4
$\begingroup$

Inspired by this question, which is asking how to deny land for several hundreds of years after a global nuclear holocaust, I would like to know, given only nuclear weapons, and especially salted bombs of any type, what is the maximum amount of time that an area can be a nuclear wasteland?

Bonus points for ways to contaminate more than just the surface, since the current mitigation of salted bombs includes bulldozing the top few inches of soil and burying the fallout.

I am not looking for land where radiation levels are very near standard background levels; I expect cancer, stillbirths, mutations that lead to malformed limbs, rickets, etc., and increased rates of heart disease, among other chronic diseases where the risk factors increase with prolonged ionizing radiation exposure. (This is WorldBuilding.se, where struggle and drama are expected, not Physics.se where people expect to go home to their air conditioned homes each night.)

The baseline that I'm starting at is, a hypothetical fission-fusion-fission device like a stereotypical doomsday cobalt salted bomb, that will produce a fallout cloud that would contaminate an area downwind with a path 10 miles wide and 50 miles long. This would be a small MIRV thermonuclear warhead or a larger configurable warhead, which are the devices that make up the bulk of superpower nuclear weapons arsenals. It would be detonated at the ground rather than an airburst to increase the amount of fallout produced and if a warhead with a configurable yield is used, it would be the setting that produces the most fallout (which I believe is typically the warhead's lowest configurable yield, but I can't find the source for that to confirm.)

To produce such a fallout cloud, the device would have to have a yield around 500 kilotons.

The device is set to produce slow neutrons rather than the largest fireball possible, would produce significant amounts of direct fission daughter products (i.e., the iodine-131, caesium-137, and strontium-90 isotopes that produce most of the delayed radioactivity in nuclear fallout). The radiation dose of these daughter products are expected to be 150,000 sieverts/hour while the ash is still falling, 350 sieverts/hour after 1 week, 50 sieverts/hour at 1 month, 10 sieverts/hour at 6 months, and 1.25 sieverts/hour at 1 year... By 2 years, it drops off quite quickly, down to about 0.01 sieverts/hour which is where I'm placing my baseline of "if a group of a couple hundred survivors are careful, knows what they're doing, and have decontaminated their land as much as is reasonable, they can start rebuilding society."

This bomb also has a sheaf of whatever material is necessary so that, through neutron activation, we produce a radioactive isotope of whatever material that we think is nasty enough to salt the earth with.

For example, if we use a cobalt-59 sheaf, then on detonation, we release a lot of cobalt-60 into the fallout cloud. We can choose whatever material we want to produce any isotope (and this is where I'm hoping for the most creativity). Again, bonus points if it's particularly resistant to decontamination, but the goal is to maintain over 0.01 sievert/hour for as long as possible.

One last constraint on the calculations: It has to be a viable weapon at the end of the day, which means that it has to be delivered through a standard delivery method, part of the nuclear triad of bombers, bunkers (ICBMs), and submarines. We can't just strap a (literal) ton of cobalt to our warhead and call it a day; it has to be a reasonable sheaf.

This means that the half-life matters not just for how long the land is dangerously contaminated, but also determines just how much radiation is released by our isotope at any given time. A reasonable cobalt sheaf, with a half-life of 5.27 years, will contaminate the area such that the radiation dose is 10 sievert/hour. If we had some material where our unstable isotope had double the half-life, to about 10.5 years, we'd cut the dose in half, to 5 sievert/hour.

So, after 10 half-lifes, land salted by a cobalt bomb becomes habitable again (with proper precautions) after 53 years, and is somewhat easily decontaminated by bulldozers.

How can we do better? (And by better, what's scarier than a cobalt bomb?)

This is not hard science... Feel free to make up your own device, as long as it can be used in either a first-strike or retaliatory strike. My goal is to stay as close to very plausible sci-fi as possible. That said, actual numbers are welcome. The numbers that I have are based on a few hours of reading through pages on various types of salted bombs on Wikipedia, so the dosage is what I currently think is plausible based only on that. I am very open to more plausible numbers if they can be found.

$\endgroup$
  • 1
    $\begingroup$ The hotter the isotope, the shorter the half-life, and vice versa. In the most general sense you won't find one that denies an area for thousands of years. The real danger is that it makes extinct even microbial life, and you basically get Sahara-style sand dunes. Soil blows away, water becomes impossible to find, and no one can live there because nothing can grow. Given the way topsoil generates, that place can probably be unlivable for millions of years... $\endgroup$ – John O Jan 8 at 22:22
  • 1
    $\begingroup$ @John O Sahara is a desert not because it has a lot of sand - it is a desert because it has little precipitation. See also African humid period. $\endgroup$ – Alexander Jan 8 at 23:02
  • 1
    $\begingroup$ @Bitterdreggs., a single warhead wouldn't be the only payload on the large ICBMs. On modern ICBMs, the payload includes countermeasures such as decoys (inflatable mylar balloons, chaff, independent heat sources), as well as a dozen other warheads with independent targets, that each need their own propulsion and guidance systems during the terminal phase. The main point of MAD is that there will always be a disproportionate and unstoppable reaction to nuclear attack, and having far more warheads than the enemy has interceptor missiles is a big part of that. The MAD doctrine needs MIRVs. $\endgroup$ – Ghedipunk Jan 8 at 23:36
  • $\begingroup$ @Bitterdreggs. The most radioactive part of the fallout cloud should be at least 10 miles wide by 50 miles long (depending on local weather patterns, of course), as stated in the question. Looking on Nukemap for the closest that produces such a cloud, the payload looks to be around 500 kilotons. I can't find it now, but I know the yield of some nukes can be configured at launch, with lower settings producing more fallout, so the smallest specific warhead would be the W88, with preference for larger warheads that can go down to 500 kiloton yields. $\endgroup$ – Ghedipunk Jan 9 at 0:13
  • $\begingroup$ @Ghedipunk Quite right, my apologies, my reading of the question was not as thorough as it should have been. $\endgroup$ – Tantalus' touch. Jan 9 at 0:21
2
$\begingroup$

