What chemical elements does a nuclear explosion release into its mushroom cloud?
According to The Preparatory Manual of Explosives, the mushroom cloud is produced by the partial vacuum left after initial expansive shock wave. That implies that it is primarily the condensation of water that makes it visible.  These clouds form in high explosive as well as in nuclear detonations near the ground. See https://books.google.com/books?id=J55D3HcgPuoC&amp;pg=PA98#v=onepage&amp;q&amp;f=false.