Potentially there's quite a bit more than one cubic meter of active corium and spent fuel at Fukushima. There are around 1000 rods at each reactor - either active or in the SF pool - and each rod holds around 200kg of uranium.

So that's around 200 tonnes. Per reactor.

If one goes, the chances of the others surviving without maintenance are not high.

And the corium includes active fission products that are both lighter and more immediately problematic than uranium.

If just one reactor goes boom, that's immensely bad news for the immediate area. But if the spike in infant mortality in the North West is confirmed, there are already obvious health effects from the current slow burn.

But I think an explosive encounter with the water table would immediately become more of a political and military problem than a scientific one.

Effectively you'd have a cloud of death, which would either rain on the US west coast or would drift east over China and Russia.

The US would shrug and let its people fry.

Russia and China, not so much.