I always figured that it was originally a Touhou meme because the right foot of the final boss, Utsuho Reiuji, from Touhou 11 (2008) is based on the Elephant's Foot from Chernobyl.

That came to mind for me too, but it's the sort of thing that always could be just a coincidence. Touching on nuclear fears is hardly unknown in Japanese media after all.

That's interesting, though I don't remember Demon Core meme to have come from Touhou. Touhou was nevertheless cultural upstream for lots of Japanese otaku community contents, as seen in Yukkuri, so it is believable if the general awareness of the incident came from there.