unanswered.
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