How to achieve an area of long-lasting radioactivity, without affecting surrounding areas?

Question

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. However, the answers to this question ( 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.

Ways of detection of radiation wastelands/spots in a technology free world?

Similar: https://worldbuilding.stackexchange.com/questions/3248/how-can-i-lightly-irradiate-30-000-km%c2%b2-for-just-3-generations — Willk, Mar 10 '21 at 18:44
@user535733: "many centuries / a couple millenia" (in the constraints section) — Qami, Mar 10 '21 at 19:54

score 5 · Answer 1 · answered Mar 10 '21 at 19:04

5

You may have an uncovered rich uranium ore releasing large amounts of Radon.

Radon is an odorless alpha-radioactive gas, which means it is very potent. It has a half-life of less than 4 days, so it would be pretty localized. You can't affect surrounding areas without moving millions of tons of uranium ore.

answered Mar 10 '21 at 19:04

alamar

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Even my Grandmother's basement needs abatement because there are mineral deposits leaking radon https://www.epa.gov/sites/production/files/2015-05/documents/hmbuygud.pdf – DWKraus Mar 10 '21 at 20:16
An ore of uranium all over a country? If we are talking about the Vatican state, all right. Otherwise it's highly unlikely. – Rekesoft Mar 11 '21 at 11:22
Regular ore, unlikely. But some specific minerals may pose a problem. – alamar Mar 11 '21 at 13:24
Common granites produce enough radon for unventilated basements to be a lung cancer risk. Also, thorium ores (much more common than uranium ores) also produce radon (in fact, are the main source of naturally occurring radon in the environment). – Zeiss Ikon Mar 11 '21 at 13:43
In the open air, radon isn't a risk because it dissipates too quickly to be a significant danger. – Keith Morrison Mar 11 '21 at 15:16
I guess it depends on its amount. One can assume a different geologic composition (a younger/older universe, etc) to maximize radon output. It's also heavy gas so it will stay near the ground. – alamar Mar 11 '21 at 20:48
It will stay near the ground and accumulate in closed spaces like basements and caves. In open air, it's subject to that extremely rare meteorological event known as "a slight breeze". – Keith Morrison Mar 11 '21 at 23:37

score 1 · Answer 2 · answered Mar 10 '21 at 20:14

Debris and scrap:

Your area is an impromptu waste dump for bits and pieces of radioactive equipment. The actual radioactive material is chunks of radioactive metal either inside equipment or sealed inside a secondary material that doesn't block radiation but does prevent oxidation (like a glass or resin). The pieces are big enough they are unlikely to wash away, the material emits radiation while not emitting particles that can spread. The radiological material is likely a mixed bag of metals, where the shielding has been salvaged but the metals still needed to be disposed of (rather improperly). It could also be inadvertent, like in an abandoned factory or hospital where non-radioactive shieldings were removed and the site was left abandoned due to the hazard of the remaining materials.

Ores and gasses can spread (due to solubility, diffusion, or erosion) while hunks of heavy metals are limited only by how solid the surface is on which they have been placed. This answer is endlessly adjustable since each metal has a specific amount of radiation it can emit and a specific half-life. Increase the concentration of metals and you increase the effective range of the radiation. Duration and amount of radiation is based on each material's half-life Increase or decrease the amount (grams to tons) or the strength (discarded watch faces vs spent nuclear fuel rods). Exposure will be mostly external exposure by gamma radiation and the amount of materials you choose to improperly dispose of.

I hope this is acceptable for the hard science tag, but it can be adjusted to any number you like, so it's hard to put an equation to it.

Metals tend to corrode and metal oxides are usually soluble, leading to ground waters contamination. — alamar, Mar 10 '21 at 20:23

score 0 · Answer 3 · answered Mar 11 '21 at 13:13

It won't be easy

Start by picking you radioactive poison of choice.

To poison a large area, you need lots of poison - ideally this means you have a relatively high yields of the desired poison from your poison source. Radon (Rn) is an example of a relatively low yield, primarily coming from the natural decay chain of U-238. U-238 has a half-life of 4.46 billion years, and its decay chain includes Rn-222 (half-life 3.8 days) Rn-218 (half-life 35 ms). Because of the very short half-lives compared to U-238 only a very small fraction of Rn exists at any given time from natural decay of U-238, Thorium decay is even worse in terms of Rn fraction.

So, how about cooking your source material in a fission reactor or nuclear bomb. Well, problem is neither method will generate significant Rn because Rn has too much mass to be a significant product from fission reactions,

Rn does have one advantage going for it, as a noble gas, it can easily be breathed in, and you don't want to inhale alpha-emitters for your continued health. As noted in many places, Rn accumulates in basements, etc. where the natural accumulation is maximized due to limited air circulation. However, even the the levels are usually too low to be more than a minor increase in cancer rates. Not exactly healthy, but far from the toxicity you require

What else is important, you need the half-life if your poison to be in the sweet spot, too short and it won't last for the necessary generations, too long, and the radioactivity level is not not high enough to be highly toxic.

Without getting into the details none of the products from fusion bombs are sufficiently long-lived for your purposes.

So, is there any hope - indeed there is, review the fission by-product yields of U-235 (the other fission sources have similar yields) and there are actually 2 good choices, Strontium-90 (Sr) and Cesium-137 (Cs) - both of which have half-lives that are conveniently about 30 years as well as having a high-yield as a fission by-product

So, given a half-life of 30 years, a century or more of highly toxic levels of radiation is possible, but millennia is not realistic.

If you are careful to drop the bombs during periods where there are essentially no prevailing winds for a week or so will prevent most of the radiation from spreading beyond the target area.

Realistically though, it will still be very difficult and expensive even using this method. Hiroshima/Nagasaki currently have radiation levels basically indistinguishable from background. You will need to do saturation bombing to end up with enough long-term radioactivity to be fatal. A majority of the radioactivity released in fission has half-lives either too short or too long for your purposes. Although the short half-life products are effective in killing the population quickly, they don't poison the area for generations. According to what I've read in the past, atom bombs probably cost about 1 million USD each when manufactured in quantity.

So, blow up enough fission bombs to cover a large area with high levels of radioactivity and you can have the needed deadly radiation levels that will last for hundreds of years. You definitely need a lot of them, even using higher yield, modern atom bombs you will need to drop dozens or hundreds per city to achieve the levels you desire (which you did not clearly define)

score 0 · Answer 4 · answered Mar 11 '21 at 14:59

Natural nuclear fission reactor

A large layer of uranium ore in Oklo, a region of Gabon, was able to act as a nuclear reactor for several hundred thousand years, using ordinary water as a moderator. The active isotope was U-235, present at 3%.

The size of the region is limited only by the size of the ore vein and its geography. The time had to be 1.7 billion years in the past given the concentration of deuterium on Earth. Planets that have lost more atmosphere can have a higher level, so it should be possible in more situations. Other moderators might be present as dissolved minerals.

Unfortunately, I can't estimate the potential exposure well from past data. The Gabon reactor safely stored its own waste (primarily xenon and krypton isotopes) in aluminosilicate minerals. It also maintained itself near criticality, shutting down when it overheated. I would like to hear of the company that could do that for a hundred thousand years. Nonetheless, the reactor used enough uranium that the ore at the site is noticeably depleted of U-235, so the potential contamination should be substantial. You need merely remove the aluminosilicate and add some nice repeating geysers to decorate your lovely tourist attraction.