A mushroom cloud just connotes a large explosion near the ground. Atomic structure may be the best to go with, since pictorial descriptions of alpha, beta and gamma radiation are found in every high-school science textbook. One issue with an advanced culture is that they may be rebuilding to the same technological state as we were around 1950, so when they find the site they go: "Cool, radioactive stuff! Might be Uranium! Just what we need to fuel our fission reactors..." Then, they discover it's merely low-level waste after building an enormous open-cast mine on the site, and polluting the countryside for miles around. I believe this was a scenario they considered, with some kind of warning marker seeded in the backfill above the underground chambers. It would show up in the tailings of any exploratory drill-holes, as chips of plastic (?) with a radiation hazard symbol.