How to achieve an area of long-lasting radioactivity, without affecting surrounding areas?
Question
(How) can we have a very long lasting source of radioactivity with strongly harmful effects within its area, without those effects dispersing into nearby areas?
Constraints
* The source of radioactivity is not being renewed by any process; it is only decaying.
* The affected area's size would be on the order of a small country or a large province/state.
* The affected area is open to the sky, with wind (and presumably dust) blowing over it and into neighbouring regions.
* Ideally, the affected area should remain uninhabitable for time on the order of many centuries/a couple millenia.
* Ideally, the neighbouring regions are so mininally affected that people living in them do not notice higher rates of cancer. (They're not doing clinical studies; they have life expectancies not usually going past 70-80).
Freedoms
* You can decide on any origin for the source of radioactivity: natural, accidental, deliberately designed and planted with antagonistic intent. (For this question, I am not concerned with the origin, but with the results.)
* You can decide how much precipitation will be a factor, and where the water will go (runs off into neighbouring regions, or the ocean, or into a "dead sea", or evaporates).
* More broadly, you can design the geography of the affected region and the neighbouring regions as you see fit. (please try to stick with "realistic" geography--use your judgement)
* I would expect that the thickness of the "boundary zone" (with a gradient of radiation running from "uninhabitable" to "unnoticeable") would be at least on the order of tens of kilometers; but feel free to adjust my expectations of what is reasonable based on your answer...
A good answer will:
* Specify the location/distribution/form of the radioactive material in the area;
* Specify any radioactive elements involved if specific ones are needed (or precluded) for the answer to work;
* Give a rough estimate of the radiation dosage and how it would drop off over distance/over time;
* Consider the effects of wind and water (as applicable) carrying sediment into and/or out of the affected area.
Context, if you care: I am intending to use, as a plot device, residual radioactivity from a long-ago event preventing settlement/general use of a relatively large area. 591
However, the answers to this question ( https://worldbuilding.stackexchange.com/questions/197365/ways-of-detection-of-radiation-wastelands-spots-in-a-technology-free-world ) challenged the notion of this being realistic. An obvious way to prolong the radioactivity is to just have more radioactive material. However, I was hoping that neighbouring areas would be relatively unaffected and be used/settled as normal, and I'm concerned that just cranking the radioactivity up to eleven might make this unrealistic. So this question is a "ranging shot" to try to scope out what's possible.
https://worldbuilding.stackexchange.com/questions/197365/ways-of-detection-of-radiation-wastelands-spots-in-a-technology-free-world 922