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I need a city-sized (10 km2 minimum) area to be declared inaccessible for 50 years minimum.

My thought was to justify this with a dirty bomb attack of some sort. Radiological dispersal devices (RDD), while very realistic, have never been used or built (as far as I'm aware).

The only examples of lasting large scale area denial I can think of are things like Chernobyl; these involve very large amount of radioactive materials which are probably not believable for a smallish device. Are there materials which can accomplish this in relatively small amounts (say 100 kg)?

NOTE: The only point relevant is area denial. The "bomb" doesn't have to be destructive or deadly, a dispersal of radioactive material that persists in the environment is more than enough.

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    $\begingroup$ ...and with a single question, every member of world builders instantly gets added to the government watch list. Oh well. It isn't like most of us weren't already listed several times over. Creative thinkers usually find their way into the suspect line. $\endgroup$ – Henry Taylor Oct 11 '17 at 14:43
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    $\begingroup$ How committed is anyone to restoring it? There are world war I sites we stay out of partially because of unexploded conventional bombs we could deal with but haven't bothered to. Also how inaccessible? Places like the big rocks in Australia or Egyptian Pyramids and Native American burial mounds are said to have curses on them. $\endgroup$ – user25818 Oct 11 '17 at 16:36
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    $\begingroup$ @HenryTaylor This is very far indeed from being the first "watchlist question" and I mean just this week. $\endgroup$ – Ash Oct 11 '17 at 16:38
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    $\begingroup$ A dirty bomb would probably have an effect similar to the radiological accident in Goiânia (see: www-pub.iaea.org/mtcd/publications/pdf/pub815_web.pdf), as it involved Caesium 137, which would be the contaminant of choice in an intentional effort to harm. $\endgroup$ – Luís Henrique Oct 11 '17 at 17:16
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    $\begingroup$ makes me wonder which is more deadly, dirty bomb or photo bomb? $\endgroup$ – user6760 Oct 12 '17 at 3:04
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enter image description here

The true answer to your question can be found on a place called Anthrax Island in Scotland.

In 1942, British Military scientists experimented with various strains of anthrax to use as biological weapons during World War II. They bombed the small island with a particularly virulent strain of anthrax called Vollum 14578.

One of the reasons why anthrax didn't become a weapon is that the spores were really durable and if they used it as a weapon it would contaminate an area and render it uninhabitable for decades. It's hard to recover and rebuild when your livestock and farmers get wiped out by a pesky bacteria.

Gruinard Island remained contaminated for over 40 years because the cost of cleaning a .75 square mile island was cost prohibitive and very dangerous. Cleanup took four years and 280 tonnes of formaldehyde solution diluted in sea water to decontaminate less than one square mile of land. A flock of sheep remain on the island to act as a "canary in a coal mine", to alert scientists they missed a spot.

Anthrax will fit your needs for your story. The best part is it's completely natural and organic. There's no pitchblende (uraninite) to collect and refine like you need for dirty bombs, just a good old fashioned deadly bacteria.

Good luck

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    $\begingroup$ Won't this spread if you do it on the mainland? I can imagine the sea blocking the anthrax, but the stuff is alive so it might travel away from your initial site. $\endgroup$ – Erik Oct 12 '17 at 8:32
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    $\begingroup$ As a side note, it's always scary to see someone ask "How can I do this truly horrible thing?" and someone answers with "Just use this real-life event as an example." $\endgroup$ – Erik Oct 12 '17 at 8:33
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    $\begingroup$ @Erik Normally people ask "How could we do this unthinkably horrible thing" and it turns out that we've done something much, much worse! $\endgroup$ – JeffUK Oct 12 '17 at 12:31
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    $\begingroup$ And the only reason it was decontaminated: 'a "team of microbiologists from two universities" had landed on the island with the aid of local people and collected 300 lb (140 kg) of soil ... threatened to leave samples of the soil "at appropriate points that will ensure the rapid loss of indifference of the government and the equally rapid education of the general public". The same day a sealed package of soil was left outside the military research facility'. $\endgroup$ – user60561 Oct 12 '17 at 18:09
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    $\begingroup$ @DavidRicherby If you're going to complain that an answer doesn't answer the question, it'd help if you read the question. Not just the title. My thought was to justify this with a dirty bomb attack of some sort. is not the same as "It needs to be a dirty bomb". Especially when coupled with: The only point relevant is area denial. $\endgroup$ – Shane Oct 13 '17 at 17:04
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Somewhat realistic. Use a Cobalt Bomb.

