How does release of radioactive potassium from coal burners compare to fission reactors?

We put this question to Dr Ian Farnan from the Department of Earth Sciences at Cambridge University:

It's not very much at all. I think the main emission from coal fire power stations is actually the uranium that's in the coal, but that would be at a very, very low level. I'm not sure I can tell you right off the bat, but it is something which is very, very insignificant.