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".