[actionfromafar]

When it comes to old nuclear material, it's more the danger of getting radiactive particles on and in you than it's about pure radiation. You can walk away from a machine but you can't walk away from that fine powder you just inhaled.

[recursivecaveat]

For a real life example: https://en.m.wikipedia.org/wiki/Goi%C3%A2nia_accident It was finally clocked as harmful 15 days after they got a hold of it, plenty of time for lots of people to get a lethal dose. And these people probably had at least some pop culture knowledge of nuclear radiation, compared to a distant primitive civilization who would have nothing.