For a realistic dirty bomb that has a has a 50 year minimum, city-sized lethality, where "realistic" is assumed to be a combination of cost-effectiveness and technological/military feasibility rather than political tenability, you'd want a Cobalt Bomb.

As defined by Wikipedia: a cobalt bomb is

a type of "salted bomb": a nuclear weapon designed to produce enhanced amounts of radioactive fallout, intended to contaminate a large area with radioactive material.

Basically a nuke wrapped with a layer of cobalt. Simple but extremely effective at area-denial because:

Areas irradiated by fallout from even a large-yield thermonuclear weapon begin to increasingly become habitable again after one to six months; a cobalt bomb's fallout on the other hand would render affected areas effectively stuck in this interim state for decades of habitable, but not safely so under constant habitation, conditions.

By not safe, this means:

After 10 half-lives (about 53 years), the dose rate would have decayed to around 10 mSv/hour. At this point, a healthy person could spend 1 to 4 days exposed to the fallout with no immediate effects.

At its worst, it has been theorized that one device

containing 510 tons of Co-60 can spread 1 g of the material to each square km of the Earth's surface (510,000,000 km2). Radiation output from 1 g of Co-60 over one half life is equivalent to 44,000 GBq, which is sufficient to kill any inhabitants. If one assumes that all of the material is converted to Co-60 at 100 percent efficiency and if it is spread evenly across the Earth's surface, it is possible for a single bomb to kill every person on Earth.

In practice, the efficiency is much lower but (using back-of-the-napkin math) we can estimate that dispersal of 100kg of cobalt at an abysmal 1% conversion will have a lethal effect over 1000km2, 100 times the minimum. Considering that New York City is only 789km2, this amount is sufficient to convert most metropolitan cities into necropolises for the next few decades.

Prior to being relieved of his duties, Douglas MacArthur promoted the idea of using Cobalt across the Korean Peninsula. As he recounted in his memoirs:

Of all the campaigns of my life, 20 major ones to be exact, [Korea was] the one I felt most sure of was the one I was deprived of waging. I could have won the war in Korea in a maximum of 10 days.... I would have dropped between 30 and 50 atomic bombs on his air bases and other depots strung across the neck of Manchuria.... It was my plan as our amphibious forces moved south to spread behind us—from the Sea of Japan to the Yellow Sea—a belt of radioactive cobalt. It could have been spread from wagons, carts, trucks and planes.... For at least 60 years there could have been no land invasion of Korea from the north. The enemy could not have marched across that radiated belt."

Note that, he's arguing not for detonating but dispersing, which is directly in line with area-denial and not necessarily with immediate mass-murder.

As for actual implementation: the U.S. investigated the idea in the 60's but chose not to move forward, for reasons not formally stated. The British gave it a go and after some setbacks, withdrew (thus the "somewhat" qualifier). Russia, on the other hand, does not have the same qualms and is rumored to have a few warheads stockpiled.

For further reading, I recommend:

https://en.wikipedia.org/wiki/Salting_the_earth

And:

http://tvtropes.org/pmwiki/pmwiki.php/Main/SaltTheEarth

Specifically, the "Real Life" section.

