Muto Nuclear Plant Collapse?

I think you maybe right. But the radiation could have spiked from just a regular earthquake, though unlikely. I think it will be caused by one of the monsters.

It was from the muto, if you look at the scenes where you watch it crumble it's the only building destroyed, possibly a tunneling muto caused an earthquake spike.

the radiation is from the leak they found

The signatures are identical! :)