50C is terribly hot for a swim.

Hot spring baths usually top out around 42-43C

Would you mind sharing a link to "the xkcd thing" if you have it?

edit: sorry for being lazy, I scrolled a bit more and found it.

Relevant: "For the kinds of radiation coming off spent nuclear fuel, every 7 centimeters of water cuts the amount of radiation in half."

On the picture, the fuel rods are indeed protected by a huge quantity of water above them. But what happens below them ? They seem to be in direct contact with the ground...

People tend to avoid casually tunneling under nuclear pools.

I would guess that in reality they are either suspended and/or there is enough concrete at the bottom