I used to favor a theory of military weapons tests involving a Sr-90 dirty bomb.
Now, I am looking at a variant of the 1079 theory. The book is excellent, and there’s a lot to it, but in a nutshell, a falling tree caused the injuries and deaths. They were discovered shortly after their deaths but presumed to be a different group that went missing. The tree falling could be attributed to blasting in the area associated with geologic exploration. Then, people made the connection that this was the group of UPI students and a very high profile search was underway. So the tent was moved up to the slope and the bodies staged to make it look like they fled from the tent and died of hypothermia, which would absolve the people involved in the geological / explosives work of any responsibility. Then the search party found the staged scene.
My speculation is that the people staging the bodies didn’t want the Ravine 4, who had the most obvious traumatic injuries due to the tree falling on them, to be found. These people were operating under a lot of pressure and made some stupid decisions, one of which was to dump the four bodies in the ravine and shovel caustic potash, or potassium hydroxide, which is a close cousin of lye, on them and bury them with snow, thinking it would dissolve the bodies. It didn’t work, and Ivanov’s team eventually found the last four bodies. But potassium is naturally radioactive due to K-40, and some of the clothes retained enough potassium hydroxide to be measurably radioactive.
I have done math to show that 4.3 grams of potassium hydroxide, if retained in the 75 cm^2 sample of Lyudmila’s brown sweater, would produce the exact results measured by the expert in Sverdlovsk.
Is this plausible? I don’t know. But it does explain the radiation, and it uses a natural source, getting around the problematic question of how unobtainable manmade isotopes contaminated the hikers.