All radioactive elements decay over time. Some faster than others.
Unfortunately for your hope however, the half-life of Uranium-235 is 713 million years, and the half-life of Plutonium-239 is 24,110 years.
So don't expect caches of either one to degrade noticeably over our lifetimes, or even your great-great-great-grandchildren's lifetimes.
What *does* decay quickly is Tritium, which has a half-life of only 12 years. This is why "baby nukes", which are salted with Tritium in a way that allows them to go "boom" even with what would otherwise be sub-critical amounts of U-235 or P-239, must be periodically "refreshed" every few years or else they'll fizzle when you eventually detonate them. So "suitcase nukes" have a limited shelf life.
But a a good old fashioned A-bomb like the types dropped on Hiroshima (15 kilotons) or Nagasaki (20 kilotons) will stay "fresh" for hundreds of years, and is far easier to manufacture (given the materials) than a "suitcase nuke". Baby nukes and really big nukes are more high-tech than the middle-of-the-road ones (10kt-50kt).
More or less correct, but I would dispute the "100's of years" figure. However, these would be inordinately large, cumbersome weapons for primitives in Afghanistan -- or even Iraq or Iran -- to manufacture. They would be much more interested in the smaller, more portable devices, and especially in devices not requiring so much fissible material! That stuff is hard to come by!
So if he's got the Suitcase Nukes, they are likely to be useless or nearly so, and I doubt he's got the fissible material for the bigger brethren.
If I had a guess, I'd say he'd extract plutonium and use it as a poison. Spread it around in America some where. Something like that.
Why are fissionable elements such as U-235 so hard to come by? Is there a finite quanity on earth? Can they be manufactured or refined?