Here's an existing marker for a nuclear burial site. This is where the SL-1 reactor blew up in the 1960s. (SL-1 was an experimental reactor at the Idaho Reactor Test Station, in the middle of nowhere for good reasons.)
Realistically, sites like that require a major mining effort to get into, and any society capable of that probably will know about radioactivity, along with all the other risks of mining.
This place is not a place of honor.
No highly esteemed deed is commemorated here.
Nothing valued is here.
This place is a message and part of a system of messages.
Pay attention to it!
Sending this message was important to us.
We considered ourselves to be a powerful culture.
This place is best shunned and left uninhabited.
The document in question is numbered Sandia 92-1382
Lots of past comments to read.
280 comments at https://news.ycombinator.com/item?id=11851871