One Million Years of Isolation

"Now, the reason that you want to isolate this material for a million years is that the spent fuel—meaning fuel that no longer supports the chain reaction that keeps reactors making electricity—contains actinides. These are metal elements, from 90 to 103 on the Periodic Table, most of which are heavier than uranium (which is 92). Actinides are generally very slow to radioactively decay into smaller atoms—which then decay more rapidly—and some of the actinides actually do remain hazardous for a million years and beyond. The trick is to isolate them for that length of time."


http://bldgblog.blogspot.com/2009/11/million-years-of-isolation-interview.html