Just in case you google these Russian nuclear lakes. I don’t actually know that there are “loads” of them. I think because “radiation hot spring” is a category, it made me think that this is common. But I assure you, it is a thing. I have a high school acquaintance who went to one of these resorts. I’ve seen the vacation pictures. But as far as I know it might be the only one.
I doubt any level of radiation can be healthy. Unless you aspire to become the next spiderman. But radon is so weak it’s almost harmless. I doubt it will heat water. Radon is somewhat of a nuisance for house and real estate owners in (i think allover) sweden. As I understand it, if the radon just stays in the ground, it’s harmless. The problem is that it sticks to dust particles and such and when you breathe those they can stick in your lungs and cause real damage over time. But I’m far from an expert on this. Anyway…
My theory is that perhaps the radon gives the water a pretty colour. People have probably bathed in these springs since before Niels Bohr. Noone knew better, but noone died either. So why shouldn’t this magic coloured water be a unique selling point. Just a wild speculation. If radon even can dissolve in water, I don’t know what colour it would have.
Edit: I read just a little bit of about the radiation hot springs. If I got it right, they claim that small amounts of radiation is proven to be beneficial. I wonder if the right amount of uranium is beneficial as well… either way, unless they are outright lying, I’ll just have to accept that it’s true. Maybe it’s not so strange… I mean, isn’t the right amount of UV-rays from the sun also proven to be beneficial?