Concentration absolutely matters, and based on the figures I've seen coal fly ash - which is what the claims about radiation released from coal power plants are based on - is about as radioactive, kg for kg, as granite is. Which isn't nothing, but it's also not exactly nuclear waste. (Speaking of nuclear waste, the comparisons with coal often also get their numbers by ignoring it and only counting radiation intentionally released to the environment from a properly-working plant. Which is basically just a way of saying that if we could solve the nuclear waste disposal problem and the humans are too incompetent to build and run nuclear power plants problem it'd be a really clean form of power. That's probsbly true, but not realistic. Also, the comparison is generally based on really old and polluting coal power plants that mostly don't exist anymore.)