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  • $\begingroup$ How do you prevent wildlife or wind from carrying fallout out of the afflicted area? $\endgroup$ – anon Oct 11 '17 at 19:07
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    $\begingroup$ @anon metals are pretty heavy and wind doesn't carry them well once they settle on the ground. Animal feet evolved not to carry much dirt either. This problem is not a big one. $\endgroup$ – Mołot Oct 12 '17 at 0:24
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    $\begingroup$ @Molot have you heard of polonium $\endgroup$ – anon Oct 12 '17 at 13:16
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It's worth noting that Chernobyl is a great example of how difficult this problem is. 30 years after the explosion you have people that live there. Tourism is growing. Etc. Due to uneven distribution of radioactive matter there are hot spots, but vast areas that are almost clean. Just over half the OP's time requirement and the worst nuclear disaster in human history already fails his needs.

Deployment

Doing this with a single explosion is unrealistic. Nagasaki and Hiroshima were re-occupied almost immediately. Chernobyl required the destruction of an entire reactor (if all you consider is just the reactor assembly itself and none of the shielding, it's still massive compared to a missle). Besides, the one-explosion solution means very dense "denial" material at the center and almost nothing at the edge of the radius.

I'd suggest a MIRV. One missle, many warheads, more even distribution.

Material

Looking at a list of radioactive isotopes I'd vote for either Caesium-137 (30 year half-life) or Titanium-44 (63 year half-life).

The Real Problem: Cleanup

Ash is completely correct that cleanup ruins your plans. If you remove the source, the remaining radioactivity decreases by the "Rule of 7" (by 90% every 7 hours). In most instances, this means your property is re-inhabitable in a dozen years because there's no actual radioactive source.

Therefore, the question is, "how do I get my radioactive source so embedded into the environment that it's not practical to clean it up?"

There's no easy way to do this. Things like RDD's will cover the surface, but the surface can be processed. Thanks to painfully nasty things we've done in the U.S. over the decades (like mining without caring a whit about the environment), our EPA has superfund cleanup procedures that go so far as to remove the top 18"-24" of dirt, haul it away to a prepared containment area, and replace it with clean dirt. In other words, the cleanup process already exists.

Off the top of my head, I don't know how you'll get around this. If the area was difficult to access or located in a 3rd-world country, then cleanup may simply never happen. But if it's in a 1st-world country, you'll have trouble keeping the area inaccessible for longer than maybe a decade. The size of the area is actually pretty small compared to superfund cleanup areas I'm familiar with, so its size will not be a deterrent. Or you'd need to figure out how to get the radioactive material deep into the ground: 3-4 feet at least. But explosions (especially RDD explosions) don't really bury things. It's a tough nut to crack.

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    $\begingroup$ It's worth noting you've misquoted the 7 / 10 Rule in both of your posts: "Thus 7 hours after explosion the fallout intensity has dropped to 10%". The drop-off follows exponential time (7^x) for each 90% reduction: 7 hours for the first, 49 hours for the 2nd. Whatever time measure you pick (hours/days) just changes the dosage R/hr / R/day you're using. $\endgroup$ – TemporalWolf Oct 11 '17 at 19:57
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    $\begingroup$ " It's true that radioactive objects make things around them radioactive, too.". NO! I am sorry but: bullshit. That answer is handwaving quite a bit and you in turn are misinterpreting. Radioactive Fallout is not contagious. Unless you are talking about neutron radiation (and no fallout produces that), then other elements are not "activated" — as it is called — by the fallout. $\endgroup$ – MichaelK Oct 12 '17 at 6:46
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    $\begingroup$ @MichaelK, and spallation, and photonuclear reactions (which creates free neutrons...). But I'll remove the sentence so you can calm down. $\endgroup$ – JBH Oct 12 '17 at 7:52
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    $\begingroup$ @JBH Also you seem to have completely missed what half-life means. As evidenced by that other post, you seem to think that half-life means "The time it takes for the radiation to go away". No, half-life is the time it takes for radiation to be halved. That is what the "half" in half-life means. So if you drop C-137 on an area, after 60 years (2 half-lives), then 25% of radiation remains. Titanium-44: 50% left after 63 years, 25% after 126 years, and so on. $\endgroup$ – MichaelK Oct 12 '17 at 8:25
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    $\begingroup$ Worth pointing out that Hiroshima and Nagasaki had quick re-population time because at the time, there was very little understanding about Radiation. In the case of Nagasaki, that bomb detonated in the industrial portion of the town which was in a valley that shielded other portions of the town from the blast and fire. Only the northern portion burned. Tvtropes has a great article that discusses the potential damage and variables that could affect the way the Radiation falls. $\endgroup$ – hszmv Oct 13 '17 at 19:49
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If the area is not an actual city but land, and was occupied, land mines are pretty effective as deterrent. While they won't make area completely unapproachable, even with extensive and expensive demining the area will be uninhabitable for general population for decades (and even after decades and extensive cleaning, people will sometimes get themselves killed or maimed by unfound ones).

