According to Wikipedia, half a million people irradiated from caesium and strontium isotopes in 1968. Sounds like it happened just as you describe. The headwate

According to Wikipedia, half a million people irradiated from caesium and strontium isotopes in 1968. Sounds like it happened just as you describe. The headwaters of the Kama, Tobol and Ural rivers run through this part of Russia. We know that caesium from Fukushima was detectable in Canada. That was a #7, this was a #6. https://en.wikipedia.org/wiki/Lake_Karachay https://en.wikipedia.org/wiki/Lake_Karachay