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.