Even if they are not in huge number, fear of not knowing where there (and news reports about people hitting them) will keep most people out.

And, opposed to most other answers, dangerous area is fixed and danger will not spread out to other locations.

(Unfortunately, lesson from my country history)

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  • $\begingroup$ Yes, conventional explosives work fine for long-term area denial. Nobody has been down this half-mile hole in the ground since 1944: geograph.org.uk/photo/3987494 $\endgroup$ – Martin James Oct 13 '17 at 20:54
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Biological and nerve agents will either decay or risk being transported elsewhere; for biological agents there's the added risk of spreading whatever pathogen is used.

I feel that this only leaves radiological agents and killerbots.

The first have the problem of dispersal; how do you contaminate the area in such a way that it is not worthwhile to have it cleaned (a city is valuable property!)? You'd need a mix of volatile dispersal (i.e. yeah, a dirty bomb) and some way of ingraining the radioactivity in the soil to such an extent that remediation simply isn't possible.

Normal soluble radioactive compounds will not penetrate very deep even when washed by either alkali or acid rain, and anyway, a city is probably mostly coated in stone, bricks and asphalt.

You would need a much more destructive approach - orbital bombardment with salted staballoy penetrators. They would be designed with a coating to ensure they'd start burning very low in the atmosphere, disseminating a part of their radioactive load like a dirty bomb, then embed deeply into the ground. There, they would contaminate the water table and ensure that unstable isotopes surface slowly in the following decades. Removing the penetrators would mean completing the destruction of the area, and simply covering the entry holes would not be an acceptable solution for long-term habitation.

Radiation background would be quite high at the start, from the deposited aerosols, then would go down (faster if the area is decontaminated e.g. with pressure jets), then up again - for example as the 228Ra or 228Th used to salt the penetrators decay into radon gas, that slowly seeps upwards - and finally slowly decay again.

Killerbots require even more advanced technology, and the time period could be that of their nuclear battery decaying until it can no longer power the bot weapons. Before that, anyone entering the no-go area would risk a laser strike to the head from a bot hidden in the shadows.

Not really all that feasible, as killer-killer-bots could be easily employed to overpower the defendants (like Roger Zelazny's Qwibbian-Qwibbian-Kel), and locating a slightly radioactive killerbot is probably not so impossible; and they might be vulnerable to EMPs, virtual-cathode oscillator bombs, near-field resonators or simply having to hide in the sewers for years.

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A dirty bomb will not do it alone

Summary: you can do it, if you...

  • take care to make the fallout set deep in buildings and soil
  • hit an area of little economic interest (i.e. budding ghost town)
  • create fear and myth about the area

Using a dirty bomb (RDD) can easily raise the level of background radiation in an area of that size, to such an amount that if is not economically viable any more. When you say:

Chernobyl [involved] very large amount of radioactive materials

...you are actually wrong. For example: the entire inventory of Iodine 131 in reactor 4 as it blew, is measured in 1-2 kg. Yeah, it accounted for a scary amount of radioactivity — measured in billions of billions of Becquerel — but its mass when compared to mundane objects is tiny.

It should perhaps be noted that 10 km2 is quite a small town... comparable to a square that is 3.3 km / 2 mi on the side. By comparison the Chernobyl Exclusion Zone is 2,600 km2

So if you steal a few medical sources, and grind these to fine aerosols, you can easily contaminate 10 km2, no problem. If you make the contamination spotty and uneven, you can cover the same area with even less.