The problem here is that Cobalt-60 hits a bit of a "sweet spot" (for want of a better term) when it comes to making salted bombs. This is because the feedstock isn't too hard to come by... Cobalt-59 is the only naturally occurring isotope of cobalt, and the means of production via nuclear weapon is similarly straightfoward... neutron activation via a nuclear reaction (it isn't totally simple, because you really want slow neutrons, so the fast neutrons released fusion aren't easily used for transmutation, but lets ignore that for now).

There aren't many other natural or synthetic radionuclides that hit the >5 year half-life you might want for a longer-lasting effect than a cobalt bomb. These are the ones I've found (lists definitely not exhaustive)

No good because they're gas phase:

  • Krypton-85, 10.7 years.

No good because they are either created in particle accelerators, or are decay products of artificial elements:

  • Californium-250, 13.1 years.
  • Curium-243, 29.1 years.

No good because they're decay products from other inconvenient to obtain, gas-phase or long-lived radionuclides:

  • Niobium-93m, 16.1 years, formed by the decay of Zirconium-93, which has a very, very long half life.
  • Lead-210, 22.3 years, no good because it is formed from a complicated decay chain.

No good because their half-lives are too long:

  • Titanium-44, 63 years, possibly from neutron capture in Ti-43.
  • Nickel-63, 100 years, possibly from neutron capture in Ni-62.

Possibly useful:

  • Tritium, 12.3 years.
  • Cadmium-113m, 14.1 years, U235 fission product.
  • Strontium-90, 28.8 years. U235 fission product.
  • Caesium-137, 30.2 years. U235 fission product.

It is possible to create tritium by wrapping your nuclear weapon in a lithium blanket. If you detonated your weapon underwater, you might be able to create tritiated water rather than just having gaseous tritium which would mostly blow away. It would be impractical to expect a body of water poisoned in this way to remain so for decades... rain and river flow would dilute it and flush the contaminated water away, and endorheic lakes that don't get flushed tend to be toxic lifeless places anyway, so humans aren't likely to drink or fish from them.

Fission products such as Cadmium-113m, Strontium-90 and Caesium-137 are potentially interesting, but as they aren't created from neutron capture you can't just slap a cheap fertile jacket on your nuke and call it a day like you can with cobalt. You only get them from fission, so in order to produce more of the stuff you'd have to make a really big U235 fission bomb, with none of the size or yield benefits that plutonium or Ulam-Teller designs would have. Also, many of these fission products are beta emitters making them much easier to shield against than the gamma rays emitted by cobalt-60. (edit as prompted by Alexander, Cs-137 decays into Ba-137m promptly releases gamma rays)

That just leaves you with cobalt-60, which is straightfoward to produce, long lived and generates a reasonable amount of highly penetrating gamma radiation. If you want a salted bomb that renders an area uninhabitable for longer, you're basically just going to have to create more cobalt by using a bigger bomb.

There's a chance that I've missed some isotope that form a material with a decade-scale half-life following absorption of a single fast neutron for which it has a large cross section. good luck finding such a material, but if you do, it might solve your problems. You'll _still_have the issue of needing a really big bomb to produce enough fallout to produce acute radiation sickess (because more stable radionuclides release less radiation in a given time), but the effects will at least last longer than cobalt-60.

If you want a persistent threat that lasts many decades, you might have to resort to some kind of biological weapon (anthrax, maybe?) that remains stable and infectious over the desired timescale, but that's a whole different set of design problems altogether.

We can't just strap a (literal) ton of cobalt to our warhead and call it a day; it has to be a reasonable sheaf.

Sure you can. Modern bombs and missiles are relatively lightweight and compact, but they haven't always been that way and don't always need to be. Things like the Titan II missile and the W53 warhead it carried represent the last of the "single giant warhead" designs deployed by the US, but there's nothing stopping you bringing them back. Similarly, though air-dropped nuclear bombs are out of favour you can make them (almost) arbitrarily large. In both cases, the weapons are more vulnerable to interception than many smaller, lighter warheads but so long as you can make them fail deadly then they can still produce and distribute the cobalt-60, albeit not necessarily in quite such a precise way.

| improve this answer | |
$\endgroup$
  • $\begingroup$ Caesium-137 is gamma emitter. $\endgroup$ – Alexander Jan 9 at 18:15
  • $\begingroup$ @Alexander to be pedantic, barium-137m is a gamma emitter, but as it is the most common decay product of Cs-137 and has a teeny tiny half life I'll take your point. $\endgroup$ – Starfish Prime Jan 9 at 18:44

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.