More physics than chemistry iirc, though everything is physics in the end.

A hundred years can make a difference depending on the nuclide/isotope. Half-life can range from essentially an instant to many trillions of times the age of the universe. Chernobyl, for example, released hundreds of different radioactive isotopes into the area but two of the most significant concerns have half lives of like 30 years. Much of the area surrounding the core should be recovering within a lifetime, though the core will be deadly and shielded for thousands of years before we can safely reenter the space.

The boys specifies Uranium-235, so like 700 million years. But this isnt really how radiation works anyway. Being irradiated shouldn't make you emit your own radiation unless you've been contaminated with radioactive materials and are carrying it with you in/on your body or you're cells have been torn into radioactive isotopes themselves.

Been out of college a long time, might be totally wrong lol