Dry cask storage will do. Uhm. It'll probably do for the full five hundred years. There are older concrete structures than that standing...

Modern concrete does not favorably compare to Roman concrete, which comprises a lot of the long standing concrete structures extant. Roman concrete used high alumina content volcanic ash as a major component. But not all volcanic ash is created equal. Again, I suspect that we could create concrete storage facilities that would last >500 years, but I am far from convinced that we will - due to lack of social competence.

"It is not necessary to have hope in order to persevere."