Earlier quoted context omitted.
Wouldn’t the best option in planning for regression be making it inaccessible and unreachable—really hard to get to without technology? Once people have technology back, then presumably they’d be able to detect radiation and avoid the contaminated location? In 100000 years the biggest threat is probably glaciation submerging very large areas of Finland.
People were trying to find a way through the artic from the 16th century, 400 years before the first Geiger–Müller tube was made.
I was thinking more along the lines of burying it deep under an inactive seabed. We’d have a hard time now digging things up. We aren’t even able to do simple floor mining well.