The problem...

...is that fallout is dust. Fallout is tiny particles that are radioactive. Once you remove the particles, simply by cleaning the dust out, you have removed the radiation with them. This is what decontamination is all about: mechanically picking up the dust and moving it away. It is physically easy to do.

So to reclaim an area that has been decontaminated is not all that hard. But you can make it harder, and not as interesting to do it.

Solutions

You need two things...

  1. make it hard (read: expensive) to remove the fallout
  2. make people not want to return

The more superficial the dust is, the easier it is to remove. You need this dust to go deep into the buildings, and into the soil. If you can make fallout turn up more than a meter down the soil, then it becomes extremely expensive to remove it.

Also, the less incentive there is to clean things up, the less likely it is that people will bother. There are plenty of "hotspots" today where there is enough radiation that you cannot actually live there, if regulations were to be followed, like some places around Ytterby Mine. But no-one bothers to decontaminate that place because it is not of any economic interest.

So if you hit a place where people are already moving out and abandoning it...

enter image description here

Packard Automotive Plant, Detroit, USA, present day

...you are greatly extending the time it will take before people will bother with cleaning the place up.

And if you employ the help of our good friends in the anti-nuclear power movement, experts at radiation scare-mongering and such propaganda... then you are set.

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A Dirty Bomb would do it.

By exploding radioactive matter such that it disperses over that area like a dust would certainly achieve your affect. This doesn't require that much matter. If you crush your radioactive matter into a fine enough dust and detonate it high enough the particles can easily cover enough of an area.

A Fungal Biological agent would work too

Create a fungal agent that can be dispersed over an area and have a lifespan of 50 years. Similar to anthrax.

I do slightly protest your notion of "deadly" whatever you use must be scary enough to deter people from intentional exposure. Radiation and anthrax are area deniers because they CAN kill you, though death isn't necessarily certain.

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    $\begingroup$ How do you prevent the fungus from spreading beyond the area you want to deny? After all, "Life uh.... finds a way". $\endgroup$ – RonJohn Oct 11 '17 at 18:55
  • $\begingroup$ @RonJohn Radiation can spread in much the same way, clinging to things that can carry it out. But you could engineer the mold to be less transmitable, though this is an inherent issue with both routes. $\endgroup$ – anon Oct 11 '17 at 19:04
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    $\begingroup$ @anon Spreading radiation just reduces the density. It doesn't actually reproduce like living things tend to do. $\endgroup$ – reirab Oct 11 '17 at 20:37
  • $\begingroup$ @reirab with genetic engineering you can control the reproduction of your organism, self terminate after x generations. As for spreading, Polonium is famous for its ability to spread from individual to individual. $\endgroup$ – anon Oct 11 '17 at 20:44
  • $\begingroup$ @anon If the area has been cordoned off — like OP wants — then there is no-one there to spread the fallout. $\endgroup$ – MichaelK Oct 12 '17 at 8:33
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It is going to take a lot of radiation and a lot of contamination to make any area irredeemable given modern clean-up techniques, so you'd want one of; A. something that throws out neutron radiation and creates secondary radioisotopes in the surrounding material and/or B. is extremely toxic in it's own right as well as being radioactive.

Coverage may be an issue, you are talking about a weapon that spreads significant quantities of heavy metal for a little over a kilometer in every direction. If that's not an airburst weapon but a bomb planted on the ground the primary explosive will have to be pretty big to create that kind of dispersion.

As to size 100kg of something like Plutonium or Polonium, both fit A and B above, would definitely be more than enough, if spread as a fine powder, to poison that much ground. This is in no way a small amount of radioactive material though, a couple of hundred grams of that stuff is a LOT.

Personally given the issues involved in spreading material explosively I would go with either A. a crop-duster or B. poisoning the city water supply directly. These methods are more reliable and just as likely to cause the area to be inaccessible for an extended period of time.

