Even Lord Kelvin, though, argued (without allowing for radioactivity) that the earth is much older (on the order of tens of millions of years) than Biblical literalism allows. Citing Kelvin doesn't do much for creationism at all (except provide another attempt to confuse people; but I guess that's what all creationism essentially does).
They used an argument along the lines of "if nuclear fission is keeping the earth warm then where is it? Why aren't we all dying of radioactive poisoning?".
Someone actually asked you that? (Every cinder block in your basement; every piece of coal burned in a power plant is a source of some radioactivity; most of it reabsorbed and emitted as heat...)
Marie Curie was a lifelong atheist. I guess that conveniently makes the theory of radioactivity bunk along with evolution.
A typical fossil-fuel power station emits more radioactivity into its immediate surroundings than a (correctly functioning) nuclear plant is what I told them. I also asked them what they thought that "radiometric" thing that they despise so much is actually measuring?