Evolution of the Nuclear Type

[Kuribat]

You're right about the gaps in logic, especially the Normal type thing (although you could argue 'normal' as the opposite of mutated from radiation? But that's a stretch) but Steel used to resist Dark up until gen 6 so it wouldn't be the first time.

Indeed, but the logic behind Darkness being weak to Steel seems less obvious to me than with Nuclear. But nothing is written in stone, I guess.