Transformer stockpiles are not sufficient (transformers are fried in sufficient GMD/solar flare or EMP), ordering additional units takes a long time (and relies on international trade working well), and in the meantime nuclear plants require lots of diesel to be trucked around keeping the backup generators running to prevent meltdown - all that in the context of widespread blackouts and likely societal chaos.
I've researched this topic thoroughly
Fukushima was ONE plant. its backup diesel generators were flooded with seawater and couldn't run, leading to the failure of the cooling system. you know the rest of that story.
a wider region with more nuclear plants (say, a quarter of the US. we have around 120 plants total) being hit with a GMD would imply keeping all those backup generators running for at least 9 months while the grid is disconnected and repaired. that entire region would be in a blackout the whole time.
even if we DID have the "spare parts lying around", that's the context within which they'd have to be "slapped back in place".
