What would happen if nuclear waste was disposed of into an open volcano?
No physical or chemical process can alter the radioactivity of the waste, so it will still be there whatever you do with it. If the volcano is active, when the next lava flow occurs the nuclear waste will be ejected with the volcanic matter, so it would be a very bad idea. The idea for long term waste disposal is to find somewhere very geologically stable where it will stay for thousands of years without any risk of returning to the earth’s surface, just the opposite of the volcano’s contents.
