Hate to rain on the parade...

There are 4000 Spent Fuel Ponds Around the Globe…

If you don’t cool the spent fuel, the temperature will rise and there may be a swift chain reaction that leads to spontaneous combustion–an explosion and fire of the spent fuel assemblies. Such a scenario would emit radioactive particles into the atmosphere. Pick your poison. Fresh fuel is hotter and more radioactive, but is only one fuel assembly. A pool of spent fuel will have dozens of assemblies.

One report from Sankei News said that there are over 700 fuel assemblies stored in one pool at Fukushima. If they all caught fire, radioactive particles—including those lasting for as long as a decade—would be released into the air and eventually contaminate the land or, worse, be inhaled by people. “To me, the spent fuel is scarier. All those spent fuel assemblies are still extremely radioactive,” Dalnoki-Veress says.

It has been known for more than two decades that, in case of a loss of water in the pool, convective air cooling would be relatively ineffective in such a “dense-packed” pool. Spent fuel recently discharged from a reactor could heat up relatively rapidly to temperatures at which the zircaloy fuel cladding could catch fire and the fuel’s volatile fission product, including 30-year half-life Cs, would be released. The fire could well spread to older spent fuel. The long-term land-contamination consequences of such an event could be significantly worse than those from Chernobyl.

http://science.time.com/2011/03/15/a-new-threat-in-japan-radioactive-spent-fuel/

The Chernobyl accident was relatively minor, involved no spent fuel ponds, and was controlled by pouring cement onto the reactor. This was breaking down so a few years back they re-entombed.

Estimates of the cancer burden in Europe from radioactive fallout from the Chernobyl accident

https://pubmed.ncbi.nlm.nih.gov/16628547/