The question can not provide that. Radioactivity is a natural phenomenon that man has taken advantage. The sun radiates what everything because the man never comes ran. So natural sources of radioactivity will always be stronger than that seen from experiments on humans.
Man concentrates Radioactive substances (in gas centrifuges) and then uses it to fission. Man thus brings natural substances in conditions which normally not found on Earth. Is quite easy, is not it?
Uranium is a natural ore, which is mined in mines. Formerly, when you did not know how dangerous uranium, you have it made tiles and even dishes. In Schneeberg, in Germany, old houses have been built with uranium-contaminated building materials. Uranium as such beam also not very strong. Only when it is enriched and you shoot with neutrons it, it gets really dangerous.