Alpha radiation (the type emitted by super heavy nuclei like uranium, basically just helium nuclei being emitted) has very low penetration depth compared to beta radiation (electrons, positrons, some accompanying photons) because the emitted particle is absorbed by the upper epidermic layers due to the sheer size of the emitted particle. Those layers of skin are shed on a regular basis anyway, so most corruptions end up being entirely irrelevant. However, alpha radiation tends to be highly energised and thus will deal substantial damage to any tissue that cannot simply be discarded, thus is highly dangerous when ingested or inhaled.