Googling "field radiography boundary line" returns a bunch of articles on medical imaging. Are there open sources (appropriately marked, I'm sure, modulo turkeys) that would have long-term consequences if we were to run too near them? Are these usually former chemistry or physics sites?

The line defines the 2mR/hr exposure boundary. Closer than that increases by the inverse square law. I do not recommend getting very close but, uh, I’ll take that over an angry turkey.