Nowhere near a nuclear phyisict, hopefully someone can clarify or correct me. When an atom splits (like in a nuclear reactor) it breaks into other elements lower on the periodic table (if it's a fission reaction). So Uranium 235 will split into Iodine and I think something else (maybe not) when it is it with a neutron. Despite the new atom being Iodine, it is unstable. Whereas the iodine that occurs naturally is stable. When an atom undergoes decay it emits radiation in the form of Alpha, Beta, X-ray or gamma particles, each have their own characteristics. Alpha rays for example are pretty much the nucleus of a helium atom but there are no electrons. They are therefore positively charged, but cannot enter your body through your skin. A regular sheet of printer paper can block them actually. The danger is when they enter through things like radioactive dust, then they begin stripping electrons from the atoms in you....which is bad.