Actually nuclear is completely unusable as standby power. Nuclear power stations can only ramp up or down slowly.
It could be that the Spanish were a bit negligent on the stability front of things. That would be a major oops. The obvious way to go would be large battery banks, pumped storage, and maybe some gasturbines.
If it turns out that it was a systemic flaw creeping up on them, then this will be a big learning case for all of Europe.