There are some issues around timing here, the best isotopes for area denial are also have some of the longest half-lives ever observed, however you could use something like Strontium-90, 20 odd year half-life, nasty health effects (it goes to the bone and replaces Calcium for a start), plus the radiation and it yields other toxic and radioactive daughter isotopes. Strontium won't however yield the same sort of neutron contamination so it's only a threat for as long as it can't be cleaned out of the area.

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    $\begingroup$ Strontium also turns flames the most amazing shade of red. Not necessarily a useful fact here, but it’s nice to know. $\endgroup$ – Joe Bloggs Oct 11 '17 at 15:02
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    $\begingroup$ You wouldn't need 100g. 10g is plenty. In Chernobyl, the exclusion zone is the area that contains half a gram of 137Cs per km^2. $\endgroup$ – CJStuart Oct 11 '17 at 15:15
  • $\begingroup$ Plutonium (Pu-239) has a half-life of 24 000 years... meaning it is really weak as a contaminant. Also it is amazingly chemically stable and non-mobile in the open, meaning it is easy to clean up. Polonium-210 is extraordinarily hard to manufacture. $\endgroup$ – MichaelK Oct 12 '17 at 8:35
  • $\begingroup$ @MichaelK What have you been reading? Plutonium is extremely reactive, especially in damp air, also many of the oxides and hydroxides of Plutonium spontaneously ignite, Plutonium is also a toxic heavy metal that happens to be radioactive. Like I pointed out both Plutonium and Polonium are hard to get in volume, Strontium-90 is relatively easy to acquire and will do the job nicely. $\endgroup$ – Ash Oct 15 '17 at 9:48
  • $\begingroup$ What I have been reading? Science. Yes pure plutonium oxidises quickly but plutonium salts are very stable. They are next to insoluble in water and you excrete plutonium dust as quickly as you eat it. The only real danger is breathing the stuff. If it gets into your lungs it may be a while before it gets out again and then its alpha radiation may become a real problem. But its specific activity is very low compared to things like Sr-90, Cs-137 and I-131. $\endgroup$ – MichaelK Oct 15 '17 at 9:54
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Area denial is mostly about risk. It is very unlikely that you can make an area totally unsurvivable for 50 years without extensively contaminating many areas around it through rain water runoff, dust, and ground water contamination. Even Chernobyl is able to be visited today, but there is long term risk to living there, so it remains a mostly abandoned area. Troops wearing protective gear could go through almost any contaminated area immediately, though PPE (personal protective equipment) makes living very difficult and wears out soldiers very quickly.

Chemical agents do not normally persist on the surface for decades (they are covered, broken down by sunlight, or stripped away by wind/rain) but may linger in interior spaces or in ground water. Biological agents are even shorter lived without a living vector to carry them, other than specialized fungal spores. Radiological agents like your dirty bomb have sufficient half-life and more importantly, generate fear which will deter folks from going through an area. Even with documented "safe" levels of radioactivity, an area affected by a radiological agent is going to be avoided, at least within living memory (as opposed to areas that experience regular floods, wild fires, or earthquakes, something about radiation generates a primal fear response).

Consider the many cancer deaths of the crew and cast from the John Wayne film "The Conquorer", which was filmed in an area with a lot of nuclear testing fallout. Even this fairly "dirty" area could still be lived in for weeks and the potential negative effects don't show up for years. So while it may be difficult to deposit enough radiologic material to acutely affect humans, if it is hot enough to be detectable then no one will live go there for fear of getting ill. Even more so if someone puts up "DANGER, RADIATION" signs all over since there may be little visible evidence of contamination.

You do have other options. An area can be mined (the US uses timed minefields for short term area denial, but other countries or older conflicts certainly did not), seeded with slow growing plant life that have toxic spores, active defensive machines that can exist in a dormant state until roused by human activity (depending on your technology level), or rendered geologically uninhabitable (for example, there are periodic releases of subterranean gas pockets that suffocate surface organisms, like CO2 gas).

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