Posting this just to explain that this is fake news by Ukraine. Spent fuel can catch fire, melt, cause a radioactive death cloud, yes - but only when it's fresh out of the reactor. The spent fuel has to be cooled to remove what is called 'decay heat' - heat released due to radioactive decay of the fission fragments. Decay heat decreases exponentially to the point that after 4 years you can safely air cool the fuel. The last Chernobyl unit was shut down in 2000 (yes the other units ran well after the accident), so the fuel's had over 20 years to cool.
This warning by Ukraine is pure bullshit.
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