So I'll break news to you: C14 and K40 do not accumulate in your body up same way like radium/uranium/heavy metals do.

"Even at low levels, these isotopes represent health hazards as they can accumulate in the human body (eg, in lungs), gradually enter the blood circulation, and deposit in bones and teeth to remain for life."

http://www.ncmedicaljournal.com/content/79/5/289.full

So there are three things that you really miss: doses are essentially permament (due to half-life), cumulative and localized in small areas of body.

And you miss problem of coal ash waste which needs to be stored and there is plenty of it.